Stellar Chromospheres

Stellar chromospheres and transition regions contain the moderate temperature (5,000-500,000 K) gas located between the photosphere and corona. These regions are best studied using the emission lines they emit in the ultraviolet. We are analyzing HST spectra of active late-type stars and binary systems to develop models for these stars consistent with the data. Of special interest are stars that flare, RS CVn-type binary systems, M dwarfs, and host stars of exoplanetary systems. The above figure shows a portion of the far-ultraviolet spectra of seven dwarf stars observed with the Far Ultraviolet Explorer (FUSE) spacecraft and a comparison solar spectrum obtained with the SUMER instrument on SOHO. Important emission lines of H I (Lyman series), He II, C II, C III, O VI, and S VI formed in the chromospheres and transition regions of these stars between 8,000 K and 300,000 K are indicated. Also shown is the Fe XVIII line at 977 A formed in the coronae of these stars. We are analyzing these data to understand the dynamics and energy balance in these stellar atmospheres.

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