Abundances in High-Velocity a Stars. I. Sextantis.
Abstract
We have compared 7 Sextantis with Sirius and find that the ratio of hydrogen to the common metals is about the same in both stars but that the ratio of helium to hydrogen in 7 Sextantis is about 10 and carbon is overabundant by a factor of 20 This can be interpreted in either of two ways. Possibly the star is a very rapid rotator seen pole-on. This could result in a spectrum that is composite of type B8 and At On the other hand, if the star has an atmosphere that is describable by a single effective temperature, then the abundance anomalies are real. In that case we interpret our results to mean that 7 Sextantis was once a metal-poor main-sequence star which has evolved to the horizontal branch. During its evolution, mass loss and possibly mixing brought the helium and carbon produced in the interior to the surface. In an appendix we have compared the very high-velocity A star, HD 161817, with Sirius and then made a rough comparison of Sirius with the sun. We find that Sirius is overabundant in iron by a factor of 5 and in titanium by a factor of 2.