[Paper Review] Stellar Wind Yields of Very Massive Stars

Erin R. Higgins, J. S. Vink+3 more|arXiv (Cornell University)|Aug 21, 2023

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper computes stellar wind yields from very massive stars (VMS, M > 100 M⊙) using MESA stellar evolution models with a physically-motivated, enhanced wind prescription for optically thick winds. It finds that VMS eject 5–10 times more processed elements (e.g., 14N, 20Ne, 23Na, 26Al) on the main sequence than standard O-star winds, making them the dominant source of 26Al in the Galaxy and key candidates for driving anti-correlations in globular clusters.

ABSTRACT

The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMS, M>100M) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50 - 500M, including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne-Na and Mg-Al cycles. VMS with enhanced winds eject 5-10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anti-correlations, such as C-N and Na-O, in globular clusters. We find that for VMS 95% of the total wind yields is produced on the main sequence, while only ~5% is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf-Rayet winds had been suggested to be responsible for Galactic 26Al enrichment. Finally, 200M stars eject 100 times more of each heavy element in their winds than 50M stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50M stars.

Motivation & Objective

• To quantify the wind yields of very massive stars (VMS, M > 100 M⊙) at solar metallicity using updated stellar evolution models.
• To investigate the impact of optically thick, enhanced mass-loss rates on nucleosynthetic yields compared to standard O-star wind prescriptions.
• To assess the contribution of VMS winds to the Galactic 26Al inventory and to globular cluster anti-correlations (e.g., C-N, Na-O).
• To evaluate the relative importance of main-sequence versus post-main-sequence wind ejection in VMS evolution.
• To determine the IMF-weighted contribution of VMS to heavy element ejection, comparing them to lower-mass O stars.

Proposed method

• Employed MESA stellar evolution code with a 92-isotope nuclear network to model VMS from 50 to 500 M⊙ at solar metallicity.
• Applied a physically-motivated wind prescription that increases mass-loss rates above the Vink & Gräfener (2012) transition point, simulating optically thick winds.
• Tracked H-burning products (14N, 20Ne, 23Na, 26Al) and isotopes (4He, 12C, 16O, 22Ne) across the main sequence and post-main sequence phases.
• Compared wind yields between enhanced (VMS-like) and standard (O-star-like) wind prescriptions to isolate the impact of wind opacity.
• Calculated ejected masses and net yields, including IMF-weighted contributions to assess relative importance in a stellar population.
• Analyzed the role of chemically homogeneous evolution (CHE) in VMS, where large convective cores ensure full mixing and surface abundance changes.

(a) Enhanced V11 optically thick winds for VMS

Experimental results

Research questions

• RQ1 How do enhanced mass-loss rates in very massive stars affect the ejection of H-burning products (e.g., 14N, 20Ne, 23Na, 26Al) on the main sequence?
• RQ2 What fraction of total wind yields is produced during the main sequence versus post-main sequence for VMS with enhanced winds?
• RQ3 To what extent do VMS contribute to the observed Galactic 26Al inventory compared to O stars or Wolf-Rayet stars?
• RQ4 Can VMS winds explain the observed C-N and Na-O anti-correlations in globular clusters, particularly given their high mass-loss rates?
• RQ5 How does the IMF-weighted contribution of VMS compare to that of lower-mass O stars in terms of heavy element ejection?

Key findings

• 95% of total wind yields from VMS are ejected during the main sequence, with only ~5% from post-main sequence phases, highlighting the dominance of early mass loss.
• VMS with enhanced winds eject 5–10 times more H-processed elements (14N, 20Ne, 23Na, 26Al) than those with standard O-star winds.
• A 200 M⊙ VMS ejects 100 times more heavy elements (e.g., 12C, 16O, 28Si) in its wind than a 50 M⊙ star, even after IMF weighting.
• VMS with initial masses ≥100 M⊙ produce the same He-ZAMS mass and surface composition regardless of initial mass due to chemically homogeneous evolution.
• VMS are the dominant source of 26Al in the Galaxy, ejecting 10–3 to 10–2 of their initial mass as 26Al, while lower-mass O stars eject none.
• The intermediate mass range (80–100 M⊙) ejects more 12C and 16O than higher-mass VMS because they avoid significant 4He loss before processing on the post-MS.

(b) Standard V01 optically thin O star winds applied to VMS