SUPERNOVA 2008bk AND ITS RED SUPERGIANT PROGENITOR

Abstract

We have obtained limited photometric and spectroscopic data for supernova (SN) 2008bk in NGC 7793, primarily at greater than or similar to 150 days after explosion. We find that it is a Type II-Plateau (II-P) SN that 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 degrees 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 H II regions near the SN that the metallicity in the environment is likely subsolar (Z approximate to 0.6Z(circle dot)). The observed SED of the star agrees quite well with synthetic SEDs obtained from model RSG 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-circle dot) = 4.57 +/- 0.06 and radius R-* = 496 +/- 34 R-circle dot at similar to 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 8-8.5M(circle dot). This mass is consistent with, albeit at the low end of, the inferred range of initial masses for SN II-P progenitors. It is also consistent with the estimated upper limit on the initial mass of the progenitor of SN 1999br, and it agrees with the low initial masses found for the RSG progenitors of other low-luminosity SNe II-P.