|Annu. Rev. Astron. Astrophys. 1997. 35:
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5.3. Type II-L Supernovae
Few SNe II-L have been observed in as much detail as SNe II-P. Figure 13 shows the spectral development of SN 1979C (Branch et al 1981), an unusually luminous member of this subclass (Gaskell 1992). Near maximum brightness the spectrum was very blue and almost featureless, with a slight hint of H emission. A week later, H emission was more easily discernible, and low-contrast P Cygni profiles of Na I, H, and Fe II appeared. By t 1 month, the H emission line was very strong but still devoid of an absorption component, while the other features clearly had P Cygni profiles. Strong, broad H emission dominated the spectrum at t 7 months, and [O I] 6300, 6364 emission was also present.
Figure 13. Montage of spectra of SN 1979C in NGC 4321 (cz = 1571 km s-1). From Branch et al (1991); reproduced with permission. Epochs (days) are given relative to the date of maximum brightness, April 15, 1979.
SN 1980K (Uomoto & Kirshner 1986), another extensively studied (and somewhat overluminous) SN II-L, also did not exhibit an absorption component of H at any phase of its development. Several authors (e.g. Wheeler & Harkness 1990, Filippenko 1991a) have speculated that the absence of H absorption spectroscopically differentiates SNe II-L from SNe II-P, but the small size of the sample of well-observed objects precluded definitive conclusions. Recently, Schlegel (1996) collected a somewhat larger set of data and formally proposed that SNe II-L and SNe II-P can be spectroscopically separated in this manner, but it is not yet clear that this is justified. As he himself admitted, the conclusion is based heavily on spectra of SNe 1979C and 1980K, yet these are not necessarily typical SNe II-L. More data are needed to convincingly demonstrate a spectroscopic difference between SNe II-L and SNe II-P.
The progenitors of SNe II-L are generally believed to have relatively low-mass hydrogen envelopes (a few solar masses); otherwise, they would exhibit distinct plateaus, as do SNe II-P. On the other hand, perhaps their envelopes are very extended, or the progenitors have more circumstellar gas than do SNe II-P, and this could give rise to the emission-line dominated spectra (see Section 5.4). They are often radio sources (Sramek & Weiler 1990); moreover, Fransson (1982, 1984) suggested that the UV excess (at 1600 Å) seen in SNe 1979C and 1980K is produced by inverse Compton scattering of photospheric radiation by high-speed electrons in shock-heated (T 109 K) circumstellar material. Finally, the light curves of some SNe II-L reveal an extra source of energy: After declining exponentially for several years after outburst, the H flux of SN 1980K reached a steady level, showing little if any decline thereafter (Uomoto & Kirshner 1986, Leibundgut et al 1991b). The excess almost certainly came from the kinetic energy of the ejecta that was thermalized and radiated owing to an interaction with circumstellar matter (Chevalier 1990, Leibundgut 1994, and references therein). Note that Swartz et al (1991) explored the possibility that SNe II-L may result from electron-capture-induced collapse of an O-Ne- Mg core, rather than by collapse of an Fe core due to photodissociation.
Fesen & Becker (1990), Leibundgut et al (1991b), Fesen & Matonick (1993, 1994; see also Fesen et al 1995) illustrated very late-time spectra of SNe II-L 1980K and 1979C. Additional objects having similar characteristics include SN 1970G (Fesen 1993) and SN 1986E (Cappellaro et al 1995b). The spectra consist of a few strong, broad emission lines such as H, [O I] 6300, 6364, and [O III] 4959, 5007. Their relative intensities and temporal changes are generally consistent with the circumstellar interaction models of Chevalier & Fransson (1994), and they can be used to further constrain the nature of the progenitor star.