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Astron. Astrophys. 339, L5-L8 (1998)

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1. Introduction

In a series of papers (Langer 1997, 1998; Langer et al. 1998) the implications of rotation for stellar mass loss and the Eddington limit were investigated recently. In these studies the classical Eddington limit was claimed not to be applicable in rotating stars. Rather a new limit for "hydrostatic stability" was derived and denoted by "[FORMULA]-limit". It consists of the existence of a critical rotation speed for the object similar to the standard critical rotation speed. Contrary to the latter, however, in its derivation radiative acceleration is taken into account with the consequence that the critical rotation speed vanishes as the star approaches the Eddington limit. Thus, any star having a finite rotation rate will - irrespective of its magnitude - critically rotate (by virtue of the [FORMULA]-limit) before reaching the Eddington limit.

Subsequently, taking a study on radiation-driven stellar mass loss in the presence of rotation (Friend & Abbott 1986) and replacing the classical by the revised critical rotation rate a rotation-dependent mass loss rate was modelled and implemented in stellar evolution calculations. The mass loss rate adopted diverges at the - revised - critical rotation rate and therefore implies stellar evolution of massive stars to be controlled by rotation and to proceed at the [FORMULA] limit for a wide range of initial conditions and over a significant fraction of the object's lifetime.

None of the studies mentioned considers the effect of gravity darkening (von Zeipel 1924, Tassoul 1978). The latter reduces the radiative acceleration at the equator and is therefore expected to influence the occurrence of the [FORMULA]-limit significantly. This is the motivation for the present investigation dealing with the conditions for critical rotation and the Eddington luminosity when gravity darkening is taken into account (Sect. 2). In Sect. 3 implications for a rotation-dependent mass loss rate will be discussed. Our conclusions follow.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998