Wolf-Rayet spin at low metallicity and its implication for black hole formation channels

¹ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland UK
e-mail: jsv@arm.ac.uk
² Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK

Received: 26 January 2017
Accepted: 28 March 2017

Abstract

Context. The spin of Wolf-Rayet (WR) stars at low metallicity (Z) is most relevant for our understanding of gravitational wave sources, such as GW 150914, and of the incidence of long-duration gamma-ray bursts (GRBs). Two scenarios have been suggested for both phenomena: one of them involves rapid rotation and quasi-chemical homogeneous evolution (CHE) and the other invokes classical evolution through mass loss in single and binary systems.

Aims. The stellar spin of WR stars might enable us to test these two scenarios. In order to obtain empirical constraints on black hole progenitor spin we infer wind asymmetries in all 12 known WR stars in the Small Magellanic Cloud (SMC) at Z = 1 / 5 Z_⊙ and within a significantly enlarged sample of single and binary WR stars in the Large Magellanic Cloud (LMC at Z = 1 / 2 Z_⊙), thereby tripling the sample of Vink from 2007. This brings the total LMC sample to 39, making it appropriate for comparison to the Galactic sample.

Methods. We measured WR wind asymmetries with VLT-FORS linear spectropolarimetry, a tool that is uniquely poised to perform such tasks in extragalactic environments.

Results. We report the detection of new line effects in the LMC WN star BAT99-43 and the WC star BAT99-70, along with the well-known WR LBV HD 5980 in the SMC, which might be undergoing a chemically homogeneous evolution. With the previous reported line effects in the late-type WNL (Ofpe/WN9) objects BAT99-22 and BAT99-33, this brings the total LMC WR sample to four, i.e. a frequency of ~10%. Perhaps surprisingly, the incidence of line effects amongst low Z WR stars is not found to be any higher than amongst the Galactic WR sample, challenging the rotationally induced CHE model.

Conclusions. As WR mass loss is likely Z-dependent, our Magellanic Cloud line-effect WR stars may maintain their surface rotation and fulfill the basic conditions for producing long GRBs, both via the classical post-red supergiant or luminous blue variable channel, or resulting from CHE due to physics specific to very massive stars.