Dusty OB Stars in SMC. I

Dusty OB Stars in the Small Magellanic Cloud - I: Optical Spectroscopy Reveals Predominantly Main-Sequence OB Stars

Abstract

We present the results of optical spectroscopic follow-up of 125 candidate main sequence OB stars in the Small Magellanic Cloud (SMC) that were originally identified in the SMC infrared imaging survey as showing an excess of emission at 24 µm indicative of warm dust, such as that associated with a transitional or debris disks. We use these long-slit spectra to investigate the origin of the 24 m emission and the nature of these stars. A possible explanation for the observed 24 µm excess, that these are emission line stars with dusty excretion disks, is disproven for the majority of our sources. We find that 88 of these objects are normal stars without line emission, with spectral types mostly ranging from late-O to early-B; luminosity classes from the literature for a sub-set of our sample indicate that most are main-sequence stars. We further identify 17 emission-line stars, 7 possible emission-line stars, and 5 other objects with forbidden-line emission in our sample. We discover a new O6 Iaf star; it exhibits strong He ii 4686 Å emission but relatively weak N iii 4640 Å emission that we attribute to the lower nitrogen abundance in the SMC. Two other objects are identified with planetary nebulae, one with a young stellar object, and two with X-ray binaries. To shed additional light on the nature of the observed 24 µm excess we use optical and infrared photometry to estimate the dust properties of the objects with normal O and B star spectra and compare these properties to those of a sample of hot spots in the Galactic interstellar medium (ISM). We find that the dust properties of the dusty OB star sample resemble the properties of the Galactic sample of hot spots. Some may be runaway systems with bow-shocks resulting from a large velocity difference between star and ISM. We further investigate the nature of these dusty OB stars in a companion paper presenting mid-infrared spectroscopy and additional imaging.

Subject headings:
dust — infrared: ISM — infrared: stars — Magellanic Clouds — planetary systems: formation — stars: early-type

1. Introduction

The Spitzer Survey of the Small Magellanic Cloud (SMC) carried out deep imaging of the main body of the SMC in all seven IRAC and MIPS bands (Bolatto et al., 2007). This survey identified about 400,000 compact sources in the infrared. Compilation of spectral energy distributions and cross-correlation against optical catalogs uncovered 193 point sources with 24-µm emission, but with optical and near-infrared colors and magnitudes all consistent with O9–B3 type main-sequence stars. Photospheres of such stars are well below the detection limit of this survey, so an additional source of infrared (IR) emission must be present, presumably warm dust with a temperature close to 150 K. Figure 1 shows the color–magnitude diagram for all objects detected at 24 µm in the SMC survey, and the box represents the and cuts on the dusty OB star sample. These sources are a puzzle, representing a few percent of all the stars in that particular color and magnitude range in the SMC. Dust in close proximity to an early B/late O star could be a remnant from the accretion process, and provide important information about the final stages of accretion and the clearing up of the original disk in massive stars. We discuss below several competing possibilities for the nature of these sources, previously presented by Bolatto et al. (2007). The discovery of these objects has been confirmed independently by the Spitzer SAGE-SMC survey (Bonanos et al., 2010, who also discuss them) and by Ita et al. (2010) using AKARI data.

If the dust is associated with the star, it must exist either in an optically thin shell surrounding the star or in a thin or transition circumstellar disk, since no appreciable reddening of the central star is detected beyond the line-of-sight extinction by our own Galaxy to the SMC. These objects have to 10, indicating that the dust is intercepting only a small fraction of the star’s light and re-radiating it in the IR. This also supports the thin shell or disk scenario. Since most of the dusty OB stars show little to no excess emission at wavelengths of 8 µm or shorter, a large central gap would be needed to explain the absence of significant emission from hotter dust located close to the star.

A clump of interstellar dust, heated by a nearby star, however, could also reproduce these characteristics. This is observed, for example, in the Pleiades (Arny, 1977; White & Bally, 1993; Herbig & Simon, 2001; Sloan et al., 2004). Vega-like stars, Herbig Ae/Be stars, and classical Be stars all have disks that are known to contain dust to varying degrees, and so we consider them, as well as cirrus hot spots in the ISM, as the possible causes for the excess emission seen around these O and B stars in the SMC survey.

Vega-like stars host debris disks, which are the remnants of planet formation. They are optically thin dust disks, mostly stripped of their primordial gas and continually replenished by collisions between planetesimals. The prototypical examples are Vega, Fomalhaut, and  Pictoris, all of which have disks that extend 100 to 1000 au from the star with L/L to 10 (Backman & Paresce, 1993, and references therein). The central region, a few au in extent, of these disks has been cleared of dust, just as in our solar system, and the mass of emitting dust is 10 to 10 M at temperatures between 50 and 125 K. The wider sample of debris disks shows an upper limit on F/F of 10 (Chen et al., 2006), temperatures up to 300 K, and dust masses as large as an Earth mass (Krivov, 2010). Note however that the known debris-disk hosts are observed, overwhelmingly, around much less massive stars with later spectral types. A gas-free disk around a very luminous star would be quickly cleared by radiation pressure, unless the dust grains are very large.

Classical Be stars are rapidly rotating main-sequence stars with an excretion disk of gaseous material (Porter & Rivinius, 2003). The disk produces IR excess in the form of free–free emission with approximately in the near- and mid-IR (Waters & Marlborough, 1994); this by itself would not explain the sudden rise observed in the spectral energy distribution (SED) of the dusty O and B stars at 24 µm. Miroshnichenko & Bjorkman (2000), however, found evidence for circumstellar, thermal dust emission in some Be stars in the far-IR, which they attribute to remnant dust from prior evolutionary processes.

Herbig Ae/Be stars, on the other hand, are pre-main sequence stars that are thought to be the more massive analogs to T Tauri stars. They are surrounded by gaseous accretion disks containing dust with temperatures as high as 1500 K, creating the IR excess (see, e.g. Hillenbrand et al., 1992; Waters & Waelkens, 1998). Herbig Ae/Be stars typically show significant excess at  µm from this hot dust. The onset of the emission at longer wavelengths indicates that the central portion of the disk nearest the star has been cleared and is free of dust. Disks with such a central hole are transition disks (e.g., Cieza et al., 2010). It is possible that more massive, early-B and O stars also go through this stage, although modeling suggests that this may be an exceedingly short phase as disks are rapidly cleared (Alexander et al., 2006). Observational evidence for circumstellar disks around massive B and O stars remains hard to come by (Cesaroni et al., 2007).

The Galactic cirrus, comprised of wispy or filamentary patches of dust throughout the disk of the Galaxy, has a typical temperature of  K (Planck Collaboration et al., 2011), but heating of the cirrus by embedded stars produces small regions of warmer dust, or hot spots, near the star, reaching temperatures of  K (van Buren & McCray, 1988). It is difficult to distinguish debris disks from hot spots in the cirrus without resolved images of either the dust emission in the IR, or the scattered light in the optical. The observed SEDs can be interpreted with either scenario, as illustrated by Martínez-Galarza et al. (2009) and Su et al. (2006); Martínez-Galarza et al. model some of the same objects in Su et al. as ISM heated by stars passing through a cloud, while Su et al. model them as debris disks. Cirrus hot spots are also sometimes the cause of the far-IR emission detected around Be stars (Miroshnichenko & Bjorkman, 2000).

Understanding the source of the excess emission in these objects is valuable because the scenarios above represent different phases in the evolution of massive stars. Detecting circumstellar material around SMC stars is interesting, because it offers the opportunity to study accretion or excretion processes in a low metallicity environment, and particularly around massive stars with an unextincted line-of-sight. Detecting debris disks in the SMC would be especially exciting, as they would offer an unprecedented opportunity to study planet formation in another galaxy. Such early-type stars are under-represented in Galactic studies.

While the photometry suggested the stellar counterparts of the IR emission to be early-type main-sequence stars, this has yet to be confirmed through spectroscopic observation. The vast majority of the 193 objects in our sample do not have spectral types reported in the literature, although a small subsample has fiber optics spectroscopy (Evans et al., 2004).

Bonanos et al. (2010) compiled a catalog of IR counterparts to massive stars spectroscopically confirmed in the literature. They identify 44 objects that are similar (OB stars with 24 m emission) but not necessarily identical to our sample. They constitute % of their sample of late-O and early-B stellar types, a fraction slightly lower but otherwise similar to that reported by Bolatto et al. (2007). The 18 spectra Bonanos et al. have in hand show nebular line emission, unlike — as we shall see — the majority of the sources in this sample. Note that the spectra used by Bonanos et al. are obtained using fibers (Evans et al., 2004, 2006), which makes sky subtraction in complex regions problematic. Long-slit spectroscopy is preferred in regions of diffuse sky emission such as found around massive stars. Clearly, further spectroscopic study is warranted. Bonanos et al. (2010) conclude that most of their sources are not found within very young regions, and suggest their IR emission is likely dust associated with cirrus or a nearby molecular cloud, rather than disks. Bolatto et al. (2007), however, noted that the majority of the sources identified in their sample are in the vicinity of active star-forming regions although not in their cores, where confusion with diffuse 24 m emission would make their identification extremely difficult.

We obtained optical long-slit spectroscopy for 125 of the 193 objects from the SMC survey in order to determine how many of them are emission-line (e.g., classical Be and Herbig Ae/Be) stars and to obtain spectral types to quantify the stellar radiation field illuminating the dust. In section 2 of this paper, we discuss the photometric and spectroscopic observations, while in section 3 we classify the stars, and in section 4 we estimate the properties of the dust around the non-emission line objects. In section 5, we discuss how well the various scenarios explain our objects and include a comparison of the dust properties to those estimated from the sample of Galactic cirrus hot spots of Gaustad & van Buren (1993). In a companion paper (Adams et al., 2013, hereafter Paper II), we discuss further infrared spectroscopic and photometric information, and comparisons to large Galactic samples based on Wide-field Infrared Survey Explorer (WISE) data.

2. Observations

2.1. Spectroscopy

The optical spectra were obtained at the 3.5-m New Technology Telescope at the European Southern Observatory at La Silla, Chile in 2007 September (programme 079.C-0485; PI J.Th.van Loon), using the EMMI instrument (Dekker et al., 1986) in RILD mode in the red arm with grism #2 and a slit. This set-up provided wavelength coverage from to 870 nm with . Biases, dome flats, and HeAr arc lamp frames were acquired for calibration. The science integrations were split into three exposures, offset along the slit, in order to correct for cosmic rays and fringing effects at wavelengths beyond 750 nm). A filter to block second-order contamination longward of 800 nm was originally planned, but it was dropped because of unwanted reduction of signal in the blue region of the spectrum.

The spectra were wavelength-calibrated with HeAr lamp exposures, using the IRAF1 identify, reidentify, and transform tasks in the noao spectroscopic reduction package. The 1D spectra were then extracted using the apsum task. The extraction was at times complicated by the background emission lines from H ii regions within which our targets were located. To remove this background, the aperture and background regions were set interactively, looking first at the profile of the star along the slit, away from any emission lines. If any [O iii] 5007 Å emission remained in the extracted spectrum, the extraction was re-done looking instead at the profile along the slit at the [O iii] line to set the aperture and background regions. The extracted spectra were summed for each object to increase the number of counts and to average out fringing effects and an electronic interference pattern that could not be removed with the bias or dark frames due to frame-to-frame variations. Lastly, the spectra were normalized to the continuum in the region between 400 and 680 nm by fitting and dividing by an order 20 polynomial.

2.2. Photometry

The photometry used to create the SEDs for each source is a combination of B, V, and I data from the OGLE II survey (Udalski et al., 1998) and the Magellanic Clouds Photometric Survey (Zaritsky et al., 2002), J, H, and K from the 2MASS survey (Skrutskie et al., 2006), and Spitzer Space Telescope (Werner et al., 2004) data from both the original SMC (Bolatto et al., 2007) survey and the combined SMC/SAGE-SMC catalogs (Gordon et al., 2011), making use of the IRAC (Fazio et al., 2004) and MIPS (Rieke et al., 2004) instruments. In the SAGE-SMC processing, combined SMC/SAGE-SMC images were created by stacking data from both surveys before performing the photometry. For the MIPS combined catalog, on the other hand, photometry was performed on the three different observing epochs (one for SMC, and two for SAGE-SMC) separately, after uniformly reprocessing the data. We searched the combined catalogs for coordinate matches to the dusty OB stars within 2″. The IRAC and 2MASS data points in the original SMC photometry were replaced with the points in the combined catalog if they existed. If more than one match was found, the closest match was used. The 24 µm points in the original SMC catalog were replaced with the weighted average of the three epochs in the combined catalog for the the closest positional match. Points were replaced as long as there was a match within 2″ in at least one of the epochs. The 70 µm data points, where they exist, must be treated with caution. The diffuse emission in the SMC is considerably brighter at 70 µm than at 24 µm, and the difference in beam size between the two channels is substantial. The beam size at 24 µm is 6″, corresponding to 2 pc at the SMC distance of 61.1 kpc (Keller & Wood, 2006), while the beam size at 70 µm is 18″, corresponding to 5 pc. Thus there is a good chance of source confusion between the two channels, and we have not searched the combined SMC/SAGE-SMC catalog for 70 µm matches. The photometry data are given in Tables 1 and 2.

3. Spectral Classification

We use our long-slit spectroscopy to classify the stars. For the classification, we employ the scheme described in Evans et al. (2004), which compares the strengths of a number of lines in the 4000–5000 Å range. We use the equivalent widths of the lines, guided by visual inspection of the spectra, to compare the line strengths, but our low resolution leaves some of the important lines blended with H (Si iv 4088 Å and Si iv 4116 Å) or with each other (e.g., He i 4471 Å and Mg ii 4481 Å). Note also that the original classification scheme was designed for supergiants, and so the Si lines for main-sequence stars may be too weak to detect in some cases. The classification scheme is summarized in Table 3. A sample spectrum of object B 107 is shown in Figure 2. Since B 107 shows no He ii absorption features, it is not an O star. It shows no evidence of Si iii 4553 Å, so it is not a B2 or B3, but the Mg ii 4481 Å appears too weak for a spectral type later than B3. Thus we conclude that most likely the Si iv lines are blended with H and that the star is a B1.

Of the 125 objects for which we obtained spectra, 87 appear to be normal main-sequence stars and one (B 167) is discussed below as a likely supergiant. Of those 87, 53 are classified via the above scheme and assigned a subtype. The remaining 34 stars were difficult to place precisely within the scheme, but they have been marked as either O7–O9, if they show He ii absorption, or B0–B2 if they lack the He ii lines. B 167 is unusual, however, in that it shows no significant He i absorption but has very strong Balmer absorption. It appears to be an A star; given its luminosity it is probably a supergiant. The spectra and SEDs of these 88 objects are shown in Figures 3 through 8, while the assigned spectral type is given in Table 4 as well as in the figures.

The remaining 37 of the 125 objects, which are not considered in the dust analysis in the next section, fall into one of four groups: 1) stars that show Balmer emission and sometimes He i/ ii emission (16 stars plus B 094), 2) stars with unusual H profiles (7 stars, including B 035 which we especifically discuss in the following section), 3) objects that show strong forbidden line emission (6 objects), and 4) spectra that are difficult to extract (8 stars).

The first group of 17 stars shows the Balmer and He i/ii emission lines characteristic of Be stars (Figure 9), while the second group of 7 stars shows only weak H absorption (Figure 10). B 094 stands out, as it is the only object among the 17 stars that shows [N ii]  6548,6583 emission. We include it here, rather than with the strong forbidden-line emission objects, because of the lack of [O iii] emission and the weakness of the Balmer emission compared to the other objects in that group. The unusually weak Balmer absorption in the second group of stars may indicate that these objects are Be stars as well. Silaj et al. (2010) modeled the H profiles of Be stars and showed that, for many combinations of base-disk density, density distribution, and viewing angle, the H emission from the circumstellar matter does not cancel out the absorption line of the star completely. This effect could explain the unusual H profiles. The definite Be stars are noted with a spectral type of OBe in Table 4, while stars with possible weak emission are noted as “OBe?” in the table.

The third group is comprised of 6 objects (5 shown in Figure 11, plus the already discussed B 094), and it includes B 001, B 028, and B 079, all of which show strong Balmer and [O iii]  4959,5007 emission, along with weaker [N ii]  6548,6583 and He ii  5876,4921, and 4471 Å emission. These spectra are typical of compact H ii regions. B 141 is dominated by Balmer emission, with weak [N ii] and very weak He i emission. B 141 also shows moderate [O iii] emission, but unlike B 001, B 028, and B 079, the forbidden-line emission is weak relative to H. B 036 is now known to be a young stellar object (YSO) (Oliveira et al., 2013); it shows strong Balmer emission, with weak [N ii] emission, very weak [O iii] emission, and little to no He i emission above the continuum level. The SEDs of these five sources are much flatter than those of the non-emission-line objects, possibly due to the predominance of the free–free emission over any stellar photospheric emission. These objects are marked in Table 4 as Em in the spectral type column.

The fourth group, shown in Figure 12, includes B 005, B 011, B 112, and B 175, all of which have low signal-to-noise spectra and, except for B 011, also have a star too nearby to set the background well. Thus it is unclear whether the H absorption appears weak because of noise, background contamination, or weak Be star emission. The other 4 objects in this group are B 006, B 093, B 108, and B 191, whose complicated background in H makes it difficult to disentangle emission from the surrounding nebula from possible Be star emission. These objects are given no spectral type in Table 4.

3.1. A newly discovered O6 Iaf star

In the “OBe?” group, B 035 is the only star that shows He ii 4686 Å emission. The spectrum is displayed in more detail in Figure 13. The appearance of the He ii 4686 Å emission indicates a very high luminosity, class Ia (Sota et al., 2011). This is commonly accompanied by emission of the N iii 4640 Å complex, but this is weak in the spectrum of B 035. We suggest that this is a metallicity effect due to the low nitrogen abundance in the SMC (van Loon et al., 2010). It does appear to be present, hence we tentatively affix the “f” suffix to the spectral type. With regard to the temperature class, the He i 4471 Å/He ii 4200 Å ratio suggests a spectral type of about O6 (Sota et al., 2011). The He i+ii 4026 Å line is weaker than that of the He ii 4200 Å line, which would suggest a spectral type O4, but it is also weaker than that of the He i 4471 Å line, which would suggest a spectral type O7 (which does not really happen, Sota et al. 2011) – we attribute this to uncertainties and/or possible binarity and arrive at a final spectral classification of O6 Iaf.

3.2. Comparison with literature

Accurate spectral classification based on high quality data was carried out by Evans et al. (2006), Martayan et al. (2007), Evans & Howarth (2008) and Hunter et al. (2008) for 23 of our 125 stars (see Table 4). Our own spectral classifications are in agreement with those determinations, with only a few deviations by one or two subclasses. The majority () of those classifications are of luminosity class IV–V, with only 6 of luminosity class II and none more luminous. This suggests that most of the stars in our sample are unevolved, main-sequence stars.

The sample, however, is a mixed bag of curious and seemingly very common types of objects. The “Em” objects include the planetary nebulae B 001 = LHA 115-N 9 and B 079 = LHA 115-N 47, the YSO B 036 (Oliveira et al., 2013), and the compact H ii region B 028 = LHA 115-N 26 (Testor, 2001). The sample further includes the X-ray binaries B 064 = SXP 214 (Coe et al., 2011) and B 085 = CXOU J005245.0722844 (Antoniou et al., 2009) and no less than 10 or 11 eclipsing binaries. We attribute the large number of eclipsing binaries to the high overall fraction of close binary systems among massive stars (Sana et al., 2012; Sana et al., 2013).

4. Constraining the Dust Properties

In order to assess the viability of the various hypotheses for the nature of the dusty OB stars and the origin of the dust, we now use the available photometry to estimate the gross properties of the dust. The primary quantities we can constrain are the dust temperature, dust mass, the fraction of the star’s light re-radiated by the dust (F/F), and the distance of the dust from the star. We also calculate the grain size below which radiation pressure will quickly remove the dust from the system in the absence of gas drag, as well as the mass of grains of that blow-out size that would be needed to account for the observed F/F.

To determine these properties, we first find the amount of expected flux from the stellar photosphere by fitting to the SED a power law of the form

(1)

with in Hz and in µm, and with the slope derived from the Kurucz stellar atmosphere model (Castelli & Kurucz, 2003) of the spectral type determined. The intercept was determined using the -band flux, or if was unavailable, to scale the models. The and fluxes were corrected for the line-of-sight extinction due to the Milky Way using and from Draine & Lee (1984) and Rieke & Lebofsky (1985), with  mag (Schlegel et al., 1998). We selected models for , closest among the Kurucz library to the typical abundance of young stars and gas in the SMC of (Pagel et al., 1978), and the suggested surface temperatures and surface gravities for specific stellar types from Martins et al. (2005) and Schmidt-Kaler (Aller et al., 1982). All the fits used models with for the surface gravity, and the surface temperature and slope used for each spectral type is given in Table 3. The temperature scale is similar – well within 10% – to that of Galactic main-sequence stars determined by Nieva (2013) and that of O-type stars in the SMC determined by Massey et al. (2005); the latter found that early-O type stars in the SMC are hotter than Galactic stars of the same spectral type, but that this difference vanishes around B0. A slope was used if the star was given a spectral type of “O7–O9”, while a slope was used for all other stars without a specific spectral type. From these fits, the excesses at 4.5, 8, and 24 µm were calculated (Table 5), along with the ratio of the excess to the expected photosphere.

4.1. Dust Temperature

In all but six cases, marked with a footnote in Table 5, no emission was detected at 70 µm, so we cannot fit a model to the 8, 24, and 70-µm excesses directly. Instead we estimate the color temperature of the dust using the 8- and 24-µm excesses, and place limits on the color temperature using the 24 µm flux and the 70 µm flux limit of 40,000 Jy, minus the expected photospheric flux. The color temperatures are estimated assuming a modified blackbody with emissivity , using the equation

(2)

where is the shorter wavelength and the longer. The color temperatures calculated for the ratios of the 8- to 24-µm excesses, for stars with a specific spectral type, are given in Table 6. Because the 8-µm band includes a major polycyclic aromatic hydrocarbon (PAH) feature, these color temperatures must be treated with caution. Note, however, that PAH emission in the SMC is considerably weaker than in the Milky Way making this less of a problem than for local samples (Sandstrom et al., 2010). The color temperatures calculated with the 24- to 70-µm ratios are very uncertain, since in most cases we have only an upper limit on the 70-µm flux, and thus are not included in the table. These values are generally in the range of 40–60 K, but they should be considered as lower limits as a 70-µm flux below the upper limit would lead to a higher derived temperature. Mid-IR spectroscopy and deeper 70-m photometry to determine the dust temperature directly are presented in Paper II.

4.2. Dust Mass

We are working in the optically thin limit at long wavelengths, so the emission of the dust is given by

(3)

where is the excess emission at 24 µm, is the solid angle subtended by the source, is the opacity due to the dust, and is the emission of a blackbody at the temperature of the dust. The opacity of the dust is

(4)

where is the mass of the dust, is the area covered by the dust, and is the dust mass absorption coefficient. Thus the dust mass is given by

(5)

Here we assume

(6)

where has units of cm g and is given in µm. This value is due to Hildebrand (1983) and within 20% of Draine & Lee (1984). We adopt it to be easily comparable to surveys of the Milky Way (for example, the Gould Belt Survey; André et al., 2010). The dust mass then is

(7)

where is given in mJy, in pc, in µm, and in erg cm s Hz sr. The dust masses calculated using temperatures the derived from 8/24-µm excess ratios are listed in Table 6.

4.3. Dust Luminosity

To calculate the fractional luminosity F/F, we first determined the infrared flux by integrating over the source function for graybody emission from 0.7 m to 100 m, rewriting Eq. 3 above as

(8)

where is in M, is the distance to the source in pc, in µm, and in erg cm s Hz. We then divided by the bolometric flux, which is the luminosity for the spectral type (Table 3), scaled to the distance of the SMC.

4.4. Distance of the Dust from the Star

The observed dust temperature and stellar luminosity constrain the location of the dust in the circumstellar environment. The distance of the dust from the star at a given temperature also depends on the size of the dust grains and the wavelength range of the light being absorbed. We can set limits on the dust distance by first assuming the relation for large, blackbody grains,

(9)

where is the stellar radius in units of R and is the dust distance in au. The stellar radii for the stars are given in Table 3 and are taken from a second-order polynomial fit in to the values given in Appendix E of Ostlie & Carroll (1996). The distances calculated with this model are given in Table 6 as the blackbody (BB) distance. Grains that instead absorb light as blackbodies, but re-radiate as a modified blackbody proportional to , can be modeled using the equation

(10)

where is in L, is in au, and is in µm. This equation is taken from Equation 2 of Backman & Paresce (1993), assuming and . We have assumed µm. Note that this last parameter is poorly constrained and it introduces significant uncertainty in . The distances calculated with this model are listed in Table 6. Smith & Wyatt (2010) note that in resolved debris disks, the actual distances of the particles are at most 3 times the distance predicted by the blackbody grain model, whereas in resolved cirrus hot-spots, the dust distance is an order of magnitude – or more – larger than that expected from the blackbody model, due to stochastic heating of small grains (Kalas et al., 2002).

We also estimate the blow-out size for grains due to radiation pressure in the absence of gas drag. Grains with will be blown out of the system by radiation pressure. We use the equation for from Burns et al. (1979),

(11)

where is the size of the grain, is the density of the grain, and is the average radiation pressure efficiency over the stellar spectrum for a given grain size. Values for for the spectral types are given in Table 3 and were derived using

(12)

where K from Schmidt-Kaler (Allen, 1973). Values for were derived by fitting a second-order polynomial in in logarithmic space to the values in Appendix E of Ostlie & Carroll (1996) and are also given in Table 3. We assume here , and we use g cm from the Draine & Lee (1984) model. Setting gives the minimum grain size in a circular orbit that can survive the radiation pressure of the host star. Grains released from parent bodies in circular orbits, such as dust created in a debris disk, however, can be ejected from the system for as low as 0.5.

We then estimate the mass of dust needed to produce the observed F/F, based on the estimated blow-out size of the grains. This mass (in units of g) is given by

(13)

where is the minimum grain size to survive radiation pressure blow-out, in cm, and is the blackbody distance in cm.

4.5. Uncertainties

We estimate the uncertainty on these parameters for the 8/24 pair of excesses using the Monte Carlo method. For each of 1000 trials per object, the color temperature is calculated, and that temperature is used in the calculation of the dust mass, F/F, and dust distance. The uncertainties on the color temperature, dust mass, F/F, and dust distance are then set according to the upper and lower values that bracket the central 67% of the results for each object. Uncertainties on the excess at 8 and 24 µm are determined via error propagation, assuming an uncertainty in slope of 0.05 for B stars and 0.01 for O stars. These ranges allow for uncertainty in the spectral type. Because the O stars all have a similar slope, the range is much smaller. We apply these uncertainties to the excesses assuming a normal distribution about the value of the excess before calculating the color temperature. The dust mass calculation also includes an uncertainty in the distance to the stars, which we model as a normal distribution centered at kpc, assuming that the objects are most likely to be located near the center of the SMC. The F/F calculation further requires the uncertainty on the bolometric luminosity, assumed to be a uniform distribution with a range set by the value for spectral sub-type. For the distance calculation, we assume uncertainties for the stellar radius and stellar temperature uniformly distributed between the values for spectral sub-type. Table 6 gives the dust parameters for those objects in Table 5 that have a measurable excess at 8 m, and to which we give a secure main sequence spectral classification.

5. Discussion

5.1. Classical Be Stars

Because the majority of our sample shows no optical emission lines, Be star activity is likely not the main cause of the 24-µm excess in these O and B stars. It is possible, however, that a few of the objects without emission lines could still be Be stars, as Be stars are known to show variability in their Balmer emission. In a study of 45 Milky Way Be stars, McSwain et al. (2009) found 23 objects with variable H emission, including six that changed from strong emission to normal B-star absorption at H or vice versa over the course of just two years. Of the original sample of 193 O and B stars with 24-µm excess identified in the SMC survey, 12 were previously catalogued as Be stars by Meyssonnier & Azzopardi (1993), and for those we did not acquire new spectra. We identify here an additional 16 stars with confirmed H emission and 7 stars with suspected weak H emission, for a total of 35 Be stars (36 if we include B 097 in the count). The remaining 88 stars in the spectroscopic sample do not display evidence of OBe activity.

To investigate the possibility of variability between the two states, we look at the 4.5-µm excesses, seen in Table 5, because Be stars, both classical and Herbig Ae/Be, are expected to show near-IR emission to some degree (see, e.g., Touhami et al., 2010). Indeed, we do see an excess ratio above the photospheric emission at 4.5 µm for 11 of the 16 OBe stars in our spectroscopic sample, as well as in 2 of the 7 “OBe?” stars. This could indicate that the other 10 OBe and “OBe?” stars were in a normal B-star phase during the IRAC observations, acquired in 2005 May. These excesses, however, are weak, at less than 4 times the expected photospheric flux, suggesting that we may just be unable to detect the excesses in our remaining spectroscopically confirmed OBe stars because of the uncertainties () in their photospheric flux in our data. Adding further uncertainty is our use of band data to scale the photosphere, because Be stars may exhibit excesses even at wavelengths as short as 1.2 µm. This can be seen in the slopes of the dashed lines in the SEDs of B 038, B 081, B 085, B 119, B 140, and B 190, for example, in Figures 9 and 10. The dashed lines are fit to and show shallower slopes than expected for the photospheric emission.

We also find 12 stars (B 023, B 033, B 047, B 115, B 128, B 135, B 136, B 142, B 154, B 156, B 160, and B 183) in Table 5 that do not show optical emission lines (see Table 4) but do have an excess ratio at 4.5 µm significant at the level. The 4.5-µm excesses are also weak, with only B 136 showing an excess/photospheric ratio greater than 1. This excess could indicate that these stars were in a Be state at the time of the IRAC photometry but settled into a normal B-star state by the time the spectra were acquired. If we assume that all of the 4.5-µm excesses are due to Be-star behavior, the fraction of transient Be-stars (counting as transient those OBe and “OBe?” optically classified stars without 4.5 µm excess, plus these 12 stars with 4.5 µm excess but no hint of H emission in their spectra) is 22/47, or about 47% of the total Be-star population in the sample. This is on par with the fraction seen in the study of Galactic Be-stars by McSwain et al. (2009). Therefore it seems unlikely that the remainder of our stars that show no Be activity in the photometry and spectra are primarily dormant Be-stars.

5.2. Herbig Ae/Be Stars

Because we do not know the ages of the Be stars in our sample, it is difficult to determine to which of the Be-star categories they belong. None of the Be stars we identified spectroscopically have the large excesses at µm that are typical of the hot dust in Herbig Ae/Be objects. Hillenbrand et al. (1992), however, identify in the Galaxy a set of objects with near-IR profiles that resemble classical Be stars, but are apparently young stars, located near reflection nebulosity. This location is more typical of Herbig Ae/Be stars. Hillenbrand et al. (1993) suggest that these objects, which they label Group III, are actually Herbig Ae/Be stars that have an optically thin circumstellar disk, rather than the optically thick disk more typical of the Herbig Ae/Be class.

The weak 4.5-µm excesses of the Be stars detected in our spectroscopic study resemble the near-IR profiles of classical Be stars as well as these Group III objects. The stars in our sample are located within regions of recent star formation, suggesting an age of  Myr. Unfortunately, 10 Myr is the approximate age at which classical Be stars begin to appear in clusters (Fabregat & Torrejón, 2000), and so we cannot be certain whether these objects are young stars or older, established main-sequence stars without more accurate age determinations. The bulk of the dusty B-star sample, with neither optical emission lines nor a near-IR excess, are unlikely to be Herbig Ae/Be stars.

5.3. Cirrus Hot-Spots Comparison

Backman & Paresce (1993) note that to be certain that debris disks are the source of the excess infrared emission, spatially resolved observations of the disk are needed, because hot spots in the ISM can mimic the color temperatures and fractional luminosities of debris disks. The morphology of the dust is the best clue as to whether the heated dust is physically associated with the star in a disk, or if it is simply a concentration of ISM material close to the star. Unfortunately, at the distance of the SMC, debris disks cannot be resolved with any available facilities, and ISM hot spots are below the resolution of the Spitzer data, especially with a 6″ resolution at 24 µm.

Since we lack resolved images of the dust emission, we instead compare the dust properties of the SMC sample to the Galactic cirrus hot-spot sample of Gaustad & van Buren (1993). A broader comparison using WISE data is carried out in Paper II. The Gaustad & van Buren (1993) sample was selected by looking for IRAS excesses at 60 µm with color temperatures above the  K temperature of the general Galactic cirrus and peaking at the location of a star, with to exclude H ii regions, and spatially extended emission that is more extended in longer wavelengths, indicating that the temperature declines with distance from the star. We note that in cases where the reported IRAS 25-µm flux is negative, the color temperatures cannot be calculated.

Whether this cirrus hot-spot sample in fact consists solely of cirrus hot-spots is, however, uncertain. Gaustad & van Buren (1993) found that in some cases, the hot spots have 12-µm emission that is brighter than the 25-µm emission, indicating a hotter dust component than the expected  K temperature of illuminated cirrus. Indeed, the color temperatures we find for the hot-spot sample from the 12/25 flux-ratios are all above 100 K. Furthermore, Kalas et al. (2002) used resolved optical images of nebulosity around three B stars that appear in both the Gaustad & van Buren (1993) and Backman & Paresce (1993) samples to model the IR emission from the objects. The authors concluded that, in all three cases, the ISM model cannot explain the 12- and 25-µm emission, requiring a source of warmer dust. The coronograph used to block the starlight for their optical images unfortunately obscures the central 100–1000 au around the star, where a circumstellar disk would be located. The warmer dust could therefore be provided by a population of either blackbody grains in a close-in circumstellar disk, or very small grains at larger distances that are heated stochastically to higher temperatures than equilibrium. In cases where the stellar velocity is large compared to the local ISM (such as runaway stars), a bow-shock could further heat the dust in addition to stellar irradiation. Distinguishing between these scenarios would require imaging at higher resolution, which we discuss further in Paper II. The 12-µm IRAS band also contains a PAH emission feature, offering another possible explanation for the excess flux at that wavelength. Illustrating the difficulty of distinguishing disks from hot-spots using SED information alone, even for nearby objects, Kalas et al. (2002) note that 31 of the 34 B-type stars in the Backman & Paresce (1993) table of debris disk candidates or Vega-like stars, appear in the Gaustad & van Buren (1993) cirrus hot-spot list.

The full hot-spot sample contains stars of spectral types O6 through B9 of all luminosity classes, but we exclude from the comparison any objects marked in the Gaustad & van Buren (1993) table as emission line stars or stars of luminosity class higher than V, to match the characteristics of the SMC dusty OB stars. We further limit the hot-spot sample to those objects that have excesses, if scaled to the SMC distance of 61.1 kpc, that we could detect at the 5- level or better at 24 µm in the SMC survey (215 Jy; Bolatto et al. 2007). This sub-sample contains 18 stars. We then calculated the dust parameters for this sub-set of the sample, using the 12/25-µm combination in place of 8/24-µm, and these values are listed in Table 7. Stars flagged in Table 7 as “known reflection nebula” are noted in Gaustad & van Buren (1993) to have extended emission that is brighter in the blue than in the red on the Palomar Sky Survey or ESO Sky Survey plates.

The comparison SMC sample contains a total of 31 stars (c.f., Table 6), and it is constituted by normal main sequence stars (no OBe, “OBe?,” or Em classification) that have a measurable 8 µm emission excess, in order to determine the temperature of the dust. While the distributions in both samples over spectral types peak at a similar type – B0 in the SMC vs. B1 in the hot-spot sub-sample – the SMC sample extends more towards earlier types while the hot-spot sub-sample extends towards later types. This is reflected in the distributions over stellar effective temperature, shown in Figure 14.

In Figure 15 we show histograms of the observable parameters, F/F or F/F, L or L, and /F or /F. The IR luminosities and fractional IR luminosities are similar; the difference in the IR ratio is largely driven by the use of the 8- vs. 12-µm bands. In Figure 16 we show histograms of the color temperature, mass, and fractional luminosity, where the samples are restricted to spectral types O9–B2, since both samples have at least one star of each of these spectral types. The histograms for the hot spots in Figures 15 and 16 are created by weighting the number of objects in that sample so that the total number of hot spots in a spectral type bin equals the total number of objects in the bin for the SMC sample. In this manner the weighting controls for the different spectral makeup of the two samples. Figure 17 shows the averages of each of the observable parameters and associated dust parameters against stellar temperature, without the restrictions on visibility at the SMC distance that are used in the histograms.

The distributions over dust temperature are very similar for both samples. they both have similar mean temperatures in the 125–130 K range, although the hot-spot sample also displays a broader range in dust temperatures, 97–226 K compared to 116–164 K for the SMC sample. The hottest color temperatures in the hot-spot sample in Figure 15 result from the coolest and most predominant spectral type, B2. The trend toward higher color temperatures in the hot-spot sample is possibly due to PAH contamination. The 12-µm PAH emission feature remains relatively unchanged over a broad range of radiation field intensities, but the thermal emission at 25 µm changes dramatically (e.g., Figure 13 in Draine & Li 2007). Thus PAHs around cooler stars would create higher color temperatures than around hotter stars; F/F would be artificially high as a result of similar amounts of PAH emission at 12 µm but reduced thermal emission at 25 µm. This is likely a lesser effect in the SMC, where PAH emission is weak (e.g. Sandstrom et al., 2010).

The distributions over dust mass are also similar for both samples, though again the distribution appears broader (both towards higher and lower dust masses) in the hot-spot sample. The SMC sample may be devoid of examples of lower dust mass as these would not have been detected. The average mass at B0 and B1, where there is the greatest number of objects in both samples, is similar for each sample (Fig. 17), but the average dust mass in the SMC sample is higher at B2, and lower for O7–O9, compared to the cirrus hot-spots. We see a trend in the hot-spots of lower dust mass for cooler stars, which is reasonable given that a hotter, more massive star could illuminate larger regions of the ISM. We do not see this dependence on the stellar temperature for the SMC objects, although the smaller temperature range of the SMC sample would make such a trend perhaps difficult to detect.

At first glance, the distributions over do also appear similar between the two samples. On closer inspection, the one for the hot-spots is clearly broader. The average for the hot-spot sample is smaller than that of the SMC, which is , although this is somewhat misleading as the averages are skewed by the extremes in the sample. The derived for both samples do fall within the typical to range of debris disks (Chen et al., 2006).

While we noted minor differences in the distributions over dust temperature, dust mass and between the SMC and hot-spots samples, these can be explained by a combination of small-number statistics, selection bias, and the opposite sense of skewedness of the distributions over spectral type. Indeed, Figure 17 shows that the trends – or absence thereof – and mean values are consistent between both samples if the breakdown over spectral types is taken into account.

Interestingly, despite the low dust-to-gas ratio in the SMC (about that of the Milky Way; Leroy et al., 2007), the SMC objects show dust masses similar to the Galactic hot-spots if common selection thresholds are applied. This suggests that, if they are hot-spots, the mass of gas being illuminated is times larger in the SMC than in the Milky Way. Assuming similar gas densities, this suggests that the typical sizes of hot-spots would be larger in the SMC than in the Milky Way.

5.4. Bow-shocks and Runaway Stars

Larger dust masses, warmer dust and thus brighter emission may arise from bow-shocks ahead of runaway stars, or stars being over-run by expanding interstellar bubbles. Stars with stronger stellar winds – i.e., of earlier type and/or at later evolutionary stages – and/or larger relative velocity with respect to the local ISM sweep up more mass; larger relative velocities also result in stronger shocks and hence greater heating. Gvaramadze et al. (2011) present a sample of 12 candidate runaway stars with bowshocks in the SMC. Curiously, there is no overlap whatsoever between their sample and ours.

The distribution of our sample on the sky (Figure 18) seems to indicate that at least a portion of them are found outside of sites of vigorous star formation. While the late-B stars may be old enough to have migrated from their birth-sites or for their natal clouds to have dispersed, the fact that some of the O-type stars are also found outside star-forming regions suggests they may be runaways. One of our “normal” OB stars, B 064 is in fact the primary in an X-ray binary2, and it is likely that the binary system will have received a high peculiar velocity when the neutron star’s progenitor star exploded as a core-collapse supernova – see Kaper et al. (1997) for the proto-typical example of a bow-shock accompanied runaway X-ray binary, Vela X-1.

We also note that the structure of the ISM in the SMC differs in important ways from that in the Milky Way disk; the SMC is dominated by expanding bubbles (Staveley-Smith et al., 1997) while gas in the Milky Way disc is more strongly entrained and collected by the recurrence of a spiral density wave. This could affect the ubiquity and properties of bow-shocks; for instance stars in the SMC might be over-run by expanding shells (see Dawson et al., 2013, for such mechanisms operating within the Large Magellanic Cloud), resulting in large relative velocities between stars and ISM without the requirement for a “kick” velocity of the star.

While proper motions relative to the systemic motion of the SMC are too small to be measured (1 mas yr 300 km s), radial velocities can be determined with much better accuracy. Our own spectroscopy, however, has neither the spectral resolving power nor velocity calibration accuracy required for such measurements. In Table 4 we list the radial velocities measured for stars in our sample by Evans et al. (2006), Martayan et al. (2007) and Evans & Howarth (2008). We also determined the velocity of the peak in the H i emission around the location of the star in question, from inspection of the datacube produced by Stanimirović et al. (1999). This differs from the intensity-weighted velocity map produced by Stanimirović et al. (2004), because we are interested in the most likely velocity difference between a star and the densest parts of the ISM. Often, more than one strong peak is seen in the H i data; in that case we list both. Of the 21 stars with radial velocity measurements, most stellar kinematics coincide with that of a strong H i peak. B 116, B 124 (and perhaps B 119) fall in between H i peaks, suggesting they might reside in the middle of a shell expanding at –20 km s. But two O7 stars, B 142 and especially B 145, have large velocity differences of and 75 km s with respect to the nearest (in velocity) H i peak. No uncertainties were quoted for the stellar radial velocities for these stars, but measurements for other stars from the same work (with the caveat that those were all later spectral type) often agreed with H i kinematics, which suggests typical errors not much larger than km s. We thus suggest that these two O7 stars may be runaways – they are located just outside of the brightest H ii region in the SMC, LHA 115-N 66 containing the massive O-star cluster NGC 346, lending further support to their large space velocities.

5.5. Disks

Statistics for circumstellar disk detections around stars with spectral types as early as the SMC sample are difficult to come by. Few surveys exist of stars younger than Myr, which encompasses most, if not all, of the main-sequence lifetimes of these late-O and early-B stars, and those surveys that do sample the appropriate age range still contain very few stars of these types (see Carpenter et al., 2009, and references therein).

Our sample of 87 objects with normal O- or B-star main sequence spectra in the SMC shows excesses at 24 µm that are larger than those of typical debris disks, in studies of somewhat less massive (late-B and A-type) stars in the Milky Way. Carpenter et al. (2009) set a limit, based on the properties of their sample of B7–A9 stars, for the ratio of observed flux to expected photospheric flux at 24 µm of for debris disks (but only a ratio of for solar-type stars). Any object with a larger ratio is considered to be a primordial disk. Hernández et al. (2006) define objects with mag as Herbig Ae/Be stars, while objects below this limit are debris disks. Hernández et al. (2006) also define any object with mag as a massive debris disk. The SMC objects have observed flux to expected photospheric ratios , with only 6 objects being less than 100 and a further 7 objects being within 1  of 100. The SMC objects also have mag, with only 11 objects below the limit of 5. Of those 11, we identify 7 as OBe stars.

The studies of Hernández et al. (2006) and Carpenter et al. (2009) only consider circumstellar matter around intermediate and low mass stars, so it is possible that these dividing limits around the more massive stars in our SMC sample should be higher. Carpenter et al. (2009), however, specifically excluded earlier type stars from consideration because, aside from being few in number, they may show a severely reduced dust luminosity due to the loss of small grains to radiation pressure. We find blowout grain sizes for the SMC objects on the order of 1 mm, in the absence of gas drag (see Table 6). The smallest dust grains are the most efficient (per unit mass) at capturing and reprocessing UV and optical radiation into infrared. This suggest that, if blowout is significant, started much larger than we currently measure in our sample.

Hernández et al. (2006) suggest two scenarios to explain massive circumstellar dust disks. They could be the result of collisions between two large planetesimals, greater than 1000 km, or they could be explained as a transition phase from Herbig Ae/Be star to debris disks, in which the disk still contains a significant fraction of primordial dust. The small number of these objects detected in the SMC sample suggests that they could be in a short-lived phase, and perhaps constitute transition disks remnants from the stellar accretion. In the original SMC sample, the 24-µm excess objects were 5% of the total number of objects that met the requirements for normal late-O and early-B stars in the specific magnitude and color cuts (Bolatto et al., 2007). Counting only the stars that have normal O- and B-star spectra, which is 87 out of the 193 objects in the original list, and also assuming that all of these objects are debris disks, rather than ISM hot spots or quiescent Be stars, gives a detection fraction of only 2.2%. Including the 45 objects for which we have no spectra to judge their emission-line star status gives a maximum of 3.4%. If the possible transition object that Hernández et al. (2006) detect is truly in this transition phase, that would give a detection fraction of 3% for transition objects in the 5 Myr cluster in which it lies. This detection fraction is consistent with that seen in the SMC sample.

Finally, it is worth noting that comparison with literature classifications shows that a significant fraction of our SMC sample consists of binaries. The effect of binary systems on the formation of circumstellar disks is unclear. One might expect the disks to be dynamically disrupted, although circumbinary planets have been recently discovered by Kepler (Doyle et al., 2011; Welsh et al., 2012; Orosz et al., 2012a, b; Schwamb et al., 2012).

6. Summary and Conclusions

We obtained long-slit spectra of 125 of the 193 dusty OB stars in the SMC survey that show large excess emission at 24 µm. These new observations cover a significant fraction of the sample, make possible accurate sky removal in the complex regions where these stars live, and allow us to draw robust conclusions on the statistics of different contributors to the sample.

We use these spectra to classify the stars and look for the signatures of the OBe activity. We find 87 objects that lack emission lines and appear to be normal, main sequence late-O/early-B main-sequence stars. For these stars, we use our spectral classification to estimate their luminosity, and calculate the dust temperature, dust mass, F/F, and equilibrium dust distance from the host star. We discuss the spectral and photometric properties of our sample in relation to several possible scenarios: classical Be stars, massive analogs of Herbig Ae/Be stars, hot-spots in the interstellar medium (possibly with contributions of bow-shocks), and disks. The results of this study are:

  • We identify 17 stars in the dusty O and B stars sample as OBe stars because of clear H emission in their spectra (this count includes B 094, the object with forbidden nitrogen emission). A further 7 stars exhibit weaker than expected H absorption and are therefore classified as possible OBe stars – one of these is an O6 Iaf star showing He ii 4686 Å emission. We also identify 12 stars that show no H emission in their spectra but do show 4.5 µm excess in the photometry that could be indicative of transient Be-star activity. If these 12 stars are dormant OBe stars, along with the 11 spectroscopically identified OBe stars that show no excess at 4.5 µm, the transient total OBe star fraction would be %. This fraction is consistent with the study of transient Be star activity by McSwain et al. (2009). Thus it seems unlikely that the remaining 75 stars, with neither 4.5 µm excesses nor H emission, are OBe stars that happened to be in normal OB star states during both the photometric and spectroscopic observations.

  • Our dusty OB stars are generally located near regions of recent star formation, similar to Herbig Ae/Be type stars, and in that respect resemble the Group III Herbig Ae/Be objects in Hillenbrand et al. (1992). Our objects have ages  Myr. Note that classical Be-star activity begins to appear in clusters around  Myr of age, once again suggesting that the majority of them cannot be explained by excretion disks with a dusty component.

  • The SMC sample exhibits broadly similar characteristics when compared to the cirrus hot-spot sample of Gaustad & van Buren (1993). The hot-spot sample shows trends for decreasing dust temperature and increasing dust mass for hotter stars, which the SMC sample does not. However, these differences likely arise from the different distributions over spectral type, and perhaps stronger contamination of the broadband colors by PAHs in the Milky Way. We show evidence that at least some of the stars in the SMC sample are runaways, including one X-ray binary; heating in a bow-shock could add to the radiative heating of the dust.

  • The SMC sample of objects show higher 24 µm excesses than typical debris disks around intermediate mass stars in the Milky Way. There are not enough debris disks known around comparably massive stars, however, to indicate what is typical for an early-B or late-O debris disk. Alternatively, if the stars are young, we may be observing the remnants of the accretion process — analogs to transition disks around massive stars. The detection fraction of these objects when compared to stars with similar optical colors and luminosities is 2.2%–3.5%, which is consistent with the expected short lifetime of a transition phase.

We analyze and discuss additional observations of the SMC dusty OB sample, as well as further comparison with a sample of massive stars in the Galaxy compiled using WISE photometry, in Paper II.

Based on observations obtained at the European Southern Observatory, programme 079.C-0485 (PI J.Th.van Loon). A. D. B. wishes to acknowledge partial support from a CAREER grant NSF- AST0955836, JPL-1433884, and from a Research Corporation for Science Advancement Cottrell Scholar award. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. A. D. B. thanks Paul Kalas and Eugene Chiang for discussions that spurred the initial stages of this research.
Figure 1.— color–magnitude diagram for all of the stars detected at 24 µm in the SMC survey. The box indicates the objects with and consistent with main-sequence late-O and early-B stars ( mag and mag). The photospheres of such stars should not be detectable at 24 µm in the SMC survey.
Figure 2.— Example optical spectrum of one of the SMC dusty OB stars, B 107. To demonstrate the classification system, the important lines are marked. The lack of He ii lines indicates that it is a B star, and the lack of Si iii lines suggests it is not a B2/B3 star. The Mg ii line, however, is too weak for a star later than B3, so we conclude that the star is a B1, with the Si iv lines blended with H.
Figure 3.— Spectra and SEDs of the objects with spectra of normal late-O and early-B stars. The solid line represents the fit of Equation 1 to the data using the slope from the spectral type, scaled by the flux, or if was unavailable. Stars with a type O7–O9 use a slope of , which is the slope for all stars in that range, while stars with type B0–B2 use a slope corresponding to B1. The dashed line is a fit of Equation 1 to the points, when available, without a fixed slope. Also plotted as a solid line is the total emission from the dust plus the photosphere, in cases where the 8/24 µm color temperature was determined. The open triangle indicates the flux limit at 70 µm, and, for the few objects with a 70 µm detection, the total dust plus photosphere emission for the 24/70 µm dust temperature is also plotted. Important lines are those marked in Figure 2, as well as H at 6563 Å, H at 4861 Å, and He i 5876 Å. The [O iii] 5007 Å line was used to gauge the effectiveness of the subtraction of the background H ii region. The fluxes have been corrected for the foreground extinction by the Milky Way.
Figure 4.— Same as Figure 3.
Figure 5.— Same as Figure 3.
Figure 6.— Same as Figure 3.
Figure 7.— Same as Figure 3.
Figure 8.— Same as Figure 3.
Figure 9.— Spectra and SEDs, as in Figure 3, of the objects that we identify as Be stars based on the Balmer emission with no [O iii] 5007 Å emission, a line whose absence we take to indicate that background subtraction is complete. The slopes of the solid lines in the SEDs correspond to a spectral type of B1, because the potential for He i/ii emission makes Be stars difficult to classify. B 094 is unusual in that it shows [N ii] emission, but no [O iii]. We place it in the Be-star group because the H emission, with the rest of the Balmer series in absorption, more closely resembles the Be stars than the group with strong Balmer and [O iii] emission.
Figure 10.— Spectra and SEDs, as in Figure 3, of the objects that show unusually little absorption at H and are potentially Be stars whose photospheric H absorption has not been filled in completely by the emission of the circumstellar matter. The slopes of the solid lines in the SEDs are set as in Figure 9.
Figure 11.— Spectra and SEDs, as in Figure 3, of the forbidden-line emission objects. The slopes of the solid lines in the SEDs correspond to a spectral type of B1. B 001, B 028, B 079, and B 141 are believed to be compact H ii regions, given the strength of the forbidden emission, while B 036 has been identified as a young stellar object (Oliveira et al., 2013).
Figure 12.— Spectra and SEDs of the objects for which background subtraction was difficult. B 005, B 011, B 112, and B 175 all suffer from low signal-to-noise due to clouds, and, except for B 011, they all have other objects nearby on the slit. B 006, B 093, B 108, and B 191 are embedded in a complex region of H emission, making it difficult to determine the appropriate background level to subtract from the stellar spectrum. The slopes of the solid lines in the SEDs again correspond to a type of B1, given the lack of a classification.
Figure 13.— Spectrum of B 035, which exhibits He ii 4686 Å emission indicative of an Ia supergiant luminosity class. The relative strength of the He i and He ii lines suggests a spectral type around O6; the N iii 4640 Å complex is weak, possibly due to the low metallicity of the SMC.
Figure 14.— Total number of objects in each stellar temperature bin for which the 8/24 or 12/25 dust temperature has been calculated in Tables 6 and 7. The SMC sample is shown in black, while the Gaustad & van Buren (1993) sample of Galactic cirrus hot-spots is shown in gray. There are 31 SMC stars and 18 hot spots.
Figure 15.— Histograms of the observable parameters displayed in Figure 17. On the left are the histograms for the SMC objects, using only those with spectral type O9–B2. On the right are the histograms for the Gaustad & van Buren (1993) sample, limited to only those objects that would have a 24 µm flux greater than  Jy at the distance of the SMC. This cutoff is chosen since it is the 5- flux limit of the SMC survey at 24 µm.
Figure 16.— Histograms of the physical parameters in Figure 17, derived from the observable parameters in Figure 15. On the left are the histograms for the SMC objects, using only those with spectral type O9–B2. On the right are the histograms for the Gaustad & van Buren (1993) sample, limited to only those objects that would have a 24 µm flux greater than  Jy at the distance of the SMC. This cutoff is chosen since it is the 5- flux limit of the SMC survey at 24 µm.
Figure 17.— Average properties for the SMC dusty OB stars (circles) and the Gaustad & van Buren (1993) cirrus hot spots (squares) as a function of stellar temperature. On the left are the observable quantities from which the physical parameters on the right are derived. The solid bars on each point represent the standard deviation, and the dashed lines connect the maximum and minimum value for each stellar temperature. The Gaustad & van Buren (1993) points are offset in stellar temperature by +250 K for clarity. The spectral types O7–B2 correspond to stellar temperatures of 37,000 to 22,000 K. Points without bars only have one or two objects included in the average, except in the case of the F/F panel. The standard deviation for this panel has a negative lower bound for all of the hot spots and for O7 (37,000 K), B0 (30,000 K), and B1 (25,000 K) in the SMC sample.
Figure 18.— Map of the 70-µm emission (SAGE-SMC, Gordon et al. 2011) with overlain the dusty O-type stars (blue), B-type stars (red) and other, mainly OB-type emission-line stars (magenta). The latter tend towards the peak of 70-µm emission but the B-type and even O-type stars often stray away from prominent star-forming regions suggesting some may be runaways (not necessarily with a bow-shock).
ID No. Name RA (J2000) Dec (J2000) B3 V4 I5 J H K
(deg) (deg) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy)
B001 J004336.56730226.80 10.90234 73.04078 2078.39 1585.23 918.67 1088.656 564.297 827.178
B003 J004511.14732111.95 11.29644 73.35332 1909.53 1424.6 780.47 387.0 278.2
B004 J004537.37731236.50 11.40574 73.21014 3743.92 2976.41 1783.03 1008 573.7 391.1
B005 J004600.13732324.54 11.50056 73.39015 1502.89 1180.57 715.75
B006 J004610.79732535.61 11.54496 73.42656 2684.89 2484.81 1688.72 1339 996.1 852.6
B007 J004611.72732325.54 11.54884 73.39043 6530.19 5655.79 3742.47 2295.569
B009 J004655.83730452.89 11.73266 73.08136 3625.15 2824.18 1596.46 768.6 523.2 261.5
B010 J004656.08731054.83 11.73367 73.18190 8190.79 6787.24 4409.19 2667 1709 1199
B011 J004657.30731808.71 11.73879 73.30242 1575.16 1221.5 682.27 353.9110
B013 J004707.66733008.17 11.78192 73.50227 5187.12 4007.68 2255.06 1099.7411 749.3812
B014 J004718.97730711.20 11.82905 73.11978 6590.62 5134.44 2956.37 1613 881.3 536.9
B015 J004726.74731316.13 11.86142 73.22115 3905.94 3351.92 1933.56 1740 1549 1125
B016 J004727.53731705.89 11.86473 73.28497 1945.03 1775.39 1073.39 477.8513
B019 J004740.20731548.63 11.91752 73.26351 2780.52 2214.59 1273.97 699.7 403.9 280.5
B021 J004742.79730009.75 11.92833 73.00271 8800.88 7009.6 4080.92 2281 1324 849.4
B022 J004746.41731809.93 11.94339 73.30276 2239.37 1744.59 988.91 518.2 280.5
B023 J004747.53731727.74 11.94806 73.29104 2592.54 2333.96 1620.16 977.4 618.7 433.2
B024 J004747.93730431.15 11.94971 73.07532 2757.57 2144.34 1229.02 580.3 379.8 317.3
B025 J004752.03725207.85 11.96681 72.86885 6393.31 5755.63 3495.9 1899 1146 700.0
B026 J004752.19732121.95 11.96750 73.35610 3806.50 2976.41 1728.06 947.3 519.4 275.1
B028 J004808.41731454.70 12.03507 73.24853 2906.2 6316.74 3064.49 5376 5601 7522
B029 J004809.68732413.06 12.04036 73.40363 9676.67 7566.47 4141.51 2310 1308 792.0
B032 J004818.08731312.79 12.07537 73.22022 2196.47 1689.25 939.2 492.6 262.5
B033 J004818.91732715.30 12.07881 73.45425 6806.53 5139.17 2852.06 1425 800.0 547.9
B034 J004830.03731809.53 12.12513 73.30265 1872.95 1424.6 900.24 466.1114 360 15
B035 J004840.43730441.87 12.16846 73.07830 3018.05 2662.52 1750.49 1133 687.9 434.0
B036 J004841.77732615.25 12.17408 73.43757 2631.03 2575.69 1516.21 1272 1047 987.9
B037 J004843.19731147.65 12.18000 73.19657 7239.79 5645.38 3084.31 1520 935.6 503.0
B038 J004853.00731410.31 12.22087 73.23620 4818.66 4011.37 2713.68 1780 1253 1029
B040 J004858.95731403.62 12.24565 73.23434 2295.76 2012.3 1234.69 790.1 482.5
B042 J004907.89730906.30 12.28289 73.15175 3180.72 2793.14 1618.67 988.316
B043 J004910.86730719.66 12.29525 73.12213 5868.49 4717.3 2555.99 1273 786.9 467.7
B044 J004916.70731631.62 12.31962 73.27545 5661.43 4410.55 2548.93 1344 791.9 521.8
B045 J004919.07731247.23 12.32949 73.21312 2638.31 2198.33 1320.56 832.7 515.6 314.4
B047 J004930.97730952.88 12.37908 73.16469 2255.93 1881.45 1102.45 636.9 346.0
B048 J004931.95725116.88 12.38313 72.85469 10827.45 8474.09 4906.35 2619 1574 903.5
B049 J004942.72731718.20 12.42801 73.28839 7470.09 6161.59 4257.54 1264.7217
B051 J004944.38725102.05 12.43493 72.85057 1212.62 979.25 586.62
B052 J004944.32732721.63 12.43470 73.45601 1274.46 1115.05 733.77 437.8 263.0
B055 J004952.37725131.60 12.46821 72.85878 2336.29 1838.62 1099.41
B057 J004957.89725154.79 12.49123 72.86522 8752.38 5766.24 3861.52 2101 1216 735.8
B059 J005000.46725512.36 12.50192 72.92010 8190.79 6133.28 3670.78 2172 1295 815.8218
B062 J005010.98730943.41 12.54577 73.16206 2399.53 1845.41 1167.24 712.0 454.1 326.2
B064 J005011.21730026.02 12.54675 73.00723 4259.18 3302.88 2129.9 1190 691.0 468.1
B065 J005016.35725239.03 12.56816 72.87751 1845.55 1444.42 875.71 477.0 306.7 255.6
B066 J005034.13725328.17 12.64223 72.89116 3523.11 2630.83 1473.53 681.2 413.3 262.7
B068 J005049.59724239.96 12.70665 72.71110 10678.89 7850.42 4454.09 2245 1317 824.0
B069 J005053.67725116.34 12.72363 72.85454 1759.24 1352.99 792.79 505.9 253.2 244.3
B070 J005058.13725141.14 12.74222 72.86143 3009.72 2194.29 1303.64 663.3 424.9 248.2
B072 J005105.15724810.08 12.77149 72.80280 2561.69 1925.28 1086.32 501.7 279.2 168.4
B073 J005105.83724057.50 12.77430 72.68264 7094.57 5177.18 2878.45 1719 933.0 632.6
B075 J005118.76733016.19 12.82818 73.50450 4881.19 3834.36 2106.49 1037 538.4 356.7
B076 J005131.66731529.01 12.88192 73.25806 2842.67 2101.32 1298.85
B077 J005137.38725713.64 12.90577 72.95379 8281.82 6469.83 3670.78 2010 1195 751.5
B079 J005158.09732031.38 12.99207 73.34205 644.65 945.57 525.72 1320 1429 2060
B080 J005202.13723930.09 13.00888 72.65836 6038.46 4165.75 2586.77 1339 745.3 541.4
B081 J005206.00725209.19 13.02502 72.86922 1918.34 1536.36 1067.48 1106 874.8 823.2
B082 J005206.44724946.84 13.02685 72.82968 5298.17 4204.29 2502.41 1153 895.2
B083 J005210.67724119.21 13.04448 72.68867 3497.24 2408.2 1561.56 989.2 560.7 346.4
B085 J005245.05722844.03 13.18771 72.47890 4366.43 4015.07 2782.01 1894 1281 855.7
B087 J005300.20724027.84 13.25087 72.67440 9014.18 6093.87 3518.51 1929 1137 766.9
B088 J005309.30725331.30 13.28879 72.89203 6812.81 4989.91 2771.78 1358 771.8 493.8
B089 J005310.59731038.38 13.29413 73.17733 4652.92 3699.07 2149.6 1130 615.3 413.4
B091 J005315.69731128.17 13.31541 73.19116 5708.55 4542.48 2656.8 1374 823.9 441.7
B092 J005342.39732320.18 13.42664 73.38894 2401.74 1881.45 1146.99 658.4 429.2 239.0
B093 J005342.42723921.52 13.42678 72.65598 2206.61 1966.5 1357.56 1379.3919 920.2420 654.0321
B094 J005353.43724828.58 13.47266 72.80794 10503.31 8003.74 5128.12 3023 1773 1116
B096 J005401.98724221.92 13.50825 72.70609 3461.99 2533.34 1495.4 777.8 468.9 308.7
B098 J005429.76722607.36 13.62401 72.43538 3956.64 3296.8 2151.59 1258.0222 678.4223 629.7924
B100 J005454.44723209.52 13.72685 72.53598 2283.11 1777.02 954.9 478.3 318.5
B101 J005455.07722537.27 13.72946 72.42702 6657.73 5087.37 2766.68 1446 842.4 489.7
B102 J005503.27725521.36 13.76366 72.92260 6145.06 4635.46 2445.45 1205.8425 709.0926 487.0727
B103 J005504.51724637.52 13.76883 72.77709 9784.22 8100.15 4541.07 2484.7828 1521.6 29 981.7430
B104 J005509.36722756.55 13.78901 72.46571 9966.12 7996.37 4528.54 2607 1531 984.3
B105 J005510.66723420.78 13.79445 72.57244 10245.33 8528.9 5185.12 3049 1862 1130
B107 J005541.20722320.14 13.92170 72.38893 5645.81 4648.29 2454.47 2984.6131 1963.8132 1216.7333
B108 J005544.60721609.94 13.93584 72.26943 3139.97 2316.82 1206.59 666.3 377.7 282.0
B109 J005545.75723817.55 13.94066 72.63821 1563.6 1189.3 653.97 422.7534
B112 J005611.62721824.26 14.04843 72.30674 7029.53 5527.05 2878.45 1383 814.1 474.2
B115 J005617.30721728.75 14.07210 72.29132 3716.43 3110.95 1636.66 928.3 576.4 357.7
B116 J005620.07722702.23 14.08366 72.45062 2260.09 1773.75 1003.6 491.7 318.2
B117 J005623.53722124.12 14.09807 72.35670 4770.08 4402.44 2252.99 1016 580.6 353.1
B118 J005627.11722542.88 14.11296 72.42858 2638.11 1499.54 864.8 521.8 319.7
B119 J005644.27722906.75 14.18448 72.48521 6328.87 5557.67 3886.5 2922 2060 1465
B122 J005656.35724906.78 14.23481 72.81855 5486.89 4282.46 2474.9 1185 739.1 389.0
B123 J005712.13723935.13 14.30056 72.65976 5293.29 4239.29 2438.7 1265 711.0 426.1
B124 J005715.21722734.30 14.31341 72.45953 4423.09 3348.83 1879.14 992.9 575.3 363.7
B125 J005719.21723112.90 14.33005 72.52025 3641.88 2938.28 1626.14 949.9 501.5 298.1
B126 J005737.27722154.32 14.40530 72.36509 8849.65 6843.73 3763.21 1959 1171 639.6
B127 J005811.84723552.65 14.54937 72.59796 7779.04 6457.93 3704.75 1941 1147 697.5
B128 J005812.38723934.81 14.55161 72.65967 6242.03 4840.53 2733.75 1421 817.9 525.7
B129 J005813.89720919.51 14.55790 72.15542 3631.83 2911.34 1606.79 790.1 470.7 340.4
B132 J005827.49721153.91 14.61457 72.19831 2827 2306.18 1413.7 795.2 492.8 297.8
B135 J005837.02721359.91 14.65429 72.23331 4173.75 3312.02 1967.7 1130 733.0 410.7
B136 J005853.66722230.75 14.72359 72.37521 1838.76 1508.32 904.4 527.8 411.1 405.8
B137 J005859.13724434.11 14.74641 72.74281 9224.14 7239.24 4114.89 2061 1244 807.5
B139 J005908.61722648.58 14.78591 72.44683 2239.37 1703.31 942.67 501.7 308.4
B140 J005912.41721617.86 14.80172 72.27163 1508.43 1215.89 713.11 679.3 461.2 379.8
B141 J005913.87720927.32 14.80780 72.15759 1675.43 1560.61 851.84 1060.9335 875.5936 530.6337
B142 J005920.70721710.89 14.83629 72.28636 9014.18 7003.14 3940.57 1990 1264 794.9
B144 J005928.82721231.57 14.87009 72.20877 4480.5 3395.42 1873.95 877.6 577.4 280.8
B145 J005931.78721334.96 14.88242 72.22638 3433.41 2580.44 1400.74 708.7 487.4 353.8
B148 J010055.17720804.66 15.22989 72.13463 6825.37 5082.68 2784.57 1474 823.9 537.9
B152 J010217.77715143.09 15.57407 71.86197 5027.2 4048.49 2189.57 1214 705.8 405.1
B154 J010241.58720036.14 15.67326 72.01004 2115.08 1686.14 910.25 458.8738
B156 J010245.09715612.26 15.68790 71.93674 6670 5603.93 3400.62 1981 1281 806.0
B159 J010300.05722738.19 15.75023 72.46061 5172.8 3816.74 2068.04 1060 519.9 373.2
B160 J010304.84715335.77 15.77017 71.89327 1500.12 1461.82 830.92 534.2 294.0 392.6439
B161 J010312.30720445.66 15.80126 72.07935 2241.26 1222.25 620.1 391.8
B163 J010315.76720251.61 15.81567 72.04767 5101.83 3695.67 1884.34 960.5 563.8 335.1
B165 J010318.91715413.35 15.82883 71.90371 5879.31 5096.75 3036.4 1845 1167 790.6
B167 J010322.54720616.59 15.84393 72.10461 1219.34 1441.76 876.51 899.7 632.6 354.8
B169 J010336.20721120.32 15.90086 72.18898 3938.46 2938.28 1594.99 775.7 440.9 260.1
B172 J010339.23723523.13 15.91347 72.58976 1749.55 1385.78 812.75 471.3 375.9
B175 J010344.37715953.37 15.93491 71.99816 1311.37 1123.29 713.77 452.6 308.4
B177 J010356.40724117.95 15.98504 72.68832 2790.78 2029.05 1045.1 436.6 274.1
B179 J010421.39715642.39 16.08913 71.94511 5851.85 3271.59 1694 952.1 631.4
B182 J010452.94715449.28 16.22062 71.91369 12397.27 8357.82 5118.69 2564 1540 973.5
B183 J010541.21720340.39 16.42175 72.06122 5031.83 3675.3 2217.98 1499 886.1 634.9
B184 J010557.47721154.24 16.48946 72.19840 5050.4 3624.88 1955.05 969.3740 564.8141 418.0242
B188 J010812.07715833.34 17.05033 71.97593 4468.13 3278.63 1694.96 885.7 583.3 292.6
B190 J011229.71731729.83 18.12382 73.29162 8632.29 4492.55 2794.85 4542 3332 2509
B191 J011348.83731805.04 18.45350 73.30140 7916.37 6651.09 4245.79 1835.2243 1034.4344 917.945
B192 J011513.70732003.29 18.80712 73.33425 8537.41 6369.32 3413.17 1636 997.9 585.5
B193 J011647.69730839.62 19.19872 73.14434 5230.29 4262.78 2337.54 1228 757.0 462.5
Table 1SMC Dusty O and B Stars: Optical and 2MASS Data
ID No. 3.6 µm 4.5 µm 5.8 µm 8.0 µm 24 µm46 47 70 µm48 49
(Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy)
B001 444.4050 2.93 198102.30 624.29
B003 58.53 2.28 44.12 2.51 930.76 26.62
B004 159.10 4.02 99.99 3.94 98.2851 8.64 1684.34 26.40
B005 189.40 7.51 168.70 6.12 382.80 23.00 666.20 33.63 4215.01 42.52
B006 263.10 14.05 238.3652 1.74 287.10 17.45 659.70 40.85 22090.02 111.60
B007 575.10 9.76 412.60 9.30 323.90 14.23 198.40 12.74 1647.90 29.82
B009 142.30 6.08 96.72 4.76 92.0053 9.48 2500.48 31.08
B010 661.70 14.72 500.50 11.84 478.50 18.28 584.80 31.21 7382.77 55.72
B011 51.80 1.96 39.78 2.78 2087.20 28.72
B013 167.60 3.61 110.60 3.62 1149.26 28.52
B014 225.30 4.66 142.00 4.02 123.7654 9.19 87.5855 11.13 1948.75 28.46
B015 485.20 14.90 319.70 12.28 238.50 15.34 158.2456 14.35 4843.77 46.41
B016 114.80 4.17 83.30 4.40 211.6057 14.35 7047.57 52.57
B019 118.30 3.81 92.86 3.76 2534.84 54.48
B021 331.90 6.01 214.00 4.20 155.40 12.08 104.8858 8.54 1093.05 27.72
B022 79.59 2.59 67.77 2.90 93.7359 9.06 186.10 11.38 6160.64 58.06
B023 178.60 3.62 162.40 5.56 229.90 13.47 790.30 27.31 39214.47 204.24
B024 88.11 2.55 58.20 1.84 55.0260 9.16 1093.05 29.29
B025 282.60 6.98 180.80 3.83 138.3261 8.68 80.9662 10.03 835.30 28.19
B026 140.80 3.77 90.96 3.72 1002.48 27.70
B028 9138.00 466.30 14030.00 513.70 37120.00 1022.00 100300.00 8321.00 1632288.0063 1264.81 11107800.0064 36590.40
B029 319.80 8.72 186.60 5.31 178.20 13.72 125.1265 13.98 5955.56 46.19
B032 76.24 2.36 48.01 2.97 3525.87 91.61
B033 208.50 4.38 133.10 3.82 119.2166 9.19 1591.69 25.90
B034 86.66 2.59 62.21 2.99 1414.72 31.51
B035 204.30 4.58 128.90 3.61 75.5367 8.78 721.3168 60.49
B036 1836.8069 27.55 983.10 39.52 3013.00 104.30 7000.00 432.70 256606.50 591.81 1687950.0070 8396.19
B037 231.70 4.55 147.90 4.19 141.9671 8.34 1380.37 28.68
B038 619.30 10.64 498.10 13.19 402.40 12.17 274.60 11.75 726.10 28.66
B040 133.80 3.67 84.04 4.17 729.74 55.17
B042 161.60 4.59 115.30 4.05 152.8872 8.34 298.70 17.57 7348.42 47.92
B043 188.00 3.99 116.30 3.88 106.4773 8.22 90.8074 12.88 3146.94 32.16
B044 208.90 4.30 134.20 3.80 102.8375 6.18 751.9176 64.57
B045 135.70 2.91 87.31 3.59 1661.44 30.26
B047 118.70 3.30 103.50 4.81 114.6677 6.97 99.3678 12.24 3603.94 37.27
B048 391.10 6.55 251.50 5.06 173.60 17.48 157.00 11.22 2900.23 35.98
B049 1217.00 25.07 922.20 19.78 664.50 17.57 478.80 14.78 3100.10 43.77
B051 47.97 1.97 34.55 2.44 563.18 41.64
B052 69.03 3.48 52.24 2.32 1175.29 24.27
B055 90.66 2.93 68.99 2.85 180.80 15.15 12804.30 117.53
B057 311.40 6.36 206.00 5.53 174.7279 9.92 192.20 12.72 7842.89 75.97
B059 339.20 9.42 215.70 4.84 197.10 18.82 105.8080 12.33 2094.49 29.79
B062 150.40 3.46 112.10 4.10 88.8281 6.36 139.8482 12.33 2556.70 32.02
B064 203.10 4.37 127.60 3.50 111.9383 8.63 45.1784 11.22 1308.54 26.49
B065 74.88 2.12 53.21 3.18 603.16 34.53
B066 108.70 3.69 65.00 2.60 90.7385 8.52 122.3686 12.24 4538.76 46.96
B068 428.00 7.25 321.90 8.50 262.60 11.26 202.10 11.29 1424.09 25.89
B069 70.29 2.74 56.89 2.86 802.61 58.30
B070 102.00 3.16 73.00 3.23 71.1287 12.24 1229.42 29.92
B072 77.25 2.10 55.66 2.23 101.2088 12.24 2370.36 29.36
B073 218.70 5.25 133.20 4.86 114.6689 6.50 97.5290 11.59 5202.92 33.44
B075 168.20 5.22 108.40 4.76 10368.36 56.60
B076 96.57 3.35 66.32 2.62 593.37 33.31
B077 287.90 8.23 180.10 4.98 1106.58 24.86
B079 3343.00 83.64 4702.00 69.86 8568.00 107.40 23370.00 213.00 123670.80 436.18
B080 199.70 3.73 129.90 4.72 109.2091 5.93 156.80 16.59 6383.41 35.59
B081 445.60 8.69 373.80 7.63 302.10 11.44 236.90 9.44 870.07 28.67
B082 173.70 3.95 119.20 4.18 98.2892 8.72 197.60 21.12 6613.47 69.29
B083 135.20 3.08 96.48 4.87 107.3893 5.76 854.04 34.96
B085 469.30 8.46 337.70 13.86 230.20 11.22 169.70 28.80 693.51 27.76
B087 308.50 5.90 199.00 5.07 160.1694 7.50 120.5295 9.20 3015.78 30.47
B088 204.30 4.36 124.70 3.38 91.0096 9.28 74.5297 11.22 1408.47 27.68
B089 179.50 4.20 115.00 2.91 84.4598 8.91 50.9799 9.08 768.26 32.27
B091 214.90 5.42 136.60 3.53 118.30100 9.28 67.90101 10.58 430.97 33.31
B092 115.40 3.08 72.07 3.09 79.81102 8.00 687.06 49.97
B093 417.76103 3.89 281.79104 1.27 535.99105 10.56 51925.08 181.97
B094 493.80 8.59 363.70 7.00 273.30 11.54 263.20 9.89 4334.72 46.56
B096 116.10 2.82 76.57 2.27 68.43106 6.29 58.60107 8.72 796.16 25.82
B098 227.20 5.41 152.40 4.26 94.76108 13.34 582.02 22.55
B100 70.68 2.20 47.07 2.25 1319.99 26.75
B101 210.60 4.30 133.70 4.17 115.57109 9.10 727.35 26.19
B102 194.80 8.24 117.40 4.12 1660.39 25.78
B103 486.08110 4.87 284.82111 1.16 264.81112 9.74 159.16113 9.38 870.28 32.01
B104 427.40 8.06 280.60 6.77 209.60 11.90 168.90 11.31 655.83 32.27
B105 482.60 8.98 345.80 7.71 211.60 12.35 136.60 13.50 2679.53 29.50 72527.40114 10291.05
B107 240.70 4.89 183.90 4.96 188.37115 8.09 225.10 14.99 1967.49 29.69 48787.20116 2308.68
B108 99.76 3.96 65.54 9.10 7553.50 53.29
B109 52.32 1.93 32.05 2.12 1467.81 24.17
B112 202.80 3.69 124.30 5.38 99.19117 7.28 101.20118 16.28 1660.39 28.11
B115 176.80 5.20 117.90 5.42 106.47119 6.99 94.76120 19.14 4382.61 36.25
B116 86.94 2.53 56.00 2.42 638.55 34.38
B117 146.20 2.59 86.68 2.69 66.52121 6.03 501.76 34.35
B118 148.90 3.87 101.70 4.08 91.91122 8.64 1270.02 25.91
B119 600.70 13.71 435.50 8.94 352.80 17.41 245.40 11.80 901.51 30.18
B122 207.50 4.53 131.20 4.25 136.50123 8.47 1361.63 23.57
B123 195.60 4.59 132.60 5.01 127.40124 8.65 766.28 24.85
B124 135.60 3.29 87.41 3.56 74.44125 8.04 624.60 34.35
B125 143.80 3.85 98.06 5.69 1465.73 29.20
B126 297.10 5.27 192.60 4.27 140.14126 6.91 81.70127 10.76 661.03 31.23
B127 628.00 10.72 447.10 8.66 360.40 14.61 263.40 9.61 1441.78 27.39
B128 206.20 5.18 135.30 4.07 144.69128 9.83 107.64129 12.97 2709.72 33.50
B129 123.20 3.59 83.92 2.78 99.19130 9.83 132.10 15.81 2466.13 28.99
B132 135.20 5.09 96.28 4.71 104.65131 9.83 190.60 20.00 12814.71 87.02
B135 189.30 4.48 118.40 3.75 131.04132 9.37 88.96133 12.33 2993.92 34.81
B136 228.80 10.10 164.80 7.40 143.78134 7.60 81.42135 11.13 579.84 31.23
B137 319.90 8.71 206.80 5.76 158.10 15.93 87.40136 12.33 754.93 25.00
B139 65.09 2.06 50.48 2.37 58.24137 6.93 52.26138 9.57 1522.98 26.40
B140 211.40 5.08 161.50 4.86 137.41139 9.83 86.02140 12.70 675.40141 62.44
B141 730.23142 1.52 9752.00143 49.40 136371.00 514.46
B142 304.40 6.22 193.40 5.10 132.30 13.08 86.11144 12.70 2992.88 31.77
B144 145.80 3.42 87.71 3.58 82.90145 8.60 102.12146 15.18 2608.75 35.49
B145 103.20 2.75 71.58 3.16 100.10147 8.22 189.20 11.01 3907.91 49.22
B148 212.30 3.71 139.20 3.62 126.49148 9.46 85.38149 14.08 927.84 27.39
B152 174.90 3.71 110.30 4.02 68.80150 8.79 2816.95 29.05
B154 101.60 2.56 81.33 3.06 73.35151 6.89 1415.76 29.00
B156 342.40 8.35 210.70 5.29 182.00152 11.38 124.80 17.24 2463.01 32.10
B159 147.20 3.55 96.16 2.31 104.65153 9.19 55.02154 8.81 1303.33 27.99
B160 87.84 2.79 67.16 2.60 240.90 12.10 9942.59 61.82
B161 87.86 1.97 56.74 2.17 64.97155 6.17 2092.41 35.31
B163 139.50 2.98 94.13 2.71 118.30156 7.46 159.70 10.80 5640.14 45.03
B165 310.70 5.42 203.70 4.86 173.81157 8.41 104.88158 12.24 1355.38 27.62
B167 183.40 4.12 118.60 3.13 1087.85 55.17
B169 118.40 4.67 79.94 2.77 1088.89 26.06
B172 98.63 4.45 81.56 4.29 537.16 33.31
B175 88.11 2.95 65.13 3.51 1009.46 26.55 75141.00159 3016.53
B177 66.51 2.35 49.52 2.59 1630.21 40.73
B179 257.40 4.99 154.90 4.56 148.33160 8.78 2327.68 29.01 41055.30161 2123.55
B182 378.10 7.24 246.80 5.58 173.30 12.55 125.12162 13.06 1099.30 23.78
B183 239.40 5.89 167.00 6.22 181.09163 10.83 280.40 13.41 12679.38 168.64
B184 141.30 6.19 93.57 4.53 711.11 36.93
B188 131.60 5.57 80.09 3.15 3050.13 29.75
B190 868.70 15.06 714.40 11.39 591.50 17.26 451.40 12.28 617.21 26.92
B191 872.64164 2.21 5805.20165 38.00 135121.80 689.04
B192 251.90 5.55 160.90 4.36 142.87166 12.19 960.74 25.23
B193 183.90 4.07 122.80 3.47 1018.72 26.20
Table 2SMC Dusty O and B Stars: Spitzer Data
SpT T Radius Mass L Slope BC Criteria
( K) (R) (M) (10 L)
O6 39 12.2 30.5 309.0 2.91 3.72 He ii 4686, 4541 Å, He i+He ii 4026 He ii 4200 Å
O7 37 11.3 27.3 213.3 2.91 3.54 He ii 4686, 4200 Å, He ii 4541 Å  He i 4471 Å
O8.5 34 9.9 22.8 118.4 2.91 3.27 He ii 4686, 4200 Å, He ii 4541 Å  He i 4387 Å
O9 33 9.5 21.5 96.3 2.91 3.18 He ii 4686, He ii 4200 Å  He i 4143 Å
B0 30 8.3 17.8 50.3 2.89 2.98 He ii 4686, 4541 Å, weak He ii 4200 Å
B0.5 27 7.2 14.5 24.8 2.86 2.60 Weak He ii 4686 Å, no He ii 4200, 4541 Å
B1/1.5 25 6.5 12.5 15.0 2.84 2.41 Si iv 4088, 4116 Å
B2/B3 22 5.6 10.0 6.6 2.79 2.11 Si iii 4553 Å
B5/B8 15 3.8 5.4 0.6 2.67 1.24 Si ii 4128/4132 Å, no Si iii 4553 Å
B9 10 2.8 3.2 0.1 2.52 0.30 Si ii 4128/4132 Å, Mg ii 4481 Å  He i 4471 Å
Table 3Main Sequence Stellar Properties and Spectral Classification Scheme
This Work Literature
No. SpT167 SpT v source168 v H i Identification
(km s) (km s)
B001 Em Planetary Nebula
B003 B0–B2
B004 O9
B005 Eclipsing binary
B006
B007 OBe
B009 B0–B2
B010 OBe Emission-line star
B011
B013 B0
B014 B0–B2
B015 OBe?
B016 OBe
B019 B0–B2
B021 B0 B1–2 II EH08 136
B022 B0
B023 O7
B024 B0–B2 Eclipsing binary
B025 B0–B2
B026 B0–B2
B028 Em Compact H ii region
B029 B0
B032 B1
B033 O7–O9
B034 B0
B035 OBe? O6 Iaf (see section 3)
B036 Em B0 V? S Young stellar object
B037 B1
B038 OBe
B040 B0–B2
B042 B0
B043 O9
B044 B0
B045 B1
B047 O7
B048 B2 B0.5 IV EH08 119, 141
B049 OBe
B051 B0
B052 B0
B055 OBe Emission-line star
B057 B0–B2 B2 S
B059 O9 B3 S
B062 OBe?
B064 B0 B1–3 III–V(e) C11 X-ray binary
B065 B1
B066 O9
B068 OBe B2? S Emission-line star
B069 O7–O9
B070 B0–B2
B072 O8.5
B073 B1 Eclipsing binary
B075 B0–B2 Eclipsing binary
B076 B1
B077 B0–B2
B079 Em Planetary Nebula
B080 O8.5 Emission-line star
B081 OBe Emission-line star
B082 B1 Emission-line star
B083 OBe? B1–5 III EH08 125, 159
B085 OBe O9–B0e A09 X-ray binary
B087 B0
B088 O7
B089 B1
B091 B0–B2
B092 B0 Eclipsing binary
B093 Eclipsing binary
B094 Em B2? S Emission-line star
B096 B0 Eclipsing binary
B098 B1
B100 B1 B0.5 V H08, E06 129, 160–180
B101 B5 B1–5 II EH08 134, 167
B102 O9
B103 OBe B1 II S
B104 OBe B1–3 II EH08 128, 163
B105 B2 B3? S
B107 B1 Emission-line star
B108 B2 IV M07 137, 165
B109 B0–B2
B112 Eclipsing binary
B115 O7–O9 O9 V M07 136, 164
B116 OBe B2 III(IVe?) 151 H08, S, E06 130, 170
B117 B0–B2 B0.5 V E06 Spectroscopic binary
B118 B0 B2 II 130 H08, E06 130, 166
B119 OBe B0 III , 159 M07, E06 129, 167
B122 O7–O9 Eclipsing binary?
B123 B0–B2
B124 B0 B1 V, B1–2 III H08, EH08 131, 168
B125 B1
B126 B1 B0 IV, B0.5 IV , M07, EH08 131, 160–180
B127 OBe
B128 O9
B129 B0–B2 B0 V 172 H08, E06 166
B132 OBe? B0.5 V 176 H08, E06 180 Emission-line star
B135 O7–O9
B136 B0
B137 B1 B1–3 II EH08 130, 146, 177 Eclipsing binary
B139 O9
B140 OBe
B141 Em B0 V H08 Emission-line star
B142 O8.5 O7 IIIn((f)) 208 H08, E06 131, 173
B144 O7–O9 X-ray source within 5″
B145 O7–O9 O7 Vn 250 H08, E06 130, 175
B148 B0
B152 O7–O9
B154 B1
B156 O7–O9 B1 S
B159 B0
B160 O7–O9 Emission object within 5″
B161 B0
B163 O7–O9
B165 OBe?
B167 A0
B169 O7–O9
B172 B0–B2
B175
B177 O7–O9 O9 V EH08 131, 185
B179 B0
B182 B0
B183 O7–O9 Emission object at 3″
B184 B0–B2 B0.5 V EH08 144, 187 Eclipsing binary
B188 OBe?
B190 OBe Emission-line star at 2″
B191 Be? S
B192 B0–B2 B1–3 II EH08 144, 179
B193 B0
Table 4SMC Dusty O and B Stars: Spectral Type, Radial Velocity and Identification
4.5 µm 8.0 µm 24 µm
No. Excess Ratio169 Excess Ratio170 Excess Ratio171 K–[24]
(Jy) (Jy) (Jy)
B001 34013 3.270.53 198098624 414365181 10.86
B003 75 0.190.15 92927 54764
B004 125 0.140.06 168126 46719 6.49
B005 1377 4.400.61 65534 60.57.3 421443 2935318
B006 11013 0.860.19 61541 13.91.8 22084112 3756387 8.44
B007 19326 0.880.21 12215 1.60.3 163830 16218
B009 239 0.320.16 6710 2.60.5 249731 74080 7.36
B010 24629 0.960.21 49633 5.60.8 737156 62965 6.88
B011 66 0.180.22 208629 1342231
B013 1212 0.120.14 114529 27533
B014 16 0.10 3412 0.60.3 194228 27428 6.31
B015 15321 0.920.21 10116 1.70.4 483646 63366 6.49
B016 388 0.820.28 19615 12.42.1 704553 3357488
B019 268 0.390.16 253254 82490 7.30
B021 1021 0.050.11 3611 0.50.2 108428 12613 5.18
B022 216 0.460.17 17012 10.91.5 615958 3139349
B023 777 0.900.10 76227 26.81.5 39211204 11244481 9.80
B024 36 0.050.11 369 1.90.6 109129 42844 6.25
B025 19 0.10 1812 0.30.2 82728 9911 5.10
B026 010 0.000.11 99828 24025 6.31
B028172 13516517 26.313.17 1001228321 561.877.6 16322641265 691387623 10.75
B029 22 0.10 5516 0.80.3 594746 68071 7.10
B032 16 0.020.13 352492 1629184
B033 96 0.070.05 158726 31211 6.07
B034 207 0.490.24 141332 801123
B035 2113 0.190.15 71660 14421 5.46
B036173 86242 7.090.91 6958433 165.020.3 256601592 459364839 10.95
B037 316 0.020.11 137429 20622 6.00
B038 32822 1.930.31 21613 3.70.5 71829 9210 4.53
B040 99 0.110.14 72655 20928
B042 2711 0.300.16 26918 9.01.3 734548 1963236
B043 55 0.050.05 5413 1.50.4 314232 69225 6.98
B044 1413 0.110.12 74765 14720 5.31
B045 89 0.100.13 165830 45351 6.72
B047 486 0.860.13 8112 4.40.7 360237 158590
B048 28 0.10 6215 0.60.2 288736 21622 6.17
B049 73628 3.960.55 41416 6.40.8 309244 36240 5.88
B051 113 0.470.18 56242 56771
B052 135 0.330.16 117424 70880
B055 216 0.440.16 16415 9.91.4 12802118 5806590
B057 521 0.030.11 12315 1.80.3 783476 84987 7.48
B059 268 0.140.05 4313 0.70.2 208730 26911 5.93
B062 448 0.650.19 11613 4.90.8 255432 81788 7.14
B064 2112 0.200.13 912 0.30.3 130426 28931 6.02
B065 86 0.170.15 60135 28736 5.84
B066 64 0.090.07 10312 5.20.7 453647 186686 8.00
B068 10724 0.500.16 12814 1.70.3 141426 14315 5.50
B069 134 0.290.09 80158 44439 6.20
B070 109 0.150.17 4913 2.20.7 122730 42160 6.65
B072 123 0.270.08 8712 5.90.9 236929 132367 7.78
B073 18 0.09 4113 0.70.3 519533 68873 7.20
B075 911 0.090.12 1036457 2276238 8.57
B076 106 0.170.13 59133 22726
B077 20 0.10 109825 12413 5.33
B079 457671 36.273.92 23326213 533.055.8 123665436 213332220 9.35
B080 136 0.110.05 11817 3.00.4 637936 133544 7.59
B081 26813 2.540.37 20010 5.50.7 86529 17819 4.97
B082 1014 0.090.14 16022 4.20.8 660869 1317162 7.08
B083 211 0.020.12 85035 19622 5.89
B085 15723 0.870.21 10730 1.70.5 68528 829 4.68
B087 2618 0.150.12 6211 1.10.3 300830 41243 6.40
B088 65 0.050.05 3511 0.90.3 140428 29011 6.05
B089 711 0.060.11 1410 0.40.3 76332 15417 5.58
B091 514 0.040.11 2212 0.50.3 42533 709 4.88
B092 137 0.220.14 68550 27535 6.06
B093 15016 1.140.26 51919182 85711054 9.66
B094 7530 0.260.13 16314 1.60.3 432147 32634 6.38
B096 78 0.100.12 359 1.50.5 79326 26930 5.94
B098 3214 0.270.14 5314 1.30.4 57623 10412 4.82
B100 16 0.030.12 131827 62771
B101 18 0.08 71726 688 5.34
B102 127 0.120.07 165626 38521 6.24
B103 4725 0.200.12 7713 0.90.2 85932 799 4.78
B104 3126 0.130.12 8214 1.00.2 64432 566 4.47
B105174 3533 0.110.12 2618 0.20.2 266430 17218 5.85
B107175 30 0.07 12618 1.30.3 195430 14916 5.43
B108 211 0.030.18 755153 2580280 8.48
B109 6 0.13 146624 790117
B112 15 0.11 5517 1.20.4 165428 27230 6.27
B115 376 0.450.09 6819 2.50.7 437936 132251 7.63
B116 96 0.190.14 63634 29536
B117 10 0.10 49734 11114 5.29
B118 249 0.310.15 126726 38741 6.41
B119 15630 0.560.16 14915 1.50.3 88930 698 4.38
B122 286 0.270.06 135724 32114 6.27
B123 1214 0.100.12 76125 13715 5.55
B124 10 0.11 62134 16520 5.50
B125 711 0.080.13 146229 35037 6.64
B126 520 0.030.11 1713 0.30.2 65231 769 4.94
B127 26221 1.410.25 19912 3.10.4 143327 16818 5.70
B128 116 0.090.05 6613 1.60.3 270533 53318 6.69
B129 88 0.110.12 10616 4.00.8 246329 71075 7.06
B132 209 0.270.15 16420 6.21.1 1281187 3669387 8.99
B135 205 0.200.06 5612 1.70.4 299035 74227 7.07
B136 1189 2.490.41 6511 4.10.9 57831 28935 5.30
B137 1021 0.050.11 1914 0.30.2 74625 829 4.84
B139 73 0.150.08 3810 2.60.7 152126 85046
B140 979 1.490.28 6313 2.80.7 67262 22532 5.53
B141 62914 6.200.98 971750 276.337.9 136366514 292693996 10.93
B142 207 0.110.05 2813 0.50.2 298632 42114 6.35
B144 114 0.140.06 7715 3.00.6 260635 83231 7.33
B145 104 0.160.07 16911 8.20.6 390549 154465 7.52
B148 714 0.050.11 4115 0.90.4 92227 16518 5.50
B152 45 0.040.05 281329 64922 7.01
B154 377 0.850.27 141429 702101
B156 387 0.220.05 6717 1.20.3 245632 34711 6.12
B159 110 0.010.11 239 0.70.3 129928 32435 6.27
B160 213 0.440.09 22512 14.51.1 994162 5216249 8.42
B161 16 0.020.12 209035 89096
B163 105 0.120.06 13211 4.70.5 563745 164572 7.97
B165 2719 0.160.12 4414 0.70.3 134728 16618 5.49
B167 15 0.09 107655 9411 6.13
B169 124 0.180.07 108626 39220 6.46
B172 377 0.810.22 53533 25933
B175176 226 0.510.18 100727 50758
B177 113 0.300.10 162941 104663
B179177 316 0.020.11 232129 36238 6.33
B182 1724 0.070.11 4815 0.60.2 109024 11212 5.04
B183 368 0.280.07 23714 5.40.4 12674169 237099 8.16
B184 112 0.010.13 70737 16621 5.49
B188 9 0.11 304630 78383 7.45
B190 28046 0.650.17 30120 2.00.3 59727 303 3.39
B191 69724 3.980.67 574439 94.412.9 135114689 167652269 10.33
B192 517 0.030.11 95425 13314 5.45
B193 1312 0.120.12