No abstract

Owing to their utility for measurements of cosmic acceleration, Type Ia supernovae (SNe) are perhaps the best-studied class of SNe, yet the progenitor systems of these explosions largely remain a mystery. A rare subclass of SNe Ia show evidence of strong interaction with their circumstellar medium (CSM), and in particular, a hydrogen-rich CSM; we refer to them as SNe Ia-CSM. In the first systematic search for such systems, we have identified 16 SNe Ia-CSM, and here we present new spectra of 13 of them. Six SNe Ia-CSM have been well-studied previously, three were previously known but are analyzed in-depth for the first time here, and seven are new discoveries from the Palomar Transient Factory. The spectra of all SNe Ia-CSM are dominated by Hα emission (with widths of ∼2000 km s −1 ) and exhibit large Hα/Hβ intensity ratios (perhaps due to collisional excitation of hydrogen via the SN ejecta overtaking slower-moving CSM shells); moreover, they have an almost complete lack of He I emission. They also show possible evidence of dust formation through a decrease in the red wing of Hα 75-100 d past maximum brightness, and nearly all SNe Ia-CSM exhibit strong Na I D absorption from the host galaxy. The absolute magnitudes (uncorrected for host-galaxy extinction) of SNe Ia-CSM are found to be −21.3 ≤ M R ≤ −19 mag, and they also seem to show ultraviolet emission at early times and strong infrared emission at late times (but no detected radio or X-ray emission). Finally, the host galaxies of SNe Ia-CSM are all late-type spirals similar to the Milky Way, or dwarf irregulars like the Large Magellanic Cloud, which implies that these objects come from a relatively young stellar population. This work represents the most detailed analysis of the SN Ia-CSM class to date.

We report on our early photometric and spectroscopic observations of the extremely luminous Type II supernova (SN) 2008es. With an observed peak optical magnitude of m V = 17.8 and at a redshift z = 0.213, SN 2008es had a peak absolute magnitude of M V = −22.3, making it the second most luminous SN ever observed. The photometric evolution of SN 2008es exhibits a fast decline rate (∼0.042 mag d −1 ), similar to the extremely luminous Type II-L SN 2005ap. We show that SN 2008es spectroscopically resembles the luminous Type II-L SN 1979C. Although the spectra of SN 2008es lack the narrow and intermediate-width line emission typically associated with the interaction of a SN with the circumstellar medium of its progenitor star, we argue that the extreme luminosity of SN 2008es is powered via strong interaction with a dense, optically thick circumstellar medium. The integrated bolometric luminosity of SN 2008es yields a total radiated energy at ultraviolet and optical wavelengths of 10 51 ergs. Finally, we examine the apparently anomalous rate at which the Texas Supernova Search has discovered rare kinds of supernovae, including the five most luminous supernovae observed to date, and find that their results are consistent with those of other modern SN searches.

We present visual-wavelength photometry and spectroscopy of supernova (SN) 2008S. Based on the low peak luminosity for a SN of M R = −13.9 mag, photometric and spectral evolution unlike that of low-luminosity SNe, a late-time decline rate slower than 56 Co decay, and slow outflow speeds of 600-1000 km s −1 , we conclude that SN 2008S is not a true core-collapse SN and is probably not an electron-capture SN. Instead, we show that SN 2008S more closely resembles a "SN impostor" event like SN 1997bs, analogous to the giant eruptions of luminous blue variables (LBVs). Its total radiated energy was ∼10 47.8 ergs, and it may have ejected 0.05-0.2 M ⊙ in the event. We discover an uncanny similarity between the spectrum of SN 2008S and that of the Galactic hypergiant IRC+10420, which is dominated by narrow Hα, [Ca ii], and Ca ii emission lines formed in an opaque wind. We propose a scenario where the vastly super-Eddington (Γ ≈ 40) wind of SN 2008S partly fails because of reduced opacity due to recombination, as suggested for IRC+10420. The range of initial masses susceptible to eruptive LBV-like mass loss was known to extend down to 20-25 M ⊙ , but estimates for the progenitor of SN 2008S (and the similar NGC 300 transient) may extend this range to 15 M ⊙ . As such, SN 2008S may have implications for the progenitor of SN 1987A.

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.