Coordinated optical and ultraviolet observations of IM pegasi

IM Peg (HR 8703, HD 216489) is a moderately active, single-lined RS CVn binary for which we have obtained contemporaneous IUE and optical observations over two seasons. The UV lines are modulated with phase such that the UV emission maxima occur simultaneously with the visible light curve minimum. The UV emission-line modulation increases as a function of height, suggesting that the emission originates in loop-like structures associated with starspot regions. Optical lines such as Hα, Hβ, and the Ca II infrared triplet show some degree of modulation as well. Hα variability is dominated by stochastic events, while Hβ shows excess absorption relative to a standard star. Thus, the hydrogen line emission may not be correlated with the star-spots responsible for the photometric and UV variability, but rather may arise from an extensive chromospheric network. The Ca II IRT shows a clear modulation with phase; the degree of the modulation suggests that it arises in the chromosphere. IM Peg appears to fit the solar paradigm generally used to explain RS CVn activity much better than more active systems, for which simple extrapolations of the Sun should be viewed with caution. For less active systems such as IM Peg, however, the visible lines alone appear to provide an excellent chromospheric diagnostic.