Evolved stars suggest an external origin of the enhanced metallicity in planet-hosting stars

PASQUINI, L.; DÖLLINGER, M. P; WEISS, A.; GIRARDI, L.; CHAVERO, C.; HATZES, A. P.; DA SILVA, L.; SETIAWAN, J.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2007 vol. 473 p. 979 - 982

0004-6361

Aims: Exo-planets are preferentially found around high metallicity main sequence stars. We investigate whether evolved stars share this property, and its implications for planet formation. Methods: Statistical tools and the basic concepts of stellar evolution theory are applied to published results as well as our own radial velocity and chemical analyses of evolved stars. Results: We show that the metal distributions of planet-hosting (P-H) dwarfs and giants are different, and that the latter do not favor metal-rich systems. Rather, these stars follow the same age-metalicity relation as the giants without planets in our sample. The straightforward explanation is to attribute the difference between dwarfs and giants to the much larger masses of giants´ convective envelopes. If the metal excess on the main sequence is due to pollution, the effects of dilution explain why this is not observed in evolved stars. Conclusions: Although we cannot exclude other explanations, the lack of any preference for metal-rich systems among P-H giants could be a strong indication of the accretion of metal-rich material. We discuss further tests, as well as some predictions and consequences of this hypothesis.