A Multiwavelength Campaign on γ Cassiopeiae. IV. The Case for Illuminated Disk-enhanced Wind Streams
On 1996 March 14-15 we conducted a campaign with the Hubble Space Telescope GHRS to observe the Si IV λλ1394, 1403 lines of the B0.5e star γ Cas at high temporal and spectral resolution. As a part of this ~22 hr campaign, the Rossi X-Ray Timing Explorer (RXTE) was also used to monitor this star's copious and variable X-ray emission. In this fourth paper of a series we present an analysis of the rapid variations of the discrete absorption components (DACs) of the Si IV doublet. The DACs attain a maximum absorption at -1280 km s-1, taper at higher velocities, and extend to -1800 km s-1. The DACs in this star's resonance lines have been shown to be correlated with a >~6 yr cycle in the Balmer line emission V/R ratio, and in 1996 this DAC strength was near its maximum. We derive hydrogen densities of 109-1010 cm-3 in the DAC material using a curve-of-growth method and find that the plasma becomes marginally optically thick near -1280 km s-1. The ``mean DAC'' probably represents a broad ``plateau'' with a volume density intermediate between the star's midlatitude wind and equatorial disk. We also follow the blueward evolution of subfeatures in the DACs. These features appear to emanate primarily from one or two discrete azimuths on the star and accelerate much more slowly than expected for the background wind, thereby exhibiting an enhanced opacity spiral stream pattern embedded within the structure forming the DAC. In the first two papers in this series, we suggested that active X-ray centers are associated with at least two major cool clouds forced into corotation. Several correlations of flickering in the Si IV DACs are found in our data, which support the idea that changes in X-ray ionizing flux cause changes in the ionization of material at various sectors along the spiral pattern. We demonstrate that similar flickering is visible in archival IUE data from 1982 and may also be responsible for earlier reports from Copernicus of rapid changes in this star's UV and optical lines. Finally, we discovered that flickering of the DAC fluxes in the 1982 data is correlated with rotation phase and shows a modulation with a 7.5 hr cyclical cessation of X-ray flares that was observed recently by RXTE. This confirms our basic picture that lulls in X-ray activity close to the star's surface cause both a lower Si V ionization fraction and an increase in Si IV variability within the DAC structures.