THE RED SUPERGIANT PROGENITOR OF SUPERNOVA 2012aw (PTF12bvh) IN MESSIER 95

We report on the direct detection and characterization of the probable red supergiant (RSG) progenitor of the intermediate-luminosity Type II-Plateau (II-P) supernova (SN) 2012aw in the nearby (10.0 Mpc) spiral galaxy Messier 95 (M95; NGC 3351). We have identified the star in both Hubble Space Telescope images of the host galaxy, obtained 17-18 yr prior to the explosion, and near-infrared ground-based images, obtained 6-12 yr prior to the SN. The luminous supergiant showed evidence for substantial circumstellar dust, manifested as excess line-of-sight extinction. The effective total-to-selective ratio of extinction to the star was R'_V ≈ 4.35, which is significantly different from that of diffuse interstellar dust (i.e., R_V = 3.1), and the total extinction to the star was therefore, on average, A_V ≈ 3.1 mag. We find that the observed spectral energy distribution for the progenitor star is consistent with an effective temperature of 3600 K (spectral type M3), and that the star therefore had a bolometric magnitude of –8.29. Through comparison with recent theoretical massive-star evolutionary tracks we can infer that the RSG progenitor had an initial mass 15 ≲ M_(ini)(M_☉) < 20. Interpolating by eye between the available tracks, we surmise that the star had initial mass ~17-18 M_☉. The circumstellar dust around the progenitor must have been destroyed in the explosion, as the visual extinction to the SN is found to be low (AV = 0.24 mag with R_V = 3.1).