Berkeley Supernova Ia Program - I. Observations, data reduction and spectroscopic sample of 582 low-redshift Type Ia supernovae

Abstract

In this first paper in a series we present 1298 low-redshift (z 0.2) optical spectra of 582 Type Ia supernovae (SNe Ia) observed from 1989 through 2008 as part of the Berkeley SN Ia Program (BSNIP). 584 spectra of 199 SNe Ia have well-calibrated light curves with measured distance moduli, and many of the spectra have been corrected for host-galaxy contamination. Most of the data were obtained using the Kast double spectrograph mounted on the Shane 3 m telescope at Lick Observatory and have a typical wavelength range of 3300-10,400Å, roughly twice as wide as spectra from most previously published datasets. We present our observing and reduction procedures, and we describe the resulting SN Database (SNDB), which will be an online, public, searchable database containing all of our fully reduced spectra and companion photometry. In addition, we discuss our spectral classification scheme (using the SuperNova IDentification code, SNID; Blondin & Tonry 2007), utilising our newly constructed set of SNID spectral templates. These templates allow us to accurately classify our entire dataset, and by doing so we are able to reclassify a handful of objects as bona fide SNe Ia and a few other objects as members of some of the peculiar SN Ia subtypes. In fact, our dataset includes spectra of nearly 90 spectroscopically peculiar SNe Ia. We also present spectroscopic host-galaxy redshifts of some SNe Ia where these values were previously unknown. The sheer size of the BSNIP dataset and the consistency of our observation and reduction methods makes this sample unique among all other published SN Ia datasets and is complementary in many ways to the large, low-redshift SN Ia spectra presented by Matheson et al. 2008 andBlondin et al. 2012. In other BSNIP papers in this series, we use these data to examine the relationships between spectroscopic characteristics and various observables such as photometric and host-galaxy properties.

Type Ia Supernovae Strongly Interacting with Their Circumstellar Medium

“…At these late times, the spectra of SNe Ia-CSM basically consist of a relatively blue "quasi-continuum," which is likely due to emission from many overlapping, relatively narrow lines of iron-group elements (IGEs), mostly Fe ii, excited by the CSM interaction, in addition to strong Hα emission and often broad Ca ii emission (e.g., Deng et al 2004). Figure 13 shows spectra of PTF11kx and SN 2005gj, along with two comparison objects: SN 2010jl (an SN IIn; Smith et al 2011a) and SN 1999aa (a somewhat overluminous SN Ia; Silverman et al 2012a). The figure is reproduced from Silverman et al (2013) and some of the major spectral features are labeled.…”

“…Both were found to be more luminous than typical SNe Ia and most SNe IIn, have spectra consisting of relatively narrow Hα emission (with P-Cygni profiles) on top of a "diluted" SN Ia spectrum, and broad, slowly evolving light curves. To the already impressive data on SN 2005gj in the literature, we add five spectra (three at early times and two at very late times) that were originally published by Silverman et al (2012a) and are displayed in Figure 1. …”

“…Unfortunately, it seems that SN 1999E was discovered well after maximum brightness; thus, the phases displayed in Figure 2 are relative to the UT date of discovery (1999 January 15; Rigon et al 2003), and are likely from a much later phase if calculated relative to maximum brightness. Silverman et al (2012a), labeled with age relative to maximum brightness. The data have had their host-galaxy recession velocity removed and have been corrected for Galactic reddening.…”

“…To further investigate the spectral characteristics of SNe Ia-CSM (specifically as compared to SNe IIn), we measure properties of the Hα, Hβ, and He i λ5876 features in all of the objects classified as SNe IIn from the Berkeley SN Group's database (Silverman et al 2012a) as well as the 63 SNe IIn discovered by PTF through 2012 August. We follow the procedure of Dilday et al (2012) and fit the spectral features with Gaussian profiles, using multiple components when necessary.…”

“…SNID classifies SN spectra by cross-correlating an input spectrum with a large database of observed SN spectra (known as "templates"). To identify SNe Ia-CSM, we created a special set of spectral templates that consisted of all 310 spectra of 163 objects classified as SNe IIn from the Berkeley SN Group's database (Silverman et al 2012a). In addition to these, we also included templates of a handful of underluminous, overluminous, and normal SNe Ia at a variety of epochs, as well as 27 spectral templates of the well-known Ia-CSM objects SNe 2002ic and 2005gj (mentioned in Section 1).…”