WO-Type Wolf-Rayet Stars: the Last Hurrah of Massive Star Evolution

Abstract

Are WO-type Wolf Rayet (WR) stars in the final stage of massive star evolution before core-collapse? Although WC-and WO-type WRs have very similar spectra, WOs show a much stronger O vi λλ3811,34 emission-line feature. This has usually been interpreted to mean that WOs are more oxygen rich than WCs, and thus further evolved. However, previous studies have failed to model this line, leaving the relative abundances uncertain, and the relationship between the two types unresolved. To answer this fundamental question, we modeled six WCs and two WOs in the LMC using UV, optical, and NIR spectra with the radiative transfer code cmfgen in order to determine their physical properties. We find that WOs are not richer in oxygen; rather, the O vi feature is insensitive to the abundance. However, the WOs have a significantly higher carbon and lower helium content than the WCs, and hence are further evolved. A comparison of our results with single-star Geneva and binary BPASS evolutionary models show that while many properties match, there is more carbon and less oxygen in the WOs than either set of evolutionary model predicts. This discrepancy may be due to the large uncertainty in the 12 C+ 4 He→ 16 O nuclear reaction rate; we show that if the Kunz et al. rate is decreased by a factor of 25-50%, then there would be a good match with the observations. It would also help explain the LIGO/VIRGO detection of black holes whose masses are in the theoretical upper mass gap.

The orbital kinematics of eta Carinae over three periastra with a possible detection of the elusive secondary's motion

“…There is also the rare WO subtype, which is similar to the WC subtype but shows more dominant O lines. The WO stars were recently shown to have higher carbon and lower helium content than the WC stars, likely representing the final stages of the WR evolution (Tramper et al 2015;Aadland et al 2022). Given the generalized characteristics of WR stars, a WN star would seem the most likely companion star if the He 4686 line is from the companion at times further from periastron.…”

Using Shell Models to Investigate Clumping in the WN4 Star HD 50896

“…Thus it is somewhat surprising how well models that use the VFF approach replicate observed spectra of WR stars (e.g. Hillier & Miller 1999;Hamann & Koesterke 1998;Crowther et al 2002a;Aadland et al 2021Aadland et al , 2022.…”