Late light curves of type I alpha supernovae

We extend earlier efforts to determine whether the late (t greater than or equal to 60 days) light curves of Type Ia SNe are better explained by the e scape of positrons from the ejecta or by the complete deposition of positro n kinetic energy in a trapping magnetic field. We refine our selection of I a SNe, using those that have extensive BV RI photometry 35 days or more aft er maximum light. Assuming that all SNe within a given Deltam(15)(B) range form a distinct subclass, we fit a combined light curve for all class membe rs with a variety of models. We improve our previous calculations of energy deposition rates by including the transport of the Comptonized electrons. Their nonlocal and time-dependent energy deposition produces a correction o f as much as 0.10 mag for Chandrasekhar-mass models and 0.18 mag for sub-Ch andrasekhar-mass models. We produce bolometric corrections, derived from measured spectra, to B, V, R, and I light curves after day 50. Comparisons of the resulting bolometric light curves with simulated energy deposition rates demonstrate that the e nergy deposition from the photons and positrons created in Co-56 --> Fe-56 decays are consistent with the observations if positron escape is assumed. This implies that there is no evidence of additional sources of energy depo sition or of a shift of emission into unobserved wavelength ranges between days 60 and 900. The V band is shown to be an accurate indicator of total e mission in the 3500-9700 Angstrom range, with a constant fraction (similar to 25%) appearing in the V band after day 50. This suggests that the V band scales with the bolometric luminosity and that the deposited energy is ins tantaneously recycled into optical emission during this epoch. We see signi ficant evolution of the colors of SNe Ia between days 50 and 170. We sugges t that this may be due to the transition from spectra dominated by emission lines from the radioactive nucleus, Co-56, to those from the stable daught er nucleus, Fe-56.