  In A- and late B-type stars, strong magnetic fields are always associated
with Ap and Bp chemical peculiarities. However, it is not clear at what point
in a star's evolution those peculiarities develop. Strong magnetic fields have
been observed in pre-main sequence A and B stars (Herbig Ae and Be stars), and
these objects have been proposed to be the progenitors of Ap and Bp stars.
However, the photospheric chemical abundances of these magnetic Herbig stars
have not been studied carefully, and furthermore the chemical abundances of
'normal' non-magnetic Herbig stars remain poorly characterized. To investigate
this issue, we have studied the photospheric compositions of 23 Herbig stars,
four of which have confirmed magnetic fields. Surprisingly, we found that half
the non-magnetic stars in our sample show lambda Bootis chemical peculiarities
to varying degrees. For the stars with detected magnetic fields, we find one
chemically normal star, one star with lambda Boo peculiarities, one star
displaying weak Ap/Bp peculiarities, and one somewhat more evolved star with
somewhat stronger Ap/Bp peculiarities. These results suggests that Ap/Bp
peculiarities are preceded by magnetic fields, and that these peculiarities
develop over the pre-main sequence lives of A and B stars. The incidence of
lambda Boo stars we find is much higher than that seen on the main sequence. We
argue that a selective accretion model for the formation of lambda Boo
peculiarities is a natural explanation for this remarkably large incidence.
