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

TL;DR

This study investigates the chemical compositions of Herbig Ae/Be stars to understand the development of chemical peculiarities and their relation to magnetic fields during stellar evolution.

Contribution

It provides the first detailed abundance analysis of a sample of Herbig stars, revealing a high incidence of lambda Boo peculiarities and insights into the evolution of magnetic and chemically peculiar stars.

Findings

Half the Herbig stars show lambda Boo peculiarities.

Magnetic Herbig stars do not show distinct chemical compositions from normal stars.

The incidence of lambda Boo peculiarities is higher in pre-main sequence stars than on the main sequence.

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 lambda 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 lambda 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 lambda Boo star formation is a natural explanation for the remarkably large number of lambda 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 lambda 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.