Nitrogen line spectroscopy in O-stars -- II. Surface nitrogen abundances for O-stars in the Large Magellanic Cloud

TL;DR

This study analyzes nitrogen line formation and measures nitrogen abundances in O-stars in the LMC, revealing early mixing processes and comparing results with stellar evolution models.

Contribution

It provides new nitrogen abundance measurements for LMC O-stars and investigates the formation of NIV lambda4058 emission, enhancing understanding of massive star evolution.

Findings

Most O-stars are strongly nitrogen-enriched.

A correlation between nitrogen and helium enrichment is observed.

Strong nitrogen enrichment indicates early efficient mixing in stellar evolution.

Abstract

This is the second paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to enable further constraints on the early evolution of massive stars. We investigate the NIV lambda4058 emission line formation, provide nitrogen abundances for a substantial O-star sample in the LMC, and compare our findings with recent predictions from stellar evolutionary models. Stellar and wind parameters are determined by line profile fitting of hydrogen, helium and nitrogen lines, based on synthetic spectra calculated by FASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzing all nitrogen lines present in the available optical spectra. The dominating process responsible for emission at NIV lambda4058 in O-stars is the strong depopulation of the lower level of the transition, which increases as a function of Mdot. Unlike the NIII triplet emission, resonance lines do not play a role for typical mass-loss rates and below. The bulk of our sample O-stars seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and helium enrichment is found. By comparing the nitrogen abundances as a function of vsini ('Hunter-plot') with tailored evolutionary calculations, we identify a considerable number of highly enriched objects at low rotation. Due to the low initial abundance, the detection of strong Nitrogen enrichment in the bulk of O-stars indicates that efficient mixing takes place already during the very early phases of stellar evolution of LMC O-stars.