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 sub([middot in circle]) + 10 M sub([middot in circle]) 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 approximately 4 M sub([middot in circle]), a final hydrogen content of approximately (3-5) x 10 super(-3) M sub([middot in circle]), 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.