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Astron. Astrophys. 359, 876-886 (2000)

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1. Introduction

The temporal evolution of a supernova's luminosity contains important information on the physical processes driving the explosion. The peak luminosity of a Type Ia Supernova (SN Ia) is directly linked to the amount of radioactive [FORMULA] produced in the explosion (Arnett et al. 1985, Branch & Tammann 1992, Höflich et al. 1997, Eastman 1997, Pinto & Eastman 2000a). The rise time of the light curve is determined primarily by the explosion energy and the manner in which the ejecta become optically thin to thermalized radiation, i.e. the opacity (Khokhlov et al. 1993). The late decline of the light curve is governed by the combination of the energy input by the radioactive material and the rate at which this input energy is converted to optical photons in the ejecta (Leibundgut & Pinto 1992).

The apparent uniformity of SN Ia light curves in photographic (pg ), B , and V filters (Minkowski 1964) prompted the adoption of standard light curve templates (e.g. Elias et al. 1985, Doggett & Branch 1985, Leibundgut et al. 1991b, Schlegel 1995). Early indications that the standard templates fail to describe the full range of SN Ia light curves came from the observations of SN 1986G, which displayed a much more rapid evolution than any other SN observed up to that time (Phillips et al. 1987). The demise of the simple standard candle treatment was brought about by the observations of the faint SN 1991bg (Filippenko et al. 1992, Leibundgut et al. 1993, Turatto et al. 1996) and the subsequent derivation of a correlation between peak luminosity and decline rate after maximum (Phillips 1993, Hamuy et al. 1996a, Riess et al. 1996a). A clear demonstration that SNe Ia do not display a uniform photometric evolution was provided earlier from the infrared J , H , and K light curves (Elias et al. 1985, Frogel et al. 1987). Observations and analyses of the near-IR R and I light curves (Suntzeff 1996, Vacca & Leibundgut 1997) confirm this variation. The red and near-infrared light curves exhibit a second maximum [FORMULA] 20 to 30 days after the B peak. This second maximum occurs at different phases and with differing strengths in individual SNe Ia and in at least one case (SN 1991bg; Filippenko et al. 1992, Leibundgut et al. 1993, Turatto et al. 1996) is altogether absent.

The light curves of SNe Ia are often described as a one-parameter family. The correlation of the BVRI light curve shapes with the peak absolute magnitudes has been employed to improve the distance measurements derived from SNe Ia (Hamuy et al. 1996a, Riess et al. 1996a, Garnavich et al. 1998, Schmidt et al. 1998, Riess et al. 1998a) and is of fundamental importance to keep the systematic uncertainties in the derivation of cosmological parameters small. Techniques that fit standard templates (e.g., Hamuy et al. 1996a), or modified versions of these templates (e.g., Riess et al. 1996a; Perlmutter et al. 1997), to the observed light curves make use of this one parameter description of the light curves. These methods have the advantage that they can be applied even to rather sparsely sampled light curves. In a more extreme form, the photometry can be supplemented by spectroscopy to provide a distance measurement with minimal data coverage. This "snapshot" method has been advocated by Riess et al. (1998b). All these methods make the assumption that SNe Ia form an "ordered class". This description is validated by the improvement in the scatter around the linear expansion line in the local Universe and also by the fact that new objects can be successfully corrected with the correlations derived from an independent sample.

While clearly useful for comparing local SNe with high redshift SNe, the template method does not allow one to investigate finer and more individual features in a large sample of SN Ia light curves. Hence, the detailed study of the explosion and radiation physics cannot be carried out with such an analysis. For data sets which are densely sampled, however, template methods are not necessary. Recent bright SNe Ia have been observed extensively and very detailed, and accurate light curves have become available. Most of these supernovae have been used as the defining objects for the templates to correct other, more sparsely observed, SNe Ia.

To analyze light curves of many SNe Ia in an individual fashion a parameter fitting method which can be applied to single filter light curves has been devised (Vacca & Leibundgut 1996 , 1997). The photometric data are approximated by a smooth fitting function. We are not fitting model light curves based on explosion physics, but simply attempt to match the data in an objective way. We have investigated well-observed SNe in a small sample to check our method. It allows us to search for correlations among various light curve parameters, to accurately fit the filter light curves at any phase, and to construct continuous bolometric light curves. The application to the light curves of SN 1994D has already provided one of the first bolometric light curves of SNe Ia (Vacca & Leibundgut 1996).

Observationally-derived bolometric light curves provide a measure of the total output of converted radiation of SNe Ia, and therefore serve as a crucial link to theoretical models and calculations of SN explosions and evolution. The total luminosity from a SN Ia is much easier to calculate from theoretical models than the individual filter light curves, which are dominated by line blending effects requiring complicated multi-group calculations (Leibundgut & Pinto 1992, Eastman 1997, Höflich et al. 1997). In addition, the observed bolometric peak luminosity of SNe Ia provides a measure of the total amount of nickel synthesized in the explosion and can be used to test various explosion models. Although no SN Ia has ever been observed in every region of the electromagnetic spectrum simultaneously, fortunately bolometric light curves can be constructed almost entirely from optical data alone (Suntzeff 1996, Leibundgut 1996).

In Sect. 2 we describe the sample of objects and the data set used. The parameter fitting method employed for our analysis is described in Sect. 3. This is followed by a discussion of the phases of the maximum epoch in different filters (Sect. 4). The construction of the bolometric light curves is presented in Sect. 5 followed by a discussion of the uncertainties in the bolometric light curves. We summarize our findings in Sect. 6; our conclusions are given in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000