Astron. Astrophys. 328, 203-210 (1997)

1. Introduction

For many years type Ia supernovae (SNe Ia) were considered very reliable standard candles. However, more accurate CCD photometry for a rapidly growing number of SNe Ia has shown the existence of a range in the properties of this type of SNe. In particular, a correlation has been found between the absolute magnitude at maximum and the shape of the light curve (Hamuy et al., 1995; Riess et al., 1995). If this relationship can be well calibrated and other parameters are found to play a negligible role, the confidence in the use of SNe Ia as distance indicators may be restored.

A more subtle problem concerns the progenitors of SNe Ia. The standard scenario for SNe Ia is that a degenerate white dwarf in a binary system accretes matter from a close companion, either red giant or white dwarf (WD), until it reaches the Chandrasekhar mass () and explodes, leaving no remnant (Nomoto et al., 1984; Woosley and Weaver, 1986). By tuning the relative masses and the separation of the stars in the binary system it is possible to obtain a very slow evolution and thus to explain the occurrence of SNe Ia among population II stars and hence in elliptical galaxies. If all SNe Ia events are triggered by the WD's mass reaching , independently of the initial conditions, it is natural to expect some homogeneity in the outcome of the explosions. One should nevertheless keep in mind the fact that type Ia supernovae occur in all types of galaxies. This theoretical view reinforced the empirical use of SNe Ia as standard candles. Lately, however, with the advent of larger samples, the observed diversity among SNe Ia appears to challenge that paradigm.

In particular, in the following we will show how the simple comparison of the late light curves of different SNe Ia requires the existence of a range in both the mass of radioactive material synthesized in the SN Ia explosions and the mass of the ejecta. Such differences can be seen not only in the well-known cases of peculiar SNe such as the bright SN 1991T and the faint SN 1991bg, but also within the group that has hitherto preserved the definition of `normal' SN Ia.

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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