3. Radioactive decay
Starting with the earliest attempts to model the SN light curves it has been clear that the fast expansion of the ejecta would result in a very rapid cooling and hence in a very rapid luminosity decline if only the thermal energy from the explosion had been available to power the SN.
On the contrary, for almost all types of SNe, the luminosity evolution is relatively slow. In particular, at epochs later than 150-200d the light curves decline almost linearly with rates in the range 0.8-1.5 mag/100d (Turatto et al., 1990b), implying the presence of a delayed energy input. The linear tail of the light curves corresponds to an exponential luminosity decline, suggesting that the radioactive decay of unstable heavy elements is a likely energy source, even though the decline rates for SNe Ia do not match the decay rates of known radioactive species.
Indeed, model calculations have shown that explosive silicon burning can produce unstable isotopes of iron group elements, in particular 56 Ni. 56 Ni decays into 56 Co, emitting -rays with an average energy per decay of 1.71 MeV (Burrows and The, 1990). Since the half-life of 56 Ni is relatively short ( days), this decay can only be important for the early light curve (the first 1-2 months).
56 Co is also unstable and decays into 56 Fe. The average energy available per decay is 3.67 MeV. Although most of this energy is released in the form of -rays, a significant fraction of the 56 Co decays (19%) produces positrons. Positrons deposit their kinetic energy in the ejecta and then annihilate with electrons, producing two photons of energy each. The positron kinetic energy accounts for about 3.5% of the total 56 Co decay energy (Arnett, 1979; Axelrod, 1980). Because of its longer half-life (77.7 days), 56 Co decay can power the light curve of SNe for at least 2-3 years.
There is now ample evidence that the radioactive decay of 56 Ni and 56 Co provides most of the energy during the first 2-3 years, as was first suggested by Pankey (1962) and then, independently and more quantitatively, by Colgate and McKee (1969). The most convincing direct evidence has been the detection of -rays lines from 56 Co decay in the type II SN 1987A (Matz et al., 1988; Arnett et al., 1989; Palmer et al., 1993) and the temporal behaviour of the [CoII] lines at (Danziger et al., 1990)and (Varani et al., 1990). In the case of many SNe Ia, modelling of the late-time spectra indicates the presence of 56 Co lines whose intensity evolution is in agreement with the prediction from the 56 Co decay (Axelrod, 1980; Kuchner et al., 1994).
In the early phases of the SN evolution the density of the ejecta is still high enough that the -rays from radioactive decay are trapped in the ejecta and completely thermalized through Compton scattering with the free electrons. With the expansion the density decreases and the mean free path of the -rays increases. In the case of SNe II (and of at least a fraction of the SNe Ib/c) the mass of the ejecta is so large () that even if their density decreases they remain optically thick to -rays for 2-3 years (Woosley, 1988). On the other hand, the ejecta become transparent to optical radiation a few months after maximum, and the observed decline of the optical luminosity reflects directly the decline of the radioactive energy supply. Indeed, the exponential tails of most SNe II light curves show the same e-folding time as that of the 56 Co energy release.
In the case of SNe Ia, the ejecta are less massive, while the Ni mass is larger ( in SNe Ia vs. in SNe II). Also, there is evidence that in some cases Ni can be found in the outer layers. This allows an increasing fraction of the -rays to escape thermalization as time goes on, so that the light curves of SNe Ia decline more rapidly than the 56 Co energy release.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998