Reading the Spectra of the Most Peculiar Type Ia Supernova 2002cx

In spite of the apparent lack of Si ii and S ii features in its spectra, SN 2002cx was classified as a peculiar Type Ia supernova (SN Ia) on the basis of its overall photometric and spectroscopic behavior. Spectra obtained near maximum light contained Fe iii features, as in SN 1991T–like events, but the blueshifts of the Fe iii absorptions were exceptionally low. The luminosity was also low. We use the supernova synthetic‐spectrum code SYNOW to study line identifications in SN 2002cx. We find that the maximum‐light spectra appear to contain weak features of Si ii, S ii, Si iii, and Ca ii, which strengthens the connection with SN 1991T–like events. We show that later spectra obtained 12, 25, and 56 days after maximum consist of P Cygni resonance‐scattering features due to permitted Fe ii and Co ii lines. SN 2002cx had been thought to have made the transition from a permitted‐line to a forbidden‐line spectrum between 25 and 56 days. Owing to the low expansion velocities, the postmaximum spectral features are narrower and easier to identify than they are in other SNe Ia. SN 2002cx will lead to improved line identifications in other SNe Ia and will clarify when the transition from a permitted‐ to a forbidden‐line spectrum occurs. In the context of current SN Ia explosion models, we suggest that the properties of SN 2002cx might be consistent with three‐dimensional deflagration models, which are not favored for normal SNe Ia.