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

2. Late light curves

We have compiled the absolute V light curves of five SNe Ia, namely SN 1991T, SN 1991bg, SN 1992A, SN 1993L and SN 1994D, combining unpublished observations with data from the literature.

In principle, bolometric light curves should be used for comparison with the models. Unfortunately, such data are available only for very few SNe, even in the best case they are limited to the UVOIR spectral region and, with the exception of SN 1987A, they do not cover late epochs. However, both observations and spectrum synthesis indicate that in SNe Ia most of the deposited radioactive energy is re-radiated in the optical region and that, at phases later than 100d, the bolometric correction is constant and small, mag (Suntzeff, 1996; Wheeler and Höflich, 1997). Therefore, in the following we simply assume that the bolometric correction for the V band is zero for all SNe Ia. Despite this crude approximation, this is not the major source of error, the uncertainties in the parent galaxies distance moduli and extinctions being in fact much larger.

The observations were retrieved from the archive of the ESO supernova monitoring programme (Turatto et al., 1990a), and are presented here for the first time, except for SN 1991bg, whose complete light curve has already been published in Turatto et al. (1996). These data have been supplemented, for the early phases, with published photometry from Phillips et al. (1992) for SN 1991T, Suntzeff et al. (1996) for SN 1992A, Filippenko et al. (1992) and Leibundgut et al. (1993) for SN 1991bg and Patat et al. (1996) for SN1994D.

Additionally, we included the photometry of SN 1992A obtained with WFPC2 on HST on Aug 2, 1994, 926 days after maximum. These observations, obtained in the framework of the SINS program (Kirshner et al. 1993) , consist of two sequences of four exposures through the F555W and the F439W filters, which are similar to the V and B bands, respectively. Exposure times for the individual frames were 900 sec for F555W and 1200 sec for F439W. The individual frames, calibrated in the standard pipeline, have been retrieved from the HST archive, properly aligned, and combined to eliminate cosmic rays. In the combined F555W image we found a stellar object whose offset from the field stars agrees to within 0.2 arcsec in both coordinates with the offset of SN 1992A as measured on the last ground-based observations (note that 1.2 arcsec west of the SN is a 23 mag background galaxy which appears as a stellar object on ground-based images). The SN magnitude, as measured by means of aperture photometry and calibrated relative to a sequence of local stars whose magnitudes were determined from the ground-based observations, is . The SN is not so evident in the combined F439W image which, however, is consistent with a colour close to 0.

The main data for the five SNe Ia are listed in Table 1, while the absolute V light curves are shown in Fig. 1. Absolute magnitudes were computed using the distance moduli given in the Tully (1988) catalog, except for SN 1993L, whose parent galaxy is not listed there. For this SN we use the distance given in the Leda extragalactic database ¹. Uncertainty in the absolute magnitudes arises mostly from the uncertainty concerning the distance scale. (both catalogs we are using adopt a Hubble constant of 75 km s^-1 Mpc^-1). Although the errors in the absolute magnitudes may be quite large (of the order of mag), the relative distances should be more reliable (typical errors are mag) and so the differences in magnitude between different SNe are expected to be real.

Table 1. SNe Ia data

Fig. 1. The absolute V light curves of 5 SN Ia are compared with 4 models with different ejecta and Ni masses. The reference epochs is the time of explosion estimated from spectrum synthesis and light curve modelling of the early photospheric phase. From top to bottom the models are characterized by , (continuous line); 1.4, 0.8 (dotted); 1.0, 0.4 (short dashed) and 0.7, 0.1 (long dashed). For all models is adopted. In the inset, the epoch near maximum is shown enlarged.

Magnitudes have been corrected for the total absorption as estimated in the references indicated in Table 1.

The reference epochs for the light curves were chosen using the times of explosion estimated from spectrum synthesis and light curve modelling of the early photospheric phase. These range between 12 and 20 days prior to B maximum. The uncertainty in the determination of the time of maximum is negligible for our discussion, which is mostly concerned with the very late epochs.

In Table 1 we also list the quantities , the difference in B magnitude from maximum to 15d, which is widely used to characterize the early light curve (col. 8), and , the difference in V magnitude from maximum to 300d (col. 9). When observations at this epoch were not available, the value of was derived by interpolation between the closest adjacent observations. Finally, we report the maximum expansion velocities of the Fe-nebula (col. 10). These are derived from models of the emission lines in spectra at epochs around 300d, except for SN 1991bg, whose spectral evolution was fast and for which a spectrum at 221d was used (Mazzali et al., 1997). The velocities tabulated correspond to the outer velocity of the Fe-nebula for which synthetic nebular spectra give a best fit to the late-time spectra of the SNe at hand (Mazzali et al., in preparation).

The SNe have been selected to represent the known range of luminosities of SNe Ia at maximum, going from the faint SN 1991bg to the `average' SNe 1992A, 1993L and 1994D and the bright SN 1991T. The full range is about 2.3 mag at maximum. In particular, it appears that SN 1994D is consistently and significantly brighter than SNe 1992A and 1993L. This was also implied by Hamuy et al. (1995) on the basis of the different decline rates. Fig.1 shows that the differences in absolute magnitude persist to the very late phases, and actually increase with time, mostly reflecting the range in the early decline rates. Differences are about 3.5 mag already 2 weeks after maximum, and reach about 4 mag at 300d (Table 1), after which they apparently stop increasing.

The exception is SN 1991T, for which the luminosity after 500d stops declining. This results from an echo formed as the light emitted by the supernova near maximum light was reflected off dust in the CSM or ISM. (Schmidt et al., 1994; Danziger et al. unpublished spectra). Since there is no direct relation between the late-time luminosity of SN 1991T and the SN remnant itself, in the following we ignore the observations of SN 1991T at phases later that 500d.

From Table 1 the correlation between the absolute magnitudes at maximum and the early decline rate () is evident. In addition, it appears that at late phases brighter SNe have larger Fe-nebula expansion velocities (as already noticed by Danziger, 1994 and Turatto et al., 1996) and slower luminosity decline rates.

Another interesting feature shown in Fig. 1 is that, although the late light curves may appear quite linear on short time intervals, the decline rate actually slows down in the long run (). Unfortunately, the evidence is based on only one observation for each of the SNe 1991bg, 1992A and, possibly, 1993L. Since observations at such late epochs are difficult, and the photometry may be contaminated by phenomena not directly related to the remnant itself (eg. echoes in the case of SN 1991T or contaminating unresolved stars) this suggestion needs further confirmation. Meanwhile, it is worth exploring the implications of the effect if real.

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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