Winds from stripped low-mass helium stars and Wolf-Rayet stars

Abstract

We present mass-loss predictions from Monte Carlo radiative transfer models for helium (He) stars as a function of stellar mass, down to 2 M . Our study includes both massive Wolf-Rayet (WR) stars and low-mass He stars that have lost their envelope through interaction with a companion. For these low-mass He stars we predict mass-loss rates that are an order of magnitude smaller than by extrapolation of empirical WR mass-loss rates. Our lower mass-loss rates make it harder for these elusive stripped stars to be discovered via line emission, and we should attempt to find these stars through alternative methods instead. Moreover, lower massloss rates make it less likely that low-mass He stars provide stripped-envelope supernovae (SNe) of type Ibc. We express our mass-loss predictions as a function of L and Z and not as a function of the He abundance, as we do not consider this physically astute given our earlier work. The exponent of theṀ versus Z dependence is found to be 0.61, which is less steep than relationships derived from recent empirical atmospheric modelling. Our shallower exponent will make it more challenging to produce "heavy" black holes of order 40 M , as recently discovered in the gravitational wave event GW 150914, making low metallicity for these types of events even more necessary.

Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity

“…Our values for L WR spec are calibrated using the lowest-luminosity WR stars, which, at least in the SMC and LMC, appear to have optically-thin winds (i.e., they are on the verge of being Of stars). Unlike typical WR stars, the mass-loss rates of such stars were shown to follow within considerable scatter theṀ − L relation derived by Vink (2017) for He stars with optically-thin winds (Shenar et al 2019). Relying on this relation (α=0.61 and β=1.36), one obtains (at a rather astonishing accuracy) L WR spec ∝ Z −1 .…”

“…Thus, one should not prematurely expect that binary interactions dominate the formation of WR stars at low metallicity, or that the WR binary fraction should increase with decreasing Z. 0.4 and the luminosity is below L WR spec (Z) we use the theoretical mass-loss rate of Vink (2017). For higher luminosities and when X H < 0.1 we follow either Hainich et al (2014) or Tramper et al (2016), depending on the surface helium mass fraction (see Yoon 2017 andWoosley 2019).…”

γCas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?

“…Key data of selected massive binary evolution models from Wellstein & Langer (1999) and Wellstein et al (2001). Besides the initial binary parameters, i.e., the initial masses of the mass donor (M 1,i ) and the mass gainer (M 2,i ), and the initial orbital period P orb,i , we give parameters of the binary and its component stars at the time where the mass donor has a central helium mass fraction of 0.8 during core helium burning, i.e., the orbital period P He+OB , both stellar masses during that stage, the corresponding luminosities and effective temperatures, and the expected stellar wind mass loss rate, velocity and mechanical wind energy production rate according to Vink (2017 ted), where spin-up to critical rotation is avoided through tidal spin-orbit coupling (de Mink et al 2013).…”

“…in the UV spectra of the rare so-called extreme helium stars (Jeffery & Hamann 2010). While helium-star wind mass loss based on Hamann et al (1982) is included in the presented binary evolution models, the present study uses the recent theoretical massloss rates by Vink (2017), which reproduce the empirical rates of Hamann et al reasonably well, but also provide a smooth transition to the mass-loss properties of the more massive Wolf-Rayet stars. As the total amount of mass lost during core helium burning is mostly very small, this does not introduce any significant inconsistency.…”

“…Figure 4 illustrates the time dependence of the helium star's mechanical wind luminosity L wind,He = 1 2Ṁ He 2 wind,He for the six evolutionary models in Table 2. Here,Ṁ He is the mass loss rate predicted by Vink (2017), and wind,He is the terminal wind velocity, for which Vink showed that it exceeds the escape speed of the helium stars by about a factor of three. It is, therefore, assumed here that wind,He = 3 √ 2GM He /R He .…”