# Helium and Nitrogen Enrichment in Massive Main Sequence Stars: Mechanisms and Implications for the Origin of WNL Stars

## TL;DR

This study models the evolution of massive main sequence stars, revealing how rotation and metallicity influence surface enrichment of helium and nitrogen, and links these processes to the origin of WNL stars.

## Contribution

It introduces a comprehensive grid of stellar evolution models that incorporate rotation, metallicity, and chemical enrichment, explaining the surface compositions of WNL stars.

## Key findings

- Rotationally-induced dredge-up causes surface He and N enrichment in MS stars.
- Surface enrichment occurs in non-rotating, metal-rich stars due to mass loss exposing fossil cores.
- Model spectra match observed WNL star spectra, supporting the proposed mechanisms.

## Abstract

The evolutionary paths taken by massive stars with $M \gtrsim 60 \, \mathrm{M}_\odot$ remain substantially uncertain. They begin their lives as main sequence (MS) O-stars. Depending on their masses, rotation rates, and metallicities, they can then encounter a wide range of evolutionary states with an equally broad set of possible surface compositions and spectral classifications. We present a new grid of calculations for the evolution of such stars that covers a broad range in mass, M/M$_\odot = 60$ to $150$, rotation rate, $v \, / \, v_{\rm crit} = 0$ to $0.6$, metallicity, $[\mathrm{Fe}/\mathrm{H}] = -4$ to $0$, and $\alpha$-element enhancement, $[\alpha/\mathrm{Fe}] = 0$ to $0.4$. We show that rotating stars undergo rotationally-induced dredge-up of nucleosynthetic products, mostly He and N, to their surfaces while still on the MS. Non-rotating metal-rich stars also reveal the products of nucleosynthesis on their surfaces because even modest amounts of mass loss expose their "fossil" convective cores: regions that are no longer convective, but which were part of the convective core at an early stage in the star's evolution. Thus surface enhancement of He and N is expected for rotating stars at all metallicities, and for non-rotating stars if they are relatively metal-rich. We calculate a stellar atmosphere for a representative model from our grid, properly accounting for He- and N-enhancement, and show that the resulting spectrum provides a good match to observed WNL stars, strongly suggesting that the physical mechanisms we have identified are the ultimate cause of the WNL phase.