Late time optical spectra from the /sup 56/Ni model for Type I supernovae

Abstract

The hypothesis that the optical luminosity of Type I supernovae 56 results from the radioactive decay of Ni synthesized and ejected by the explosion has been investigated by numerical simulation of the optical spectrum resulting from a homologously expanding shell com-56 posed initially of pure Ni. This model, which neglects the cc effects of material external to the Ni core, is expected to pro vide a reasonable representation of the supernova at late times when the star is nearly transparent to optical photons. The numerical simulation determines the temperature, ionization state, and non-LTE level populations which result from energy deposition by the radio-56 56 active decay products of Ni and Co. The optical spectrum includes the effects of both allowed and forbidden lines. The optical spectra resulting from the simulation are found to be sensitive to the mass and ejection velocity of the Ni shell. A range of these parameters has been found which results in good agree ment with the observed spectra of SN1972e over a considerable range of time. In particular, evidence for the expected decaying abundance of Co 56 has been found in the spectra of SN1972e. These results are 56 used to assess the validity of the Ni model and set limits on the mass and explosion mechanism of the Type I progenitor. The possibili ties for improvement of the numerical model are discussed and future atomic data requirements defined. II. Numerical Modelling of the Nebular Phase of SNI 8 III. Energy Deposition by Decay Products 17 A. Thi Deposition Rate 17 B. Atomic Processes Induced by Decay Products 23 IV. The Ionization and Thermal Balances in Steady State 45 A. The Ionization Balance 45 B. The Thermal Balance 61 V. The Emergent Spectrum 89 VI. Application to Real SNI: The Optical Light Curve 123 VII. Application to Real SNI: The Optical Spectrum of SN1972e. . .148 A. Comparison of Calculated and Observed Spectra 151 B. Atmospheric Absorption Lines 155 C. Uncertainties in Atomic Data D. Evidence For Co 56 E. Limits on the Ni 56 Mass and Velocity VIII. Conclusion: Constraints on the Progenitors of SNI References 195 Appendix. Atomic Data 201

Optical Light Curves of Supernovae

“…This has been interpreted as the optically thin phase when the ejecta nebula captures fewer and fewer γ-rays and the optical and NIR light curves decline faster than the 56 Co decay rate (see, e.g., [49,59,85]). After about 150 days the light curves change slope once more (see, e.g., [58]) when the importance of the positron channel in the 56 Co decay sets in (see, e.g., §1, [5,75]. The decline at these phases should tell us about the magnetic fields in the explosion as they determine whether the positrons are captured or escape the ejecta [22,75,94].…”

Spectra of Supernovae in the Nebular Phase

“…The effective opacity is around κ e+ =10 cm 2 g −1 [Colgate et al, 1980, Axelrod, 1980 for positrons, and similar for electrons.…”

“…A simplified formal solution is the continuousslowing-down approximation [Axelrod, 1980], which is quite accurate for the primary electrons, but not for the secondaries.…”

SNIa Diversity: Theory and Diagnostics

“…1 we compare spectra of SN 1991T and SN 1991aa corresponding to a similar phase. Nebular spectra of SNe la were first modelled by Axelrod (1980) and Meyerott (1980) Concerning the nature of the companion of the exploding WD, there are possible diagnostics for the cases in which the companion has a H-rich envelope. In particular, Chugai (1986) predicted that the fraction of the envelope stripped by the explosion would appear as low-velocity material in the nebular spectrum of the SN.…”