| Abstract | We have obtained limited photometric and spectroscopic data for supernova (SN) 2008bk in NGC7793, primarily at ≳ 150 days after explosion. We find that it is a Type II-Plateau (II-P) SN that most closely resembles the low-luminosity SN1999br in NGC4900. Given the overall similarity between the observed light curves and colors of SNe 2008bk and 1999br, we infer that the total visual extinction to SN2008bk (AV = 0.065mag) must be almost entirely due to the Galactic foreground, similar to what has been assumed for SN1999br. 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 8m 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 SN1987A. From a combination of photometry of the Gemini images with that of archival, pre-SN, Very Large Telescope JHKs 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 0.6 Z ⊙). The observed SED of the star agrees quite well with synthetic SEDs obtained from model RSG atmospheres with effective temperature T eff = 3600 50K. We find, therefore, that the star had a bolometric luminosity with respect to the Sun of log (L bol/L ⊙) = 4.57 0.06 and radius R ⊙ = 496 34 R ⊙ 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 ⊙. 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 SN1999br, and it agrees with the low initial masses found for the RSG progenitors of other low-luminosity SNe II-P. |
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