Wolf-Rayet stars in the Small Magellanic Cloud

Abstract

Context. Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Aims. Following our comprehensive studies of WR stars in the Milky Way, M 31, and the LMC, we derive stellar parameters and massloss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. Methods. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. Results. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10 5.5 to 10 6.1 L . The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M 31, and the LMC. Conclusions. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past.

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

“…Given that, firstly, the Hainich et al empirical rates were derived for a data set showing a significant scatter when expressed as a function of L, which suggests the Hainich et al (2015) study misses a relevant parameter and, secondly, that our exponent was derived over a far wider range in Z than the Hainich et al data, we consider our lower exponent of 0.61 to be more likely close-to-correct.…”

The Evolution and Physical Parameters of WN3/O3s: A New Type of Wolf–Rayet Star*

“…This nomenclature has been assigned to the Galactic stars WR3 (HD9974), WR7, WR10, WR18 and WR128, the LMC stars BAT99-18 and BATAT-63, and seven SMC WRs, all of which show WR-like emission lines with some hydrogen absorption lines (see, e.g., Martins et al 2009Martins et al , 2013. cmfgen and PoWR modeling of these stars (by Marchenko et al 2004, Martins et al 2009, 2013, and Hainich et al 2015 has constrained their physical parameters 8 . While there are many similarities between the physical properties of these WRha stars and the WN3/O3s, there are also a few key differences.…”

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

“…These fractions are broadly consistent with the reported Milky Way (MW) WR binary fraction of 40% (van der Hucht 2001), as well as with those reported for the super-solar environments of the galaxies M31 and M33 . Hainich et al (2014Hainich et al ( , 2015 and Shenar et al (2016Shenar et al ( , 2017Shenar et al ( , 2018Shenar et al ( , 2019 performed a spectral analysis of the apparently-single and binary (or multiple) WN stars in the SMC and the LMC. Contrary to expectation, the distribution of the apparently-single and binary WN stars on the Hertzsprung-Russell diagram (HRD) was found to be comparable.…”