Carnegie Supernova Project: The First Homogeneous Sample of Super-Chandrasekhar-mass/2003fg-like Type Ia Supernovae
C. Ashall, Jing Lü, E. Y. Hsiao, P. Hoêflich, M. M. Phillips, L. Galbany, C. R. Burns, C. Contreras, K. Krisciunas, N. Morrell, M. Stritzinger, N. B. Suntzeff, F. Taddia, J. Anais, E. Baron, P. J. Brown, L. Busta, A. Campillay, S. Castellón, C. Corco, Scott Davis, G. Folatelli, F. Förster, Wendy L. Freedman, C. González, M. Hamuy, S. Holmbo, R. Kirshner, Sahana Kumar, G. H. Marion, P. A. Mazzali, Tomoki Morokuma, P. Nugent, S. E. Persson, Anthony L. Piro, M. Roth, F. Salgado, David J. Sand, J. Serón, Melissa Shahbandeh, B. J. Shappee
Abstract
Abstract We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δ m 15 ( B ) < 1.3 mag), and peak absolute B -band magnitudes of −19 < M B < −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical B and V bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one i -band maximum, which peaks after the epoch of the B -band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above M H = −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000 km s −1 ) in Si ii λ 6355 velocities at maximum light with no rapid early velocity decline, and no clear H -band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C ii at maximum light have lower Si ii λ 6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.