Metallic fingers and metallicity excess in exoplanets host stars : the accretion hypothesis revisited

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, is it 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 one 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” may be created which dilute the accreted matter inside the star. Introducing this effect may reconcile the overabundances expected in case of accretion with the observations in stars of different masses. Subject headings: exoplanets; stars : abundances; stars: accretion; diffusion; double-diffusive convection ; thermohaline convection