Long-term periodic variability in UV absorption lines of the Be star γ Cassiopeiae: on the relation with V/R variations in the Hβ line.

Abstract

We present a quantitative study of the variability in ultraviolet resonance lines of NV, SiIV and CIV of the Be star γ Cas. For this purpose we used IUE spectra obtained over a period of eleven years. Variability occurs in the form of discrete absorption components (DACs), which are formed in the fast-outflowing radiatively driven part of the stellar wind. We constructed a template spectrum from spectra containing no or minor extra absorption due to DACs and modelled the isolated DACs in the obtained quotient spectra. Besides the frequently observed narrow components (v_t_ typically <=250 km/s) at high velocity, we found several broad components occurring at low and intermediate wind velocities. We confirm the finding of Doazan et al. (1987) who reported that the number of observed DACs is associated with the cyclic V/R variability of the Balmer-emission lines. We show that when V/R<1 the central optical depth of DACs is significantly lower than when V/R>1. In our interpretation this is due to a correlation between the column density associated with the DACs and the phase of the V/R cycle. We find that the Hβ observations of Doazan et al. are consistent with a model in which the cyclic V/R variability is due to a global, one-armed oscillation moving through an equatorial disc. We suggest that the higher column density of DACs in phases of V/R>1 is the result of the higher density in the region of their origin, namely close to or in the part of the equatorial disc which is rotating towards the observer.