This is the third report appearing in these newsletters of a optical/UV/X-ray campaign carried out on the prototypical B0.5e star Gamma Cas about two years ago. The thrust of the first part of our work was to show that the X-rays from this source can be easiest understood as originating from near the surface of the Be star itself, with no hypothetical companion needed (Paper I). Below we argue that magnetospheric lobes exist over the star. The work described in the former and current report have been submitted to the ApJ for publication and is currently in the refereeing process. The authors of this Paper II are Smith, R. Robinson, and A. Hatzes. If further interested, you may also consult a Web site of a press release on this subject at: http://oposite.stsci.edu/pubinfo/pr/1998/07/pr.html. An article for Mercury magazine is further planned for later this year. Simultaneous X-ray and UV observations over a full day on 14-15 March 1996 have been made of Gamma Cas using the RXTE 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 1394-1403 doublet, also permitted the construction of a light curve from a nearby ``pseudo-continuum'' region. The continuum UV and X-ray light curves from this effort reveal a pair of X-ray maxima ~10 hours apart which coincide in time with UV continuum flux ``dips'' of 1%. In Paper I of this series we attributed the UV dip features to magnetic activity sites on the star's surface which undergo rotational modulation (RM) on a ~1.125 day period. In the current study we find that flux and color curves generated from a 33-hour sequence of IUE echellograms obtained in January 1996 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 datasets 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 Gamma Cas over is modulated by rotation of long-lived structures close to its surface. Analysis of the pseudo-continuum high-quality, monochromatic light curve constructed from the GHRS spectra and lower quality color curves constructed from the IUE echellograms over a broad range of wavelengths shows at least two surprising characteristics for the flux dips: (1) the dips last only ~0.3 cycles. This duration is too brief for rotational modulation of surface features, and (2) their amplitudes increase from long to short wavelengths, attaining a maximum near 1206 Angstroms. The IUE color curves are shown as Figure 1.
FIGURE 1. Flux ratio curves as a function of wavelength for Gamma Cas, taken during 1996 Jan. 18-19 (zero pts. arbitrary). The character of the variations of the photospheric Si IV line profiles is also unexpected in that the profile 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. Except for an interval when they showed a pair of general weakenings and strengthenings, the photospheric Si IV line profiles do not show clear patterns. During this same interval these variations showed a correlated time-lag as one samples fluxes redward across the line profile. We suggest that these amorphous variations are not caused on the surface but rather by wind inhomogeneities from active regions on the star impacting at the inner edge of the star's circumstellar disk. We have modeled the attenuation of flux with wavelength due to an occulting ``cloud'' which transits Gamma Cas with Hubeny's suite of ``CIRCUS/SYNSPEC'' model atmosphere program. The result of these simulations is shown in Figure 2. The solid lines in this figure depict the flux attenuations caused by three models a cloud having a temperature of 9000K, 7000K, or 5000K. The uncertainties in the observed amplitudes at various wavelengths (triangles) decrease from about the size of the symbol at short wavelengths to about half the size at long wavelengths.
FIGURE 2. Comparison of observed vs. theoretical line-blanketed color variations for a cool occulting cloud. We interpret the monotonically increasing absorption toward short wavelengths, as being due to the strength of a strong Lyman-alpha line which can occur only in a cool, essentially recombined, gas. Such cool gas might co-exist near a hot source like a B0.5 star in the presence of thermal instabilities, similar to those occurring in active coronal canopies of cool stars; these are fed from the surface by a nonthermal energy sources. Finally, assuming a tilt of the rotational axis of 45o to the observer's line of sight from optical interferometry, our model simulations of the two major dips in the UV light curves (see our last report) 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. In all, these results support Paper I's conclusion that Gamma Cas is a member of a small group of OB stars which have magnetospheres associated with X-ray activity. |
