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

5.4. Type IIn Supernovae

During the past decade, there has been the gradual emergence of a new, distinct subclass of SNe II (Filippenko 1991a, b, Schlegel 1990, Leibundgut 1994) whose ejecta are believed to be strongly interacting with dense circumstellar gas (see Chevalier 1990 for an overview of this process). The derived mass-loss rates for the progenitors can exceed 10^-4 M year^-1 (Chugai 1994b). In these objects, the broad absorption components of all lines are weak or absent throughout their evolution. Instead, their spectra are dominated by strong emission lines, most notably H, that have a complex but relatively narrow profile. Although the details differ among objects, H typically exhibits a very narrow component (FWHM 200 km s^-1) superposed on a base of intermediate width (FWHM 1000-2000 km s^-1; sometimes a very broad component (FWHM 5000-10,000 km s^-1) is also present. Schlegel (1990) christened this subclass "Type IIn," the "n" denoting "narrow" to emphasize the presence of the intermediate-width or very narrow emission components. Representative spectra of five SNe IIn are shown in Figure 14, with two epochs for SN 1994Y.

Figure 14. Montage of spectra of SNe IIn. The objects, UT dates of observation, parent galaxies, and adopted redshifts (kilometers per second) are as follows: SN 1994Y (September 1, 1994 and January 26, 1995; NGC 5371; 2553), SN 1994W (October 1, 1994; NGC 4041; 1234), SN 1994ak (January 26, 1995; NGC 2782; 2562), SN 1988Z (April 27, 1989; MCG+03-28-022; 6595), and SN 1995N (May 24, 1995; MCG +02-38-017; 1534). Epochs are given relative to the estimated dates of explosion rather than maximum brightness; the rise times to maximum can differ substantially among SNe IIn.

The early-time continua of SNe IIn tend to be bluer than normal. Occasionally He I emission lines are present in the first few spectra [e.g. SN 1994Y in Figure 14 and SN 1987B (see Figure 1.22 of Harkness & Wheeler 1990)]. Very narrow Balmer absorption lines are visible in the early-time spectra of some of these objects, often with corresponding Fe II, Ca II, O I, or Na I absorption as well (e.g. SNe 1994W and 1994ak in Figure 14). Some of them are unusually luminous at maximum brightness, and they generally fade quite slowly, at least at early times. The equivalent width of the intermediate H component can grow to astoundingly high values at late times.

One of the first extensively observed SNe IIn was SN 1987F (Filippenko 1989a, Wegner & Swanson 1996). Initially, broad H emission was superposed on a luminous (M_V -19.3 mag), nearly featureless continuum, but its profile did not have the characteristic P Cygni shape, and its centroid was blueshifted by 1500 km s^-1 with respect to the systemic velocity of the parent galaxy. Many months later, the broad H in SN 1987F was more luminous and had much larger equivalent width; Fe II, Ca II, and O I emission were detected as well (see also SN 1994Y in Figure 14). Forbidden lines, normally prominent at this phase, were very weak; Filippenko (1989a, 1991a) concluded that the ejecta had high electron density (n_e 10⁹ cm^-3). The narrow component of H, initially quite luminous, was now much weaker. At early times it may have been produced by material previously lost from the progenitor, but this gas was eventually engulfed by the expanding SN ejecta. At 10 months after maximum, SN 1987F was ~ 2-mag more luminous than typical SNe II-P (Cappellaro et al 1990). Chugai (1991) modeled the data according to an interaction of the SN ejecta with dense circumstellar matter.

Another example is SN 1988Z (Filippenko 1991a, b, Stathakis & Sadler 1991, Turatto et al 1993b, Chugai & Danziger 1994). At early times, SN 1988Z showed very narrow (FWHM 100 km s^-1) [O III] 4363 and [O III] 4959, 5007 emission lines whose relative intensities indicated n_e 10⁷ cm^-3. They were almost certainly produced by circumstellar gas released by the progenitor prior to exploding and then photoionized by the intense flash of UV radiation emitted at the time of shock breakout. A resolved, intermediate-width (FWHM 2000 km s^-1) component of H appeared less than two months after discovery and steadily grew stronger. At H, this component was superposed on a much broader emission line (FWHM 15,000 km s^-1; see Figure 14). Nearly a year later, the intermediate-width component completely dominated the optical spectrum (Filippenko 1991a, b, Turatto et al 1993b). Its Balmer decrement was very steep, possibly indicating "Case C" recombination conditions (e.g. Xu et al 1992) in which the gas is optically thick to the Lyman and Balmer series, although collisional excitation may have also contributed to the peculiar line intensity ratios. We were probably seeing shock-induced emission from dense clumps in a wind emitted by the progenitor star (Chugai & Danziger 1994). As in SN 1987F, forbidden lines were weak or absent, and very strong lines of Fe II, Ca II, and O I emerged (Figure 14). The blend of O I 8446 and the Ca II near-IR triplet, in particular, became stronger than the very broad component of H, yet little or no [Ca II] 7291, 7324 was present, indicating high density. However, Chugai & Danziger (1994) argued that the envelope was not massive, and hence the progenitor itself may have had a relatively low mass, in contrast with the conclusion of Stathakis & Sadler (1991).

The late-time optical spectra of SN 1988Z closely resembled those of SN 1986J, an object that was discovered at radio wavelengths long after its optical outburst (Rupen et al 1987, Leibundgut et al 1991b). Accordingly, Filippenko (1991a) predicted that SN 1988Z should eventually become very luminous at radio wavelengths, as did SN 1986J. SN 1988Z was indeed subsequently detected at radio wavelengths with a luminosity comparable to that of SN 1986J, and analysis of the radio light curves suggested a high mass-loss rate (Van Dyk et al 1993). SN 1988Z was also detected as an X-ray source (Fabian & Terlevich 1996). Another similar object is SN 1978K (Ryder et al 1993, Chugai et al 1995), which was luminous at radio and X-ray energies, although its Balmer decrement was not unusually steep and suggests Case B recombination.

Type IIn supernovae exhibit considerable heterogeneity. For example, objects like SNe 1986J, 1988Z, 1993N (Filippenko & Matheson 1993, 1994), and 1995N (Pollas et al 1995, Garnavich et al 1995a, Van Dyk et al 1996b), whose spectra were for many years completely dominated by H emission of FWHM 1000 km s^-1, became strong radio and X-ray sources. They seem to have the densest circumstellar material. Of these objects, the ones observed at early times (SNe 1988Z and 1993N) had relatively featureless blue continua with almost no H emission. Other SNe IIn, however, are distinct from the SN 1988Z flavor; they exhibit strong H emission right from the start (e.g. SN 1994Y in Figure 14), and they don't become luminous radio sources (Van Dyk et al 1996c). Even among these latter objects there is considerable heterogeneity: Witness the presence of narrow absorption lines in SN 1994W (and also SN 1994ak) but not in SN 1994Y (Figure 14). Moreover, as illustrated by Cumming & Lundqvist (1997), the brightness of SN 1994W dropped precipitously after an age of four months, while SN 1994Y remained quite bright for several years after outburst (AV Filippenko, unpublished data). As another example, the early-time spectrum of SN 1987B (Harkness & Wheeler 1990, Filippenko 1991b, Schlegel et al 1996) closely resembled that of SN 1994Y (Figure 14), with Balmer emission lines and He I that had broad bases; on the other hand, a few months later, the spectrum of SN 1987B exhibited only a hint of broad emission and was instead dominated by relatively narrow absorption lines (Filippenko 1991b), while that of SN 1994Y showed broad Balmer and Fe II emission lines (Figure 14).

SN 1983K (Niemela et al 1985), despite its classification as a SN II-P by Phillips et al (1990), might also be a variant of SNe IIn and further illustrates the diversity of this subclass: Emission lines of N III 4651 and He II 4686, as well as hydrogen Balmer emission lines, were superposed on a very blue continuum in spectra obtained about 10 days prior to maximum brightness, but by maximum brightness the spectrum showed only a few weak and narrow absorption lines. In the case of SN 1984E (Henry & Branch 1987), there was evidence that circumstellar material had been ejected from the progenitor in a relatively discrete event less than 30 years before the explosion (Gaskell & Keel 1988, but see Dopita et al 1984). Recently there have been a substantial number of other SNe IIn discovered (as documented in IAU Circulars), and they seem to exhibit a great variety of properties that should provide clues to the nature of mass loss in evolved massive stars.