Supernova 2008bk and Its Red Supergiant Progenitor
  We have obtained limited photometric and spectroscopic data for Supernova
(SN) 2008bk in NGC 7793, primarily at >~ 150 d after explosion. We find that it
is a Type II-Plateau (II-P) SN which most closely resembles the low-luminosity
SN 1999br in NGC 4900. Given the overall similarity between the observed light
curves and colors of SNe 2008bk and 1999br, we infer that the total visual
extinction to SN 2008bk (A_V=0.065 mag) must be almost entirely due to the
Galactic foreground, similar to what has been assumed for SN 1999br. We confirm
the identification of the putative red supergiant (RSG) progenitor star of the
SN in high-quality g'r'i' images we had obtained in 2007 at the Gemini-South 8
m telescope. Little ambiguity exists in this progenitor identification,
qualifying it as the best example to date, next to the identification of the
star Sk -69 202 as the progenitor of SN 1987A. From a combination of photometry
of the Gemini images with that of archival, pre-SN, Very Large Telescope JHK_s
images, we derive an accurate observed spectral energy distribution (SED) for
the progenitor. We find from nebular strong-intensity emission-line indices for
several HII regions near the SN that the metallicity in the environment is
likely subsolar (Z~0.6 Z_Sun). The observed SED of the star agrees quite well
with synthetic SEDs obtained from model red supergiant atmospheres with
effective temperature T_eff=3600+/-50 K. We find, therefore, that the star had
a bolometric luminosity with respect to the Sun of log(L_bol/L_Sun)=4.57+/-0.06
and radius R_star=496+/-34 R_Sun at ~6 months prior to explosion. Comparing the
progenitor's properties with theoretical massive-star evolutionary models, we
conclude that the RSG progenitor had an initial mass in the range of 8--8.5
M_Sun. (Abridged.)
