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 Iasupernova (SN Ia), taken 227 and 277 days past maximum light. Astonishingly,the spectra are not dominated by the forbidden emission lines of iron that area hallmark of thermonuclear supernovae in the nebular phase. Instead, weidentify numerous P-Cygni profiles of Fe II at very low expansion velocities ofabout 700 km/s, which are without precedent in SNe Ia. We also report thetentative identification of low-velocity O I in these spectra, suggesting thepresence of unburned material near the center of the exploding white dwarf. SN2002cx is the prototype of a new subclass of SNe Ia, with spectralcharacteristics that may be consistent with recent pure deflagration models ofChandrasekhar-mass thermonuclear supernovae. These are distinct from themajority of SNe Ia, for which an alternative explosion mechanism, such as adelayed detonation, may be required.Comment: 18 pages, 5 figures, to appear in The Astronomical Journal; minor revisions to match accepted versio