Stripped-envelope stars in different metallicity environments

Abstract

Massive stars that become stripped of their hydrogen envelope through binary interaction or winds can be observed either as Wolf-Rayet stars, if they have optically thick winds, or as transparent-wind stripped-envelope stars. We approximate their evolution through evolutionary models of single helium stars, and compute detailed model grids in the initial mass range 1.5−70 M⊙ for metallicities between 0.01 and 0.04, from core helium ignition until core collapse. Throughout their lifetimes some stellar models expose the ash of helium burning. We propose that models that have nitrogen-rich envelopes are candidate WN stars, while models with a carbon-rich surface are candidate WC stars during core helium burning, and WO stars afterwards. We measure the metallicity dependence of the total lifetimes of our models and the duration of their evolutionary phases. We propose an analytic estimate of the wind’s optical depth to distinguish models of Wolf-Rayet stars from transparent-wind stripped-envelope stars, and find that the luminosity ranges at which WN-, WC-, and WO-type stars can exist is a strong function of metallicity. We find that all carbon-rich models produced in our grids have optically thick winds and match the luminosity distribution of observed populations. We construct population models and predict the numbers of transparent-wind stripped-envelope stars and Wolf-Rayet stars, and derive their number ratios at different metallicities. We find that as metallicity increases, the number of transparent-wind stripped-envelope stars decreases and the number of Wolf-Rayet stars increases. At high metallicities WC- and WO-type stars become more common. We apply our population models to nearby galaxies, and find that populations are more sensitive to the transition luminosity between Wolf-Rayet stars and transparent-wind helium stars than to the metallicity-dependent mass loss rates.

Density Profiles of Collapsed Rotating Massive Stars Favor Long Gamma-Ray Bursts

“…This same class of SN has been observationally linked to lGRBs, suggesting that W-Rs or other stripped-envelope stars are the progenitors of lGRBs. These stars may include ones that have undergone chemically homogeneous evolution (CHE) without developing the optically thick winds that are characteristic of W-Rs, which are observationally distinguished by their emission lines (e.g., Szécsi et al 2015;Aguilera-Dena et al 2022). Massive stripped-envelope stars that fail to form BHs may also successfully power lGRBs and Type Ic-bl SNe through the protomagnetar model (Metzger et al 2011;Shankar et al 2021;Song & Liu 2023).…”

A Metallicity Dependence on the Occurrence of Core-collapse Supernovae

“…This would go against stellar evolution studies and models that suggest that black hole formation is more common at lower-progenitor metallicities (e.g., Heger et al 2003;Eldridge & Tout 2004;O'Connor & Ott 2011;Ertl et al 2016), although the metallicity dependence on the expected outcome of failed SNe has yet not been explored in depth (e.g., Kochanek et al 2008;Piro 2013). Some studies suggest that the lower mass limit for CCSN decreases as a function of metallicity (for those metallicity ranges probed in the current study; e.g., Eldridge & Tout 2004;Ibeling & Heger 2013), and that metallicity can also affect mass loss and the populations of different SNe (for a recent analysis see, e.g., Aguilera-Dena et al 2022. The effect of metallicity could be particularly stronger in binary systems.…”

Wolf-Rayet stars: recent advances and persisting problems

“…2). Hence, Shenar et al (2020a) estimate that in the Milky Way, stars initially less massive than ≈ 20 M would not appear as WR stars after stripping, while in the SMC, this limit grows almost to ≈ 40 M (see also Aguilera-Dena et al 2022). This means that, at low Z, neither of the WR formation channel is effective, and it is not obvious that a metallicity trend should be present.…”