The Colours and Chemical Composition of the G Dwarf HR 72

Abstract

Model atmosphere techniques have been used to study element abundance ratios in the G dwarf HR 72. The abundances have been determined relative to abundances in HR 483, a G dwarf with almost solar abundances. Our results confirm the conclusion of Spinrad and Luebke that the iron to hydrogen ratio in HR 72 is twice that of HR 483, or about two and a halftimes the solar ratio. The fourteen other elements which we have studied are also overabundant in HR 72. Apart from the general excess of heavy elements (or deficiency of hydrogen) relative abundances in HR 72 resemble solar abundances. Since the overabundance of all metals relative to hydrogen may be caused by reduction in hydrogen content and increase in helium content, we have compared the curve of growth shifts predicted by super-metal-rich and helium rich atmospheres. We have also computed narrow- and broad-band colours from our model atmospheres. The colours of HR 72 are consistent either with it being a He-rich star which has evolved away from the He-rich main sequence or, alternatively, with it being a super-metal-rich star.