Long term changes in ultraviolet resonance lines in gamma CAS

Astronomy and Astrophysics – Astronomy

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Absorption Spectra, B Stars, Iue, Shell Stars, Ultraviolet Astronomy, Carbon, Cassiopeia Constellation, Nitrogen, Silicon, Stellar Mass Ejection, Stellar Winds, Variable Stars

Scientific paper

High resolution ultraviolet spectra of the B 0.5 IVe star gamma Cas, obtained with International Ultraviolet Explorer (IUE) show spectacular changes in the profiles of the resonance doublets of C4, N5 and Si4. In addition to the broad assymetric low velocity absorption lines which seem to be always present, narrow, blue-shifted absorption components, with velocities up to 1500 km/s were observed on 4 occasions. The time dependent high velocity features are interpreted in terms of spherical high density shells, which travel outwards with a terminal velocity not exceeding 1500 km/s. To explain the origin of these shells intermittent enhancement of the stellar wind mass loss rate (duration tau = 1 day, enhancement factor rno greater than 2) is proposed. It is predicted that the column density of the narrow lines will fade out, and the absorptions will last not longer than a few weeks. Earlier observations (by Copernicus) suggest that UB shells probably occur in all early type stars, independent of spectral class. In this respect, gamma Cas might be not unique.

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