Evidence for a Disk in the Wind of {HD93521}: UV Line Profiles from an Axisymmetric Model

Based on the {C iv} UV resonance line profile of {HD93521} ({O9.5V}), Massa (1992) and Howarth & Reid (1993) have suggested that there is a high speed component in the polar outflow from the star as well as a low speed component in the equatorial regions. We present theoretical calculations of the line profiles that would be produced by such a model. We find from HST observations of {HD93521} that the observed {C iv} profile can be produced if the star has an inclination angle very close to 90(deg) and if the star is surrounded by a thin disk, whose half width is approximately 3(deg) in latitude. The geometry of this disk is similar to that predicted by the Wind-Compressed Disk model of Bjorkman & Cassinelli (1993). In addition to the {C iv} resonance line, we examine both the {Si iv} and {N v} resonance lines. The {Si iv} line exhibits low velocity absorption that is similar to the {C iv} absorption, but the emission is much weaker. The absorption component is well fit by the disk-geometry, and it appears that the weaker emission from the polar component arises because {Si iv} is one stage below the dominant state, {Si v}. On the other hand, the {N v} line has weaker absorption and much stronger emission than the {C iv} line. This implies that {N v} is probably one stage above the dominant state. The correlation of the emission strength of these three ions with their ionization state suggests that there are latitudinal ionization gradients that occur because the density increases from the pole to the equator. Apart from fitting individual line profiles, we also examine the differences between two HST observations of {HD93521}. We find evidence for a pair of narrow absorption components, as well as evidence for a discrete emission feature in the blueshifted absorption cores of the lines. This blueshifted emission can cause what instead appears to be an interstellar absorption line. Without multiple observations that can reveal the temporary emission, one must be very careful when determining interstellar column densities to stars like {HD93521}.