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Astron. Astrophys. 324, 1071-1082 (1997)

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Analytical 2-D solutions for hydrodynamic, thermally and radiatively driven, astrophysical outflows. Applications to B stars

Alexander Kakouris 1, 2 and Xenophon Moussas 1

1 Space Group, Section of Astrophysics, Astronomy and Mechanics, Physics Department, National University of Athens,
Panepistimiopolis GR-15783, Athens, Greece (akakour@atlas.uoa.gr, xmoussas@atlas.uoa.gr)
2 Hellenic Air Force Academy, Dekelia, Attiki, Greece

Received 25 March 1996 / Accepted 31 January 1997

Abstract

In this work, we deal with the two-dimensional problem of steady plasma outflow from rotating central astrophysical gravitational objects. Considering the stellar atmosphere optically thin and the radiative force (due to the central object's luminosity) radial, we obtain fully analytical solutions for thermally and radiatively driven outflows. The flow is helicoidal and axisymmetric and the plasma inviscid and non-polytropic. First, we generalize the Kakouris & Moussas 1996 solution imposing a generalized geometry in the flow velocity which implies differential fluid rotation. The solutions are of four types with velocity maxima either along the equator or along the polar axis of the central body. The dependence of the solutions upon the radial distance R is similar to Kakouris & Moussas 1996 case for each type of solutions. Just assuming that the radiative acceleration is a function of the distance we obtain analytical 2-D solutions for thermally and radiatively driven outflows. The inclusion of the radiative force helps the generation of the stellar wind giving higher maximum radial velocities. The velocity asymmetry between the equator and the poles varies with radial distance and it is sensitive upon the relative strength of the adopted radiative force as well as upon the degree of the fluid differential rotation. Several possible dependences of the radiative acceleration upon the radial distance are considered, and the deduction of the wind transition from a strong radiative driving to a pure non-radiative thermally driven outflow is presented by the given applications.

The incorporation of the radiative force in the hydrodynamic equations is very important in massive winds of early and late type main sequence stars and evolved giants and supergiants. We present analytical 2-D solutions for thermally plus radiatively driven stellar winds and we apply one kind of them to B5I type supergiants in order to understand the observed winds of these stars under a thermal (coronal) plus a radiative mechanism of ejecting stellar plasma in the interstellar medium. Maximum outflow velocities and mass loss rates, close to the observed, are easily obtained.

Key words: hydrodynamics – stars: winds – atmospheres – flows – radiation pressure – methods: analytical – B supergiants: winds – atmospheres – mass-loss

Send offprint requests to: A. Kakouris

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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