Title: Wolf-Rayet spin at low metallicity and its implication for Black Hole formation channels,
Abstract: 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, as well as
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), the other invokes classical
evolution through mass loss in single and binary systems. WR spin rates 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 Zsun, as well as within a
significantly enlarged sample of single and binary WR stars in the Large
Magellanic Cloud (LMC at Z = 1/2 Zsun), tripling the sample of Vink (2007).
This brings the total LMC sample to 39, making it appropriate for comparison to
the Galactic sample. We measure WR wind asymmetries with VLT-FORS linear
spectropolarimetry. We report the detection of new line effects in the LMC WN
star BAT99-43 and the WC star BAT99-70, as well as the famous WR/LBV HD 5980 in
the SMC, which might be evolving chemically homogeneously. 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 4, 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. 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 (RSG) or luminous blue variable (LBV) channel, as well as
resulting from CHE due to physics specific to very massive stars (VMS).