THE IDENTIFICATION OF EXTREME ASYMPTOTIC GIANT BRANCH STARS AND RED SUPERGIANTS IN M33 WITH 24 μm VARIABILITY

Abstract

We present the first detection of 24 μm variability in 24 sources in the Local Group galaxy M33. These results are based on 4 epochs of Multiband Imaging Photometer for Spitzerobservations, which are irregularly spaced over 750 days. We find that these sources are constrained exclusively to the Holmberg radius of the galaxy, which increases their chances of being members of M33. We have constructed spectral energy distributions (SEDs) ranging from the optical to the submillimeter to investigate the nature of these objects. We find that 23 of our objects are most likely heavily self-obscured, evolved stars, while the remaining source is the Giant H II region, NGC 604. We believe that the observed variability is the intrinsic variability of the central star reprocessed through their circumstellar dust shells. Radiative transfer modeling was carried out to determine their likely chemical composition, luminosity, and dust production rate (DPR). As a sample, our modeling has determined an average luminosity of (3.8 0.9) 10 4 L e and a total DPR of ´-(2.3 0.1) 10. Most of the sources, given the high DPRs and short wavelength obscuration, are likely extreme asymptotic giant branch (XAGB) stars. Five of the sources are found to have luminosities above the classical AGB limit (M bol <−7.1 mag, L > 54,000 L e ), which classifies them as probable red supergiants (RSGs). Almost all of the sources are classified as oxygen-rich. As also seen in the LMC, a significant fraction of the dust in M33 is produced by a handful of XAGB and RSG stars.

Massive young stellar objects in the Local Group spiral galaxy M 33 identified using machine learning

“…2.1.1), none of these sources have a counterpart within 1 arcsec in the near-IR catalogue of Javadi et al (2015). In fact within 30 arcsec of the centre of NGC 604 (01:34:32.1, +30:47:01; Montiel et al 2015), the near-IR catalogue contains only 27 sources, of which five are classified by the PRF analysis (as WRs, consistent with the young nature of the region). Likewise the Spitzer-IRS pointings described in Martínez-Galarza et al (2012) are all located in this region of sparse near-IR point sources.…”

AGB population as probes of galaxy structure and evolution

“…We modelled SEDs of variables stars with at least two measurements in near-IR bands and two mid-IR bands using the publicly available dust radiative transfer code dusty (Ivezić & Elitzur 1997). In addition 24-µm sources from Montiel et al (2015) were modelled, because they contribute a large fraction to the total dust and mass return (Table 1, Fig. 3).…”