High-Velocity Features: A Ubiquitous Property of Type Ia Supernovae

Evidence of high-velocity features such as those seen in the near-maximumspectra of some Type Ia Supernovae (eg SN 2000cx) has been searched for in theavailable SNIa spectra observed earlier than one week before B maximum. Recentobservational efforts have doubled the number of SNeIa with very early spectra.Remarkably, all SNeIa with early data (7 in our RTN sample and 10 from otherprogrammes) show signs of such features, to a greater or lesser degree, in CaIIIR, and some also in SiII 6255A line. High-velocity features may be interpretedas abundance or density enhancements. Abundance enhancements would imply anouter region dominated by Si and Ca. Density enhancements may result from thesweeping up of circumstellar material by the highest velocity SN ejecta. Inthis scenario, the high incidence of HVFs suggests that a thick disc and/or ahigh-density companion wind surrounds the exploding white dwarf, as may be thecase in Single Degenerate systems. Large-scale angular fluctuations in theradial density and abundance distribution may also be responsible: this couldoriginate in the explosion, and would suggest a deflagration as the more likelyexplosion mechanism. CSM-interaction and surface fluctuations may coexist,possibly leaving different signatures on the spectrum. In some SNe the HVFs arenarrowly confined in velocity, suggesting the ejection of blobs of burnedmaterial.Comment: 12 pages, 2 figures, ApJ Letters in pres