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

TL;DR

This study reveals a population of slowly-rotating B-type dwarfs with nitrogen enrichment and boron depletion, challenging existing models, and suggests magnetic fields may influence their surface chemical peculiarities.

Contribution

It provides the first detailed NLTE abundance analysis showing chemical peculiarities in slowly-rotating B dwarfs, indicating magnetic fields could play a significant role.

Findings

Presence of nitrogen-rich, boron-depleted slowly-rotating B dwarfs

Higher incidence of chemical peculiarities in stars with magnetic fields

Magnetic phenomena may influence surface abundances in early B dwarfs

Abstract

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.