Astron. Astrophys. 341, 560-566 (1999)

1. Introduction

Abundance anomalies are observed in many different types of star - the photospheres of stars with effective temperatures hotter than K may have undergone chemical separation (Michaud 1970). It is generally accepted that this may be explained via the force on atoms by radiation: if this is larger than gravity, then the element will diffuse toward the surface, else it will settle. The force due to radiation pressure on elements in a star has been discussed at length by e.g. Michaud et al. (1976) Vauclair et al. (1978), Aleican & Artru (1990) and Gonzalez et al. (1995). Abundance anomalies have been discussed in a variety of chemically peculiar stars eg. Am and Fm stars (Charbonneau & Michaud 1991, Alecian 1996),  Bootis stars (Michaud & Charland 1986) and magnetic Ap-Bp stars (Alecian & Vauclair, 1981, Michaud et al. 1981). When the star is also rotating, the surface abundance anomalies generated by radiation pressure may be destroyed by the action of turbulent motions within the star (Charbonneau 1992, Talon & Zahn 1997). These arise from shear motions created by the meridional motions which arise due to gravity darkening (see eg. von Zeipel 1924, Tassoul & Tassoul 1982, Zahn 1992)

There is some observational evidence for latitudinal abundance variations of heavy elements in planetary nebulae (Balick et al. 1994). This is possibly explained by more highly processed material being ejected faster but at later times than the rest of the shell. However, the observations raise an interesting question: it is possible that there are stars which are chemically inhomogeneous in both radial and latitudinal directions?

Here a non-magnetic mechanism which generates latitudinal abundance variations is suggested and investigated. It is found that a significant latitudinal radiative acceleration on heavy elements exists in rotating stars due to gravity darkening (von Zeipel 1924), which causes ionic species to diffuse toward the equatorial plane. Competing with this equatorward drift are the meridional circulation currents, and turbulence generated via shear motions. The aim of this paper is solely to assess whether any latitudinal drift of heavy elements may occur in rotating stars.

The transport of chemicals is discussed in Sect. 2. In Sect. 3 the radiative force due to gravity darkening is calculated and in Sect. 4 the drift velocity of metals around an equipotential is derived. Numerical results of the diffusion velocity in stellar envelopes are presented in Sect. 5, and by considering the relevant timescales, Sect. 6 identifies the important regions within a star for diffusion to occur. A discussion is presented in Sect. 7, and conclusions given in Sect. 8.

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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