Jeffrey M. Silverman, Jason J. Kong, Alexei V. Filippenko
In this second paper in a series we present measurements of spectral features
of 432 low-redshift (z < 0.1) optical spectra of 261 Type Ia supernovae (SNe
Ia) within 20 d of maximum brightness. The data were obtained from 1989 through
the end of 2008 as part of the Berkeley SN Ia Program (BSNIP) and are presented
in BSNIP I (Silverman et al., submitted). We describe in detail our method of
automated, robust spectral feature definition and measurement which expands
upon similar previous studies. Using this procedure, we attempt to measure
expansion velocities, pseudo-equivalent widths (pEW), spectral feature depths,
and fluxes at the centre and endpoints of each of nine major spectral feature
complexes. A sanity check of the consistency of our measurements is performed
using our data (as well as a separate spectral dataset). We investigate how
velocity and pEW evolve with time and how they correlate with each other.
Various spectral classification schemes are employed and quantitative spectral
differences among the subclasses are investigated. Several ratios of pEW values
are calculated and studied. The so-called Si II ratio, often used as a
luminosity indicator (Nugent et al. 1995), is found to be well correlated with
the so-called "SiFe" ratio and anticorrelated with the analogous "SSi ratio."
Furthermore, SNe Ia that show strong evidence for interaction with
circumstellar material or an aspherical explosion are found to have the largest
near-maximum expansion velocities and pEWs, possibly linking extreme values of
spectral observables with specific progenitor or explosion scenarios.
[Abridged]
View original:
http://arxiv.org/abs/1202.2129
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