Spitzerinfrared spectrograph point source classification in the Small Magellanic Cloud

Abstract

The Magellanic clouds are uniquely placed to study the stellar contribution to dust emission. Individual stars can be resolved in these systems even in the mid-infrared, and they are close enough to allow detection of infrared excess caused by dust. We have searched the Spitzer Space Telescope data archive for all Infrared Spectrograph (IRS) staring-mode observations of the Small Magellanic Cloud (SMC) and found that 209 Infrared Array Camera (IRAC) point sources within the footprint of the Surveying the Agents of Galaxy Evolution in the Small Magellanic Cloud (SAGE-SMC) Spitzer Legacy programme were targeted, within a total of 311 staring mode observations. We classify these point sources using a decision tree method of object classification, based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information. We find 58 asymptotic giant branch (AGB) stars, 51 young stellar objects (YSOs), 4 post-AGB objects, 22 Red Supergiants (RSGs), 27 stars (of which 23 are dusty OB stars), 24 planetary nebulae (PNe), 10 Wolf-Rayet (WR) stars, 3 H ii regions, 3 R Coronae Borealis (R CrB) stars, 1 Blue Supergiant and 6 other objects, including 2 foreground AGB stars. We use these classifications to evaluate the success of photometric classification methods reported in the literature.

Characterizing the Population of Bright Infrared Sources in the Small Magellanic Cloud

“…Along with the typical silicate dust features, three of the unambiguous RSGs in our sample, MSX SMC 096, 101, and 109, clearly show the 11.3 µm PAH feature (Figure 6), as was noted by Ruffle et al (2015) for 096 and 109. None have strong features at the other expected wavelengths of PAHs, although there is a hint at 12.7 µm in MSX SMC 109.…”

“…Additional, multiply used references: Sanduleak (1989); Elias et al (1985); Massey & Olsen (2003); Levesque et al (2006). c R15 Class: class from Ruffle et al (2015), with the marked sources changed as discussed in the text.…”

“…They are primarily from Ruffle et al (2015), with modifications for a few objects described in §3.2 and §5.…”

“…Jones et al (2012) had calculated M bol = −7.1 for HV 12122 and called it an RSG. Ruffle et al (2015), though, classified it as an early-type O-rich AGB star (O-EAGB), a class of long-period variables (LPVs) with weak infrared excess but no distinct dust features. As they noted, Li absorption has been detected in the optical (Smith et al 1995), which is an indicator of hot bottom burning (HBB).…”

“…MSX SMC 181 has also been classified as both an AGB and an RSG (Ruffle et al 2015). Elias et al (1980) first classified it as an M5e I based on an optical spectrum taken near maximum light, but they also suggest that it could be an unusual type of LPV rather than a real supergiant.…”