Evolutionary status of isolated B[e] stars
Key Words.:Infrared: stars – Submillimeter: stars – Stars: emission-line, Be – circumstellar matter – evolution
Aims: We study a sample of eight B[e] stars with uncertain evolutionary status to shed light on the origin of their circumstellar dust.
Methods: We performed a diagnostic analysis on the spectral energy distribution beyond infrared wavelengths, and conducted a census of neighboring region of each target to ascertain its evolutionary status.
Results: In comparison to pre-main sequence Herbig stars, these B[e] stars show equally substantial excess emission in the near-infrared, indicative of existence of warm dust, but much reduced excess at longer wavelengths, so the dusty envelopes should be compact in size. Isolation from star-forming regions excludes the possibility of their pre-main sequence status. Six of our targets, including HD 50138, HD 45677, CD24 5721, CD49 3441, MWC 623, and HD 85567, have been previously considered as FS CMa stars, whereas HD 181615/6 and HD 98922 are added to the sample by this work. We argue that the circumstellar grains of these isolated B[e] stars, already evolved beyond the pre-main sequence phase, should be formed in situ. This is in contrast to Herbig stars, which inherit large grains from parental molecular clouds. It has been thought that HD 98922, in particular, is a Herbig star because of its large infrared excess, but we propose it being in a more evolved stage. Because dust condenses out of stellar mass loss in an inside-out manner, the dusty envelope is spatially confined, and anisotropic mass flows, or anomalous optical properties of tiny grains, lead to the generally low line-of-sight extinction toward these stars.
B-type stars embrace a diversified stellar class. In addition to the main sequence population, which itself covers a wide range of stellar luminosities and masses, some B-type stars exhibit emission lines in the spectra, show rapid rotation, or have gaseous/dusty envelopes. Yet because B-type stars evolve rapidly, it is often challenging to investigate their evolutionary status. B[e] stars, characterized by additional forbidden lines in the spectra, and large near-infrared excess, are particularly elusive. It is believed that their peculiarities originate from latitude-dependent mass loss with dusty disk-like structure (Zickgraf, 2003).
The B[e] phenomena are observed in heterogeneous stages of stellar evolution, from pre-main sequence (PMS) Herbig stars to evolved supergiants, compact planetary nebula or symbiotic stars (Lamers et al., 1998). Apart from these B[e] stars with established evolutionary status, there is a subclass still not well known, the ”unclassified B[e] stars” (Lamers et al., 1998), which have dust in their envelopes, yet lack cold dust components (Zickgraf & Schulte-Ladbeck, 1989). Sheikina et al. (2000) and Miroshnichenko (2007) proposed that the unclassified B[e] stars with little cold dust, collectively called FS CMa stars, are binary systems undergoing mass transfer. The evolutionary stage of a star is relevant when its circumstellar dust is studied. In Herbig stars, the grains have progressively grown in size since in molecular clouds (”old dust around a young star”). In contrast, the dust condensed out of the expanding envelope of an evolved star must start with, and grow from, tiny grains (”fresh dust around an old star”). This paper aims to address the evolutionary status of some of the unclassified B[e] stars.
Diagnostics of stellar maturity may not be always conclusive for single massive stars. Occasionally a star is presumed young on the basis of its large infrared excess alone. While such excess emission manifests existence of dust, it is a necessary, but not sufficient condition for stellar infancy. Spectroscopic detection of enriched elements by nuclear synthesis in advanced stages of stellar evolution may differentiate a post-main sequence star from being in an earlier phase. For example, the CO bandhead emission seen in the infrared K Band has been used as indicative of an evolved status for a B[e] star (Kraus, 2009). Even though a few FS CMa stars show the feature, hence should be relatively evolved, some have evolutionary statuses that remain ambiguous.
We started with a sample of nine Be stars with near-infrared excess rivaling to that of Herbig stars. All our targets exhibit forbidden line spectra, therefore are B[e]-type stars, though in the literature, they might have also been classified as FS CMa or Herbig stars. In Section 2 we revisit the issue that thermalized dust in the circumstellar envelopes, instead of plasma free-free radiation, is responsible for the elevated infrared excess emission in Be stars. In Section 3 we show the spectral energy distributions (SEDs) of our targets extending from mid-infrared to millimeter wavelengths to contrast the compact envelope sizes with those seen in typical Herbig stars. In Section 4 we present an elaborative census of each target in order to rule out the possibility of stellar youth for eight B[e] stars, adding support to their evolved status. In Section 5 we discuss the implication of evolutionary status and dust-formation mechanism in isolated B[e]-type stars.
2 Selection of targets
Be stars typically have only moderate infrared excess arising from free-free emission, with the majority being slightly redder than early-type main sequence stars, mag and mag (Zhang et al., 2005; Yu et al., 2015), as shown in Figure 1. Some Be stars, however, exhibit near-infrared excess as large as that of Herbig stars, mag, so must contain dust in their envelopes (Allen, 1973).
Figure 1 also shows the effect on near-infrared colors by adding free-free emission to a B0 V photosphere, assuming an envelope gas temperature . The envelope size together with electron number density alters the total emerging infrared flux, and hence also the and colors. Adopting typical values of an emitting radius cm and electron density cm (Gehrz et al., 1974; Neto & de Freitas-Pacheco, 1982), each cross in the figure represents the free-free intensity in orders from typical (bottom left) to (top right), a value unreasonably large for a Be star. The corresponding and colors change from mag, to an asymptotic value of mag, consistent with the results by Allen (1973).
Therefore, we selected among the compilation by Zhang et al. (2005) Be stars with and both . These include HD 45677, HD 259431, HD 50138, CD24 5721 and CD49 3441, HD 85567, HD 98922, HD 181615 and MWC 623. Note that except HD 181615, all have been listed as Herbig stars Thé et al. (1994). Moreover, other than HD 181615 and HD 98922, the other six are considered FS CMa stars (Miroshnichenko, 2007). Even though not selected by the forbidden line feature, our targets all turn out to be B[e] stars (Miroshnichenko, 2007; Acke et al., 2005; Finkenzeller, 1985; Kipper & Klochkova, 2012). Our sample thus was chosen with no a priori assumption of the evolutionary status. Table 1 summarizes the stellar parameters of our targets, collected from the literature, including the star name, coordinates, spectral type, reddening, and 2MASS J, H, and K magnitudes.
|Star||R.A. (J2000)||Decl. (J2000)||SpType||J||H||K|
|name||h m s||(mag)||(mag)||(mag)||(mag)|
|HD 45677||06 28 17.42||13 03 11.1||B2||0.2||7.24||6.35||4.78|
|HD 259431||06 33 05.20||10 19 20.0||O9||0.42||7.45||6.67||5.73|
|HD 50138||06 51 33.40||06 57 59.5||B7 III||0.12||5.86||5.09||4.15|
|CD24 5721||07 39 06.17||24 45 04.9||B1.5||0.73||9.24||8.49||7.42|
|CD49 3441||08 14 20.50||50 09 46.5||B7||0.3||8.98||8.22||7.41|
|HD 85567||09 50 28.53||60 58 03.0||B2||0.73||7.47||6.68||5.77|
|HD 98922||11 22 31.67||53 22 11.4||B9V||0.14||6.00||5.23||4.28|
|HD 181615/6||19 21 43.63||15 57 18.2||O9 VB8p I||0.2||4.15||3.39||2.62|
|MWC 623||19 56 31.55||31 06 20.2||B4 IIIK4 I/II||1.4||7.10||6.10||5.38|
2MASS Skrutskie et al. (2006); Cidale et al. (2001); Sheikina et al. (2000); Voroshilov et al. (1985); Friedemann (1992); Ellerbroek et al. (2015); Miroshnichenko et al. (2003); Miroshnichenko et al. (2001); Caratti o Garatti et al. (2015); Malfait et al. (1998); Malkov et al. (2006); Dyck & Milkey (1972); Zickgraf (2001)
3 Spectral energy distributions
The SEDs of our targets have all been presented in the literature (de Winter & van den Ancker, 1997; Malfait et al., 1998; Miroshnichenko et al., 2003, 2001; Zickgraf & Schulte-Ladbeck, 1989; Netolický et al., 2009; Wheelwright et al., 2013; Kraus et al., 2008b, a; Ellerbroek et al., 2015). At long wavelengths, HD 50138 lacked data, so we observed it at 225.8 GHz with the sub-millimeter array (SMA) on February 13, 2012. The observations were carried out with an extended configuration of baselines ranging from 44 meters to 225 meters. The radio sources 0530135 and 0607085 were used as antenna complex gain calibrators, whereas 3C 84 and Callisto were employed for bandpass and flux calibration, respectively. The typical uncertainty in the absolute flux scale is 20%. With an on-source integration time of 1.25 hours, a total bandwdith of 8 GHz in the double-side-band mode provided by the digital correlator for continuum imaging, and typical system temperatures ranging between 160 K and 250 K, an rms noise of about 0.9 mJy/beam was reached. Figure 2 shows the 1.3 mm continuum image of HD 50138, with the synthesized beam of obtained with natural weighting. The source was unresolved with a peak flux of 20.75 mJy/beam, yielding an S/N greater than 20.
Figure 3 shows the SEDs of our targets, each typified with a near-infrared excess, followed by a steep decrease toward mid- and far-infrared wavelengths. The only exception is HD 259431, which shows prominent excess emission extending to millimeter wavelengths. For each target, an atmospheric model (Kurucz, 1993) reddened by the observed value (Zhang et al., 2005; Neckel et al., 1980), was used to approximate the stellar photosphere with its adopted spectral class. Two additional components, each of a blackbody radiation, were added to represent the inner and outer edges of the envelope, to approximate each SED at long wavelengths.
Star F1565 F1965 F2365 F2740 U B V R I L     name (W/m/nm) (W/m/nm) (W/m/nm) (W/m/nm) (mag) (mag) (mag) (mag) (mag) (mag) (Jy) (Jy) (Jy) (Jy) HD 45677 1.26E13 7.84E14 6.94E14 5.43E14 7.86 8.52 8.5 8.11 8.01 2.15 146.00 143.00 24.80 5.87 HD 259431 2.95E14 2.56E14 1.95E14 1.94E14 8.48 9.01 8.73 8.72 8.61 4.58 12.54 20.19 109.40 1.13 HD 50138 2.12E13 1.69E13 1.20E13 1.14E13 6.26 6.66 6.64 6.58 6.59 2.72 70.30 62.50 13.30 2.81 CD24 5721 11.53 11.04 10.90 10.13 5.86 3.28 2.67 0.49 2.86 CD49 3441 10.35 10.3 10.30 10.31 5.35 4.67 1.77 3.29 HD 85567 2.97E14 3.16E14 1.72E14 2.03E14 8.14 8.69 8.57 8.51 8.46 6.39 5.81 1.40 8.84 HD 98922 6.62 6.82 6.78 6.74 40.16 27.24 6.19 7.69 HD 181615/6 2.98E13 4.10E13 3.60E13 2.60E13 4.18 4.71 4.61 4.54 4.52 137.00 44.20 8.15 2.67 MWC 623 11.99 10.5 10.34 9.79 3.85 9.16 3.85 2.56 3.48 \tablebib Thompson et al. (1978); Morel & Magnenat (1978); Helou & Walker (1988); Monet et al. (2003); Herbst et al. (1982); Mermilliod (1987a, b); Ammons et al. (2006); DENIS Consortium (2005); Myers et al. (2001); Perryman (1997)
4 Census of neighboring regions
Because a Herbig star, unlike a T Tauri star, lacks unambiguous youth discriminant such as the lithium absorption in the spectrum, it is difficult to distinguish a PMS Herbig star from an early-type main sequence star. Nonetheless, the short PMS lifetime, for example, 10 Myr at most for a B-type star (Lejeune & Schaerer, 2001), means a runaway Herbig star cannot transverse far from its parental cloud. While an early-type star associated with star formation can already be on the main sequence, one that is isolated from any nebulosity, dark clouds, or young stellar population must have evolved beyond the PMS phase. Therefore, we conducted a thorough neighborhood census of our targets in order to resolve their evolutionary status, in response to the question raised by Herbig (1994) “are there stars which look like Ae/Be’s yet are not pre-main-sequence?”
HD 259431: This star, with a known companion away (Baines et al., 2006), is seen near the center of the reflection nebula NGC 2247, with many nearby star formation regions (SFRs) projected within a few deg, for example, NGC 2245, DOG 105, IC 2167, LDN 1607, and NGC 2264. The spectral type reported in the literature ranges from O9 to B6 (Voroshilov et al., 1985; Finkenzeller, 1985; Hernández et al., 2004), so its PMS phase, in any case, must be short. Its SED shows copious excess emission leveling to far-infrared, the most prominent, and hence with the most extended outer dust envelope, among our targets. The star has a Hipparcos distance of 173 pc (van Leeuwen, 2007). The cluster NGC 2247 has a distance 800 pc, estimated by the CO line radial velocity, similar to that of Mon OB1 projected 2 deg away (Oliver et al., 1996). HD 259431 therefore should be in the foreground of NGC 2247. However, given the uncertain distance determination of, and the line-of-sight alignment with, NGC 2247, we cannot rule out the possible association of the star with the nebulosity. HD 259431 may well be a PMS Herbig (Alecian et al., 2013) or a young main sequence star, so is excluded in our final sample of relatively evolved B[e] stars.
We emphasize that our targets were selected on the basis of the level of infrared excess. The Be star sample by Zhang et al. (2005) precludes Herbig stars known at the time, but is not conclusive. That is, Herbig stars may still be included, and some FS CMa stars historically recognized as Herbig stars need further scrutiny. Except HD 259431 just presented, the other eight targets turn out to be isolated B[e] stars. Their neighborhood census follows.
HD 50138: Also known as MWC 158, this is a B7 III star (Ellerbroek et al., 2015) at a distance 392 pc measured by Hipparcos (van Leeuwen, 2007). There are some distant SFRs, projected about one degree away, including the H ii regions LBN 1040 and BSF 60, the reflection nebula GN 06.47.6.01, and the T Tauri star J064958540738522, all at distances more than 1 kpc (Quireza et al., 2006; Fich & Blitz, 1984; Magakian, 2003; Vieira et al., 2003). Additional SFRs with no distance information include Paramian 16, Gn 06.54.8.01, Gn 06.54.8.03, and 220.92.5A. There are four open clusters, M 50, NGC 2302, NGC 2306 and NGC 2309, within 3 deg and with ages 53 Myr (Loktin & Beshenov, 2001), 309 Myr (Kharchenko et al., 2005), 800 Myr (Kharchenko et al., 2005) and 250 Myr (Piatti et al., 2010), respectively. Among these, NGC 2302 is only 8 arcmin off with a heliocentric distance about 1200 pc (Kharchenko et al., 2005), whereas other clusters are all beyond 4.5 deg away and at a distance 1150 pc. The spatially extended CMa OB1 (age 3 Myr, Clariá, 1974) is seen about 4.5 deg away and at a distance 1150 pc. We cannot rule out HD 50138 being a possible main sequencemember escaped from one of these clusters, given the main sequence lifetime of some five hundred Myr with its spectral type. The star has a measured rotational speed , and is inferred to have a disk inclined at (Borges Fernandes et al., 2011, 2012). Using the spectrointerferometric technique, Ellerbroek et al. (2015) resolved its au-scale disk with a measured photocenter offset, possibly caused by an asymmetric structure of the disk or a binary companion. These authors favored a post-main sequence status for HD 50138, because of the presence of high transition lines, such as Paschen, Bracket and Pfund series less commonly observed in PMS stars, and because the star is apparently not associated with an SFR, supported by our assessment.
HD 45677: The spectral type reported for this star ranges from B0, B2 IV/V[e] to B3 (Tucker et al., 1983; Cidale et al., 2001; Bohlin et al., 1994). At a distance 279 pc (van Leeuwen, 2007), this star has the largest near-infrared excess among our targets. Zickgraf (2003) inferred that HD 45677 is nearly pole-on based on the high resolution spectroscopic line profiles. Its infrared spectra indicate existence of not only silicate grains (Molster et al., 2002a, b), but maybe also calcium silicate hydrate, that is, cementitious nano-particles, as suggested by modeling the 10 and 18 m features by Bilalbegović et al. (2014), rendering evidence of ultra-small particles in the envelope.
Herbig (1994) described this star as “projected upon completely unobscured fields”. The nearest SFR WV 340 is projected some away but with no distance information. Two other SFRs, CMa OB1 and Mon R2, are eight deg away and at distances more than 800 pc (Mel’Nik & Dambis, 2009; Herbst, 1975), so too far to be related to HD 45677. Besides, both these SFRs are a few Myr old (Clariá, 1974; Andersen et al., 2006), too young for such an early-type star to be an escaped member. Hence, HD 45677 should not be a PMS Herbig star. The star, also called FS CMa, characterized by spectral forbidden lines and strong Balmer emission (e.g., de Winter & van den Ancker, 1997), is the prototype of the subclass of Be stars studied by Miroshnichenko (2007).
CD24 5721: This B1.5 V star is at a distance 3.5 kpc (Miroshnichenko et al., 2003). Zickgraf (2003) reported narrow Fe ii absorption lines resembling classical Be stars, yet claimed the star as being an unclassified B[e] star because of the presence of P Cygni profiles in Balmer lines and He i line variability. There are only three small clouds, LDN 1668, TGU H1615 and 240.251.75 (May et al., 1997), plus one open cluster C 0739242 (Drilling, 1991; Cidale et al., 2001), seen within 1 deg, and a large ( square degrees) dark cloud LDN 1667 about 2.5 deg to the south-west. None of these have distance information in the literature. The star is some 4 deg away from the large OB association, Pup-CMa (520 pc, Gyulbudaghian & May, 1999) and is 1.5 deg away from the open cluster M 93, which is 400 Myr old (Hamdani et al., 2000) and at a distance of 1037 pc (Kharchenko et al., 2005). Another open cluster NGC 2467, with an age 12.6 Myr (Huang & Gies, 2006) and at a distance 1355 pc (Kharchenko et al., 2005), is 3.25 deg away. Both these clusters should not be related to CD24 5721, so the star should not be young.
CD49 3441: Also known as Hen 3140, this star has an uncertain spectral type. With high resolution spectroscopy, Miroshnichenko et al. (2001) inferred a B7 type in terms of the intensity ratios of He lines, but a B2 type in terms of the Balmer line wings. Based on radial velocity and interstellar extinction, the estimated distance is approximately 2 kpc (Miroshnichenko et al., 2001). Even with a late-B type, the star as a dwarf would be at 1 kpc. There are molecular cloud BHR 31 and reflection nebula BRAN 137, both about 1 deg away at pc (Henning & Launhardt, 1998; Stark & Brand, 1989). The open cluster NGC 2547, with an age 38.5 Myr and a distance 361 pc (Naylor & Jeffries, 2006), is 1 deg away. All these objects are within the Gum nebula which itself extends about 120 pc in radius at an average distance 400 pc (Sushch et al., 2011), so not related to CD49 3441.
MWC 623: This is a binary system at kpc (Zickgraf, 2001) with a B4 III star and a K4 I/II companion (Liermann et al., 2014). The binary period is uncertain with long-term radial velocity monitoring (Zickgraf, 2001; Polster et al., 2012), and only a minimum period of 25 years was suggested (Polster et al., 2012). Among our targets, only HD 181615/6 and MWC 623 are known with spectral types for both the primary and secondary. MWC 623 is 1.26 deg above the Galactic plan, with eight unresolved sources within 20 arcmin (Taylor et al., 1996), plus two young stellar objects and one supernova remnant projected within 20–40 arcmin. None of these has a distance estimation except a very rough 7–17 kpc estimated for the supernova remnant (Hui & Becker, 2009), but their small sizes suggest likely being more distant than, thus unrelated to, MWC 623.
Additionally, the prominent H ii region W 58 and the young stellar object IRAS 195923302, both more than 2 deg away, are associated with the Perseus arm or Cygnus arm, which are at 8.3 kpc and 13.38 kpc, respectively (Thompson et al., 2006; Yang et al., 2002), so not related to MWC 623. In addition to SFRs, one old open cluster (about 800 Myr) Kronberger 52 is away, but is at 3.6 kpc (Szabó et al., 2010), so too far for MWC 623 to be an escaped member. Accordingly, MWC 623 should not be a PMS star. The cool stellar companion should contribute to the near-infrared excess, but in no way could extend beyond mid-infrared. MWC 623 therefore should be a secure case of a non-PMS dusty Be star. The observed extinction of MWC 623, , the largest among our targets, is almost entirely interstellar (Sheikina et al., 2000), suggesting little dust extinction along the line of sight to the star.
HD 181615/6: Also known as Sgr, this star is an evolved spectroscopic binary (Schoenberner & Drilling, 1983; Campbell, 1899) consisting of a luminous supergiant and an “invisible” but hotter and more massive dwarf with a binary period 138 d (Wilson, 1915) and mass ratio (Dudley & Jeffery, 1990). The spectral type reported for the supergiant primary ranges from B5 II, A2 Ia to F2 I (Bonneau et al., 2011; Abt et al., 1979; Maury, 1925), and for the dwarf secondary from O9 V to B 3V (Malkov et al., 2006; Maury & Pickering, 1897). With a close distance of 595 pc (van Leeuwen, 2007), the circumbinary geometry of HD 181615 was well determined by the optical and mid-infrared interferometric techniques (Bonneau et al., 2011; Netolický et al., 2009): the radius of the bright companion (), the H formation region (12 au), the inner radius of the geometrically thin disk (6 au), the angle of disk inclination (50°) and orientation (position angle ). There are no obvious SFRs within 10 deg from the star, with only four small or unresolved radio sources, OV133, OV136, PMN J19221525 and PMN J19241549 (Ehman et al., 1970; Griffith et al., 1994), projected about 0.5 away. None of the them have distance estimation in the literature. There is little doubt that this is an evolved system, so the dust should be freshly produced in situ, likely as a result of mass transfer or wind-wind collision.
HD 85567: This B2 star is at a distance 1.5 kpc (Miroshnichenko et al., 2001). Wheelwright et al. (2013) found no indication of close binary within 100 AU using near-infrared spectrointerferometry. These authors found the inner radius of the dusty disk to be undersized ( au) given the stellar luminosity, and attributed it to the presence of dense gas in the interior of the disk. Moreover, they suggested the cold dust in the outer disk to have been photoevaporated so its SED shows a strong near-infrared excess but little far-infrared excess. Our SED analysis indeed shows an inner envelope size less than 1 au with a temperature 1600 K, and an outer envelope edge of 15 au at 300 K.
There are two high-velocity clouds (Wakker & van Woerden, 1991; Putman et al., 2002) seen within 50′, but they are small in angular sizes so likely in the background and not physically associated with HD 85567. The open cluster NGC 3114, at a distance of 820 pc (González & Lapasset, 2001) and with an age of 160 Myr, is some 1.3 deg away, is too old to harbor any Herbig stars. Therefore, HD 85567 is very unlikely to be in the PMS phase.
HD 98922: This star is the most isolated among our targets, with only the compact dark nebula G291.117.86 and the reflect nebula BRAN 350 projected 1.4 deg away, neither with distance information. The prominent SRF, the Carina Nebula, projected 7 deg away, is at a distance 2.7 kpc (Tapia et al., 2003).
HD 98922 was considered a Herbig star on the basis of its infrared excess Thé et al. (1994), which has been allegedly propagated through the literature, with no further evidence of stellar youth. Hales et al. (2014) detected tenuous but extended (more than 600 au across) molecular gas. Little reddening to the star, – (Malfait et al., 1998; Hales et al., 2014), indicates non-spherical distribution of circumstellar dust. It is puzzling how the star could form out of a completely isolated cloud, keep the surplus gas and dust, and maintain active accretion (Alecian et al., 2013).
We note that the distance determination, 400 or 500 pc, for neither a B9 V nor an A2 III spectral type (Caratti o Garatti et al., 2015; Hales et al., 2014) can reconcile with the Hipparcos measurement (1150 pc van Leeuwen, 2007; Alecian et al., 2013). Using high-dispersion spectroscopy, Hales et al. (2014) derived a gravity () and effective temperature ( K) that put the star much above even the 1 Myr isochrone in the HR diagram. This inconsistency can be vindicated if this star is in the post-main sequence phase. Adopting the effective temperature and stellar luminosity (Hales et al., 2014) the star is indeed consistent with being a M star along the giant branch, likely a luminosity class II bright giant. In any case, HD 98922 should not be a PMS object.
Apart from positional isolation, our targets should not be associated with any SFRs in terms of space motions. Table 2 lists the heliocentric distances, proper motions, and radial velocities of our targets. The three stars having complete proper motion and radial velocity measurements have Galactic space motions (, , ), respectively of for HD 45677, for HD 259431, and for HD 50138, all in km s, which are within the range for field disk-population stars.
HD 85567 and CD49 3441 are intrinsically high-velocity stars. HD 85567 has a relative fast km s. Notably, CD49 3441, with its appreciable proper motion mas yr (Høg et al., 2000) and a large distance yield a tangential space velocity close to 200 km s alone. Plus a radial velocity 33 km s (Miroshnichenko et al., 2001), it is therefore not impossible that CD49 3441 was escaped from one of the regions projected on the sky or along the line of sight.
|name||(pc)||mas yr||mas yr||km s|
|HD 45677||279||2.0 0.9||0.3 0.9||21.6 2.0||1, 2|
|HD 259431||173||2.4 1.1||2.7 0.9||19 4.1||1, 3|
|HD 50138||392||3.3 0.6||4.1 0.5||34.2 2.0||1, 4|
|CD24 5721||3500||1.1 2.7||2.0 2.4||5, 6|
|CD49 3441||2000||11.5 1.7||18.0 1.6||33 2||6, 7|
|HD 85567||1500||9.3 0.6||6.2 0.5||0 2||1, 7|
|HD 98922||1190||9.0 0.4||0.8 0.3||1|
|HD 181615/6||546||1.3 0.2||6.3 0.2||8.9 0.9||1, 4|
|MWC 623||2000||0.6 2.1||3.9 2.1||5, 8|
Whether a young star can be in isolation from star-forming activity is an open issue. Grady et al. (1993) suggested that the isolated B[e] stars with prominent infrared excess could be in the PMS phase. However, most of the prototypical isolated Herbig stars are actually not far from SFRs. For example, UX Ori is only some 40 from the nearby luminous cloud NGC 1788 and a group of T Tauri stars. Another well known example, HD 163296 (Thé et al., 1985) is, when a large field was scrutinized, associated with dark clouds as a part of a prominent H ii region and cloud complex (Lee & Chen, 2011). We note that HD 163296 clearly shows elevated mid-infrared emission, signifying cold dust (see Figure 4; Kraus et al., 2008a). Even if isolated star formation could have been possible (Grinin et al., 1991; Alecian et al., 2013), a new-born star collapsing out of a molecular core should still keep a distributed dust, warm and cold, in the envelope; this is not the case for our targets. We therefore conclude that the eight stars reported here, namely HD 50138, HD 45677, CD24 5721, CD49 3441, MWC 623, HD 181615/6, HD 85567, and HD 98922, represent a well defined FS CMa sample. We note that the two newly identified FS CMa stars reported here, HD 98922 and HD 181615/6, have IRAS colors and , respectively, indeed consistent with the known ranges of this class (Miroshnichenko, 2007). Observationally the FS CMa stars show B[e] spectra and prominent excess emission in near-infrared, but only moderate at longer wavelengths. They have evolved beyond the PMS phase, so the dust should not be surplus from star-forming processes.
Hillenbrand et al. (1992) divided Herbig stars into three groups by their level of near- to mid-infrared excess relative to the stellar photospheric emission: Group I (moderate), Group II (prominent), and Group III (weak). Groups I and II show flat or rising SEDs toward mid-infrared, whereas Group III sources have steep flux decrease toward longer wavelengths. These authors classified HD 259431, the only Herbig star in our sample, as a Group I source. In comparison, the eight FS CMa stars we present here have comparable near-infrared excesses as Group I Herbig stars, but with mid-infrared excess clearly even weaker than Group III. In at least two stars in our sample, in HD 45677 (Di Francesco et al., 2008) and HD 50138 (this work), the flux continues to decrease beyond far-infrared. Millimeter measurements of HD 45677, included in the SCUBA legacy catalogues (Di Francesco et al., 2008), indicated an emission size 14, comparable to the SCUBA resolution FWHM at 850 m, that is, the emission was unsolved. For our SMA observations of HD 50138 at 1.3 mm, shown in Figure 2, with an angular resolution of , the emission was unresolved in either the image or the visibility domain. The fall-off of the SEDs toward submillimeter wavelengths suggests a lack of cold grains, thus more compact dusty envelopes in size than those in Herbig stars. Alternatively, photoevaporation would erode the outer parts of a circumstellar disk earlier than the inner part, thereby exacerbating the disk dispersal within timescales of a few Myr (Gorti et al., 2015). The classification scheme for Herbig stars based on infrared excess therefore is not directly applicable to the B[e] stars in our sample because of the different origins of circumstellar grains.
Existence of thermonuclear products would provide a diagnostic of an evolved atmosphere. Kraus (2009) proposed that an evolved B[e] star should have a C-enriched equatorial environment because of the disk-forming winds in massive stars, locking the isotope in the form of molecules, for example, CO seen in high-dispersion Kband spectra. Oksala et al. (2013) and Liermann et al. (2014) found CO emission in four FS CMa stars, including MWC 137, HD 327083, GG Car, and Hen 3298, to justify the stellar seniority. We found related literature only for two stars in our sample. Wheelwright et al. (2013) detected no CO emission in HD 85567, but the spectral resolution of the data was not sufficient to dismiss the post-MS status. In MWC 623 CO is in absorption, which may be attributable to the cool supergiant companion (Liermann et al., 2014). In any case a sample of FS CMa stars in star clusters would better firm up their evolutionary status. Recently, a few FS CMa candidates were found as members of two young clusters, in Mercer 20 and Mercer 70 (de la Fuente et al., 2015), though uncertainty in mid-infrared fluxes owing to cluster crowding requires further clarification.
Correlation between the level of infrared excess and intensity of Balmer emission lines has been known in Be stars (e.g., Feinstein & Marraco, 1981; Neto & de Freitas-Pacheco, 1982; Dachs & Wamsteker, 1982; Ashok et al., 1984; Goraya & Rautela, 1985; Dachs et al., 1988; Kastner & Mazzali, 1989; van Kerkwijk et al., 1995; Touhami et al., 2010), in Herbig stars (Corcoran & Ray, 1998; Manoj et al., 2006) and in T Tauri stars (Cabrit et al., 1990). For Be stars, those with H and H both in emission exhibit large near-infrared excess (Lee et al., 2011). All our targets belong to this group. In comparison, those with H in emission, but H and higher Balmer lines in absorption have moderate near-infrared excess, with and up to 0.8 mag. The Be stars with H and H both in absorption, show little near-infrared excess, with the observed and .
The correlation between the dust emission and emission lines supports the two-component model for FS CMa stars, one of an enhanced stellar mass loss, and the other of a dusty envelope (Zickgraf, 2003). The copious hot plasma produces the prominent H emission lines in the vicinity of the stars, and also the forbidden lines in the tenuous regions further away. The expanding gas cools off and condenses to form dust grains, which reprocess starlight to give rise to the excessive infrared emission. Because dust is formed out of stellar mass loss, proceeding inside out to the envelope, it is confined preferentially in the vicinity of the star. This is in contrast to a Herbig star which harbors existing large grains distributed on scales of the parental molecular core.
All isolated B[e] stars in our sample suffer comparatively small intrinsic extinction (Table 1), despite the prominent near-infrared excess signifying existence of warm dust in quantity. Possible explanations include an anisotropic dust distribution or ineffective dust extinction. Dust formation in an asymmetric mass outflow, for example, as a consequence of colliding massive stellar winds, ejection in mass transfer in a binary system, or fast rotation to shed the atmosphere of a single star, results in a dusty region close in to the star and a relatively clear line of sight. The mass loss consequently gives rise to the P Cygni profiles commonly seen in the B[e] spectra.
Alternatively, tiny grains are effective in thermal emission but otherwise not in attenuation of visible light. Freshly condensed grains should be tiny in size. Grain growth in an FS CMa envelope may proceed efficiently, but with ample supplies of tiny grains, such as nanosized particles, which are known to have anomalous specific heat capacity (van de Hulst, 1957; Purcell, 1976).
It has been suggested that binarity may be responsible for the FS CMa phenomena (Polcaro, 2006), and dust is formed as a consequence of binary interaction. Interestingly, among our targets, HD 181615/16 and MWC 623 have ascertained spectral types for both binary components, whereas HD 45677, HD 50138, HD 85567, and HD 98922 are found to be spectro-astrometric binaries, that is, the photocenter of a star is wavelength dependent (Baines et al., 2006).
Dust formed out of stellar mass loss, therefore, has properties distinct from that in the interstellar medium or around a young star. Caution must be taken, then, to derive dust contents in different environments based on interstellar grain properties. The current paradigm is that most cosmic dust is produced in expanding cool atmospheres of post-main sequence stars. Our work supports the notion that FS CMa stars may serve as additional suppliers of dust in space (Miroshnichenko, 2007).
We present a sample of eight B[e]-type stars that show prominent near-infrared excess, but steep flux drop toward mid-infrared and longer wavelengths, indicating the dusty envelopes being compact in size and lacking cold grains. On the basis of their isolation from star-forming activity or young stellar populations, we argue that these stars have evolved beyond the PMS phase, so the grains must be produced in situ, starting with very tiny sizes. Anisotropic mass loss, or properties of tiny grains, lead to the phenomena observed in our targets. The properties of dust, such as the size, composition, and spatial distribution in these isolated B[e] stars certainly deserve further investigation.
Acknowledgements.Support for this work was provided by the the Ministry of Science and Technology of Taiwan through grant 103- 2112-M-008-024-MY3.
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