The type Ia supernova 1994D in NGC 4526: The early phases

The type Ia supernova SN 1994D in NGC 4526 was extensively observed as a target of opportunity at ESO. In this paper, we present and discuss the UBVRI photometry and the spectra collected during this campaign from 11 days before maximum to about two months afterwards. Several absorption components of the Na I D lines arising in our Galaxy, in NGC 4526 and in high-velocity clouds located in between have been identified in a high-resolution spectrum. The total amount of reddening related to these features has been estimated as E(B-V)=0.06. The light curve shapes are similar, but not identical, to those of SN 1992A. The onset of the secondary maximum in the I band occurs around day +20. The SN reached at maximum B=11.84 and showed a distinctly blue colour [(B-V)= -0.08]. SN 1994D is therefore the bluest SN Ia observed to date, SN 1994D appears to depart from the previously reported correlation between maximum brightness and rate of decay. The implications of this for the absolute magnitudes and colour dispersion of type Ia SNe are discussed. The spectroscopic evolution of SN 1994D followed that of standard objects like SN 1992A. The excellent time coverage allowed us to show a sudden break in the decline rate of the expansion velocity deduced from the Si II lambda 6355 line at around day - 6, probably indicating that after this phase this strong feature originates well above the photosphere. Finally, physical parameters and line identifications have been obtained modelling the day -4 spectrum by means of a spectrum synthesis code. Overall, the spectrum is successfully fitted as a normal SN Ia, although the very blue colour cannot be reproduced. The observed weakness of the Co II absorption around 3350 Angstrom and its possible bearing on the amount of radioactive Ni produced during the explosion are discussed.