Nitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs

T. Morel (1,2), S. Hubrig (3) and M. Briquet (1)

1 - Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium; 2 - Institut d'Astrophysique et de Geophysique, Liege University, Belgium; 3 - European Southern Observatory, Chile

Evolutionary models for massive stars, accounting for rotational mixing effects, do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. Contrary to theoretical expectations, we present a detailed and fully-homogeneous, NLTE abundance analysis of 20 early B-type dwarfs and (sub)giants that reveals the existence of a population of nitrogen-rich and boron-depleted, yet intrinsically slowly-rotating objects. The low-rotation rate of several of these stars is firmly established, either from the occurrence of phase-locked UV wind line-profile variations, which can be ascribed to rotational modulation, or from theoretical modelling in the pulsating variables. The observational data presently available suggest a higher incidence of chemical peculiarities in stars with a (weak) detected magnetic field. This opens the possibility that magnetic phenomena are important in altering the photospheric abundances of early B dwarfs, even for surface field strengths at the one hundred Gauss level. However, further spectropolarimetric observations are needed to assess the validity of this hypothesis.

Reference: Accepted by A&A.
Status: Manuscript has been accepted


Comments: 11 pages, 4 figures (some in colour).