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

Abstract

The HST Treasury Program "Advanced Spectral Library Project: Cool Stars" was designed to collect representative, high quality ultraviolet 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 M2Iab 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 for numerous other cool, low-gravity stars for years to come.

ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

“…circumstellar CO fourth-positive absorption bands and chromospheric emission lines. Figure 5 shows that starting from 1700 Å, the observed T Rad slowly increases toward shorter wavelengths (Carpenter et al 2018), as is observed for the Sun (Avrett & Loeser 2008, Fig . 1), albeit at lower values.…”

“…olution Spectrograph (GHRS;Carpenter et al 1994 and the Space Telescope Imaging Spectrograph (STIS) of the HST (Carpenter et al 2018), and it is interesting to compare these observations with the FUV continuum generated by empirically derived VLA and ALMA radio temperature profiles. To do so, we performed time-independent nonlocal thermal equilibrium (NLTE) spherical radiative transfer computations for the formation of the Si I continua to the ground and first excited terms, which are expected to dominate the bound-free opacity between 1350 < λ(Å) < 1680.…”

ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

“…The FUV radiation field of Betelgeuse has been observed at high signal-to-noise ratio (S/N) with the Goddard High Resolution Spectrograph (GHRS) (Carpenter et al 1994) and the Space Telescope Imaging Spectrograph (STIS) of the HST (Carpenter et al 2018), and it is interesting to compare these observations with the FUV continuum generated by empirically derived VLA and ALMA radio temperature profiles. To do so, we performed time-independent nonlocal thermal equilibrium (NLTE) spherical radiative transfer computations for the formation of the Si I continua to the ground and first excited terms, which are expected to dominate the bound-free opacity between 1350 < λ(Å) < 1680.…”

Constraining Stellar Parameters and Atmospheric Dynamics of the Carbon AGB star V Oph

“…Alfvén waves have been found to be efficient in driving winds and produce clumpy mass loss from giant and supergiant stars (e.g. Vlemmings 2014; see also Hartmann & MacGregor 1982;Airapetian et al 2000;Suzuki 2007;Airapetian et al 2010Airapetian et al , 2015Yasuda et al 2019;Carpenter et al 2018, andFigure 7 in Rau et al 2018). In a weakly ionized but collisional shell, a propagating Alfvén wave would also involve the motion of the neutral species that are present in the shell.…”