The properties of the peculiar type Ia supernova 1991bg - I. Analysis and discussion of two years of observations

Abstract

Observations of the peculiar type Ia supernova 1991bg in NGC 4374 collected at ESO La Silla and Asiago are presented and discussed. The photometric coverage extends for 530 d and the spectroscopy for the first 7 months after the explosion. The broad-band light curves in the early months have a narrower peak and a luminosity decline that is faster than other SNe Ia [14.6 and 11.7 mag × (100d)-1 in B and V respectively]. The R and I light curves do not show the secondary peak typical of normal SNe Ia. The SN is intrinsically very red [(B -V)max = 0.74] and faint (Bmax= -16.54). The light curves flatten with age but remain significantly steeper [2.0 and 2.7 mag × (100d)-1 in B and V between 70 and 200 d] than the average. Consequently the uvoir bolometric light curve of SN 1991bg is fainter with a steeper decline than that of the normal SN Ia (e.g. 1992A). This object enhances the correlation that exists between the peak luminosity of SNe Ia, the decline rate and the kinetic energy. Peculiarities are evident in the spectra at various phases. The continuum at maximum is very red and the photospheric expansion velocity extremely low. There are a number of unusual spectral features, in particular a broad absorption between 4200 and 4500 Å which is attributed to Tin and the appearance, as early as one month after maximum, of nebular emission of possibly [Co III] λ5890-5908. Nevertheless, contrary to previous claims, the spectral evolution retains a general resemblance to that of other SNe Ia until the latest available observation (day 203). At this epoch one sees the typical emission features of SNe Ia at late times although they are significantly narrower (FWHM∼2300 km s-1). This facilitates the identification of most lines with forbidden emission of [Fe II], [Fe III] and [Co III]. The emission feature centred at about λ6590 is difficult to reconcile with the previous identification as Hα, unless asymmetries in the ejecta or ad hoc binary configurations are invoked. This work suggests that the explosion energy was probably a factor of 3 to 5 lower than in normal SNe Ia. Whether this resulted from an explosion of a subChandrasekhar mass white dwarf (WD) is not unambiguously established.