Rotational Mixing in Be Stars: Nitrogen Abundances for a Sample of Be Stars from the MiMeS Survey

TL;DR

This study measures nitrogen abundances in Be and B stars to investigate rotational mixing effects, finding some stars enriched in nitrogen while accounting for observational biases, with results comparable to solar abundance but with larger dispersion in Be stars.

Contribution

First detailed nitrogen abundance analysis of Be stars considering gravitational darkening and disk effects, revealing insights into rotational mixing and abundance dispersion.

Findings

Approximately one third of Be stars show nitrogen enrichment.

Corrections for gravitational darkening and disk contamination are minimal.

Be stars exhibit larger nitrogen abundance dispersion than B stars.

Abstract

Photospheric nitrogen abundances for a sample of 26 Be stars and 16 normal B stars were found using high-resolution spectra from the Magnetism in Massive Stars (MiMeS) spectroscopic survey. Nitrogen abundances were obtained using non-LTE equivalent widths and line profiles, and Monte Carlo simulation was used to determine the error bounds of the measured nitrogen abundances due to uncertainties in the adopted stellar parameters, continuum normalization, and atomic data. In addition, the effects of the gravitational darkening and disk contamination on the measured Be star nitrogen abundances were investigated. About one third of the sample Be stars and half of the normal B-type stars have nitrogen enrichments that may be due to rotational mixing in these rapidly rotating objects. Corrections for gravitational darkening and disk contamination do not significantly change the overall results for the Be star sample. The average nitrogen abundance of the B and Be star samples coincide with the solar abundance, although the dispersion in the nitrogen abundances is much larger in the Be star sample. The Be star sample also has a significant fraction of sub-solar nitrogen abundance objects which are not present in significant numbers in the B star sample. This may point to yet unresolved systematic errors in the analysis of the Be stars