## Turbulence, mass loss and H emission by stochastic shocks in the hypergiant Cassiopeiae
^{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
The hypergiant Cas is known for its variable rate of mass loss, with an average value of about , and the supersonic value for the line-of-sight component of the microturbulent velocity, about 11 . Emission components in H 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 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: . 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 ), as well as the high value for the quasi-microturbulence (we calculated 12 ). 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 . Non-LTE calculations of the H line profile show that the shocks, in association with the observed time-dependent variation of can reproduce aspects of the variable emission in H . These three aspects of this star,
* Research Assistant of the Fund for Scientific Research - Flanders (Belgium)
## Contents- 1. The extreme properties of the hypergiant Cas
- 2. Input data for the 'smooth background atmosphere'
- 3. Motion field between shocks
- 4. Stochastic spectrum of shock waves
- 5. Temperature distribution in a shocked atmosphere
- 6. Shocked atmospheric model on a -scale
- 7. Rate of mass loss
- 8. Quasi-turbulence: the notion of microturbulence in a shocked atmosphere
- 9. Application to the case of Cas
- 10. Calculated H line profile from a shocked atmosphere
- 11. Conclusions
- Acknowledgements
- References
© European Southern Observatory (ESO) 1997 Online publication: April 28, 1998 |