We describe observed properties of the Type Iax class of supernovae (SNe Iax), consisting of SNe observationally similar to its prototypical member, SN 2002cx. The class currently has 25 members, and we present optical photometry and/or optical spectroscopy for most of them. SNe Iax are spectroscopically similar to SNe Ia, but have lower maximum-light velocities (2000 |v| 8000 km s −1 ), typically lower peak magnitudes (−14.2 ≥ M V,peak −18.9 mag), and most have hot photospheres. Relative to SNe Ia, SNe Iax have low luminosities for their light-curve shape. There is a correlation between luminosity and light-curve shape, similar to that of SNe Ia, but offset from that of SNe Ia and with larger scatter. Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies, there are several indications that the progenitor stars are white dwarfs (WDs): evidence of C/O burning in their maximum-light spectra, low (typically ∼ 0.5 M ⊙ ) ejecta masses, strong Fe lines in their late-time spectra, a lack of X-ray detections, and deep limits on massive stars and star formation at the SN sites. However, two SNe Iax show strong He lines in their spectra. The progenitor system and explosion model that best fits all of the data is a binary system of a C/O WD that accretes matter from a He star and has a deflagration. At least some of the time, this explosion will not disrupt the WD. The small number of SNe in this class prohibit a detailed analysis of the homogeneity and heterogeneity of the entire class. We estimate that in a given volume there are 31 +17 −13 SNe Iax for every 100 SNe Ia, and for every 1 M ⊙ of iron generated by SNe Ia at z = 0, SNe Iax generate ∼0.036 M ⊙ . Being the largest class of peculiar SNe, thousands of SNe Iax will be discovered by LSST. Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia.

We study the observables of 158 relatively normal Type Ia supernovae (SNe Ia) by dividing them into two groups in terms of the expansion velocity inferred from the absorption minimum of the Si II λ6355 line in their spectra near B-band maximum brightness. One group ("Normal") consists of normal SNe Ia populating a narrow strip in the Si II velocity distribution, with an average expansion velocity v = 10, 600 ± 400 km s −1 near B maximum; the other group ("HV") consists of objects with higher velocities, v 11, 800 km s −1 . Compared with the Normal group, the HV one shows a narrower distribution in both the peak luminosity and the luminosity decline rate ∆m 15 . In particular, their B − V colors at maximum brightness are found to be on average redder by ∼0.1 mag, suggesting that they either are associated with dusty environments or have intrinsically red B − V colors. The HV SNe Ia are also found to prefer a lower extinction ratio R V ≈ 1.6 (versus ∼2.4 for the Normal ones). Applying such an absorption-correction dichotomy to SNe Ia of these two groups remarkably reduces the dispersion in their peak luminosity from 0.178 mag to only 0.125 mag.

We present extensive photometry at ultraviolet (UV), optical, and near-infrared (NIR) wavelengths, as well as dense sampling of optical spectra, for the normal Type Ia supernova (SN Ia) 2005cf. The optical photometry, performed at eight different telescopes, shows a 1σ scatter of 0.03 mag after proper corrections for the instrument responses. From the well-sampled light curves, we find that SN 2005cf reached a B-band maximum at 13.63 ± 0.02 mag, with an observed luminosity decline rate Δm 15 (B) = 1.05 ± 0.03 mag. The correlations between the decline rate and various color indexes, recalibrated on the basis of an expanded SN Ia sample, yield a consistent estimate for the host-galaxy reddening of SN 2005cf, E(B − V ) host = 0.10 ± 0.03 mag. The UV photometry was obtained with the Hubble Space Telescope and the Swift Ultraviolet/Optical Telescope, and the results match each other to within 0.1-0.2 mag. The UV light curves show similar evolution to the broadband U, with an exception in the 2000-2500 Å spectral range (corresponding to the F220W/uvm2 filters), where the light curve appears broader and much fainter than that on either side (likely owing to the intrinsic spectral evolution). Combining the UV data with the ground-based optical and NIR data, we establish the generic UVoptical-NIR bolometric light curve for SN 2005cf and derive the bolometric corrections in the absence of UV and/or NIR data. The overall spectral evolution of SN 2005cf is similar to that of a normal SN Ia, but with variety in the strength and profile of the main feature lines. The spectra at early times displayed strong, high-velocity (HV) features in the Ca ii H&K doublet and NIR triplet, which were distinctly detached from the photosphere (v ≈ 10,000 km s −1 ) at a velocity ranging from 20,000 to 25,000 km s −1 . One interesting feature is the flatbottomed absorption observed near 6000 Å in the earliest spectrum, which rapidly evolved into a triangular shape and then became a normal Si ii λ6355 absorption profile at about one week before maximum brightness. This premaximum spectral evolution is perhaps due to the blending of the Si iiλ6355 at photospheric velocity and another HV absorption component (e.g., an Si ii shell at a velocity ∼18,000 km s −1 ) in the outer ejecta, and may be common in other normal SNe Ia. The possible origin of the HV absorption features is briefly discussed.