Berkeley Supernova Ia Program – V. Late-time spectra of Type Ia Supernovae

In this work we analyse late-time (t > 100 d) optical spectra of low-redshift\n(z < 0.1) Type Ia supernovae (SNe Ia) which come mostly from the Berkeley\nSupernova Ia Program dataset. We also present spectra of SN 2011by for the\nfirst time. The BSNIP sample studied consists of 34 SNe Ia with 60 nebular\nspectra, to which we add nebular spectral feature measurements of 20 SNe Ia\nfrom previously published work (Maeda et al. 2011; Blondin et al. 2012),\nrepresenting the largest set of late-time SN Ia spectra ever analysed. The full\nwidth at half-maximum intensity (FWHM) and velocities of the [Fe III]\n{\\lambda}4701, [Fe II] {\\lambda}7155, and [Ni II] {\\lambda}7378 emission\nfeatures are measured in most observations of spectroscopically normal objects\nwhere the data have signal-to-noise ratios >20 px^-1 and are older than 160 d\npast maximum brightness. The velocities of all three features are seen to be\nrelatively constant with time, increasing only a few to ~20 km/s/d. The nebular\nvelocity (v_neb, calculated by taking the average of the [Fe II] {\\lambda}7155\nand [Ni II] {\\lambda}7378 velocities) is correlated with the\nnear-maximum-brightness velocity gradient and early-time ejecta velocity.\nNearly all high velocity gradient objects have redshifted nebular lines while\nmost low velocity gradient objects have blueshifted nebular lines. No\ncorrelation is found between v_neb and {\\Delta}m_15(B), and for a given\nlight-curve shape there is a large range of observed nebular velocities. The\ndata also indicate a correlation between observed (B-V)_max and v_neb.\n