High‐ResolutionChandraSpectroscopy of γ Cassiopeiae (B0.5e)

gamma Cas has long been famous for its unique hard X-ray characteristics. Wereport herein on a 53 ks Chandra HETGS observation of this target. Aninspection of our spectrum shows that it is quite atypical for a massive star,with abnormally weak Fe XXV, XXVI lines, Ly-alpha lines of H-like species fromFe XVII, XXIII, XXIV, S XVI, Si XIV, Mg XII, Ne X, O VII, VIII, and N VII.Also, line ratios of the rif-triplet of for a few He-like ions XVII areconsistent with the dominance of collisional atomic processes. Yet, thepresence of Fe and Si fluorescence K features indicates that photoionizationalso occurs in nearby cold gas. The line profiles indicate a mean velocity atrest and a broadening of 500 km/s. A global fitting analysis of the line andcontinuum spectrum finds that there are 3-4 plasma emission components. Thedominant hot (12 keV) component and has a Fe abundance of 0.22 solar. Somefraction of this component (10-30%) is heavily absorbed. The other 2-3components, with temperatures 0.1, 0.4, 3 keV, are "warm," have a nearly solarcomposition, a lower column absorption, and are responsible for most otheremission lines. The strength of the fluorescence features and the dual-columnabsorption model for the hot plasma component suggest the presence near the hotsites of a cold gas structure with a column density of 10^23 cm^-2. Since thisvalue is consistent with theoretical estimates of the vertical disk column ofthis star, these attributes suggest that the X-rays originate near the star ordisk. It is possible that the Fe anomaly in the hot component is related to theFirst Ionization Potential effect found in coronal structures around activecool stars. This would be yet another indication that the X-rays -rays areproduced in the immediate vicinity of the Be star.Comment: 32 pages, 4 figures (Fig. 3 colorized.) To be published in 01/10/04 Astrophysical Journal, Main Journal; included figures and updated formattin