EUVE Observations of 44i Boo: Coronal Structure and Variability

The EUV spectrum of 44i Boo (HD 133640), a W UMa-type contact binary observed with the \EUVE\ spectrometers during May 2-7 1994, shows strong emission from highly ionized Fe. Like the emission measure distributions of \Capella\ and other RS CVn binaries observed with \EUVE, 44i Boo requires a continuous distribution of plasma temperatures to account for the high temperature \EUVE\ spectra. A local enhancement of emission measure at about T_e = 8 x 10(6) K is reminiscent of the narrow feature found in \Capella\ (Dupree et al./ 1993, Ap. J., 418, L41), though less pronounced and at a somewhat higher temperature. Line ratio diagnostics from four ionization stages of Fe indicate extremely high densities (N_e ~ 5 x 10(13) cm(-3) ). The light curve obtained with the \EUVE\ Deep Survey (DS) instrument during the 6-day observation ( ~ 20 continuous epochs) shows variability with orbital phase with a single EUV light minimum near phase 0.5, as well as an erratic component. During the observation, fluxes from both the DS and the integrated SW spectrometer exhibited enhancements near phase 1.0. Since the DS flux originates predominantly from high temperatures ( ~ 10(7) K), as determined from the short wavelength (SW) spectra with a similar passband, this observation demonstrates for the first time that the coronal emission has a rotational component. The high density and the EUV light curve are consistent with small emitting volumes associated with one star.