SN 2009md: Another faint supernova from a low mass progenitor
  We present adaptive optics imaging of the core collapse supernova (SN)
2009md, which we use together with archival \emph{Hubble Space Telescope} data
to identify a coincident progenitor candidate. We find the progenitor to have
an absolute magnitude of $V = -4.63^{+0.3}_{-0.4}$ mag and a colour of $V-I =
2.29^{+0.25}_{-0.39}$ mag, corresponding to a progenitor luminosity of log
$L$/L$_{\odot}$ $\sim4.54\pm0.19$ dex. Using the stellar evolution code STARS,
we find this to be consistent with a red supergiant progenitor with $M =
8.5_{-1.5}^{+6.5}$ M$_{\odot}$. The photometric and spectroscopic evolution of
SN 2009md is similar to that of the class of sub-luminous Type IIP SNe; in this
paper we compare the evolution of SN 2009md primarily to that of the
sub-luminous SN 2005cs. We estimate the mass of $^{56}$Ni ejected in the
explosion to be $(5.4\pm1.3) \times 10^{-3}$ M$_{\odot}$\ from the luminosity
on the radioactive tail, which is in agreement with the low $^{56}$Ni masses
estimated for other sub-luminous Type IIP SNe. From the lightcurve and spectra,
we show the SN explosion had a lower energy and ejecta mass than the normal
Type IIP SN 1999em. We discuss problems with stellar evolutionary models, and
the discrepancy between low observed progenitor luminosities (log
$L$/L$_{\odot}$ $\sim4.3-5$ dex) and model luminosities after the
second-dredge-up for stars in this mass range, and consider an enhanced carbon
burning rate as a possible solution. In conclusion, SN 2009md is a faint SN
arising from the collapse of a progenitor close to the lower mass limit for
core-collapse. This is now the third discovery of a low mass progenitor star
producing a low energy explosion and low $^{56}$Ni ejected mass, which
indicates that such events arise from the lowest end of the mass range that
produces a core-collapse supernova (7-8 M$_{\odot}$).
