Core-Collapse Supernovae from the Palomar Transient Factory: Indications
  for a Different Population in Dwarf Galaxies
  We use the first compilation of 72 core-collapse supernovae (SNe) from the
Palomar Transient Factory (PTF) to study their observed subtype distribution in
dwarf galaxies compared to giant galaxies. Our sample is the largest
single-survey, untargeted, spectroscopically classified, homogeneous collection
of core-collapse events ever assembled, spanning a wide host-galaxy luminosity
range (down to M_r ~ -14 mag) and including a substantial fraction (>20%) of
dwarf (M_r >= -18 mag) hosts. We find more core-collapse SNe in dwarf galaxies
than expected and several interesting trends emerge. We use detailed
subclassifications of stripped-envelope core-collapse SNe and find that all
Type I core-collapse events occurring in dwarf galaxies are either SNe Ib or
broad-lined SNe Ic (SNe Ic-BL), while "normal" SNe Ic dominate in giant
galaxies. We also see a significant excess of SNe IIb in dwarf hosts. We
hypothesize that in lower metallicity hosts, metallicity-driven mass loss is
reduced, allowing massive stars that would have appeared as "normal" SNe Ic in
metal-rich galaxies to retain some He and H, exploding as Ib/IIb events. At the
same time, another mechanism allows some stars to undergo extensive stripping
and explode as SNe Ic-BL (and presumably also as long-duration gamma-ray
bursts). Our results are still limited by small-number statistics, and our
measurements of the observed N(Ib/c)/N(II) number ratio in dwarf and giant
hosts (0.25_{-0.15}^{+0.3} and 0.23_{-0.08}^{+0.11}, respectively; 1 sigma
uncertainties) are consistent with previous studies and theoretical
predictions. As additional PTF data accumulate, more robust statistical
analyses will be possible, allowing the evolution of massive stars to be probed
via the dwarf-galaxy SN population.
