Constraining the progenitor evolution of GW 150914
  One of the largest surprises from the LIGO results regarding the first
gravitational wave detection (GW 150914) was the fact the black holes (BHs)
were "heavy", of order 30 - 40 solar masses. The most promising explanation for
this obesity is that the BH-BH merger occurred at low metallicity (Z): when the
iron (Fe) contents is lower this is expected to result in weaker mass loss
during the Wolf-Rayet (WR) phase. We therefore critically evaluate the claims
for the reasons of heavy BHs as a function of Z in the literature. Furthermore,
weaker stellar winds might lead to more rapid stellar rotation, allowing WR and
BH progenitor evolution in a chemically homogeneous manner. However, there is
as yet no empirical evidence for more rapid rotation amongst WR stars in the
low Z environment of the Magellanic Clouds. Due to the intrinsic challenge of
determining WR rotation rates from emission lines, the most promising avenue to
constrain rotation-rate distributions amongst various WR subgroups is through
the utilisation of their emission lines in polarised light. We thus provide an
overview of linear spectro-polarimetry observations of both single and binary
WRs in the Galaxy, as well as the Large and Small Magellanic Clouds, at 50% and
20% of solar Z, respectively. Initial results suggest that the route of
chemically homogeneous evolution (CHE) through stellar rotation is challenging,
whilst the alternative of a post-LBV or common envelope evolution is more
likely.
