Surface chemical composition of single WNh stars

Abstract

Wolf-Rayet (WR) stars of the WNh category contain a significant fraction of hydrogen at their surface. They can be hydrogen-burning, very massive stars or stars in a post-main sequence phase of evolution. Also, WNh stars are sometimes not included in population synthesis models. We aim to better characterise the properties of single WNh stars in the Galaxy and the Magellanic Clouds. In particular, we want to constrain their surface chemistry beyond the hydrogen content by determining the helium, carbon, and nitrogen surface abundances. We perform a spectroscopic analysis of 22 single WNh stars. We fit their ultraviolet and/or optical spectra using synthetic spectra computed with the code CMFGEN. We determine the main stellar parameters (temperature, luminosity, mass-loss rates) and the surface H, He, C, and N mass fractions. We investigate the ability of current evolutionary models to reproduce all parameters at the same time. We find that all WNh stars show the signatures of CNO-cycle material at their surface: they are carbon-depleted and nitrogen-rich. A clear trend of higher nitrogen content at higher metallicity is observed, as expected. The amount of hydrogen (X) varies significantly from one star to another, independently of luminosity. Values of X larger than 0.4 are not exceptional. The majority of Galactic WNh stars can be explained by evolutionary models, provided sufficient fine-tuning of the input parameters of evolutionary calculations. At lower metallicity, most stars escape predictions from evolutionary models. This has been noted in the literature but constraints on the surface nitrogen content exacerbate this severe issue. Our study highlights the need to refine the treatment of WR stars in both stellar evolution and population synthesis models.

16 pages, 12 figures + appendix. Accepted in Astronomy & Astrophysics

References in corpus (39)

• Physical Properties of Wolf-Rayet Stars
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• A distance to the Large Magellanic Cloud that is precise to one per cent
• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
• The Effects of Stellar Rotation. II. A Comprehensive Set of Starburst99 Models
• JADES NIRSpec Spectroscopy of GN-z11: Lyman-$\alpha$ emission and possible enhanced nitrogen abundance in a $z=10.60$ luminous galaxy
• The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
• Mass loss from late-type WN stars and its Z-dependence: very massive stars approaching the Eddington limit
• The most massive stars in the Arches cluster
• The Galactic WC stars: Stellar parameters from spectral analyses indicate a new evolutionary sequence
• The evolution of rotating very massive stars with LMC composition
• Nitrogen enhancements 440 Myr after the Big Bang: super-solar N/O, a tidal disruption event or a dense stellar cluster in GN-z11?
• Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
• The Galactic WN stars revisited. Impact of Gaia distances on fundamental stellar parameters
• The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
• N-enhancement in GN-z11: First evidence for supermassive stars nucleosynthesis in proto-globular clusters-like conditions at high redshift ?
• On the role of the WNH phase in the evolution of very massive stars: Enabling the LBV instability with feedback
• Extreme N-emitters at high-redshift: signatures of supermassive stars and globular cluster or black hole formation in action?
• Metallicity-dependent wind parameter predictions for OB stars
• The very massive binary NGC3603-A1
• Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
• Grids of stellar models with rotation VI: Models from 0.8 to 120 $M_{\odot }$ at a metallicity Z = 0.006
• X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description
• Mass loss and the Eddington parameter: a new mass-loss recipe for hot and massive stars
• A first orbital solution for the very massive 30 Dor main-sequence WN6h+O binary R145
• Spectroscopic evolution of very massive stars at Z = 1/2.5 Zsun
• A synthetic population of Wolf-Rayet stars in the LMC based on detailed single and binary star evolution models
• Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC346 in the SMC galaxy
• Physics and evolution of the most massive stars in 30 Dor. Mass loss, envelope inflation, and a variable upper stellar mass limit
• WO-Type Wolf-Rayet Stars: the Last Hurrah of Massive Star Evolution
• Inferring the presence of very massive stars in local star-forming regions
• A Modern Search for Wolf-Rayet Stars in the Magellanic Clouds. III. A Third Year of Discoveries
• Stromlo Stellar Tracks: non-solar scaled abundances for massive stars
• Very Massive Star Models: I. Impact of Rotation and Metallicity and Comparisons with Observations
• HR-pyPopStar: high wavelength-resolution stellar populations evolutionary synthesis model
• Exploring the influence of different velocity fields on Wolf-Rayet star spectra
• GN-z11 in context: possible signatures of globular cluster precursors at redshift 10
• Stellar Wind Yields of Very Massive Stars

Cited by in corpus (5)

• Evidence for Very Massive Stars in extremely UV-bright star-forming galaxies at $z\sim 2.2-3.6$
• An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud: implications for mass loss and black hole masses at low metallicity
• WR + O binaries as probes of the first phase of mass transfer
• A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18
• Evolution of stars with 60 and 200 Msun: predictions for WNh stars in the Milky Way