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

The spectral variability of the cool hypergiant Cassiopeiae ^*

A. Lobel ^{** 1}, G. Israelian ^{1, 2, 4}, C. de Jager ³, F. Musaev ⁵, J.Wm. Parker ⁶ and A. Mavrogiorgou <sup>1

1</sup> 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 Cas from combined analyses of optical spectra of 1993-95 and IUE spectra of 1979-81. We find an upper range for 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 displays variable distortions of filling-in by emission with the pulsation. Our theoretical line profile fits yield a value for v sini of 25 .

We report the finding of two different causes for the splitting of absorption line cores in the spectra of 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 . These violet absorptions originate about 2.5 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 . We also report the finding of an increase of the supersonic wind velocity by 15 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 .

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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