A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars

• Published in: The Astrophysical journal 2005-03, Vol.622 (1), p.629-652
• Main Authors: Dupree, A. K, Lobel, A, Young, P. R, Ake, T. B, Linsky, J. L, Redfield, S
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/428111

The Far Ultraviolet Spectroscopic Explorer (FUSE) ultraviolet spectra of eight giant and supergiant stars reveal that high-temperature (3 x 10 super(5) K) atmospheres are common in luminous cool stars and extend across the color-magnitude diagram from alpha Car (F0 II) to the cool giant alpha Tau (K5 III). Emission present in these spectra includes chromospheric H Ly beta , Fe II, C I, and transition region lines of C III, O VI, Si III, and Si IV. Emission lines of Fe XVIII and Fe XIX signaling temperatures of similar to 10 super(7) K and coronal material are found in the most active stars, beta Cet and 31 Com. A short-term flux variation, perhaps a flare, was detected in beta Cet during our observation. Stellar surface fluxes of the emission of C III and O VI are correlated and decrease rapidly toward the cooler stars, reminiscent of the decay of magnetically heated atmospheres. Profiles of the C III lambda 977 lines suggest that mass outflow is underway at T similar to 80,000 K and the winds are warm. Indications of outflow at higher temperatures (3 x 10 super(5) K) are revealed by O VI asymmetries and the line widths themselves. High-temperature species are absent in the M supergiant alpha Ori. Narrow fluorescent lines of Fe II appear in the spectra of many giants and supergiants, apparently pumped by H Ly alpha , and formed in extended atmospheres. Instrumental characteristics that affect cool star spectra are discussed.