J an 2 00 4 Direct Analysis of Spectra of the Peculiar Type Ia Supernova 2000 cx

The Type Ia SN 2000cx exhibited multiple peculiarities, including a lopsided B–band light–curve peak that does not conform to current methods for using shapes of light curves to standardize SN Ia luminosities. We use the parameterized supernova synthetic–spectrum code Synow to study line identifications in the photospheric–phase spectra of SN 2000cx. Previous work established the presence of Ca II infrared–triplet features forming above velocity ∼20,000 km s, much higher than the photospheric velocity of ∼10,000 km s. We find Ti II features forming at the same high velocity. High–velocity line formation is partly responsible for the photometric peculiarities of SN 2000cx: for example, B–band flux blocking by Ti II absorption features that decreases with time causes the B light curve to rise more rapidly and decline more slowly than it otherwise would. SN 2000cx contains an absorption feature near 4530 Å that may be Hβ, forming at the same high velocity. The lack of conspicuous Hα and Pα signatures does not necessarily invalidate the Hβ identification if the high–velocity line formation is confined to a clump that partly covers the photosphere and the Hα and Pα source functions are elevated relative to that of resonance scattering. The Hβ identification is tentative. If it is correct, the high–velocity matter must have come from a nondegenerate companion star. Subject headings: supernovae: general – supernovae: individual (SN 2000cx) Department of Physics and Astronomy, University of Oklahoma, Norman, Oklahoma 73019, USA; [email protected] Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720–8158 Department of Astronomy and Research Center for the Early Universe, University of Tokyo, Bunkyo-ku, Tokyo, Japan Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720–3411 Space Science Applications Laboratory, The Aerospace Corporation M2/266, P. O. Box 92957, Los Angeles, CA 90009