High-Velocity Features: a ubiquitous property of Type Ia SNe

Evidence of high-velocity features such as those seen in the near-maximum spectra of some Type Ia Supernovae (e.g. SN 2000cx) has been searched for in the available SN Ia spectra observed earlier than one week before B maximum. Recent observational efforts have doubled the number of SNe Ia with very early spectra. Remarkably, all SNe Ia with early data (7 in our RTN sample and 10 from other programmes) show signs of such features, to a greater or lesser degree, in Ca ii IR, and some also in Si ii λλ 6255Å line. High-velocity features may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disc INAF-Osserv. Astron., Via Tiepolo, 11, 34131 Trieste, Italy Max-Planck Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85748 Garching, Germany INAF-Osserv. Astron. di Padova, 35122 Padova, Italy Department of Astronomy, U. Barcelona, Barcelona, Spain INAF-Osserv. Astron. di Capodimonte, Napoli, Italy Lab. de Physique Nucléaire et de Haute Energies, CNRS-IN2P3, Univ. Paris VI and VII, Paris, France Dept. of Physics, Stockholm Univ., SE-106 91 Stockholm, Sweden Blackett Laboratory-Imperial College, London, England European Southern Observatory, 85748 Garching, Germany Stockholm Obs., SE-106 91 Stockholm, Sweden Dept. of Astronomy, Oxford University, Oxford, England Mt.Stromlo and Siding Spring Obs., Canberra, Australia Institute of Astronomy, Cambridge, England