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

C.P. Folsom, S. Bagnulo, G.A. Wade, E. Alecian, J.D. Landstreet, S.C. Marsden, I.A. Waite

Figure 1. H-R diagram for the stars in this study. Evolutionary tracks (solid lines) are labelled by mass in solar masses. Isochrones (dashed lines) are labelled by age in Myr. The birth line (right dot-dashed line) for an accretion rate of 10^òÈÒ5 M_☉ yr^òÈÒ1, and the zero-age main sequence line (ZAMS, left dot-dashed line) are also shown. Circles are chemically normal stars, squares are λ Boo stars, the diamond is the possible Bp star V380 Ori A. Open symbols are stars with confirmed magnetic field detections.

Abstract

The photospheres of about 10-20% of main sequence A- and B-type stars exhibit a wide range of chemical peculiarities, often associated with the presence of a magnetic field. It is not exactly known at which stage of stellar evolution these chemical peculiarities develop. To investigate this issue, in this paper we 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 of which have confirmed magnetic fields), and one dusty young star. We have found that half the stars in our sample show λ Boo chemical peculiarities to varying degrees, only one star shows weak Ap/Bp peculiarities, and all the remaining stars are chemically normal. The incidence of λ Boo chemical peculiarities we find in Herbig stars is much higher than what is seen on the main sequence. We argue that a selective accretion model for λ Boo star formation is a natural explanation for the remarkably large number of λ Boo stars in our sample. We also find that the magnetic Herbig stars do not exhibit a range of chemical compositions remarkably different from the normal stars: one magnetic star displays λ Boo chemical peculiarities (HD 101412), one displays weak Ap/Bp peculiarities (V380 Ori A), and one (HD 190073) is chemically normal. This is completely different from what is seen on the main sequence, where all magnetic A and cool B stars show Ap/Bp chemical peculiarities, and this is consistent with the idea that the magnetic field precedes the formation of the chemical peculiarities typical of Ap and Bp stars.

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Last Revised: 2012 February 9th