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Astron. Astrophys. 325, 714-724 (1997)

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

Cornelis  de  Jager 1, Alex  Lobel * 1, 2 and Garik  Israelian 2, 3

1 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2 Astronomy Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
3 Instituto 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 H [FORMULA] 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 H [FORMULA] 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 H [FORMULA] line profile show that the shocks, in association with the observed time-dependent variation of [FORMULA] can reproduce aspects of the variable emission in H [FORMULA].

These three aspects of this star, viz. the observed rate of mass loss, the observed supersonic 'microturbulence', as well as the H [FORMULA] 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

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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