Supernova 2002bo: inadequacy of the single parameter description

We present optical/near-infrared photometry and spectra of the type Ia SN2002bo spanning epochs from -13 d before maximum B-band light to +102 d after. The pre-maximum optical coverage is particularly complete. The extinction deduced from the observed colour evolution and from interstellar Na I D absorption is quite high, namely 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) similar to 0.30]. In some respects, SN 2002bo behaves as a typical 'Branch normal' type Ia supernova (SN Ia) at optical and infrared wavelengths. We find a B-band risetime of 17.9 +/- 0.5 d, a Deltam(15)(B) of 1.13 +/- 0.05, and a dereddened M-B = 19.41 +/- 0.42. However, comparison with other type Ia supernovae having similar Deltam(15)(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 [Deltam(15)(B) = 1.19], lower velocities and a generally more structured appearance are found in SNe 1990N, 1994D and 1998bu. For supernovae having Deltam(15)(B) > 1.2, we confirm the variation of R(Si II) with Deltam(15)(B). However, for supernovae such as SN 2002bo, with lower values of Deltam(15)(B) the relation breaks down. Moreover, the evolution of R(Si II) for SN 2002bo is strikingly different from that shown by other type Ia supernovae. The velocities of SN 2002bo and 1984A derived from S II 5640 Angstrom, Si II 6355 Angstrom and Ca II H and 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 similar Deltam(15)(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.