Chemical abundances of magnetic and non-magnetic Herbig Ae/Be stars

摘要

The photospheres of about 10-20% of main sequence A- and B-type stars exhibita wide range of chemical peculiarities, often associated with the presence of amagnetic field. It is not exactly known at which stage of stellar evolutionthese chemical peculiarities develop. To investigate this issue, in this paperwe study the photospheric compositions of a sample of Herbig Ae and Be stars,which are considered to be the pre-main sequence progenitors of A and B stars.We have performed a detailed abundance analysis of 20 Herbig stars (three ofwhich have confirmed magnetic fields), and one dusty young star. We have foundthat half the stars in our sample show lambda Boo chemical peculiarities tovarying degrees, only one star shows weak Ap/Bp peculiarities, and all theremaining stars are chemically normal. The incidence of lambda Boo chemicalpeculiarities we find in Herbig stars is much higher than what is seen on themain sequence. We argue that a selective accretion model for lambda Boo starformation is a natural explanation for the remarkably large number of lambdaBoo stars in our sample. We also find that the magnetic Herbig stars do notexhibit a range of chemical compositions remarkably different from the normalstars: one magnetic star displays lambda Boo chemical peculiarities (HD101412), one displays weak Ap/Bp peculiarities (V380 Ori A), and one (HD190073) is chemically normal. This is completely different from what is seen onthe main sequence, where all magnetic A and cool B stars show Ap/Bp chemicalpeculiarities, and this is consistent with the idea that the magnetic fieldprecedes the formation of the chemical peculiarities typical of Ap and Bpstars.