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 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_-0.4mag and a colour of V-I= 2.29^+0.25_-0.39mag, corresponding to a progenitor luminosity of log L/L_⊙~ 4.54 ± 0.19 dex. Using the stellar evolution code STARS, we find this to be consistent with a red supergiant progenitor with M= 8.5^+6.5_-1.5 M_⊙. 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 ⁵⁶Ni ejected in the explosion to be (5.4 ± 1.3) × 10^-3 M_⊙ from the luminosity on the radioactive tail, which is in agreement with the low ⁵⁶Ni masses estimated for other sub-luminous Type IIP SNe. From the light curve 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_⊙~4.3-5 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 ⁵⁶Ni ejected mass, which indicates that such events arise from the lowest end of the mass range that produces a core-collapse SN (7-8 M_⊙).