Abstract

Models for the spectra and the light curve (LC), in the photospheric as well as in the late nebular phase, are used to infer the properties of the very radio-bright, broad-lined type IIb supernova (SN) 2003bg. Consistent fits to the LC and the spectral evolution are obtained with an explosion that ejected 4 M of material with a kinetic energy of 5 × 1051 erg. A thin layer of hydrogen, comprising 0.05 M, is inferred to be present in the ejecta at the highest velocities (v ? 9000 km s-1), while a thicker helium layer, comprising 1.25 M, was ejected at velocities between 6500 and 9000 km s-1. At lower velocities, heavier elements are present, including 0.2 M of 56Ni that shape the LC and the late-time nebular spectra. These values suggest that the progenitor star had a mass of 20-25 M, (comparable to, but maybe somewhat smaller than that of the progenitor of the X-ray flashes/SN 2008D). The rather broad-lined early spectra are the result of the presence of a small amount of material (0.03 M) at velocities >0.1c, which carries 10% of the explosion kinetic energy. No clear signatures of a highly aspherical explosion are detected. © 2009 The American Astronomical Society. All rights reserved.