Chemical signatures of planet formation in field and open cluster stars

Abstract

In this thesis, I have conducted a strictly line-by-line differential abundance analysis using high resolution, high signal-to-noise ratio spectra of field stars (e.g., stellar binaries, terrestrial planet hosts etc.) and open cluster stars (e.g., the Hyades stars) in order to identify the chemical signatures of planet formation. The three main results from this thesis are: First, we present a detailed differential abundance analysis of the HAT-P-1 stellar binary. The secondary star in this double system is known to host a transiting giant planet while no planets have yet been detected around the primary star. The derived elemental abundances of the primary and secondary stars are identical within the errors. The striking similarity in the chemical compositions of the two stellar components in HAT-P-1 indicates that the formation of giant planets does not necessarily imply differences in the chemical abundances of the host stars. Secondly, we conduct a detailed differential abundance analysis of the terrestrial planet host Kepler-10 and 14 of its stellar twins. Stellar parameters and elemental abundances of Kepler-10 and its stellar twins were obtained with very high precision. When compared to the majority of thick disc twins, Kepler-10 shows a depletion in the refractory elements relative to the volatile elements, which could be due to the formation of terrestrial planets in the Kepler-10 system. The average abundance pattern corresponds to roughly 13 Earth masses, while the two known planets in Kepler-10 system have a combined mass of 20 Earth. Although our results demonstrate that several factors (e.g., planet signature, stellar age, stellar birth location and Galactic chemical evolution) could lead to or affect abundance trends with condensation temperature, we find that the trends give further support for the planetary signature hypothesis. Thirdly, we present a high-precision differential abundance analysis of 16 solar-type stars in the Hyades open cluster. We derived stellar parameters and differential abundances for 19 elements with total uncertainties as low as 0.01 - 0.02 dex. Our main results include: (1) there is no clear chemical signature of planet formation detected among the sample stars, i.e., no correlations in elemental abundances versus condensation temperature; (2) the observed abundance dispersions are a factor of about 2 times larger than the average measurement errors for most elements; (3) there are positive correlations, of high statistical significance, between the abundances of at least 90 per cent of pairs of elements. We demonstrate that none of these findings can be explained by errors in the inferred stellar parameters. Our results reveal that the Hyades is chemically inhomogeneous at the 0.02 dex level. Possible explanations for the abundance variations include (1) inhomogeneous chemical evolution in the proto-cluster environment, (2) supernova ejection in the proto-cluster cloud, and (3) pollution of metal-poor gas before complete mixing of the proto-cluster cloud. Our results provide significant constraints on the chemical compositions of open cluster stars and for Galactic archeology, especially the concept of chemical tagging.