http://arxiv.org/abs/1604.01044
We examine the evolution of late-time, optical nebular features of Type Ia supernovae (SNe Ia) using a sample consisting of 160 spectra of 27 normal SNe Ia taken from the literature as well as unpublished spectra of SN 2008Q and ASASSN-14lp. Particular attention was given to nebular features between 4000$-$6000 \AA\ in terms of temporal changes in width and central wavelength. Analysis of the prominent late-time 4700 \AA\ feature shows a progressive central wavelength shift from $\sim$4600 \AA\ to longer wavelengths out to at least day +300 for our entire sample. We find no evidence for the feature’s red-ward shift slowing or halting at an [Fe III] blend centroid of 4701 \AA\ as has been proposed. The width of the feature also steadily increases with a FWHM $\sim$170 \AA\ at day +100 growing to 200 \AA\ or more by day +350. Two weaker adjacent features around 4850 and 5000 \AA\ exhibit very similar red shifts to that of the 4700 \AA\ feature but show no change in width until very late times. We discuss possible causes for the observed red-ward shifting of these late-time optical features including contribution from [Co II] emission at early nebular epochs and the emergence of additional features at later times. We conclude that the ubiquitous red shift of these common late-time, nebular SN Ia spectral features is not mainly due to a decrease in a blueshift of forbidden Fe lines but the result, in part, of decreasing velocity and/or optical depth of permitted Fe lines.
C. Black, R. Fesen and J. Parrent
Wed, 6 Apr 16
6/63
Comments: 12 pages, 9 figures
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