The properties of the peculiar Type Ia SN 1991 bg . Analysis and discussion of two years of observations . ⋆

Observations of the peculiar type Ia SN 1991bg in NGC 4374 collected at ESO–La Silla and Asiago are presented and discussed. The photometric coverage extends for 530 days and the spectroscopy for the first seven months after the explosion. The broad– band light curves in the early months have a narrower peak and a luminosity decline faster than other SNIa (14.6 and 11.7 mag×(100d) in B and V respectively). The R and I light curves do not show the secondary peak typical of normal SNIa. 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) in B and V between 70 and 200 day) than the average. Consequently the uvoir bolometric light curve of SN 1991bg is fainter with a steeper decline than that of the normal SNIa (e.g. 1992A). This object enhances the correlation which exists between the peak luminosity of SNIa, 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 TiII and the appearance, as early as one month after maximum, of nebular emission of possibly [CoIII] λ5890 − 5908. Nevertheless, contrary to previous claims (Ruiz-Lapuente et al 1993), the spectral evolution retains a general resemblance to that of other SNIa until the latest available observation (day 203). At this epoch one sees the typical emission features of SNIa at late times although they are significantly narrower (FWHM∼ 2300 km s). This facilitates the identification of most lines with forbidden emission of [FeII], [FeIII] and [CoIII]. The emission feature centered 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 3 to 5 lower than in normal SNIa. Whether this resulted from an explosion of a sub–Chandrasekhar mass WD is not unambiguously established.