AAS 207th Meeting, 8-12 January 2006
Session 182 Stellar Winds, Circumstellar Matter, and Activity
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

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[182.13] The Physical Properties of Red Supergiants in the Magellanic Clouds: An Abundance of Effects

E. M. Levesque (Lowell Observatory/MIT), P. Massey (Lowell Observatory), K. A. G. Olsen (Cerro Tololo Inter-American Observatory), B. Plez (Universite de Montpellier II), A. Maeder, G. Meynet (Geneva Observatory)

A long-standing mystery has been the difference found in the distribution of spectral subtypes for red supergiants (RSGs) in the SMC, LMC, and Milky Way, an effect first noted by Elias et al. (1985, ApJS, 57, 91) and recently confirmed with more complete data by Massey & Olsen (2003, AJ, 126, 2867). The distributions shift with metallicity: RSGs in the relatively metal-poor SMC show the earliest average type (K5-7 I), followed by the LMC (M1 I), with the Milky Way averaging the latest spectral type (M2 I).

In order to investigate the dependence of the physical properties of these evolved massive stars on metallicity, we modeled moderate-resolution (5Å) optical spectrophotometry of 36 RSGs in the LMC and 40 RSGs in the SMC using data obtained with the CTIO 4-m telescope. By comparing the TiO band strengths to those predicted by the MARCS stellar atmosphere models, we determined new effective temperature scales for these stars in both Clouds. Here we present these temperature scales and compare them to the effective temperature scale for Galactic RSGs (Levesque et al. 2005, ApJ, 628, 973).

We find that a K5-7 I star in the SMC actually has about the same effective temperature as an M2 I in the Milky Way, and that hence the differences in metallicity fully explain the variations in spectral type distribution. The new scale results in good agreement with the Geneva evolutionary models for the LMC RSGs. For the SMC RSGs, some stars agree well, while others appear to be too warm. This is consistent with the study of SMC A-type supergiants by Venn (1999, ApJ, 518, 405), who found an under-abundance of alpha elements, including Ti and O.

This work was partially funded by National Science Foundation Grants 00-093060 to P. M. and 99-88007 (NAU/REU).

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