ar X iv : a st ro - p h / 04 11 05 9 v 2 3 J an 2 00 5 The diversity of Type Ia Supernovae : evidence for systematics ?

The photometric and spectroscopic properties of 26 well observed Type Ia Supernovae (SNe Ia) were analyzed with the aim to explore SN Ia diversity. The sample includes (Branch-)normal SNe as well as extreme events like SNe 1991T and 1991bg, while the truly peculiar SN Ia, SN 2000cx (Li et al. 2001) and SN 2002cx (Li et al. 2003) are not included in our sample. A statistical treatment reveals the existence of three different groups. The first group (FAINT) consists of faint SNe Ia similar to SN 1991bg, with low expansion velocities and rapid evolution of SiII velocity. A second group consists of " normal " SNe Ia, also with high temporal velocity gradient (HVG), but with brighter mean absolute magnitude < M B >= −19.3 and higher expansion velocities than the FAINT SNe. The third group includes both " normal " and SN 1991T-like SNe Ia: these SNe populate a narrow strip in the SiII velocity evolution plot, with a low velocity gradient (LVG), but have absolute magnitudes similar to HVGs. While the FAINT and HVG SNe Ia together seem to define a relation between R(SiII) and ∆m 15 (B), the LVG ones either do not conform with that relation or define a new, looser one. The R(SiII) pre-maximum evolution of HVGs is strikingly different from that of LVGs. The impact of this evidence on the understanding of SN Ia diversity, in terms of explosion mechanisms, degree of ejecta mixing, and ejecta-CSM interaction, is discussed.