Supernova Remnant Progenitor Masses in M31
  Using HST photometry, we age-date 59 supernova remnants (SNRs) in the spiral
galaxy M31 and use these ages to estimate zero-age main sequence masses (MZAMS)
for their progenitors. To accomplish this, we create color-magnitude diagrams
(CMDs) and use CMD fitting to measure the recent star formation history (SFH)
of the regions surrounding cataloged SNR sites. We identify any young coeval
population that likely produced the progenitor star and assign an age and
uncertainty to that population. Application of stellar evolution models allows
us to infer the MZAMS from this age. Because our technique is not contingent on
precise location of the progenitor star, it can be applied to the location of
any known SNR. We identify significant young SF around 53 of the 59 SNRs and
assign progenitor masses to these, representing a factor of 2 increase over
currently measured progenitor masses. We consider the remaining 6 SNRs as
either probable Type Ia candidates or the result of core-collapse progenitors
that have escaped their birth sites. The distribution of recovered progenitor
masses is bottom heavy, showing a paucity of the most massive stars. If we
assume a single power law distribution, dN/dM proportional to M^alpha, we find
a distribution that is steeper than a Salpeter IMF (alpha=-2.35). In
particular, we find values of alpha outside the range -2.7 to -4.4 inconsistent
with our measured distribution at 95% confidence. If instead we assume a
distribution that follows a Salpeter IMF up to some maximum mass, we find that
values of M_max greater than 26 Msun are inconsistent with the measured
distribution at 95% confidence. In either scenario, the data suggest that some
fraction of massive stars may not explode. The result is preliminary and
requires more SNRs and further analysis. In addition, we use our distribution
to estimate a minimum mass for core collapse between 7.0 and 7.8 Msun.
