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Keywords:

  • methods: data analysis;
  • techniques: spectroscopic;
  • supernovae: general;
  • cosmology: observations;
  • distance scale

ABSTRACT

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 to the end of 2008 as part of the Berkeley Supernova Ia Program (BSNIP) and are presented in BSNIP I by Silverman et al. 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 (pEWs), spectral feature depths and fluxes at the centre and endpoints of each of nine major spectral feature complexes. 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, is found to be well correlated with the so-called SiFe ratio and anticorrelated with the analogous ‘SSi ratio’, confirming the results of previous studies. 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. We find that purely spectroscopic classification schemes are useful in identifying the most peculiar SNe Ia. However, in almost all spectral parameters investigated, the full sample of objects spans a nearly continuous range of values. Comparisons to previously published theoretical models of SNe Ia are made and we conclude with a brief discussion of how the measurements performed herein and the possible correlations presented will be important for future SN surveys.