Comparative Direct Analysis of Type Ia Supernova Spectra. IV. Postmaximum - AstrophysicsReport as inadecuate

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Abstract: A comparative study of optical spectra of Type Ia supernovae SNe Iaobtained near 1 week, 3 weeks, and 3 months after maximum light is presented.Most members of the four groups that were defined on the basis of maximum lightspectra in Paper II core normal, broad line, cool, and shallow silicondevelop highly homogeneous postmaximum spectra, although there are interestingexceptions. Comparisons with SYNOW synthetic spectra show that most of thespectral features can be accounted for in a plausible way. The fits show that 3months after maximum light, when SN Ia spectra are often said to be in thenebular phase and to consist of forbidden emission lines, the spectra actuallyremain dominated by resonance scattering features of permitted lines, primarilythose of Fe II. Even in SN 1991bg, which is said to have made a very earlytransition to the nebular phase, there is no need to appeal to forbidden linesat 3 weeks postmaximum, and at 3 months postmaximum the only clearidentification of a forbidden line is Ca II 7291, 7324. Recent studies of SNIa rates indicate that most of the SNe Ia that have ever occurred have been-prompt- SNe Ia, produced by young 100,000,000 yr stellar populations, whilemost of the SNe Ia that occur at low redshift today are -tardy-, produced by anolder several Gyrs population. We suggest that the shallow silicon SNe Iatend to be the prompt ones.

Author: David Branch, David J. Jeffery, Jerod Parrent, E. Baron, M. A. Troxel, V. Stanishev, Melissa Keithley, Joshua Harrison, Christoph


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