35 6 v 1 10 N ov 2 00 6 Nearby Supernova Factory Observations of SN 2006 D : On Sporadic Carbon Signatures in Early Type Ia Supernova Spectra The Nearby Supernova Factory

We present four spectra of the Type Ia supernova (SN Ia) 2006D extending from −7 to +13 days with respect to B-band maximum. The spectra include the strongest signature of unburned material at photospheric velocities observed in a SN Ia to date. The earliest spectrum exhibits C II absorption features below 14,000 km s, including a distinctive C II λ6580 absorption feature. The carbon signatures dissipate as the SN approaches peak brightness. In addition to discussing implications of photospheric-velocity carbon for white dwarf explosion models, we outline some factors that may influence the frequency of its detection before and around peak brightness. Two effects are explored in this regard, including depopulation of the C II optical levels by non-LTE effects, and line-ofsight effects resulting from a clumpy distribution of unburned material with low volume-filling factor. Physics Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 Institut de Physique Nucléaire de Lyon, UMR5822, CNRS-IN2P3; Université Claude Bernard Lyon 1, F-69622 Villeurbanne France Laboratoire de Physique Nucléaire et des Hautes Energies IN2P3 CNRS Universités Paris VI et Paris VII, 4 place Jussieu Tour 33 Rez de chaussée 75252 Paris Cedex 05 Centre de Recherche Astronomique de Lyon, 9, av. Charles André, 69561 Saint Genis Laval Cedex University of California, Space Sciences Laboratory, Berkeley, CA 94720-7450 Department of Physics, University of California, Berkeley, CA 94720 Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637 Department of Physics, Yale University, New Haven, CT 06250-8121 Homer L. Dodge Department of Physics and Astronomy, 440 W. Brooks Street, University of Oklahoma, Norman, OK 73019