The evolved B[e] star HD 87643: observations and a radiation-driven disc wind model for B[e] stars

Abstract

New high resolution spectroscopic and medium resolution spectropolarimetric data, complemented with optical broad and narrow band imaging, of the B[e] star HD 87643 are presented. The spectrum of HD 87643 exhibits the hybrid characteris- tics well known to be representative of the group of B[e] stars; a fast wind with an expansion velocity in excess of 1000 km s−1 is measured in the hydrogen and helium lines, while a slower component is traced by lower excitation lines and forbidden lines. Clues to the geometry of the rapidly expanding circumstellar shell are provided by the startling polarization changes across H . Comparison with published schematic calculations indicates that the polarizing material is located in a slowly rotating, ex- panding disk structure. A hydrodynamical model is then presented whose results are consistent with the original two-wind concept for B[e] stars and exhibits kinematic properties that may well explain the observed spectral features in HD 87643. The model calculations use as input a B star undergoing mass loss, surrounded by an op- tically thick disk. The resulting configuration consists of a fast polar wind from the star and a slowly expanding disk wind. The model also predicts that the stellar wind at intermediate latitudes is slower and denser than in the polar region.