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Astron. Astrophys. 325, 714-724


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Turbulence, mass loss and H emission by stochastic shocks
in the hypergiant Cassiopeiae

Cornelis de Jager1, Alex Lobel1,2,*, and Garik Israelian2,3

1SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2Astronomy Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
3Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Canary Islands, Spain

Received 17 February 1997 / Accepted 28 March 1997

Abstract

The hypergiant FORMULA Cas is known for its variable rate of mass loss, with an average value of about FORMULA , and the supersonic value for the line-of-sight component of the microturbulent velocity, about 11 FORMULA . Emission components in HFORMULA suggest the presence of a thermally excited outer atmospheric region.

Since hydrodynamical turbulence in a stellar atmosphere turns rapidly into a field of shock waves, and shock waves are known to be able to initiate a stellar wind and heat stellar atmospheric layers, we have tried to predict the rate of mass loss, the microturbulent velocity component and the observed HFORMULA profile by assuming a stochastic field of shock waves. To that end we adopted a Kolmogoroffian spectrum of shock waves, characterized by only one parameter: the maximum Mach number in front of the shocks: FORMULA . Behind every shock a thin hot region originates. Spectroscopically, the thermal motions in these sheetlike regions cannot be distinguished from the stochastic hydrodynamic (shock wave) motion component, and therefore these hot regions add to the line broadening and will also contribute to the observed 'microturbulence'.

We find that it is indeed possible to explain the observed rate of mass loss (we derived log FORMULA ), as well as the high value for the quasi-microturbulence (we calculated FORMULA 12 FORMULA ). The hot sheets behind the shocks appear to be responsible for the observed 'microturbulence'; this thermal contribution is much larger than that of the hydrodynamic (shock) motions, which is only 0.4 to 0.5 FORMULA . Non-LTE calculations of the HFORMULA line profile show that the shocks, in association with the observed time-dependent variation of Teff can reproduce aspects of the variable emission in HFORMULA .

These three aspects of this star, viz. the observed rate of mass loss, the observed supersonic 'microturbulence', as well as the HFORMULA line profile can be simulated by one parameter only: viz. FORMULA =1.06 to 1.08, a value that characterizes a fairly weak shock-wave field.

Key words: stars: atmospheres - stars: supergiants - shock waves - turbulence - stars: mass loss

*Research Assistant of the Fund for Scientific Research - Flanders (Belgium)

Send offprint requests to: C. de Jager


© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998
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