Supernova 2002bo: inadequacy of the single parameter description

We present optical photometry and optical/near-infrared spectra of the type Ia SN 2002bo spanning epochs from -13 days before maximum B-band light to +102 days after. The pre-maximum optical coverage is particularly complete. The extinction de- duced from the observed colour evolution and from interstellar NaID absorption is quite high viz. E(B V ) = 0.43 ± 0.10. On the other hand, model matches to the observed spectra point to a lower reddening (E(B V ) � 0.30). In some respects, SN 2002bo behaves as a typical "Branch normal" type Ia supernova (SN Ia) at opti- cal and IR wavelengths. We find a B-band risetime of 17.9±0.5 days, am15(B) of 1.13±0.05, and a de-reddened MB = 19.41±0.42. However, comparison with other type Ia supernovae having similarm15(B) values indicates that in other respects SN 2002bo is unusual. While the optical spectra of SN 2002bo are very similar to those of SN 1984A (�m15(B) = 1.19), lower velocities and a generally more structured ap- pearance are found in SNe 1990N, 1994D and 1998bu. For supernovae havingm15(B) > 1.2, we confirm the variation of R(SiII) (Nugent et al. 1995) withm15(B). How- ever, for supernovae such as SN 2002bo, with lower values ofm15(B) the relation breaks down. Moreover, the evolution of R(SiII) for SN 2002bo is strikingly differ- ent from that shown by other type Ia supernovae. The velocities of SN 2002bo and 1984A derived from SII 5640u SiII 6355u and CaII H&K lines are either much higher and/or evolve differently from those seen in other normal SNe Ia events. Thus, while SN 2002bo and SN 1984A appear to be highly similar, they exhibit behaviour which is distinctly different from other SNe Ia having similarm15(B) values. We suggest that the unusually low temperature, the presence of high-velocity intermediate-mass elements and the low abundance of carbon at early times indicates that burning to Si penetrated to much higher layers than in more normal type Ia supernovae. This may be indicative of a delayed-detonation explosion.