The Physical Properties of Red Supergiants: Cool, But Not As Cool As We Thought!

Details The Physical Properties of Red Supergiants: Cool, But Not As Cool As We Thought! The Physical Properties of Red Supergiants: Cool, But Not As Cool As We Thought!

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.4002l&link_type=abstract

American Astronomical Society Meeting 210, #40.02; Bulletin of the American Astronomical Society, Vol. 39, p.153

Astronomy and Astrophysics

Astronomy

Scientific paper

Red supergiants (RSGs) are an important, short-lived, and poorly understood evolutionary phase of massive stars nearing the ends of their lives. We have used moderate-resolution optical spectrophotometry and the new generation of MARCS stellar atmosphere models to investigate the physical properties of these stars in the Milky Way and the Magellanic Clouds. Our newly determined parameters brought these RSGs into excellent agreement with stellar evolutionary theory, although for the Small Magellanic Cloud there are still some slight differences, with the RSGs remaining somewhat cooler than the predictions allow. We found that the most luminous RSGs show substantial amounts of circumstellar extinction (up to several magnitudes at V), consistent with the observed dust mass-loss rates. Although RSGs contribute only a small fraction of the dust production locally (which is dominated by AGBs), they should dominate in starburst galaxies or galaxies at large look-back times. We don't know much about the properties of such dust, although VY Cma offers a cautionary tale that in some stars the circumstellar dust may be considerably more “grey” than the dust in the ISM. We also examined metallicity’s role in the average RSG spectral type of these galaxies; although the distribution of spectral subtypes shifts with the galaxies’ metallicities, there are a few RSGs in the Clouds that have much later types than expected. Further studies found that these stars exhibited several traits previously unassociated with RSGs: variable V magnitudes, variable spectral subtypes, and placement in the “forbidden” region of the H-R diagram to the right of the Hayashi track, HV 11423 being the most extreme example. We suggest that these stars are in a unique, unstable, and short-lived evolutionary state. Work in progress extends these studies to RSGs of high metallicity (M31), and future work will extend to the lowest metallicity RSGs known, in WLM.

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