Session 25 - Stars in the Ultraviolet.
Display session, Tuesday, June 10
South Main Hall,

## [25.11] ORFEUS Far Ultraviolet Observations of Cool Stars

A. K. Dupree, N. S. Brickhouse (Harvard-Smithsonian CfA), M. Hurwitz (Space Sciences Lab., UC Berkeley)

Far ultraviolet spectra (\lambda\lambda910--1216) of cool stars were obtained by the Berkeley spectrograph aboard the ORFEUS telescope under NASA's Guest Investigator program carried out during a 14-day ORFEUS-SPAS II mission in November/December 1996. Target stars included the single stars: \beta Dra (HD 6536, G2Ib-IIa), \alpha TrA (HD 150798, K2IIb-IIIa), \alpha Aqr (HD 209750, G2Ib), and the binary systems: UX Ari (HD 21242, G5V/K0IV), HR 1099 (HD 22468, G5IV/KIIV), and 44\iota Boo (HD 133640, G2V/G2V). The short period W UMa-type binary 44\iota Boo was observed at two orbital phases. The spectral resolution (\sim90 km\ s^-1) is sufficient to resolve emission from the two components of this system. Emission from chromospheric species of C III (\lambda977, \lambda1176), O VI (\lambda1032, \lambda1038), and Si III (\lambda1206) is present in all of the stars. Emission at H Ly-\beta (\lambda1025) is most likely produced both by the target star and the geocorona. Acquisition of a simultaneous spectrum of the nearby sky facilitates extraction of the stellar flux. Electron densities in the lower transition regions can be inferred from ratios of the C III transitions; the size of emitting regions can be estimated from the emission measure distribution. Models of the atmospheres will be presented in conjunction with coronal models.
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