SN 2011dh and the progenitors of Type IIb supernovae

Abstract: Core-collapse supernovae (SNe) are the observed events following the collapse of the core of evolved massive stars. The gravitational energy released creates a powerful shock that disrupts the star and ejects the heated material into the surrounding circumstellar medium. The observed properties depend on the mass lost by the star, e.g. through stellar winds or mass transfer in binary systems, and the subject of this thesis is the class of Type IIb SNe, which are thought to have lost most, but not all of their hydrogen envelopes. A quite unique set of observations has recently been obtained for the Type IIb SN 2011dh, which was followed to more than a thousand days after the explosion, and observed by several groups at a wide range of wavelengths. In this work, the bulk portion of the ultraviolet to mid-infrared observations, as well as pre-explosion images of the progenitor star, are presented, discussed, and analysed. Lightcurve and spectral modelling of the SN observations, presented in this and related works, all suggest a progenitor of modest initial mass (<15 solar masses) with an extended and low-mass hydrogen envelope, consistent with what is found from the pre-explosion observations. Although mass-loss rates for single stars are uncertain, they are likely too weak to expel the hydrogen envelope for stars in this mass range. Therefore, an appealing alternative is mass-loss by Roche-lobe overflow in a binary system, as was likely the case for the Type IIb SN 1993J. Post-explosion observations have revealed a blue compact companion star blended with the fading SN 1993J, and a similar result has been claimed for SN 2011dh. The fact that some SNe arise from binary systems is not surprising given the large binary fraction observed for massive stars, and in this work, a grid of hydrodynamical SN models is used to infer modest initial masses (<15 solar masses) for most Type IIb SNe documented in the literature, suggesting that binary systems actually dominate the production of Type IIb SNe.