WO-type Wolf-Rayet Stars: The Last Hurray of Massive Stars

The most massive stars spend their helium burning lives as Wolf-Rayet (WR) stars, whose spectra are characterized by broad, strong emission lines. The majority of WRs are classified as either WN- or WC-type, depending upon whether their spectra show the products of hydrogen burning (helium and nitrogen) or helium burning (carbon and oxygen) at their surfaces. This represents an evolutionary sequence, with WCs being more highly evolved than WNs. A few percent of WRs, however, are classified as WO-type. Spectroscopically, these WO-type WRs are nearly indistinguishable from high-excitation WC types, but contain strong OVI 3811,34 emission. It has long been thought that this high-excitation line is indicative of high oxygen abundances, which would suggest that the WOs are even more evolved than the WCs, and hence likely the last stage before the core-collapse supernova stage. Alternatively, the WOs might be simply hotter versions of WCs. No previous studies have successfully fit the OVI 3811,34 feature, and therefore results on the abundances are suspect. We have completed the analysis of six WCs and two WOs (all in the LMC) using high quality UV, optical, and NIR spectra. Using the latest version of the radiative transfer code CMFGEN, we now obtain excellent fits to all of the lines, including the OVI 3811,34 doublet. We find that indeed the WOs have less helium and more carbon than the WCs, indicating that they are more evolved. However, ironically, their strong OVI 3811,34 lines are due to higher temperatures, and not more oxygen. 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 evolutionary model predicts. This discrepancy may be due to the large uncertainty in the 12C(alpha)16O nuclear reaction rate. We find that if the commonly adopted rate is decreased by a factor of 25-50%, then there would be a good match with the observations. The WOs are indeed more evolved than the WCs based on their lower helium content, and they are near the end of their nuclear-burning lives.

This work comprises the PhD thesis of co-author Erin Aadland, who unfortunately could not be here to present the results herself. The study has been supported primarily through the NASA ADAP grant 80NSSC18K0729, with the ground-based observing supported through the National Science Foundation grant AST-1612874. Partial support was also provided by the Lowell Slipher Society funds. We also acknowledge support from STScI/NASA, through GO-5460, GO-5723, GO-12940, and GO-13781, AR-14568, and AR-16131