The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity

Emily M. Levesque (1, 2), Philip Massey (1), K. A. G. Olsen (3), Bertrand Plez (4), George Meynet (5), and Andre Maeder (5)

(1) Lowell Observatory; (2) MIT; (3) CTIO/NOAO; (4) GRAAL CNRS, Univ de Montpellier II; (5) Geneva Observatory

We present moderate-resolution optical spectrophotometry of 36 red supergiants (RSGs) in the LMC and 39 RSGs in the SMC. Using the MARCS stellar atmosphere models to fit this spectrophotometry, we determine the reddenings, effective temperatures and other physical properties, such as bolometric luminosity and effective stellar radii, and compare these to stellar evolutionary models. As a self-consistency check, we also compare the broad-band colors
(V-K)_0 and (V-R)_0 with the models. The (V-R)_0 results are in good agreement with those from fitting the optical spectrophotometry, but the (V-K)_0 results show metallicity-dependent systematic differences, amounting to 3-4% in effective temperature, and 0.2 mag in bolometric luminosity, at the metallicity of the SMC; we conclude that this is likely due to the limitations of static 1D models, as spectra of RSGs in the optical and IR may reflect different atmospheric conditions due to the large surface granulation present in these stars. We adopt the scales indicated by the optical spectrophotometry and (V-R)_0 colors, but accept that there is still some uncertainty in the absolute temperature scales. We find that the effective temperature scales for the LMC and SMC K-type supergiants agree with each other and with that of the Milky Way, while for M-type supergiants the scales are cooler than the Galactic scale by 50 K and 150 K, respectively. This is in the sense that one would expect: since the spectral classification of RSGs is based on the line strengths of TiO, stars with lower abundances of these elements have to be cooler in order to have the same strength. However, this effect is not sufficient to explain the shift in average RSG spectral type between the three galaxies. Instead, it is the effect that metallicity has on the coolest extent of the evolution of a star that is primarily responsible. Our new results bring the RSGs into much better agreement with stellar evolutionary theory, although the SMC RSGs show a considerably larger spread in effective temperatures at a given luminosity than do the LMC stars. This is expected due to the larger effects of rotational mixing in lower-metallicity stars, as higher helium abundance at the surface would lead to higher effective temperatures in the RSG phase. We also find that the distribution of reddening of RSGs in the Clouds is skewed significantly towards higher values, consistent with our recent finding that Galactic RSGs show extra extinction due to circumstellar dust.

Reference: ApJ, in press
Status: Manuscript has been accepted


Comments: Accepted by the Astrophysical Journal