ABSTRACT. While the metallicity excess observed in the central stars of planetary systems is confirmed by all recent observations, the reason for this excess is still a subject of debate: is it primordial, or the result of accretion, or both? The basic argument against an accretion origin is related to the mass of the outer convective zones, which varies by more than 1 order of magnitude among the considered stars, while the observed overabundances of metals are similar. We show here that in previous discussions a fundamental process was forgotten: thermohaline convection induced by the inverse μ-gradient. "Metallic fingers" might be created that dilute the accreted matter inside the star. Introducing this effect may reconcile the overabundances expected in cases of accretion with the observations of stars of different masses.

Subject headings: accretion, accretion disks; convection; diffusion; planetary systems

Print publication: Issue 2 (2004 April 20)
Received 2003 September 11, accepted for publication 2004 January 9