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

Abstract

We used Spitzer's Infrared Spectrograph (IRS) to observe stars in the Small Magellanic Cloud (SMC) selected from the Midcourse Space Experiment (MSX) Point Source Catalog. We concentrate on the dust properties of oxygen-rich evolved stars, which show less alumina than Galactic stars. This difference may arise from the SMC's lower metallicity, but it could be a selection effect: the SMC sample includes more stars which are brighter and thus more massive. The distribution of SMC stars along the silicate sequence looks more like that of Galactic red supergiants than asymptotic giant branch stars (AGBs). While many are definitively AGBs, several SMC stars show evidence of hot bottom burning. Other sources show mixed chemistry (oxygen-rich and carbon-rich features), including supergiants with PAH emission. MSX SMC 134 may be the first confirmed silicate/carbon star in the SMC, and MSX SMC 049 is a post-AGB candidate. MSX SMC 145, previously a candidate OH/IR star, is actually an AGB star with a background galaxy at z=0.16 along the same line-of-sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of Spitzer's higher sensitivity and resolution, and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the James Webb Space Telescope (JWST). Color-color diagrams using the IRS spectra and JWST mid-infrared filters show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different YSO classes.

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

“…Ruffle et al (2015) classify it as O-rich due to the presence of strong crystalline emission features at 23, 28, and 33 µm. Kraemer et al (2017; They refer to the object as MSX SMC 049.) confirmed its optical C-rich spectrum, noted its dual C/O chemistry in the mid-infrared, and suggested that it may be a post-AGB object.…”

“…At least two published papers have already investigated which filter combinations look to be most useful for distinguishing and characterizing different classes of objects. Kraemer et al (2017) examined the sample of SMC objects observed by the IRS, using the spectra to confirm the classifications. They found that the 5.6, 7.7, and 21 µm filters best separated C-rich from O-rich stars, while the 5.6, 10, and 21 µm filters best separated young stellar objects (YSOs) from planetary nebulae (PNe).…”

A dearth of OH/IR stars in the Small Magellanic Cloud

“…Instead, two-thirds of the sample were found to be carbon stars, and no heavily obscured oxygen-rich stars were detected, with the remaining third composed of RSGs, post-AGB stars, YSOs, PN, and oxygen-rich stars without significant dust. Other recent searches have discovered a few candidate OH/IR stars, some of which have been observed in the IR (Jones et al 2015;Kraemer et al 2017), and some at radio frequency, but only a few deeply embedded evolved stars like those within the LMC or our galaxy, either carbon-or oxygen-rich, have been found in the SMC (van Loon, Marshall & Zijlstra 2005;van Loon et al 2008;Lagadec et al 2007). It is not a requirement for OH/IR stars to have a high self-extinction, but as the maser luminosity is dependent on the dust, the most luminous maser sources are expected to.…”

“…After observing BMB-B75, Jones et al (2015) found that the far-IR emission detected by Herschel was due to a spatially coincident galaxy. The mid-IR IRS spectrum of the source shows red-shifted (z ∼ 0.16) emission peaks of Polycyclic Aromatic Hydrocarbons (PAHs) at 6.3 and 11.3 µm and Ne iii at 12.8 and 15.5 µm from the background galaxy (Lebouteiller et al 2011;Kraemer et al 2017), but the silicate emission detected from the source appears unaffected. This source has also been found to have 6 cm continuum (Wong et al 2012), emission likely emanating from the background galaxy.…”

Mass-Loss Rates and Luminosities of Evolved Stars in the Magellanic Clouds

“…Figure 2 shows the FWHM range of 29 NIRCAM and MIRI filters compared to the SEDs of two C and M stars with increasing MLR. Kraemer et al (2017) and Jones et al (2017) have already investigated which filter combinations appear to be the most useful for distinguishing and characterizing different classes of objects.…”