The Wolf-Rayet Content of M33
  Wolf-Rayet stars (WRs) are evolved massive stars, and the relative number of
WC-type and WN-type WRs should vary with metallicity, providing a sensitive
test of stellar evolutionary theory. The observed WC/WN ratio is much higher
than that predicted by theory in some galaxies but this could be due to
observational incompleteness for WN-types, which have weaker lines. Previous
studies of M33's WR content show a galactocentric gradient in the relative
numbers of WCs and WNs, but only small regions have been surveyed with
sufficient sensitivity to detect all of the WNs. Here we present a sensitive
survey for WRs covering all of M33, finding 55 new WRs, mostly of WN type. Our
spectroscopy also improves the spectral types of many previously known WRs,
establishing in one case that the star is actually a background quasar. The
total number of spectroscopically confirmed WRs in M33 is 206, a number we
argue is complete to approximately 5%, with most WRs residing in OB
associations, although approximately 2% are truly isolated. The WC/WN ratio in
the central regions (<2 kpc) of M33 is much higher than that predicted by the
current Geneva evolutionary models, while the WC/WN ratios in the outer regions
are in good accord, as are the values in the SMC and LMC. The WC/WN ratio and
the WC subtype distribution both argue that the oxygen abundance gradient in
M33 is significantly larger than found by some recent studies, but are
consistent with the two-component model proposed by Magrini et al.
