Comprehensive observations of the bright and energetic Type lax SN 2012Z: Interpretation as a Chandrasekhar mass white dwarf explosion

Abstract

We present ultraviolet through near-infrared (NIR) broadband photometry, and visual-wavelength and NIR spectroscopy of the Type lax supernova (SN) 2012Z. The data set consists of both early- and late-time observations, including the first late phase NIR spectrum obtained for a spectroscopically classified SN lax. Simple model calculations of its bolometric light curve suggest SN 2012Z produced similar to 0.3 M-circle dot of Ni-56, ejected about a Chandrasekhar mass of material, and had an explosion energy of similar to 10(51) erg, making it one of the brightest (M-B = -18.3 mag) and most energetic SN Iax yet observed. The late phase (+269d) NIR spectrum of SN 2012Z is found to broadly resemble similar epoch spectra of normal SNe Ia; however, like other SNe Iax, corresponding visual-wavelength spectra differ substantially from all supernova types. Constraints from the distribution of intermediate mass elements, e.g., silicon and magnesium, indicate that the outer ejecta did not experience significant mixing during or after burning, and the late phase NIR line profiles suggests most of the Ni-56 is produced during high density burning. The various observational properties of SN 2012Z are found to be consistent with the theoretical expectations of a Chandrasekhar mass white dwarf progenitor that experiences a pulsational delayed detonation, which produced several tenths of a solar mass of Ni-56 during the deflagration burning phase and little (or no) Ni-56 during the detonation phase. Within this scenario only a moderate amount of Rayleigh-Taylor mixing occurs both during the deflagration and fallback phase of the pulsation, and the layered structure of the intermediate mass elements is a product of the subsequent denotation phase. The fact that the SNe lax population does not follow a tight brightness-decline relation similar to SNe Ia can then be understood in the framework of variable amounts of mixing during pulsational rebound and variable amounts of Ni-56 production during the early subsonic phase of expansion.