Near-infrared spectroscopy of candidate red supergiant stars in clusters. Based on observations collected at the European Southern Observatory (ESO Programme 60.A-9700(E), and 089.D-0876, and on observations collected at the UKIRT telescope (programme ID H243NS). MM is currently employed by the MPIfR. Part of this work was performed at RIT (2009), at ESA (2010), and at the MPIfR.

Near-infrared spectroscopy of candidate red supergiant stars in clusters. 1 2

Key Words.:
Stars: mass-loss, dust: extinction, Stars: supergiants, Stars: late-type, Galaxy: stellar content

Abstract

Context:Clear identifications of Galactic young stellar clusters farther than a few kpc from the Sun are rare, despite the large number of candidate clusters.

Aims: We aim to improve the selection of candidate clusters rich in massive stars with a multiwavelength analysis of photometric Galactic data that range from optical to mid-infrared wavelengths.

Methods:We present a photometric and spectroscopic analysis of five candidate stellar clusters, which were selected as overdensities with bright stars (K mag) in GLIMPSE and 2MASS images.

Results:A total of 48 infrared spectra were obtained. The combination of photometry and spectroscopy yielded six new red supergiant stars with masses from 10 M to 15 M. Two red supergiants are located at Galactic coordinates (l,b)=(167,) and at a distance of about kpc; four other red supergiants are members of a cluster at Galactic coordinates (l,b)=(493,) and at a distance of kpc.

Conclusions: Spectroscopic analysis of the brightest stars of detected overdensities and studies of interstellar extinction along their line of sights are fundamental to distinguish regions of low extinction from actual stellar clusters. The census of young star clusters containing red supergiants is incomplete; in the existing all-sky near-infrared surveys, they can be identified as overdensities of bright stars with infrared color-magnitude diagrams characterized by gaps.

1 Introduction

ID RA(J2000) DEC(J2000) radius DM( DM() DM(red clump) DM(Sp)
[ ] []
cl1.5 17 53 45.09 -28 11 08.29 73      
cl9.5 18 10 54.34 -21 12 37.67 60      
cl16.7 18 23 34.40 -14 39 10.86 76  
cl49.3 19 19 31.53 14 54 08.57 130
cl59.8 19 46 18.57 23 22 30.23 85        
  • Notes: Positions are derived as the centroid of surface brightness maps from 2MASS K band images.  is the average A of the spectroscopically observed late-type stars in the field.  DM() is the distance modulus obtained from the A values of detected late-type stars and the model of Drimmel et al. (2003) is the average A of the detected RSGs.  DM() is the distance modulus obtained from the A values of cRSGs and the model of Drimmel et al. (2003) DM(red clump) is the distance modulus obtained from the A values of cRSGs and field red clump stars.  DM(Sp): spectro-photometric distance modulus of star 39, by assuming a giant case (O9.5-B3III). For a dwarf case, DM would be mag.

Table 1: List of candidate cluster positions, apparent radii, A, and modulus distances (DM).

Stellar clusters are the best approximation in nature of a simple stellar population, they are the clocks of the Universe, and the building blocks of galaxies. More than 90% of massive stars are found in young and massive stellar clusters (de Wit et al. 2005).

In the inner regions of the Galactic disk, the identification of stellar clusters is a difficult task because of their irregular shapes and density profiles, and because of high and patchy interstellar extinction. About 3000 new candidate stellar clusters have been identified as significant peaks of stellar counts in the 2MASS and GLIMPSE catalogs, or with visual inspection of images (e.g., Ivanov et al. 2010; Mercer et al. 2005; Dutra et al. 2003; Froebrich et al. 2007; Borissova et al. 2011, 2014). However, only a few of these candidates have been confirmed with follow-up spectroscopic and photometric studies, mostly on the near-side of the Galactic plane (e.g., Messineo et al. 2009; Davies et al. 2012; Chené et al. 2013). A multiwavelength photometric screening/selection of candidate massive clusters is mandatory for the best use of the observing facilities, since spectroscopic follow-up can only be accessible for a limited number of candidate clusters. Typically, 50% of the detected overdensities are found to be spurious (Froebrich et al. 2007). For example, overdensities may result from regions of low interstellar extinction (e.g., Dutra et al. 2002). An overdensity can be defined as a cluster of stars if it is made of stars at the same distance and approximately the same age. Depending on the dynamical status, a cluster can be a bound cluster or an association. Typically, cluster members have similar interstellar extinction, and may be recognized on color magnitude diagrams (CMDs) as specific sequences.

A new class of stellar clusters has recently been discovered, the red supergiant clusters (RSGCs), which are characterized by a large number of red supergiants (RSGs) (Figer et al. 2006; Davies et al. 2007; Clark et al. 2009; Negueruela et al. 2010, 2011; González-Fernández & Negueruela 2012). RSGC1, RSGC2, RSGC3, RSGC4, and RSGC5 contain 14, 26, , , and 7 RSGs, respectively, or collectively 15% of all known RSGs in the Galaxy (Messineo et al. 2012). These newly discovered RSGCs are all concentrated between longitude  and , i.e., close to the near end of the Galactic bar, where the bar appears to meet the Scutum-Crux spiral arm. However, their census is incomplete (e.g., Messineo et al. 2012); further searches for RSGCs are needed to investigate Galactic structure. RSGCs are dominated by RSGs, which are intrinsically bright at infrared wavelengths. Their near-infrared CMDs are characterized by gaps of several magnitudes between the RSGs and the blue supergiant members. RSGCs are detected as overdensities of solely infrared bright stars, so bright (typically K mag at 6 kpc) that they can be detected throughout the Galactic plane; infrared dimmer main sequence members are hard to identify in the glare of bright RSGs.

In this paper, we analyze five candidate clusters rich in infrared bright stars that show different types of CMDs by means of a quantitative analysis of near-infrared spectra of their brightest stars. In Sect. 2, we describe the observed candidate stellar clusters; in Sect. 3, we report on the available infrared spectroscopic and photometric data. Spectral and photometric classifications are given in Sect. 4. The cluster properties are discussed in Sect. 6. Finally, our findings are summarized in Sect. 7.

2 Targeted candidate clusters

Figure 1: GLIMPSE 3.6 m images of the candidate clusters listed in Table 1: cl1.5, cl9.5, cl16.5, cl49.3, and cl59.8. Spectroscopic targets are labeled as in Tables 3 and 4. Diamonds indicate red giants, squares Mira-like AGB stars, crosses RSG stars, and triangles early- and yellow-type stars. The coordinates of the image centers are in degrees.

A random sample of candidate stellar clusters were selected among overdensities of infrared-bright stars in both GLIMPSE and 2MASS images (see, e.g., Ivanov et al. 2010, 2002). The candidate clusters were selected to have different types of CMDs, with gaps and without gaps (see Sect. 6). The observed candidate clusters are listed in Table 1. The list reports their coordinates, which are the flux weighted centroids of  K band images from 2MASS. The overdensities of infrared-bright stars are shown in Fig. 1, and in the star counts of Table 2.

ID Center/Control fields
Nstar Nstar Nstar
( mag) ( mag) ( mag)
cl1.5 57/24 8/1 1/1
cl9.5 21/06 4/1 1/1
cl16.7 15/09 7/2 4/1
cl49.3 27/14 7/5 5/1
cl59.8 9/02 5/1 3/1
Table 2: Counts of stars with K, 8.5, and 7.0 mag in the fields listed in Table 1, and, as a comparison, in control fields of equal area.

3 Data

3.1 Near-IR spectroscopy

2MASS DENIS GLIMPSE MSX WISE NOMAD
ID RA(J2000) DEC(J2000) J H I J [3.6] [4.5] [5.8] [8.0] A D C D W1 W2 W3 W4 B V R
[hh mm ss] [deg mm ss] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag] [mag]
1 17 53 60.00 -28 10 50.19 7.48 5.88 4.86               1.56 0.67 0.70 0.01              
2 17 53 45.50 -28 11 07.12 9.68 8.17 7.35       6.94 7.00 6.69 6.46 5.88       6.74 6.72 5.72 4.78      
3 17 53 45.80 -28 10 48.35 10.57 8.91 8.09       7.45 7.55 7.20 7.08         7.69 7.41 6.73 6.11      
4 17 53 46.14 -28 11 11.64 10.57 9.20 8.54       8.18 8.28 8.06 8.01                 17.89   17.41
5 17 53 36.99 -28 11 25.62 10.84 9.43 8.81       8.52 8.72 8.51 8.48         7.83 7.80 8.10       18.65
6 17 53 44.85 -28 11 13.42 11.07 9.57 9.01       8.69 8.80 8.65 8.63                     19.54
7 17 53 58.30 -28 10 54.96 10.95 9.67 9.07       8.80 8.84 8.59 8.73         8.86 9.18       13.15  
8 17 54 03.84 -28 10 46.62   10.71 10.06       9.68 9.77 9.42 9.47         9.64 9.74   6.40 18.81   23.26
9 17 53 35.93 -28 11 25.89 11.00 10.30 10.08       10.02 10.06 10.00 9.99                 14.39 13.28 13.24
10 17 54 01.74 -28 10 49.68 12.15 10.78 10.23       9.89 10.04 9.53 9.67                 17.28 16.42  
11 17 53 47.05 -28 11 11.16 12.48 12.28 12.24       12.10 12.18 12.16 12.15                 13.37 12.75 13.10
12 18 10 53.31 -21 12 34.75 8.93 7.18 6.39 14.69 9.33 6.84 6.35 6.04 5.75 5.54 5.15       5.54 5.80 4.89 3.81      
13 18 10 54.09 -21 12 25.57 7.99 7.96 7.83 9.03 8.52 8.28 7.78 7.79 7.74 7.80         7.72 7.71 7.63 7.82 9.01 8.79 8.66
14 18 10 53.81 -21 12 52.15 10.74 8.90 8.02 17.59 11.14 8.54 7.48 7.61 7.31 7.27         7.56 7.48 6.87 5.65      
15 18 10 54.78 -21 12 30.55 10.91 9.05 8.13   10.80 8.05 7.51 7.79 7.41 7.31         7.74 7.71 6.96 6.00      
16 18 10 54.99 -21 12 22.67 11.44 9.63 8.84 16.16 11.72 9.23 8.20 8.12 7.91 7.79         8.40 8.19 7.89 7.51      
17 18 10 52.89 -21 13 37.62 11.96 10.34 9.49 16.77 12.43 9.79                              
18 18 10 54.75 -21 11 19.81 11.73 10.18 9.60 15.89 12.01 9.88 9.30 9.40 9.13 9.15         9.19 9.27 8.69 7.14     18.08
19 18 10 54.82 -21 10 49.14 12.05 11.88 11.76 12.92 12.63 12.31 11.66 11.51 11.65 12.02                 12.97 12.87 11.95
20 18 10 55.33 -21 10 15.97 12.38 12.02 11.77 13.41 13.06 12.02 11.55 11.50 11.58 11.59                 13.90 13.64 13.33
21 18 10 54.50 -21 11 42.23 14.22 12.96 12.49   14.83 13.06 12.11 12.11 11.88 12.57                      
22 18 23 35.07 -14 38 40.87 6.38 5.15 4.55 9.41 6.22 3.97     4.21 4.23 4.07       4.27 4.15 4.21 4.00 15.40 13.70 12.21
23 18 23 35.33 -14 39 28.95 7.01 5.68 5.06 11.25 6.78 4.22 4.65 4.75 4.35 4.18 4.02 3.18 3.02   4.75 4.14 3.66 2.60 18.98 16.47 15.04
24 18 23 33.30 -14 39 50.57 9.74 7.34 5.91 13.77 8.93 4.80     4.07 3.72 2.96 1.97 1.88 1.58 4.56 3.30 2.43 1.32 16.09 15.33 13.40
25 18 23 34.43 -14 39 47.33 9.11 7.43 6.64 13.64 9.03 6.46 6.71 6.34 6.08 6.09         6.15 6.41 6.45 6.45      
26 18 23 31.14 -14 39 57.17 13.12 12.52 12.10 14.55 13.00 11.89 11.86 11.92 11.60                   16.20 15.36 15.00
27 19 19 33.81 14 53 55.68 5.49 4.52 4.12       3.83 3.91 3.60 3.37 3.10 2.61 2.67 1.41 3.69 3.39 2.90 2.06 14.55 17.18 10.84
28 19 19 27.91 14 53 51.64 7.40 5.98 5.41       4.89 5.31 4.92 4.72 4.57       5.04 4.93 4.53 3.76   15.00  
29 19 19 25.58 14 54 21.07 7.88 6.54 5.97         5.91 5.57 5.46 5.49       5.62 5.69 5.44 4.83 16.92 15.09 12.95
30 19 19 28.68 14 54 21.98 8.41 6.93 6.38       5.72 6.23 5.96 5.93         5.99 6.11 5.97 5.58 16.67 15.07 13.25
31 19 19 16.76 14 53 46.72 8.35 7.49 7.16       6.97 7.16 7.00 6.97         6.93 7.08 7.06 7.31 14.17 12.82 10.74
32 19 19 33.77 14 50 19.19 10.18 8.48 7.40       6.12 5.99 5.63 5.29 5.22       6.31 5.76 4.62 3.79 13.72 13.08 12.01
33 19 19 15.15 14 50 55.29 11.09 9.11 7.93       6.79 6.65 6.22 5.89 5.46       6.52 5.98 4.65 3.74      
34 19 19 23.18 14 56 26.01 9.14 8.35 8.06       7.93 8.10 7.95 7.87         7.61 7.84 7.89 8.64 14.22 13.18 10.74
35 19 19 11.58 14 52 43.78 8.86 8.28 8.09       8.02 8.08 7.96 7.96         7.97 8.02 7.98 8.45 12.51 11.52 10.92
36 19 19 26.44 14 53 25.01 10.19 8.86 8.39       8.06 8.23 8.04 8.01         8.05 8.17 8.28        
37 19 19 29.23 14 53 10.32 12.00 10.60 10.07       9.74 9.88 9.69 9.65         9.76 9.86 10.38 8.52      
38 19 19 33.30 14 52 54.58 12.76 11.01 10.24       9.87 10.00 9.74 9.74         9.91 9.99         20.68
39 19 19 28.80 14 53 35.01 12.10 11.50 11.19       10.94 10.97 10.79 10.93                 17.47   13.82
40 19 19 15.15 14 49 54.25 13.55 11.99 11.29                                 17.92  
41 19 19 28.78 14 52 57.50 12.56 11.76 11.48                               17.22   14.07
42 19 46 18.93 23 22 10.72 8.95 7.32 6.49       6.01 6.01 5.68 5.49 5.25       6.02 5.82 5.07 4.14     19.86
43 19 46 19.70 23 22 29.28 8.58 7.29 6.73       6.39 6.67 6.40 6.37         6.43 6.57 6.41 6.18 18.16 16.02 14.31
44 19 46 17.50 23 22 22.46 8.05 7.24 6.97       6.85 7.01 6.90 6.82         6.80 6.94 6.86 6.54 14.46 11.40 11.58
45 19 46 15.45 23 22 36.41 11.10 9.13 8.22       7.47 7.51 7.17 7.03 6.74       7.47 7.21 6.48 5.40      
46 19 46 18.38 23 22 58.52 10.24 8.86 8.39       8.14 8.30 8.09 8.03         8.13 8.20 8.06 7.27 18.44 16.72 15.43
47 19 46 16.87 23 21 45.87 9.63 8.75 8.54       8.44 8.52 8.46 8.42         8.37 8.45 8.46 7.90 14.24 12.96 12.04
48 19 46 22.48 23 22 49.50 10.67 9.45 9.03       8.80 8.97 8.76 8.74         8.73 8.80 8.74 7.57 18.29 16.65 15.32
  • () Upper-limit magnitudes were removed.  () Measurements with signal-to-noise ratio smaller than 2 were removed.  () For star 41, the adopted , , and measurements are from the UKIDSS survey.

Table 3: Infrared counterparts of the spectroscopically observed targets.

UIST spectra

A set of spectroscopic data was taken with the UKIRT 1-5 micron Imager Spectrometer (UIST; Ramsay Howat et al. 1998) on Mauna Kea under program ID H243NS (PI: Kudritzki) on 2008 July 24. We used the long-K grism, which covers from 2.204 m to 2.513 m at a resolving power (R) of 1900. Integration times varied from 10 s to 45 s per exposure, and the number of exposures varied from 8 to 20.

Pairs of adjacent frames at different nod-positions were subtracted from each other and flat-fielded, wavelength-calibrated with Ar arc lines, and corrected for atmospheric absorption and instrumental response. Curves of atmospheric absorption and instrumental response were generated by dividing the observed spectra of standard stars (with spectral types from B2 to B9) by blackbody curves. Linear interpolation was used to remove Br lines and possible He I lines from the spectra of the standards. A total of 22 stars were observed with UKIRT, and are listed in Tables 3 and 4; their spectra are shown in Fig. 2.

Figure 2: Long- spectra taken with UIST. Identification numbers refer to Table 4. All are late-type stars, with the exception of star 13.

SOFI spectra

For four of the targeted candidate clusters, additional low-resolution and medium-resolution spectra were obtained with the SofI spectrograph mounted on the NTT telescope (Moorwood et al. 1998). Data were taken under program 60.A-9700(E) at Paranal-La Silla Observatory on 2010 August 3, and under program 089.D-0876 on 2012 June 1. The pixel scale is 0288 pix. The low-resolution Red grism in combination with the slit yielded R 980 over the range 1.50m to 2.49m. A few spectra were taken with the HR grism, the K filter, and a -wide slit (R ).

Typically, two slit positions per cluster were observed. The rotator angle was set to optimize the number of stars falling onto the slit. For each slit, frames were taken in a nodding sequence ABBA, which included a small jittering between positions, with detector integration times (DITs) from 1.18 s to 120 s. B-type stars were selected as standards, and observed with the same settings as the targets.

Data reduction was carried out with IDL programs, and the Image Reduction and Analysis Facility (IRAF), using the NOAO/onedspec package3. Pairs of subsequent images at different nod-positions were subtracted and flat-fielded. Wavelength calibration was performed with observations of arc lamps. Only spectra with a signal-to-noise ratio above 30 were used. For each star, typically, four spectra were combined and then corrected for atmospheric transmission and instrumental response. Hydrogen and helium lines were removed from the spectra of the standard stars with a linear interpolation; the intrinsic continuum slopes of the standard spectra were eliminated by dividing them by blackbody curves (calculated at the T of the standard stars). A total of 28 low-resolution spectra and 13 medium-resolution spectra were obtained (see Table 4, Fig. 3, and Fig. 4).

Figure 3: Low-resolution and spectra taken with SofI. Identification numbers are taken from Table 4. Late-type stars are shown in panel (a). Early-type stars are shown in panel (b) together with a few low-resolution template spectra from the library of Lancon & Rocca-Volmerange (1992), and medium-resolution template spectra from the IRTF library (Rayner et al. 2009).
Figure 4: Medium-resolution spectra taken with SofI. Identification numbers are taken from Table 4. All stars are late-type stars, with the exception of star 39.

3.2 Photometric data

For the target stars, simultaneous photometric measurements in bands , , and K were available in the Two Micron all Sky Survey (2MASS) catalog (Skrutskie et al. 2006), with 10 resolution; for two fields, additional near-infrared measurements (simultaneous , , and K) were available from the Deep Near-Infrared Survey (DENIS) catalog (Epchtein et al. 1999). Deep , , and photometry was obtained from the UKIDSS Galactic Plane Survey (Lucas et al. 2008; Hodgkin et al. 2009).

Mid-infrared data were available from the Galactic Legacy Infrared Mid–Plane Survey Extraordinaire (GLIMPSE) with the Spitzer Space Telescope, from the Midcourse Space Experiment (MSX) Spatial Infrared Imaging Telescope (SPIRIT III), and from the Wide-field Infrared Survey Explorer (WISE) satellite (Egan et al. 2003; Benjamin et al. 2003; Fazio et al. 2004; Wright et al. 2010). The SPITZER/IRAC camera acquired simultaneous images in the four channels at 3.6, 4.5, 5.8, and 8.0 m, with a spatial resolution of and a sensitivity of 2 mJy. MSX covered from 8 m to 21 m, with a pixel scale of 183, and a sensitivity of 0.1 Jy at the short-wavelength. WISE bands are centered at 3.4, 4.6, 12, and 22 m; detectability limits are 0.08, 0.11, 1, and 6 mJy; spatial resolutions are , , , and , respectively.

DENIS and 2MASS data were cross-matched using a search radius of ; 2MASS astrometry was retained. Mid-infrared counterparts from the MSX catalog were searched within a radius of 5; matches from the WISE and GLIMPSE catalogs within a radius of 2. In addition, we checked for possible visual counterparts in the NOMAD catalog of Zacharias et al. (2004), using a search radius of 2.

Photometric measurements of the spectroscopic targets are listed in Table 3.

UKIDSS photometry

We used JHK photometry from the UKIDSS Galactic Plane Survey (GPS). For every position, JHK images were obtained from the UKIDSS archive. Raw frames were processed by the standard UKIDSS pipeline (Lucas et al. 2008); corrections for linearity, dark, flat-fielding, decurtaining (the removal of a pseudo-periodic ripple), defringing (the removal of interference fringes due to atmospheric emission lines), sky-subtraction, and cross-talk were applied.

The original WFCAM pixel scale is of 04 pix; however, GPS observations were performed with a micro-stepping technique, which improved the sampling; a microstep of pixels was used. Each observation comprises eight individual exposures of duration 10, 10, and 5 s to make up integration times on sources of 80, 80, and 40 s in the , , and filters, respectively. We resampled the exposures of each observation in a finer grid () in order to reduce the image noise, and to improve the detectability of faint sources; we used a bilinear interpolation. Exposures were combined with a three clipping to eliminate possible cosmic rays. The astrometric distortion of the final mosaic was corrected by using a set of unsaturated stars with 2MASS photometry, and performing a polynomial spatial de-warping of third order. Source extraction was performed using the DAOPHOT PSF-fitting algorithm by Stetson (1987). For each mosaic, a set of bright and isolated unsaturated stars were selected, and a invariable PSF was modeled. The detection threshold was set to 4 (the standard deviation). Finally, individual , , and catalogs were photometrically calibrated with overlapping 2MASS sources, positionally cross-correlated, and combined.

For saturated stars that could be uniquely associated with a 2MASS point source, 2MASS magnitudes were retained.

3.3 Information available on SIMBAD

The SIMBAD database reports information only for one target. We classified star 13 as a B0-F0 star; the star coincides with HD166307, which has been classified as an A0III star at optical wavelength (Houk & Smith-Moore 1988).

4 Stellar classification

We detected candidate RSGs in two fields. In order to confirm RSGs in the direction of the inner disk of the Milky Way, a complex procedure is required. Spectroscopic observations alone cannot firmly distinguish between RSGs and red giant stars. When analyzing RSGs, it is crucial to combine spectroscopic information (see Sect. 4.1) with photometric quantities (see Sects. 4.1.3 and 4.1.4), and to estimate their fundamental parameters (effective temperatures, T, and luminosities). Distances of late-type stars are usually estimated in two ways, either approximated with kinematic distances, or estimated using interstellar extinction as an indicator of distance (e.g., Habing et al. 2006; Messineo et al. 2005; Drimmel et al. 2003). In the inner Galaxy kinematic distances alone cannot be trusted as they rely on the assumption of circular orbits that is not valid in the inner 3.5 kpc (e.g., de Vaucouleurs 1964); our estimates of distances are based on interstellar extinction (Sect. 4.2). Luminosities are described in Sect. 4.3.

Photometry Spectroscopy
ID Q1 Q2 var EW(CO) sp_RSG sp_giant HO Comment Instrument & mode
UIST SofIl SofIm UIST SofIl SofIm UIST SofIl SofIm
[mag] [mag] [Å] [Å] [Å] [%]
1 0.24 0.12      0   53     M1.5     M7   39 sp_Mira_AGB SofIl
2 0.04 0.09 0.07 0.09 0 51 52   M1.5 M1.5   M7 M7   15 sp_Mira_AGB SofIl, UIST
3 0.20 0.09 0.26 0.12 0 44     K5.5     M7       UIST
4 0.17 0.13 0.62 0.13 0 42 46   K5 M0   M6.5 M7   7 SofIl, UIST
5 0.30 0.21 1.13 0.24 0   44     K5.5     M7   7 SofIl
6 0.48 0.13 1.03 0.14 0   32     K2     M1.5   4 SofIl
7 0.21 0.10 0.95 0.15 0   36     K3     M3.5   4 SofIl
8           0   38     K4     M4.5   3 SofIl
9 0.30 0.11 0.68 0.15 0                     SofIl
10 0.40 0.11 0.40 0.23 0   33     K2.5     M2.5   1 SofIl
11 0.11 0.20 0.01 0.51 0                     SofIl
12 0.32 0.14 0.26 0.12 0 43 47   K5 M0   M7 M7   6 SofIl, UIST
13 0.20 0.18 0.07 0.16 1                     SofIl, UIST
14 0.24 0.08 0.71 0.11 1 42 45   K5 M0   M7 M7   2 SofIl, UIST
15 0.23 0.08 0.57 0.11 0 49 45   M1 M0   M7 M7   3 SofIl, UIST
16 0.40 0.09 0.22 0.16 0 30 33   K2 K2.5   M0.5 M2   2 SofIl, UIST
17 0.08 0.13      1   37     K3.5     M4   3 SofIl
18 0.51 0.09 0.90 0.14 0   32     K2.5     M1.5   4 SofIl
19 0.04 0.13 1.01 0.66 1                     SofIl
20 0.09 0.16 0.13 0.29 1                     SofIl
21 0.44 0.18 1.94 0.97 1                     SofIl
22 0.16 0.08 0.96 0.10 0 42 41   K5 K4.5   M7 M6   4 SofIl, UIST
23 0.22 0.10 0.44 0.11 0 44     K5.5     M7       UIST
24 0.15 0.09 2.08 0.12 1 56 29   M3 K1.5   M7 M0   26 sp_Mira_AGB SofIl, UIST
25 0.28 0.14 0.98 0.11 0 41 39   K5 K4   M6 M5   1 SofIl, UIST
26 0.16 0.21      0                     SofIl
27 0.25 0.70 0.66 1.42 0   39 31   K4 K2   M5 M0.5 3 SofIl, SofIm
28 0.39 0.13 0.14 0.10 0 51 50   M1.5 M1   M7 M7   1 SofIl, UIST
29 0.32 0.12 0.53 0.10 0 43 47   K5.5 M0   M7 M7   4 SofIl, UIST
30 0.49 0.11 0.84 0.12 0 42 44   K5 K5.5   M7 M7   1 SofIl, UIST
31 0.28 0.14 0.67 0.11 0     22     K0     K3   SofIm
32 0.24 0.14 2.90 0.11 0     28     K1.5     K5.5   SofIm
33 0.15 0.12 2.31 0.09 0     27     K1     K5   SofIm
34 0.28 0.14 0.57 0.13 1     21     K0     K2   SofIm
35 0.23 0.13 0.44 0.10 0     6     K0     K0   SofIm
36 0.48 0.10 0.80 0.12 0     24     K0     K3.5   SofIm
37 0.46 0.09 0.79 0.12 0     24     K0.5     K3.5   SofIm
38 0.36 0.10 1.17 0.12 0     30     K2     M0   SofIm
39 0.03 0.10 0.19 0.20 0                     SofIm
40 0.30 0.18      0     22     K0     K3   SofIm
41 0.30 2.78      0     15     K0     K0   SofIm
42 0.11 0.12 0.22 0.11 0 44     K5.5     M7       UIST
43 0.28 0.09 0.89 0.10 0 37     K3.5     M4       UIST
44 0.32 0.14 0.66 0.12 0 28     K1.5     K5.5       UIST
45 0.34 0.09 0.32 0.13 0 45     M0     M7       UIST
46 0.52 0.09 0.90 0.12 0 33     K3     M2.5       UIST
47 0.51 0.09 0.78 0.12 0 24     K0.5     K4       UIST
48 0.46 0.09 0.85 0.13 0 29     K1.5     M0       UIST
  • () EW(CO)s from low-resolution SofI spectra were multiplied by a factor 1.4.  () sp_RSG: spectral type by assuming a supergiant class.  () sp_giant: spectral type by assuming a giant class.  () SofIl= low-resolution mode of SofI; SofIm= medium-resolution mode of SofI.

Table 4: List of stars spectroscopically observed with UKIRT/UIST and NTT/SofI. Identification numbers are as in Table 3.

4.1 Spectroscopic classification

Late-type stars

We defined as late-type stars those stars with effective temperatures lower than 4500 K (red giant, asymptotic giant branch stars (AGBs), and RSGs). We define a star as a RSG when it has a T lower than 4500 K and a luminosity of , which corresponds to an initial mass of M (Ekström et al. 2012). Late-type stars can easily be identified with band spectroscopy, because of their strong CO band-head at 2.29 (e.g., Kleinmann & Hall 1986; Wallace & Hinkle 1996; Rayner et al. 2009; Ivanov et al. 2004). Near-infrared spectra of late-type stars (mostly Mira AGBs) may show continuum absorption by water. Water absorption affects both ends of the band spectrum, from m to m, and from m to m, and the blue side of the band from 2.0 m to 2.1 (e.g., Comerón et al. 2004; Blum et al. 2003; Frogel & Whitford 1987; Alvarez et al. 2000; Rayner et al. 2009). Water absorption in band is most easily detected, as a curved (pseudo)-continuum. Highly variable water absorption is found in Mira AGBs because of their pulsation (Matsuura et al. 2002). In Fig. 3, the stellar continua of stars 1 and 24 clearly show absorption by water.

For red giants and RSGs, the equivalent width of the CO band-head, EW(CO), can be used as a temperature indicator because it increases linearly with decreasing . Red giants and RSGs follow two different relations (Blum et al. 2003; Frogel & Whitford 1987; Figer et al. 2006); for a given temperature, RSGs have stronger CO bands than red giant stars. EW(CO)s of Miras are variables, do not correlate with values, and their EW(CO)s may be as large as those of late RSGs (Blum et al. 2003). Water absorption decreases with increasing stellar luminosity (Blum et al. 2003; Frogel & Whitford 1987). All these considerations mean that combined information on water absorption and EW(CO) is useful for estimating luminosity classes.

We measured the EW(CO)s using the feature and continuum regions that are specified in Table 5; we obtained spectral types for the targets by comparing their EW(CO)s with those of reference spectra from the atlas of Kleinmann & Hall (1986); the reference spectra were smoothed to match the spectral resolution of the targets (e.g., Figer et al. 2006). Twelve targets were observed with both UIST and SOFI detectors; by comparing their resulting EW(CO)s, the typical accuracy of spectral types is found within two subclasses; star 24 is a Mira-like star that went from (UIST run) to M0 type (SOFI run). For spectra taken with the medium-resolution mode of SofI, we selected a narrower bandpass for the CO feature (see Table 5). A scaling factor of 1.4 was measured between the EW(CO)s from the medium-resolution SofI and those from UIST. For star 27, which was observed with both the low and medium modes of SofI, we obtained a RSG-type of K4 and K2, respectively. The degeneracy between RSGs and red giants disappears for EW(CO) values larger than Å. Miras stars, with their erratic behaviors, may also have EW(CO)s larger than 43 Å. Stars 1, 2, 15, 24, and 28 in Table 4 have EW(CO) values larger than 48; these stars are candidate AGBs or RSGs.

Figure 5: indexes versus the EW(CO)s of targeted late-type stars and comparison samples of AGBs (Mira and semi-regular variables) and RSGs from the IRTF library (Rayner et al. 2009).
Type Band Continuum References Comment
[m] [m]
2.0525-2.0825 1.68-1.72 1
2.20-2.29 1
CO 2.285-2.315 2.28-2.29 UIST, SofI Low
CO 2.285-2.307 2.28-2.29 SofI medium
  • References: (1) Blum et al. (2003).

Table 5: Spectroscopic indexes. Spectral regions used as features and continuum are specified.

Water indexes were obtained from the de-reddened low-resolution SofI spectra, which cover and bands. We used a water index with continuum and absorption regions defined as in Blum et al. (2003). We measured the water index with a quadratic fit to two continuum regions (see Table 5); is the average ratio of observed flux densities in the water region and estimated flux densities (with a fit). For comparison, we estimated the EW(CO)s and water indexes of a sample of known AGBs and RSGs with spectra available from the IRTF library (Rayner et al. 2009). In Fig. 5, we plot water indexes versus EW(CO)s of the targets, as well as of reference spectra. A combination of water indexes and EW(CO)s allows us to distinguish between RSGs and Mira AGBs; we find that known Mira AGBs have water indexes greater than 15 Å, as in Blum et al. (2003). This confirms that targets 1, 2, and 24 are Mira-like stars. Water indexes for known RSGs and semi-regular AGBs (SR, e.g., Alard et al. 2001) are typically negligible; SR AGBs have EW(CO)s as large as Å; RSGs up to Å (see Fig. 5 and Table 4). SR AGBs contaminate the sample of spectroscopic candidate RSGs (EW(CO) Å); luminosities are crucial for classifying RSGs.

Early-type stars

Low-resolution infrared spectra of O-, B-, A-, and F-type stars are characterized by hydrogen (H) lines (e.g., Hanson et al. 1996, 1998; Lancon & Rocca-Volmerange 1992; Rayner et al. 2009). The low-resolution SofI spectra of stars 9, 11, 13, 19, 20, 21, and 26 show H I lines in absorption at 1.555 m, 1.570 m, 1.588 m, 1.611 m, 1.641 m, 1.681 m, 1.737 m, and 2.166 m (see Fig. 3). The strengths of the hydrogen lines indicate types from B0 to F0 for stars 11, 13, 20, and 26; the presence of weak CO bands imply types from F5 to G5 for stars 9, 19, and 21.

A spectrum of star 39 was obtained with the medium-resolution mode SofI in band. A Br in absorption and a He I at 2.1127 m are detected. The presence of the He I line implies a B0-8 I, or a O9.5-B3V.

The coordinates and spectral types of early-type stars are listed in Tables 3 and 4.

Photometric variability index

Mira AGBs are characterized by periodic photometric variations that can reach up to mag in K (Messineo et al. 2004). In contrast, only a small fraction of RSGs (about 20%) are known to vary, and typically RSGs have amplitudes of a few tenths of a magnitude in band (Yang & Jiang 2011).

Since the two DENIS and 2MASS filters are similar, we used both measurements for identifying candidate variable stars, i.e., stars with (DENIS) mag, and (DENIS)(2MASS) mag, or with (DENIS) mag, and (DENIS) (2MASS) mag. For nonvariable stars, (DENIS) (2MASS) and (DENIS)(2MASS) are within 0.15 mag (Schultheis et al. 2000; Messineo et al. 2004). We also flagged as variables those targets with indication of variability from the shortest WISE band. The late-type stars 14, 17, 24, and 34 are candidate variables, as well as the early-types 13 and 20, and the - stars 19 and 21. Star 24 is a Mira-like AGB, as inferred from the water index; the other variable late-type stars could be SR AGBs (Alard et al. 2001). None of the RSGs found has the variability flag on.

Target intrinsic colors and interstellar extinction

For red giants and RSGs, we assumed the intrinsic , , and colors per spectral-type provided by Lejeune & Schaerer (2001) and Koornneef (1983). For each spectral type from K0 to M5, the (or ) colors of red giants and RSGs agree within 0.28 mag (or 0.09 mag), respectively (Koornneef 1983); this quantity is comparable to the color change between two neighboring subspectral types of the same luminosity class. We obtained intrinsic colors by interpolating a colored-isochrone of 6 Gyr and solar metallicity (Pietrinferni et al. 2004). The obtained colors of late-type giants agree with those of Lejeune & Schaerer (2001) for RSGs (4-30 Myr old) within 0.05 mag. The Koornneef photometric system agrees with the 2MASS system within 0.09 mag (Carpenter 2001).

For early-type stars, infrared-colors were taken from Bibby et al. (2008), Humphreys & McElroy (1984), Wegner (1994), Koornneef (1983), and Wainscoat et al. (1992).

For every target, we estimated A with the assumed intrinsic colors, and a power-law extinction curve with an index of (Messineo et al. 2005). This law yields an excellent agreement between estimates of A from the observed (-), (K), and (K) colors. For eight stars with DENIS and 2MASS measurements, we obtained with =0.14 mag; with =0.09 mag.

Typically, we adopted the A values from the (K) colors. For every field, an average extinction (A) was measured, as the average of the A values of the observed late-type stars (with the exclusion of AGB stars). The field results are listed in Table 1. Stars with a value of A that differs from the average extinction by more than three standard deviations were classified as foreground (background) sources, as indicated in Table 6. We used the shape of the continuum (quantified by the water index) to distinguish between objects with circumstellar envelopes (e.g., Mira AGB) and obscured, but naked, luminous objects.

4.2 Interstellar extinction as a distance indicator

Distances were derived by matching the target A value with a known curve of A values versus distances (along the target line of sight). The targets are distributed in five fields along the Galactic plane, at galactic longitudes of 15, 95, 16