The Advanced Spectral Library (ASTRAL): Reference Spectra for Evolved M Stars

The HST Treasury Program Advanced Spectral Library Project: Cool Stars was designed to collect representative, high-quality UV spectra of eight evolved F-M type cool stars. The Space Telescope Imaging Spectrograph (STIS) echelle spectra of these objects enable investigations of a broad range of topics, including stellar and interstellar astrophysics. This paper provides a guide to the spectra of the two evolved M stars, the M2 Iab supergiant α Ori and the M3.4 giant γ Cru, with comparisons to the prototypical K1.5 giant α Boo. It includes identifications of the significant atomic and molecular emission and absorption features and discusses the character of the photospheric and chromospheric continua and line spectra. The fluorescent processes responsible for a large portion of the emission-line spectrum, the characteristics of the stellar winds, and the available diagnostics for hot and cool plasmas are also summarized. This analysis will facilitate the future study of the spectra, outer atmospheres, and winds, not only of these objects but of numerous other cool, low-gravity stars, for years to come.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.