The Supernova Progenitor Mass Distributions of M31 and M33: Further
  Evidence for an Upper Mass Limit
  Using Hubble Space Telescope (HST) photometry to measure star formation
histories, we age-date the stellar populations surrounding supernova remnants
(SNRs) in M31 and M33. We then apply stellar evolution models to the ages to
infer the corresponding masses for their supernova progenitor stars. We analyze
33 M33 SNR progenitors and 29 M31 SNR progenitors in this work. We then combine
these measurements with 53 previously published M31 SNR progenitor measurements
to bring our total number of progenitor mass estimates to 115. To quantify the
mass distributions, we fit power laws of the form $dN/dM \propto M^{-\alpha}$.
Our new, larger sample of M31 progenitors follows a distribution with $\alpha =
4.4\pm 0.4$, and the M33 sample follows a distribution with $\alpha =
3.8^{+0.4}_{-0.5}$. Thus both samples are consistent within the uncertainties,
and the full sample across both galaxies gives $\alpha = 4.2\pm 0.3$. Both the
individual and full distributions display a paucity of massive stars when
compared to a Salpeter initial mass function (IMF), which we would expect to
observe if all massive stars exploded as SN that leave behind observable SNR.
If we instead fix $\alpha = 2.35$ and treat the maximum mass as a free
parameter, we find $M_{max} \sim 35-45M_{sun}$, indicative of a potential
maximum cutoff mass for SN production. Our results suggest that either SNR
surveys are biased against finding objects in the youngest (<10 Myr old)
regions, or the highest mass stars do not produce SNe.
