Late-Type Red Supergiants: Too Cool for the Clouds?

Details Late-Type Red Supergiants: Too Cool for the Clouds? Late-Type Red Supergiants: Too Cool for the Clouds?

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.9306l&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #93.06; Bulletin of the American Astronomical Society, V

Astronomy and Astrophysics

Astronomy

Scientific paper

It has long been known that the median spectral types of red supergiants change from M2 I in the Milky Way to M1 I in the Large Magellanic Cloud (LMC) and to K5-7 I in the Small Magellanic Cloud (SMC) (Elias et al 1985, Massey & Olsen 2002). This is now understood in terms of the shifting of the evolutionary tracks to warmer temperatures with decreasing metallicity. Stars falling below the temperatures of these tracks would no longer be in hydrostatic equilibrium. This region of the H-R diagram is known as the Hayashi "forbidden zone". Early work identified supergiants no later than M2 I in the SMC, while in the Milky Way supergiants of spectral class M4 I and later abound.
However, our work has identified seven red supergiants in the LMC and four red supergiants in the SMC, all of which have spectral types that are considerably later than the average type observed in their parent galaxy. We find that these stars have radial velocities which are consistent with membership in the Clouds. By fitting our moderate-resolution spectrophotometry of these stars with MARCS stellar atmosphere models of the appropriate metallicities, we also determine their physical parameters and place them on the H-R diagram for comparison with the predictions of current stellar evolutionary tracks. We find that these stars are colder and more luminous than allowed by the predictions of stellar evolutionary theory at these low metallicities. Unsurprisingly, these stars also exhibit unusual variability in V. We then suggest that these stars have such unusual properties because they are in an unstable (and short-lived) phase of their evolutionary lives. This work was supported by the National Science Foundation through AST-0604569 to PM.

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