Late-time spectroscopy of SN 2002cx: The prototype of a new subclass of type Ia supernovae

We present Keck optical spectra of SN 2002cx, the most peculiar known Type Ia supernova ( SN Ia), taken 227 and 277 days past maximum light. Astonishingly, the spectra are not dominated by the forbidden emission lines of iron that are a hallmark of thermonuclear SNe in the nebular phase. Instead, we identify numerous P Cygni profiles of Fe (II) at very low expansion velocities of similar to 700 km s(-1), which are without precedent in SNe Ia. We also report the tentative identification of low-velocity O (I) in these spectra, suggesting the presence of unburned material near the center of the exploding white dwarf. SN 2002cx is the prototype of a new subclass of SNe Ia, with spectral characteristics that may be consistent with recent pure deflagration models of Chandrasekhar-mass thermonuclear SNe. These are distinct from the majority of SNe Ia, for which an alternative explosion mechanism, such as a delayed detonation, may be required.