C. P. Folsom, S. Bagnulo, G. A. Wade, E. Alecian, J. D. Landstreet, S. C. Marsden, I. A. Waite
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.
View original:
http://arxiv.org/abs/1202.1845
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