The early spectral evolution of SN 2004dt

¹ Department of Astronomy, University of Barcelona, Martí i Franqués 1, 08028 Barcelona, Spain
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: giuseppe.altavilla2@unibo.it
³ Max–Planck–Institut für Astrophysik, Karl–Schwarzschild–Straße 1, 85741 Garching, Germany
⁴ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁵ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica, Santiago de Chile, Chile
⁶ INAF–Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁷ Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁸ Physics Department, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
⁹ European Southern Observatory, Karl–Schwarzschild–Straße 2, 85748 Garching, Germany
¹⁰ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611 Canberra, Australia
¹¹ Australian National University, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611 Canberra, Australia

Received: 16 March 2007
Accepted: 8 August 2007

Abstract

Aims.We study the optical spectroscopic properties of Type Ia Supernova (SN Ia) 2004dt, focusing our attention on the early epochs.

Methods.Observation triggered soon after the SN 2004dt discovery allowed us to obtain a spectrophotometric coverage from day –10 to almost one year (~353 days) after the B band maximum. Observations carried out on an almost daily basis allowed us a good sampling of the fast spectroscopic evolution of SN 2004dt in the early stages. To obtain this result, low–resolution, long–slit spectroscopy was obtained using a number of facilities.

Results.This supernova, which in some absorption lines of its early spectra showed the highest degree of polarization ever measured in any SN Ia, has a complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is present, moving at velocities as high as ~16 700 km s^-1, with some intermediate–mass elements (Mg, Si, Ca) moving equally fast. Modeling of the spectra based on standard density profiles of the ejecta fails to reproduce the observed features, whereas enhancing the density of outer layers significantly improves the fit. Our analysis indicates the presence of clumps of high–velocity, intermediate–mass elements in the outermost layers, which is also suggested by the spectropolarimetric data.