Chemical signatures of planets: beyond solar-twins

摘要

Context. Elemental abundance studies of solar twin stars suggest that the solar chemical composition contains signatures of the formation of terrestrial planets in the solar system, namely small but significant depletions of the refractory elements. Aims. To test whether these chemical signatures of planets are real, we study stars which, compared to solar twins, have less massive convective envelopes (therefore increasing the amplitude of the predicted effect) or are, arguably, more likely to host planets (thus increasing the frequency of signature detections). Methods. We measure relative atmospheric parameters and elemental abundances of two groups of stars: a “warm” late-F type dwarf sample (52 stars), and a sample of “metal-rich” solar analogs (59 stars). The strict differential approach that we adopt allows us to determine with high precision (errors ~0.01?dex) the degree of refractory element depletion in our stars independently of Galactic chemical evolution. By examining relative abundance ratio versus condensation temperature plots we are able to identify stars with “pristine” composition in each sample and to determine the degree of refractory-element depletion for the rest of our stars. We calculate what mixture of Earth-like and meteorite-like material corresponds to these depletions. Results. We detect refractory-element depletions with amplitudes up to about 0.15?dex. The distribution of depletion amplitudes for stars known to host gas giant planets is not different from that of the rest of stars. The maximum amplitude of depletion increases with effective temperature from 5650?K to 5950?K, while it appears to be constant for warmer stars (up to 6300?K). The depletions observed in solar twin stars have a maximum amplitude that is very similar to that seen here for both of our samples. Conclusions. Gas giant planet formation alone cannot explain the observed distributions of refractory-element depletions, leaving the formation of rocky material as a more likely explanation of our observations. More rocky material is necessary to explain the data of solar twins than metal-rich stars, and less for warm stars. However, the sizes of the stars’ convective envelopes at the time of planet formation could be regulating these amplitudes. Our results could be explained if disk lifetimes were shorter in more massive stars, as independent observations indeed seem to suggest. Nevertheless, to reach stronger conclusions we will need a detailed knowledge of extrasolar planetary systems down to at least one Earth mass around a significant number of stars.