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M. A. Smith (CSC/STScI/MAST), R. D. Robinson (IACS/CUA)
To understand the cause of reported rapid UV variations from the B0.5e star gamma Cas, we obtained continuous HST/GHRS large-aperture spectra centered on the SiIV 1394A, 1403A lines over a timespan of 22.5 hours and at a cadence of 1 per minute. In previous work we found that the star has multiple UV/X-ray activity centers which rotate into view every rotation cycle, estimated as ~1.12 days. An ultraviolet light curve constructed from our spectra shows two ~1 clouds forced into corotation. We examine herein spectral absorption line variations apart from the photospheric/DAC SiIV lines. These features vary on timescales from minutes to several hours. From Hubeny model atmosphere codes, we identify these features as optically thick absorptions from several ions formed in ``cool" (T < 10,000K), ``warm" (10,000--18,000K), and ``hot" (> 30,000K) sources. The cool/hot plasma lines vary in phase with the light curve whereas the light curves from warm ions lead this curve by 3 hours. All these lines exhibit either of two types of kinematical behavior: (1) blue-to-red motion consistent with plasma trapped incorotating clouds or (2) uniform motion of hot plasma over several hours at either low or high (~+1500 km/s) velocities.
The existence of cool/hot plasmoids above gamma Cas implies the importance of surface magnetic processes. We suggest that the high constant-velocity features arise from mass ejections triggered by interactions between magnetic loops from the star and the circumstellar decretion disk. This picture might explain the hot, flaring character of this star's X-rays and predicts the existence of other gamma Cas-analogs: magnetic, X-ray emitting Be stars with dense disks.
The author(s) of this abstract have provided an email address for comments about the abstract: msmith@stsci.edu