|Annu. Rev. Astron. Astrophys. 1992. 30:
Copyright © 1992 by Annual Reviews. All rights reserved
2.3 Peculiar Sipernovae
With the removal of SN 1954A, SN 1962L, and SN 1964L from the ranks of ``peculiar'' SNe Ia, only a few with definite peculiarities (other than high or low expansion velocity) remain to be discussed.
SN 1885A in M31 occasionally is listed as a type Ia, but the classification is not established by the assembled descriptions of its optical spectrum or its reconstructed light curve (de Vaucouleurs & Corwin 1985). In fact, the spectrum seems to have lacked the red Si II absorption feature, the light curve too fast for SN Ia, and the color at maximum light was too red. Chevalier & Plait (1988) have proposed that SN 1885A was an off-center detonation of an accreted helium layer on an underlying carbon-oxygen white dwarf. Fesen et al (1989) have rediscovered SN 1885A in an image made with a narrow band filter centered on the wavelength of a strong Fe I absorption line.
SN 1980I, which occurred between NGC 4374, NGC 4387, and NGC 4406 in the Virgo cluster, appeared to have a nearly ordinary type Ia spectrum (Smith 1981) including the usual red Si II absorption, but with an unidentified absorption near 6680 Å which has not been reported in the early spectra of any other SN Ia. In the absence of an identification the significance of the extra feature is not at all clear.
SN 1939B is sometimes listed as an SN Ia because it occurred in an elliptical galaxy, but no spectrum is available. Its blue light curve, as published by Minkowski (1964) and attributed to Baade, appears to have decayed extremely quickly (like that of SN 1885A), but it seems that Baade merely passed on to Minkowski the data of Shapley (1939) which were possibly of low quality. The steep rise and fast decline could be due to a single scale error in the photometric standard sequence. Indeed, a recent reanalysis of (some of) the original photographic plates has yielded a B light curve that follows the template fairly well (Leibundgut 1991a).
The optical spectrum of SN 1986G, which appeared in the dust lane of NGC 5128 (Centaurus A), revealed a low expansion velocity but was not otherwise peculiar. The ultraviolet spectrum near maximum light was, however, definitely different from other SNe Ia (Panagia & Gilmozzi 1991). The initial decline of the well observed UBV light curves (Phillips et al 1987) was somewhat fast for SNe Ia, although the later, slower decline had the conventional slope. The JHK light curves (Frogel et al 1987) matched those of neither SN Ia nor SN Ib. In all three infrared bands the somewhat undulant initial decline was too slow. In particular, the fast initial decline and the secondary maximum in the J band were missing. SN 1986G was very faint and red. It was 4 mag fainter than SN 1972E in NGC 5253, which is believed to be in the same group of galaxies, and with (B - V) = 0.96 it is the reddest SN Ia observed so far. Rich (1987), Phillips et al (1987), and di Serego Alighieri & Pons (1987) have argued for a very high reddening, E(B-V) = 0.8. Correcting for extinction with a standard value RB = 4 would make the maximum almost as bright as normal, but if a smaller value of RB is appropriate (Section 2.5) then SN 1986G was intrinsically quite dim.
A striking example of a peculiar SN Ia is the very recent SN 1991T (Filippenko et al 1992a, Ruiz-Lapuente et al 1992, Jeffery et al 1992, Phillips et al 1992). The premaximum optical spectrum, which did not resemble that of any previous supernova, was dominated by lines of iron-group elements. Si II and Ca II features appeared near maximum light, but much more weakly than in ordinary SNe Ia. Only during the later phases, when the spectrum developed strong Fe II lines, did SN 1991T begin to closely resemble ordinary SNe Ia. In addition, SN 1991T may have been overluminous, although the issue is somewhat clouded by strong extinction as indicated by the strengths of the interstellar sodium lines and by the uncertain membership of the host galaxy, NGC 4527, to the Virgo cluster. Binggeli et al (1985) do not list NGC 4527 as a certain cluster member. For a Virgocentric model (Kraan-Korteweg 1985) the redshift of NGC 4527 formally puts it either in the cluster or at a distance 1.7 times farther, but the possibility that it is a foreground galaxy falling into the Virgo cluster cannot be ruled out. The blue maximum of SN 1991T occurred near April 29, with mB = 11.65, brighter by 1.3 than the mean in Table 2. The value of (B - V)0 is 0.15. Assuming (B - V)00 = -0.15 and RB = 1.5 (Section 2.5), then mB00 becomes 11.20, 3.2 brighter than the mean. The spectroscopy and the possible overluminosity appear to be consistent with the hypothesis that the explosion of SN 1991T incinerated more of the ejected matter to nuclear statistical equilibrium than do ordinary SNe Ia.
SN 1991bg also failed to conform to the standard behavior of an SN Ia (Filippenko et al 1992b). It was intrinsically red and intrinsically subluminous. Its maximum-light and post-maximum spectra were perhaps more like those of SN Ia than like those of other types, but very peculiar. SN 1991bg is the first outburst in an elliptical galaxy to show definite peculiarities.
None of the peculiar supernovae discussed in this section necessarily pose serious challenges to the uniformity of SNe Ia. It is not yet clear whether they really represent variations on the standard SN Ia theme, or are the first examples of what will become recognized as distinct subclasses. In any case, with reliable data it is possible to weed them out.