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

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1. Introduction

Mass loss from early type stars is thought to be of radiative origin. In many cases, the spectral lines of such stars indicate emission and form P Cygni profiles. This phenomenon is found in main-sequence stars and giants (Be stars) as well as supergiants (B[e] stars) (Doazan 1982). Moreover, IUE observations have shown very clearly that "shell" or "blob" characteristics appear frequently in spectral analyses of early type stars (e.g. Cassatella et al. 1979, Lamers et al. 1985, Lamers et al. 1988, Scuderi et al. 1994). The effects of these structures on the stellar spectrum have been reported by Lamers (1994). These shells are considered either stable or variable and in many cases the respective stars exhibit variability in spectral features. The formation of shells, shown in the spectrum, is also known for late-type supergiants (Weymann 1963, Bernat 1977). In these stars the radiation pressure effect is also important.

In this work we study the shell formation dynamics in a stellar atmosphere as a steady state hydrodynamic (HD) procedure. In fact, we apply the analytical 2-D solutions for thermally and radiatively driven stellar winds of Kakouris & Moussas (1997) assuming the stellar atmosphere optically thin. In addition, we used the analyses of Chen & Marlborough (1994) (hereafter CM) and Lamers (1986) for the radiative force based on optically thin spectral lines.

Kakouris & Moussas stellar wind solutions are thermo-radiative (i.e. thermally and radiatively driven). In Kakouris & Moussas (1997) (hereafter Paper II) the thermally driven solutions of Kakouris & Moussas (1996) (hereafter Paper I) were generalized to incorporate the differential rotation of the fluid and the optically thin radiative force (also Kakouris 1997). These solutions are global 2-D and self-consistent to HD, and [FORMULA] - self similar (Tsinganos & Sauty 1994).

The rest of this article is organized as follows: In Sect. 2 we introduce physical considerations for a HD steady state shell. In Sect. 3 we present briefly both thermal and radiative mechanisms which drive stellar winds and the Paper II model which is applied. In Sect. 4 we present solutions with shells for a hypothetical early type superluminous object, for a hypothetical late-type supergiant and for hypergiant P Cygni. Finally, in Sect. 5 we discuss the results and physical characteristics of the model.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998