A Multiwavelength Campaign on γ Cassiopeiae. II. The Case for Corotating, Circumstellar Clouds

Abstract

Simultaneous X-ray/UV observations over a full day on 1996 March 14-15 have been made of the prototypical B0.5e star γ Cas using the Rossi X-Ray Timing Explorer satellite and the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. The GHRS spectra, taken in the region of the Si IV λλ1394-1403 doublet, also permitted the construction of an extremely precise light curve from a nearby "pseudocontinuum" region. The continuum UV and X-ray light curves reveal a pair of X-ray maxima ~10 hr apart that coincide in time with UV continuum flux "dips" of ~1%. In the first paper in this series we attributed the long-term X-ray variations to magnetic activity sites on the surface of the star that undergo rotational modulation on a ~1.125 days period. In the current study we find that flux and color curves generated from a 33 hr sequence of International Ultraviolet Explorer (IUE) echellegrams obtained in 1996 January display dip features similar to those in the GHRS data. Comparing the timings of the continuum flux dips and the Si IV line strength variations in both the GHRS and IUE data sets gives a slightly revised period of 1.123 days for both the UV and X-ray activities. This strengthens the argument that high-energy activity on γ Cas is modulated by rotation of long-lived structures close to its surface. Analysis of the pseudocontinuum light curves constructed from the GHRS and IUE light curves shows at least two surprising characteristics for the flux dips: (1) the dips last only ~0.3 cycles, which is too brief for rotation modulation of surface features, and (2) their amplitudes increase from long to short wavelengths, which attain a maximum near 1206 Å. The character of the variations of the photospheric Si IV line profiles is unexpected in that the equivalent width fluctuations do not correlate with the slow undulations of the continuum flux. Moreover, the profile variations do not show an expected blue-to-red migration of microfeatures. We show that the continuum characteristics and absence of migration of features in the Si IV lines can be explained by the presence of very cool, optically thin clouds that corotate with the star. Assuming a tilt of the rotational axis of +45° to the observer's line of sight, our model simulations of the two major dips in the UV light curves indicate that the clouds have radii of a few tenths of a stellar radius and are attached to points on the surface at low to mid-latitudes on the near hemisphere. These findings support the conclusion of the first paper in this series that γ Cas is a member of a small group of OB stars that have magnetospheres associated with X-ray activity.