SN 1991bg - A type IA supernova with a difference

Abstract

Artículo de publicación ISI.We present 13 spectra and 31 photometric observations covering the first 150 days of SN 1991bg in NGC 4374 (M 84). Although SN 1991bg was a type Ia supernova displaying the characteristic Si II absorption at 6150 angstrom near maximum and the Fe emission lines at late phases, it varied from the well-defined norm for SNe Ia in several important respects. The peculiarities include faster declines in the B and V light curves after maximum, a distinct color evolution, a very red B-V color near maximum, relatively faint peak luminosity, a distinct spectral evolution, and a short peak phase. The narrow peak of the luminosity and the rapid declines of the light curves suggest a smaller mass in the ejecta and larger energy losses than for most SNe Ia. The unusually red color at maximum is not a result of normal extinction, since SN 1991bg was as blue as other SNe Ia at late times and no narrow interstellar lines are observed in the spectra. The faint absolute magnitude of SN 1991bg is established beyond doubt by comparison with SN 1957B, another type Ia supernova in the same galaxy, which was approximately 2.5 magnitudes brighter than SN 1991bg. The spectral evolution reveals minor differences near maximum compared to other well-observed SNe Ia, mainly in relative line strengths. At later phases several wavelength regions display discrepancies when compared to spectra of normal SNe Ia. Although other SNe Ia, such as SN 1986G and SN 1939B, have light curves with fast decline rates, SN 1991bg is unique, with deviations in both light curves and spectra. In particular SN 1991bg is the only SN Ia observed to date with a distinct spectrum at approximately 40 days past maximum. Although SN 1991bg is an extreme case, with unusual photometric and spectroscopic properties, we believe it can be understood in the context of exploding white dwarf models, and is properly grouped with type Ia. SN 1991bg demonstrates the need for detailed observations of SNe Ia as part of their use as standard candles for cosmology. While there is a well-defined prototype with homogeneous properties, unusual cases like SN 1991bg must be identified and separated to avoid misleading results

DEVIATIONS FROM AXISYMMETRY REVEALED BY LINE POLARIZATION IN THE NORMAL TYPE Ia SUPERNOVA 2004S

“…This velocity is actually quite low for a normal SN Ia at this epoch and is comparable to the lowest-velocity objects in the sample of Benetti et al (2005). Only peculiar subluminous objects such as SNe 1991bg and 2002cx have shown lower Si ii velocities at a similar age (Leibundgut et al 1993;Li et al 2003).…”

A reddening-free method to estimate the56Ni mass of Type Ia supernovae

“…We tested our method on SN 2014J, a heavily reddened SN Ia in the nearby galaxy M 82, and find good agreement between the estimates from the γ-ray observations (Churazov et al 2014;Diehl et al 2015, see Table 4). Faint, 91bg-like SNe Ia, which show typically lower luminosities (Filippenko et al 1992;Leibundgut et al 1993), do not display a second maximum in their NIR light curves and are not in our sample. Therefore, the true dispersion in peak luminosity and M56 Ni for SN Ia will likely be larger than what is derived here.…”

Towards an understanding of Type Ia supernovae from a synthesis of theory and observations

“…SN 1991bg, SN 1992K, SN 1999by and SN 2005bl are well-studied examples for red, fast, and subluminous supernovae with a typical ∆m 15 value of about 1.8 and B-band peak absolute magnitudes around -17, roughly one magnitude fainter than their 'normal' counterparts [59,[69][70][71][72][73]. Their V , I, and R-band light curves decline unusually quickly, skipping the second maximum in I, and their spectra show a high abundance of intermediate mass elements (including Ti ii) with low expansion velocities but only little iron.…”