Light Element abundances (Li and Be) in nearby galactic and extragalactic stars



Thesis Supervisors: Luca Pasquini (ESO), Rodolfo Smiljanic (CAMK/PAN, Poland)

Abstract

Evidences triggered by the GAIA DR2 release show that a large fraction of the stars of the nearby halo and thick disk originated in an accreted satellite, while others were born ‘in situ’. The accreted stars occupy defined regions in the dynamical and orbital spaces, and show element abundances characteristic of dwarf galaxies in the local group. The student is asked to identify a substantial sample of accreted stars and compare their abundances with an equivalent sample of stars formed in situ. For this s/he will use literature data, archive spectra and will propose new observations, if needed. The project is to concentrate mostly on the light elements Li and Be . In relatively metal poor, hot main sequence stars these elements are not expected to be destroyed in the stellar interior (but could suffer of diffusion), so their atmospheric abundances should principally follow the chemical enrichment of the galaxies where they were born. Li and Be are extremely interesting: Li is one of the few elements synthetized in the Big Bang, and it can be enriched in the life of a galaxy through several mechanism, that include Nova production and cosmic ray spallation. Beryllium can only be produced through cosmic ray spallation, and it has been proposed to be used as a cosmo-chronometer in the Galaxy. Li and Be have been extensively studied in halo and in thick disk stars, but the evidence that a major fraction of them is of extragalactic origin changes radically the perspective of the chemical evolution for these elements.