Extreme Ultraviolet ExplorerObservations of the W Ursae Majoris Contact Binary 44i Bootis: Coronal Structure and Variability

Abstract

EUVE observations of 44i Boo (HD 133640), the first W UMa-type active binary observed with EUVE, give evidence for localized coronal structure. An EUV light curve has been obtained with the EUVE Deep Survey (DS) detector and Lexan/boron filter during the 6 day observation (19.5 continuous epochs) in 1994. A period search on the DS data stream reveals a period corresponding to 0.272652 ± 0.00141 days, longer than the optical period (0.267817 days) by 428 s. Since the DS flux originates predominantly from high-temperature plasma (T_e ~ 10⁷ K), this observation demonstrates for the first time an orbital phase-dependent component of the hot coronal emission of a contact binary, indicates the presence of an active region on the primary star, and signals a different rotation rate of the corona with respect to the photosphere. The high-temperature plasma shows no correspondence with the two partial eclipses per orbit found in the optical and in the chromospheric and transition-region emission. A flaring enhancement appears associated with maximum brightness. The EUV spectrum shows strong emission from highly ionized Fe. Like the emission measure distributions of Capella and other RS CVn binaries observed with EUVE, the spectrum of 44i Boo requires a continuous distribution of plasma temperatures to account for the relative Fe emission line fluxes. A local enhancement of emission measure at about T_e ~ 8 × 10⁶ K is reminiscent of the narrow feature found in Capella at T_e ~ 6 × 10⁶ K, although substantially broader. Line ratio diagnostics indicate extremely high densities, N_e ~ 2 × 10¹³ cm^-3—higher than has been observed for other EUVE coronal sources. We place a hard lower limit on the Fe/H abundance ratio of Fe/H > 0.46 with respect to the solar photospheric value. We compare our results with those derived from the ROSAT PSPC and conclude that the soft X-ray band in ROSAT observations does not give reliable abundances or low-temperature emission measures. The high electron density and peak emission measure together imply a small emitting volume, L³ (L ~ 0.004R). If all the EUV emission arises from such a volume, the EUV light curve places tight constraints on the possible location of a "hot spot," namely that it must be located at high latitudes (>72°) on the primary star.