ON STEADY SHELL FORMATION IN STELLAR ATMOSPHERES - I - ANALYTICAL 2-DCALCULATIONS UNDER AN OPTICALLY THIN THERMO-RADIATIVE MECHANISM

In this work we study a possible physical mechanism which is able to c reate steady state shells around astrophysical objects. This mechanism is thermo - radiative and it is applied to optically thin stellar atm ospheres. An outflow deceleration region separates the rest of the ste llar envelope into inner and outer acceleration regions. The shell is formed in the supersonic region of the outflow. Studying the dynamic n ature of the shells, it is found that the shell distance depends on th e thin spectral line opacity and the number of thin lines. The shape o f the shell depends on the differential rotation of the fluid. It is f ound that the mass concentration may look like a shell or a double blo b 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) fo r the acceleration mechanism in the envelope of P Cygni. The shell cha racteristics are deduced through applications to superluminous early t ype supergiants. Applications to late type supergiants as well as P Cy gni are shown. It is found that superluminous supergiants are expected to form steady shells and an example for P Cygni illustrates such a s hell at similar to 5 stellar radii. The present three - zone envelope for P Cygni resembles that of Nugis, Kolka & Luud (1979) but uses diff erent driving outflow mechanisms. The parameters used in this article are in accordance with previous works and several observational data a re reproduced successfully by the model.