A BINARY PROGENITOR FOR THE TYPE IIb SUPERNOVA 2011dh IN M51

We perform binary stellar evolutionary calculations following the simultaneous evolution of both stars in the system to study a potential progenitor system for the Type IIb supernova 2011dh. Pre-explosion photometry as well as light-curve modeling has provided constraints on the physical properties of the progenitor system. Here, we present a close binary system (CBS) that is compatible with such constraints. The system is formed by stars of solar composition with 16 M-circle dot + 10 M-circle dot on a circular orbit with an initial period of 125 days. The primary star ends its evolution as a yellow supergiant with a mass of approximate to 4 M-circle dot, a final hydrogen content of approximate to(3-5) x 10(-3) M-circle dot, and with an effective temperature and luminosity in agreement with the Hubble Space Telescope (HST) pre-explosion observations of SN 2011dh. These results are nearly insensitive to the adopted accretion efficiency factor beta. At the time of explosion, the companion star has an effective temperature of 22,000-40,000 K, depending on the value of beta, and lies near the zero-age main sequence. Considering the uncertainties in the HST pre-SN photometry, the secondary star is only marginally detectable in the bluest observed band. CBSs, as opposed to single stars, provide a natural frame to explain the properties of SN 2011dh.