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Astron. Astrophys. 330, 659-675 (1998)


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The spectral variability of the cool hypergiant [FORMULA] Cassiopeiae *

A. Lobel ** 1, G. Israelian 1, 2, 4, C. de Jager 3, F. Musaev 5, J.Wm. Parker 6 and A. Mavrogiorgou 1

1 Astronomy Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
2 Chatterton Astronomy Department and RCfTA, School of Physics A28, University of Sydney, 2006 NSW, Australia
3 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands, Spain
5 Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz, Stavropol'skii Kraii, 357147, Russia
6 Southwest Research Institute, Suite 429, 1050 Walnut Street, Boulder, CO 80302, USA

Received 30 January 1997 / Accepted 12 August 1997

Abstract

We checked the change of the effective temperature with the atmospherical pulsation of [FORMULA] Cas from combined analyses of optical spectra of 1993-95 and IUE spectra of 1979-81. We find an upper range for [FORMULA] 750 K over a period of 16-17 months. We present a thorough study of the related absorption line profile changes from which a velocity stratification with excitation energy could not be detected for the metallic lines. The distinct evolution of H [FORMULA] displays variable distortions of filling-in by emission with the pulsation. Our theoretical line profile fits yield a value for v sini of 25 [FORMULA].

We report the finding of two different causes for the splitting of absorption line cores in the spectra of [FORMULA] Cas. For the phase near highest effective temperature we have analyzed the development of very far shortward extended line wings assuming velocities up to 150 [FORMULA]. These violet absorptions originate about 2.5 [FORMULA] above the photosphere in a cool and variable supersonic wind from which we determine an upper value of the mass-loss rate of 9.2 [FORMULA]. We also report the finding of an increase of the supersonic wind velocity by [FORMULA] 15 [FORMULA] per electronvolt decrease, measured from the maximum velocities of these violet absorptions for Fe I lines.

A separate analysis of the IUE spectra reveals tremendous changes of the Balmer continuum flux by a factor 4 to 5 in only 26 hours, clearly distinct from the long-term variations of H [FORMULA].

Key words: stars: oscillations; variables; atmospheres; supergiants; ae Cas – line: identification – line: profiles

* Partly based on observations taken at the La Palma Observatory, the Special Astrophysical Observatory and with the International Ultraviolet Explorer
** Research Assistant of the Fund for Scientific Research - Flanders (Belgium)

Send offprint requests to: A. Lobel (alexl@ster1.vub.ac.be)

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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