|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
3.1. Spectral Evolution
The first thorough long-term set of spectra of a SN Ia was that of SN 1937C, obtained photographically by Minkowski (1939), with an intensity calibration provided by Greenstein & Minkowski (1973). The use of linear detectors having high quantum efficiency (especially CCDs) led to the publication of better sequences of spectra of "normal SNe Ia" (Section 3.3), specifically those of SN 1972E (Kirshner et al 1973, Kirshner & Oke 1975), SN 1981B (Branch et al 1983), SN 1989B (Barbon et al 1990, Wells et al 1994), and SN 1994D (Meikle et al 1996, Patat et al 1996, Filippenko 1997a). Wells et al (1994) discuss in some detail the temporal changes in their densely sampled spectra of SN 1989B. A representative set of spectra of SN 1994D is illustrated in Figure 4.
Figure 4. Montage of spectra of SN Ia 1994D in NGC 4526 (cz = 850 km s-1), based on data from Patat et al (1996; reproduced with permission) and Filippenko (1997a). Epochs (days) are given relative to maximum B brightness (March 20.5, 1994). The last two spectra are of the similar SN Ia 1987L in .
The early-time spectra of SNe Ia exhibit prominent broad peaks and valleys. Extensive computer modeling of the expanding ejecta has resulted in reliable identifications for most features, after decades of uncertainty. In general, at very early times they are attributed to lines of neutral and singly ionized intermediate-mass elements (O, Mg, Si, S, Ca), with some contribution from iron-peak elements (Fe, Co) especially at near-UV wavelengths (Branch et al 1983, 1985, Harkness 1986, 1991, Kirshner et al 1993, Mazzali et al 1993). The strongest features are Si II 6355 and Ca II H&K 3934, 3968. Convincing evidence for the presence of helium has never been shown, although the results of Meikle et al (1996) are intriguing and warrant further scrutiny.
The relative contribution of iron-group elements quickly increases as the photosphere recedes into the ejecta. By t 2 weeks the spectrum is dominated by lines of Fe II, which is consistent with an iron-rich core (Harkness 1991), but some lines of intermediate-mass elements are still present (e.g. Si II, Ca II). Thereafter the spectral changes are more gradual, although forbidden emission lines of Fe (and some Co, most prominent at ~ 5900 Å) eventually dominate in the nebular phase (Axelrod 1980), which begins roughly one month past maximum brightness. Nevertheless, Ca II remains visible, primarily in absorption (Ca II H&K and the near-IR triplet of 8498, 8542, 8662). Qualitatively, Axelrod (1980) was able to show that the cobalt lines decrease with time in a manner suggestive of radioactive decay. More recently, compelling evidence that the late-time tail of SNe Ia is powered by the decay of radioactive Co56 (initially from radioactive Ni56) was found from the temporal changes in the intensity ratio of two relatively unblended [Co III] and [Fe III] emission lines (Kuchner et al 1994; see also Varani et al 1990 for the case of SN 1987A). Also, Spyromilio et al (1992) demonstrated the presence of a large amount (0.4-0.7 M) of iron in the ejecta of SN 1991T. Incidentally, SN 1991T is the only SN Ia to have been observed spectroscopically more than two years past maximum, but the data were dominated by an echo produced by foreground dust reflecting the near-maximum spectrum (Schmidt et al 1994c).
Early-time photospheric expansion velocities, determined by measuring the positions of Doppler-shifted absorption minima in the P Cygni profiles of strong, fairly unblended lines such as Si II 6355, are typically 10,000 km s-1 (Pskovskii 1977, Branch 1981). Different lines do not have identical velocities: for example, Si II (10,000-12,000 km s-1) and Ca II H&K (13,000-15,000 km s-1) at maximum brightness. Initially the velocity obtained from the Si II and Ca II H&K lines decreases rapidly with time, as shown most recently by Wells et al (1994), Patat et al (1996); see Figure 5. Patat et al (1996) note that there is a sudden break in the decline rate around t = -6 days, most easily visible in the Si II line (Figure 5).
Figure 5. Evolution of the expansion velocity as deduced from the minima of the Si II 6355 (left panel) and Ca II H&K (right panel) absorption troughs for SNe Ia 1994D, 1992A, 1990N, 1989B, and 1981B. From Patat et al (1996); reproduced with permission.
Note that the broad emission near 6500 Å, which begins to develop around t = 2 weeks and remains thereafter with some changes in shape (Figure 4), is not H but rather Fe II and later [Fe II]. It is occasionally incorrectly identified as H by observers attempting to classify spectra of SN candidates. Similarly, the strong Si II 6355 line at early times is sometimes attributed to H. In both cases, erroneous classifications of Type II are made, especially when uncalibrated spectra are quickly examined at the time of observation.