The Diversity of Type Ia Supernovae: Evidence for Systematics?

The photometric and spectroscopic properties of 26 well observed Type IaSupernovae (SNeIa) were analyzed with the aim to explore SNIa diversity. Thesample includes (Branch-)normal SNe as well as extreme events like SNe 1991Tand 1991bg, while the truly peculiar SNIa, SN2000cx and SN2002cx are notincluded in our sample . A statistical treatment reveals the existence of threedifferent groups. The first group (FAINT) consists of faint SNeIa similar toSN1991bg, with low expansion velocities and rapid evolution of SiII velocity. Asecond group consists of ``normal'' SNeIa, also with high temporal velocitygradient (HVG), but with brighter mean absolute magnitude =-19.3 andhigher expansion velocities than the FAINT SNe. The third group includes both``normal'' and SN1991T-like SNeIa: these SNe populate a narrow strip in theSiII velocity evolution plot, with a small velocity gradient (SVG), but haveabsolute magnitudes similar to HVGs. While the FAINT and HVG SNeIa togetherseem to define a relation between RSi(II) and Dm15(B), the SVG ones either donot conform with that relation or define a new, looser one. The RSi(II)pre-maximum evolution of HVGs is strikingly different from that of SVGs. Theimpact of this evidence on the understanding of SNIa diversity, in terms ofexplosion mechanisms, degree of ejecta mixing, and ejecta-CSM interaction, isdiscussed.