The Wind Outflow of zeta Aurigae: A Model Revision Using Hubble Space Telescope Spectra

Abstract

Using high-resolution spectra taken with the Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) and with the International Ultraviolet Explorer (lUE) satellite, we reconsider the wind model of ζ Aurigae (K4 Ib + B5 V). The high quality of the HST data permits a critical test of the simplifying assumptions previously used to analyze lUE spectra. We find the line profiles observed with the GHRS show a complex structure, which seems to be inconsistent with the model of a spherically expanding envelope.

With detailed line profile calculations, we show that these observations are essentially compatible with a spherical wind outflow in a global sense. However, the distribution of ions must be corrected for ionization effects induced by the B star UV radiation field. We are able to retain the "classical" model by introducing some ad hoc modifications and confirm the earlier wind analyses of ζ Aur. We obtain a mass-loss rate of 5 x 1O^-9 M_sun yr^-1 and a terminal wind velocity of 70 km s^-1. The deduced line- broadening velocities clearly decrease with the radial position. The microturbulence is about 20 km s^-1 in the inner envelope (r ≥ 1.2R_sg) and reaches a minimum of 8 km s^-1 at large distances (r ≥ 25R_sg).

To demonstrate some important effects and to support the reliability of our results, we present a series of model calculations that show the sensitivity of the line formation to the large-scale wind properties. The emission components of strong resonance lines are formed in a region extending up to 50 K star radii and are only slightly dependent upon the global wind geometry. We show that resonance scattering influences considerably the line formation and may distort all analyses that assume pure absorption.