The properties of the peculiar type la supernova 1991bg - II. The amount of56Ni and the total ejecta mass determined from spectrum synthesis and energetics considerations

Abstract

The photometric and spectroscopic data presented in Paper I are used, together with spectrum synthesis models and energetics considerations, to derive the properties of the peculiar type la supernova (SN) 1991bg. The analysis of the light curve shows that SN 1991bg was a subluminous SN Ia. Spectrum synthesis during the photospheric epoch independently confirms that this SN was intrinsically cool and faint. The red colour and the presence of a Ti II feature near 4000 Å at early times are consequences of the low temperature. The possibility that SN 1991bg was a highly reddened but otherwise normal SN la is ruled out since model spectra are then too hot to reproduce the observed spectral features. The Fe-group elements were underabundant in the outer part of the ejecta, indicating that a large fraction of the mass of the progenitor was unburned. The observed behaviour of the O I 7774-Å line, which does not evolve to lower velocities with time, suggests that an outer oxygen shell, containing unburned material from the progenitor, is observed directly for the first time in an SN Ia. The outer location of oxygen also rules out an He-shell white dwarf as a progenitor of SN 1991bg. The width of the nebular lines (the narrowest ever observed in SNe Ia) and the profile of the [Co III] 5890-Å line, together with the light curve, imply that the mass of synthesized 56Ni was small (∼0.07 M⊙), the Ni distribution was centrally peaked, and the total ejecta mass was also small (0.4-1.0 M⊙). Models for the nebular spectra were also computed. They are consistent with a small mass of Fe-peak material and with its confinement to low velocities. A similarly low value for the Ni mass (0.07 Ṁ) was obtained from spectrum synthesis, by summing the Ni mass derived from the photospheric epoch models for the outer part and from the nebular models for the inner part. It appears that not all SNe la are the result of the explosion and complete disruption of white dwarfs at the Chandrasekhar mass limit. © 1997 RAS.