THE SPECTROSCOPIC DIVERSITY OF TYPE Ia SUPERNOVAE

Abstract

We present 2603 spectra of 462 nearby Type Ia supernovae (SNe Ia), including 2065 previously unpublished spectra, obtained during 1993-2008 through the Center for Astrophysics Supernova Program. There are on average eight spectra for each of the 313 SNe Ia with at least two spectra. Most of the spectra were obtained with the FAST spectrograph at the Fred Lawrence Whipple Observatory 1.5 m telescope and reduced in a consistent manner, making this data set well suited for studies of SN Ia spectroscopic diversity. Using additional data from the literature, we study the spectroscopic and photometric properties of SNe Ia as a function of spectroscopic class using the classification schemes of Branch et al. and Wang et al. The width-luminosity relation appears to be steeper for SNe Ia with broader lines, although the result is not statistically significant with the present sample. Based on the evolution of the characteristic Si ii λ6355 line, we propose improved methods for measuring velocity gradients, revealing a larger range than previously suspected, from ∼0 to ∼400 km s −1 day −1 considering the instantaneous velocity decline rate at maximum light. We find a weaker and less significant correlation between Si ii velocity and intrinsic B − V color at maximum light than reported by Foley et al., owing to a more comprehensive treatment of uncertainties and host galaxy dust. We study the extent of nuclear burning and the presence of unburnt carbon in the outermost layers of the ejecta and report new detections of C ii λ6580 in 23 early-time SN Ia spectra. The frequency of C ii detections is not higher in SNe Ia with bluer colors or narrower light curves, in conflict with the recent results of Thomas et al. Based on nebular spectra of 27 SNe Ia, we find no relation between the FWHM of the iron emission feature at ∼4700 Å and Δm 15 (B) after removing the two low-luminosity SN 1986G and SN 1991bg, suggesting that the peak luminosity is not strongly dependent on the kinetic energy of the explosion for most SNe Ia. Finally, we confirm the correlation of velocity shifts in some nebular lines with the intrinsic B − V color of SNe Ia at maximum light, although several outliers suggest a possible non-monotonic behavior for the largest blueshifts.

Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T

“…Instead, early spectra show unusual Fe III features (Filippenko et al 1992;Ruiz-Lapuente et al 1992;Jeffery et al 1992), requiring high ionisation and Fe abundance at high velocity. Moreover, SNe 91T events are more luminous by about 0.2-0.3 mag than the width-luminosity relation predicts for normal SNe Ia (Blondin et al 2012).…”

“…Mazzali et al 1995;Fisher et al 1999; but see Sasdelli et al 2014). Furthermore, the fact that only a few per cent of SNe Ia are classified as 1991T-like (Li et al 2011;Silverman et al 2012;Blondin et al 2012) would be naturally explained by the relative scarcity of the single-degenerate scenario compared to the violent-merger or double-detonation scenarios (e.g. Ruiter et al 2009Ruiter et al , 2011Ruiter et al , 2013Fisher & Jumper 2015).…”

“…SN 1991T is the best characterised exemplar of a sub-class (SNe 91T) of SNe Ia that make up a few percent of all observed SNe Ia (Li et al 2011;Silverman et al 2012;Blondin et al 2012). In contrast to normal SNe Ia, SNe 91T are known to occur preferentially in late-type galaxies (Hamuy et al 2000;Howell 2001), indicating an origin in young stellar populations.…”

Observational probes of cosmic acceleration

“…For example Foley et al (2011) find a correlation between intrinsic color and the ejecta velocity inferred from the line width (see also Blondin et al 2012;Foley 2012). However, Silverman et al (2012), considering a variety of spectral indicators, find only marginal evidence for a diagnostic that improves Hubble residuals, and Walker et al (2011) find similarly ambiguous results.…”

Optical and near-IR observations of the faint and fast 2008ha-like supernova 2010ae

“…14), while it is typically not present in late spectra of thermonuclear SNe Ia (e.g. Blondin et al 2012;Silverman et al 2013). Recently, however, [O i] has been observed in the sub luminous Type Ia SN 2010lp , and is expected to be observed in turbulent deflagration explosions of C/O white dwarfs, the possible progenitor candidates for bright 2002cx-like SNe Iax (e.g.…”