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

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, $v/v_{\mathrm{crit}}=0$ 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.

21 pages, 18 figures, 2 tables, accepted for publication in MNRAS, in press

References in corpus (17)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Binary interaction dominates the evolution of massive stars
• Physical Properties of Wolf-Rayet Stars
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• Presupernova evolution and explosive nucleosynthesis of rotating massive stars in the metallicity range -3 <=[Fe/H]<= 0
• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
• The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
• The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming Galaxies
• The most massive stars in the Arches cluster
• Stellar and wind properties of massive stars in the central parsec of the Galaxy
• The evolution of rotating very massive stars with LMC composition
• The spectroscopic Hertzsprung-Russell diagram
• The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
• Emission-line diagnostics of nearby HII regions including interacting binary populations
• Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
• Testing massive star evolution, star-formation history and feedback at low metallicity : Spectroscopic analysis of OB stars in the SMC Wing
• The impact of stars stripped in binaries on the integrated spectra of stellar populations

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• Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840
• HeII emitters at cosmic noon and beyond. Characterising the HeII λ1640 emission with MUSE and JWST/NIRSpec
• The Predicted Properties of Helium-Enriched Globular Cluster Progenitors at High Redshift
• Kinematics of Wolf-Rayet Stars in the LMC: Clues to Subtype Origins
• Spectroscopic Line Modeling of the Fastest Rotating O-type Stars