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Astron. Astrophys. 333, 678-686 (1998)

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On steady shell formation in stellar atmospheres

I. Analytical 2-D calculations under an optically thin thermo-radiative mechanism

Alexander Kakouris 1, 2 and Xenophon Moussas 1

1 Section of Astrophysics, Astronomy and Mechanics, Physics Department, National University of Athens, Panepistimiopolis GR-15784, Athens, Greece (akakour@atlas.uoa.gr; xmoussas@atlas.uoa.gr)
2 Hellenic Air Force Academy, Dekelia, Attiki, Greece

Received 17 September 1997 / Accepted 30 January 1998

Abstract

In this work we study a possible physical mechanism which is able to create steady state shells around astrophysical objects. This mechanism is thermo - radiative and it is applied to optically thin stellar atmospheres. An outflow deceleration region separates the rest of the stellar envelope into inner and outer acceleration regions. The shell is formed in the supersonic region of the outflow. Studying the dynamic nature of the shells, it is found that the shell distance depends on the thin spectral line opacity and the number of thin lines. The shape of the shell depends on the differential rotation of the fluid. It is found that the mass concentration may look like a shell or a double blob over the poles of the central object. The present thermo - radiative mechanism is based on the analytical 2 - D, hydrodynamic solution of Kakouris & Moussas (1997) and the analysis of Chen & Marlborough (1994) for the thin radiative force as well as the work of Lamers (1986) for the acceleration mechanism in the envelope of P Cygni. The shell characteristics are deduced through applications to superluminous early type supergiants. Applications to late type supergiants as well as P Cygni are shown. It is found that superluminous supergiants are expected to form steady shells and an example for P Cygni illustrates such a shell at [FORMULA] stellar radii. The present three - zone envelope for P Cygni resembles that of Nugis, Kolka & Luud (1979) but uses different driving outflow mechanisms. The parameters used in this article are in accordance with previous works and several observational data are reproduced successfully by the model.

Key words: hydrodynamics – methods: analytical – stars: atmospheres – supergiants – stars: mass-loss

Send offprint requests to: A. Kakouris

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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