The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMSs, M>100 $\rm {\rm M}_{\odot }$) 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 50–500 $\rm {\rm M}_{\odot }$, 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 anticorrelations, such as C–N and Na–O, in globular clusters. We find that for VMS 95 per cent of the total wind yields is produced on the main sequence, while only ∼ 5 per cent 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, 200 $\rm {\rm M}_{\odot }$ stars eject 100 times more of each heavy element in their winds than 50 $\rm {\rm M}_{\odot }$ stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 $\rm {\rm M}_{\odot }$ stars.
We present follow-up spectroscopy of 21 cataclysmic variables (CVs) with evolved secondaries and ongoing or recently terminated mass transfer. Evolutionary models predict that the secondaries should have anomalous surface abundances owing to nuclear burning in their cores during their main-sequence evolution and subsequent envelope stripping by their companion white dwarfs. To test these models, we measure sodium (Na) abundances of the donors from the Fraunhofer ‘D’ doublet. Accounting for interstellar absorption, we find that all objects in our sample have enhanced Na abundances. We measure 0.3 dex ≲ [Na/H] ≲ 1.5 dex across the sample, with a median [Na/H] = 0.956 dex, i.e. about an order of magnitude enhancement over solar values. To interpret these values, we run Modules for Experiments in Stellar Astrophysics binary evolution models of CVs in which mass transfer begins just as the donor leaves the main sequence. These generically predict Na enhancement in donors with initial donor masses $\gtrsim 1\, {\rm M}_{\odot }$, consistent with our observations. In the models, Na enrichment occurs in the donors’ cores via the NeNa cycle near the end of their main-sequence evolution. Na-enhanced material is exposed when the binaries reach orbital periods of a few hours. Donors with higher initial masses are predicted to have higher Na abundances at fixed orbital period owing to their higher core temperatures during main-sequence evolution. The observed [Na/H] values are on average ≈0.3 dex higher than predicted by the models. Surface abundances of evolved CV donors provide a unique opportunity to study nuclear burning products in the cores of intermediate-mass stars.
more » « less- NSF-PAR ID:
- 10429997
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 524
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 740-758
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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