Annu. Rev. Astron. Astrophys. 1997. 35: 309-355 Copyright © 1997 by Annual Reviews. All rights reserved

4.2. SNe Ib: Spectral Evolution

Genuine helium-rich SNe Ib appear to be rather rare objects. At present, the most complete series of published spectra of a SN Ib is that of Harkness & Wheeler (1990) for SN 1984L, and even this case includes only the first two months past maximum brightness. As shown in Figure 9, there is no strong evidence of hydrogen, and the 6150-Å trough of SNe Ia is weak or absent at all times. Although some of the absorption lines can be attributed to He I in the earliest spectra, other alternatives exist (e.g. Na I D for the feature near 5800 Å). However, the gradual strengthening of blueshifted (~ 7500 km s^-1) lines corresponding to He I 4471, 5876, 6678, and 7065 makes the helium identification unambiguous a few weeks past maximum brightness (e.g. the spectrum at t = 20 days). This differs markedly from the spectral development of SNe Ia (Figure 4).

Figure 9. Montage of spectra of SN Ib 1984L in NGC 991 (cz = 1532 km s-1), from Harkness et al (1987). The last spectrum is of SN Ib 1983N in NGC 5236 (cz = 516 km s-1), from Gaskell et al (1986). Epochs (days) are given relative to maximum B brightness (September 7, 1984, for SN 1984L; July 17, 1983, for SN 1983N). Reproduced with permission.

The seventh spectrum in Figure 9, obtained about two months past maximum brightness, appears to exhibit weak [O I] 6300, 6364 and especially [O I] 5577, indicating an early onset of the nebular phase. Strong, broad (FWHM 4500 km s^-1) lines of [O I] and [Ca II] 7291, 7324 are visible in the spectra of SN 1984L obtained by Schlegel & Kirshner (1989) 13-14 months past maximum, as is somewhat narrower Mg I] 4571, but the data are noisy. The last plot in Figure 9 instead shows the late-time spectrum (t 8 months) of a similar SN Ib, SN 1983N (from Gaskell et al 1986).

Early-time spectra of SN 1983N were presented by Richtler & Sadler (1983), before the SN Ib subclass had been recognized. The blueshifted minimum of the He I 5876 line corresponds to a photospheric velocity of 18,200 km s^-1 about 15 days prior to maximum brightness, and 10 days later this decreased to 13,100 km s^-1 (Wheeler & Harkness 1990). The velocity given by this line was only ~ 10,000 km s^-1 at maximum brightness (Harkness et al 1987), further evidence of the retreat of the photosphere into deeper, more slowly moving layers.