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Paper: EUVE Observations of 44 i Boo}: Evidence for Localized Emission
Volume: 154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop
Page: 993
Authors: Brickhouse, N. S.; Dupree, A. K.
Abstract: EUVE has obtained an EUV light curve and spectrum of 44 i Boo (HD 133640), a W UMa-type contact binary. A period search on the Deep Survey (DS) data obtained during the 6-day observation (19.5 epochs) reveals a period which is longer than the optical period by 1.8%. Unlike the optical light curve, with its two minima roughly corresponding to the geometrical eclipses of both stars, the DS light curve is a smooth, nearly sinusoidal modulation with only one minimum. The DS light curve cannot be produced by plasma predominantly in the neck region connecting the equatorial regions of the two stars, nor can it be produced by diffuse global emission closely associated with the optical surfaces of the two stars. The emission measure distribution derived from emission lines of highly ionized Fe is reminiscent of the Capella emission measure distribution (Dupree et al. 1993). The corona of 44 i Boo shows a narrow enhancement of emission measure at about 8 x 10^6 K. Density-sensitive line ratios suggest the presence of electron densities higher than 2 x 1013 cm^{-3}. We interpret the EUVE data as evidence for a localized emitting region of high density formed at high latitudes on the primary star, the coronal counterpart of polar active regions indicated by optical studies of rapidly rotating binaries (see Donati & Collier Cameron 1997; Vogt et al. 1997).
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