|Annu. Rev. Astron. Astrophys. 1997. 35:
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Supernovae occur in at least three, and possibly four or more, spectroscopically distinct varieties. The two main classes, Types I and II, were firmly established by Minkowski (1941, but see Popper 1937). Type I SNe are defined by the absence of obvious hydrogen in their optical spectra, except for possible contamination from superposed H II regions. SNe II all prominently exhibit hydrogen in their spectra, yet the strength and profile of the H line vary widely among these objects. Until recently, most spectra of SNe have been obtained near the epoch of maximum brightness, but in principle the classification can be made at any time, as long as the spectrum is of sufficiently high quality. Only occasionally (Section 5.5) do SNe metamorphose from one type to another, suggesting the use of hybrid designations.
The early-time (t 1 week) spectra of SNe are illustrated in Figure 1. The lines are broad owing to the high velocities of the ejecta, and most of them have P Cygni profiles formed by resonant scattering above the photosphere. SNe Ia are characterized by a deep absorption trough around 6150 Å produced by blueshifted Si II 6347, 6371 (collectively called 6355). Members of the Ib and Ic subclasses do not show this line. The presence of moderately strong optical He I lines, especially He I 5876, distinguishes SNe Ib from SNe Ic (Wheeler & Harkness 1986, Harkness & Wheeler 1990).
Figure 1. Spectra of SNe, showing early-time distinctions between the four major types and subtypes. The parent galaxies and their redshifts (kilometers per second) are as follows: SN 1987N (NGC 7606; 2171), SN 1987A (LMC; 291), SN 1987M (NGC 2715; 1339), and SN 1984L (NGC 991; 1532). In this review, the variables t and represent time after observed B-band maximum and time after core collapse, respectively. The ordinate units are essentially "AB magnitudes" as defined by Oke & Gunn (1983).
The late-time (t 4 months) optical spectra of SNe provide additional constraints on the classification scheme (Figure 2). SNe Ia show blends of dozens of Fe emission lines, mixed with some Co lines. SNe Ib and Ic, on the other hand, have relatively unblended emission lines of intermediate-mass elements such as O and Ca. Emission lines in SNe Ib are narrower (Filippenko et al 1995b) and perhaps stronger (Wheeler 1980) than those in SNe Ic, but these conclusions are based on the few existing late-time spectra of SNe Ib, and no other possibly significant differences have yet been found. At this phase, SNe II are dominated by the strong H emission line; in other respects, most of them spectroscopically resemble SNe Ib and Ic, but the emission lines are even narrower and weaker (Filippenko 1988). The late-time spectra of SNe II show substantial heterogeneity, as do the early-time spectra.
Figure 2. Spectra of SNe, showing late-time distinctions between various types and subtypes. Notation is the same as in Figure 1. The parent galaxy of SN 1987L is NGC 2336 (cz = 2206 km s-1); others are listed in the caption of Figure 1. At even later phases, SN 1987A was dominated by strong emission lines of H, [O I], [Ca II], and the Ca II near-IR triplet, with only a weak continuum.
At ultraviolet (UV) wavelengths, all SNe I exhibit a very prominent early-time deficit relative to the blackbody fit at optical wavelengths (e.g. Panagia 1987). This is due to line blanketing by multitudes of transitions, primarily those of Fe II and Co II (Branch & Venkatakrishna 1986). The spectra of SNe Ia (but not of SNe Ib/Ic) also appear depressed at IR wavelengths (Meikle et al 1997). The early-time spectra of most SNe II, in contrast, approximate the single-temperature Planck function from UV through IR wavelengths, with occasionally even a slight UV excess. SN 1987A was an exception: The earliest IUE spectra showed a strong UV deficit relative to the blackbody curve defined at optical wavelengths (Danziger et al 1987), as in SNe I.