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

5.5. Links Between SNe II and SNe Ib/Ic

5.5.1. SN 1987K    SN 1987K (Figure 15) appears to be a link between SNe II and SNe Ib (Filippenko 1988). Near maximum brightness, it was undoubtedly a SN II but with rather weak photospheric Balmer and Ca II lines. Many months after maximum, the broad H emission that dominates the late-time spectra of other SNe II was weak or absent. Instead, broad emission lines of [O I] 6300, 6364, [Ca II] 7291, 7324, and the Ca II near-IR triplet were the most prominent features, just as in SNe Ib. Such a metamorphosis was, at that time, unprecedented in the study of SNe.

Figure 15. Montage of spectra of SN 1987K in NGC 4651 (cz = 817 km s-1), showing the dramatic transformation from a SN II to a SN Ib. Narrow emission lines, produced by superposed H II regions, have been excised in the late-time spectra. Adapted from Filippenko (1988). Epochs are given relative to the date of maximum brightness, July 31, 1987.

SN 1987K provides very strong evidence for a physical continuity between the progenitors and explosion mechanisms of SNe II and at least some SNe Ib/Ic. The simplest interpretation is that SN 1987K had a meager hydrogen atmosphere at the time it exploded (but see Harkness & Wheeler 1990); it would naturally masquerade as a SN II for a while, and as the expanding ejecta thinned out the spectrum would become dominated by emission from deeper and denser layers. It is noteworthy that, aside from the H line, the early-time spectrum of SN 1987K was actually quite similar to those of SNe Ic 1983I and 1983V (Wheeler et al 1987). The progenitor was probably a star that, prior to exploding via iron core collapse, lost almost all of its hydrogen envelope either through mass transfer onto a companion or as a result of stellar winds.

5.5.2. SN 1993J    The data for SN 1987K (especially its light curve) were rather sparse, making it difficult to model in detail; moreover, it was an apparently unique object and hence perhaps somewhat of a fluke. Consequently, there may have been some room for skepticism regarding the connection between SNe II and SNe Ib (dubbed "SNe IIb" by Woosley et al 1987, who had proposed a similar preliminary model for SN 1987A before it was known to have a massive hydrogen envelope; see Arnett et al 1989). Fortunately, the Type II SN 1993J in NGC 3031 (M81) came to the rescue. Bright (V 10.7 mag), nearby (d = 3.6 Mpc; Freedman et al 1994), and well placed in the night sky (circumpolar for many northern observatories), it was studied in greater detail than any SN since SN 1987A. Its early history of observations and modeling is documented by Wheeler & Filippenko (1996).

The unusual nature of SN 1993J first became apparent through its light curves (Benson et al 1994, Lewis et al 1994, Richmond et al 1994, 1996a, Barbon et al 1995, Prabhu et al 1995). At visual wavelengths, it rose to maximum brightness in just a few days, then plummeted by ~ 1.3 mag over the next week, and subsequently climbed for two weeks to a second peak of brightness comparable to the first. Thereafter it declined, initially about as rapidly as the second rise. By ~ 40 days after the explosion, it had settled onto an exponential tail of ~ 0.02 mag day^-1. This behavior differed substantially from that of either the "plateau" or "linear" SNe II (Barbon et al 1979, Doggett & Branch 1985). The rapid initial rise and the twin peaks were reminiscent of SN 1987A, but the time scale of SN 1993J's second peak was much shorter.

A number of independent groups quickly concluded that the progenitor of SN 1993J probably had a relatively low-mass hydrogen envelope (Nomoto et al 1993, Podsiadlowski et al 1993, Ray et al 1993, Bartunov et al 1994a, Utrobin 1994, Woosley et al 1994); otherwise, the second peak would have resembled the more typical plateau of SNe II, since stored energy slowly diffuses out of a massive envelope. Indeed, Nomoto et al (1993) pointed out that the second rise and decline closely resembled the light curve of SN 1983N (Clocchiatti et al 1996b), a prototypical SN Ib. Most groups found that the light curve could be modeled well by assuming that the progenitor was a ~ 4 M He core having a low-mass (0.1-0.6 M) "skin" of hydrogen; the explosion mechanism was the standard iron core collapse of SNe II.

The likely progenitor of SN 1993J was identified as a G8I-K5I star with a bolometric magnitude of about -7.8 (Aldering et al 1994, Garnavich et al 1997). The general consensus (for a dissenting view, see Höflich et al 1993) is that its initial mass was ~ 15 M. A star of such low mass cannot shed nearly its entire hydrogen envelope without the assistance of a companion star. Thus, the progenitor of SN 1993J probably lost most of its hydrogen through "Case C" mass transfer (in the asymptotic giant phase, after core He burning) to a bound companion 3- to 20-AU away. In addition, part of the gas may have been lost from the system.

A specific prediction made by Nomoto et al (1993), Podsiadlowski et al (1993) was that the spectrum of SN 1993J should evolve to resemble those of SNe Ib/Ic, as in the case of SN 1987K described in Section 5.5.1. Nearly simultaneously with the submission of these papers, Filippenko et al (1993) identified prominent absorption lines of He I in SN 1993J, confirming the prediction. Instead of growing progressively more prominent with time (relative to other features), the emission component of H developed a distinct notch identified as blueshifted He I 6678 (see Figure 16). Swartz et al (1993a) came to essentially the same conclusion: The spectrum of SN 1993J was transforming itself into that of a SN Ib together with a bit of hydrogen.

Figure 16.Montage of spectra of SN 1993J in NGC 3031 (cz = -34 km s-1), adapted from Filippenko et al (1994). Epochs (days) are given relative to the estimated date of explosion, March 27.5, 1993. There are a few telluric features in the first spectrum.

Filippenko et al (1993) suggested that many months after the explosion, the spectrum of SN 1993J would closely resemble the late-time spectra of SNe Ib-dominated by strong emission lines of [O I], [Ca II], and Ca II, with H weak or absent (cf SN 1987K). This was confirmed by Filippenko et al (1994; see also Finn et al 1995), as illustrated in Figure 16. Although H remained visible throughout the evolution of SN 1993J, it was weak relative to the neighboring [O I] line at = 6-10 months, whereas in normal SNe II it dominates the optical spectrum at these epochs (e.g. Figure 12). The emission lines were considerably broader than those of normal SNe II at comparable phases, which is consistent with the progenitor having lost a majority of its hydrogen envelope prior to exploding.

The photometric behavior during the first four months (twin peaks; 0.02-mag day^-1 exponential decline), together with the spectral evolution, strongly suggests that the progenitor of SN 1993J had only a low-mass skin of hydrogen. Moreover, the V-band decline rate at very late times (~ 0.014 mag day^-1 at 300 days; Richmond et al 1996a) was substantially steeper than that of normal SNe II (0.009 mag day^-1) and comparable to that of SNe I (Turatto et al 1990), again indicating a low mass for the ejecta. Had the progenitor lost essentially all of its hydrogen prior to exploding, it would have had the optical characteristics of SNe Ib. Consequently, there is now little doubt that most SNe Ib, and probably SNe Ic as well, result from core collapse in stripped massive stars rather than from the thermonuclear runaway of white dwarfs. In addition, it seems likely that a good fraction of the progenitors lost their mass largely via transfer to a bound companion rather than with winds. These conclusions have broad implications for the chemical evolution of galaxies, the expected number density of compact remnants, the potential detectability of neutrino bursts, and the origin of X-ray binary stars.

SN 1993J held several more surprises, however. Observations at radio (Van Dyk et al 1994) and X-ray (Suzuki & Nomoto 1995) wavelengths revealed that the ejecta were interacting with relatively dense circumstellar material (Fransson et al 1996), probably ejected from the system during the course of its pre-SN evolution. Optical evidence for this interaction also began emerging at 10 months: The H emission line grew in relative prominence, and by 14 months it had become the dominant line in the spectrum (Filippenko et al 1994, Finn et al 1995, Patat et al 1995), consistent with the model of Chevalier & Fransson (1994). Its profile was very broad (FWHM 17,000 km s^-1; Figure 16) and had a flat top, but with prominent peaks and valleys whose likely origin is Rayleigh-Taylor instabilities in the cool, dense shell of gas behind the reverse shock (Chevalier et al 1992). (The late-time emergence of H is the main reason SN 1993J is listed as a "SN II-pec" in Figure 16; otherwise, it is a prime example of SNe IIb.) Radio VLBI measurements showed that the ejecta are circularly symmetric but with significant emission asymmetries (Marcaide et al 1995), which are possibly consistent with the asymmetric H profile.

5.5.3. IS THERE HYDROGEN IN OTHER SNe Ib/Ic?    In view of the prominence of H in the first few spectra of SNe 1987K and 1993J, and the resemblance between the late-time spectra of these two SNe II and SNe Ib/Ic, the search for hydrogen in the spectra of SNe Ib/Ic is of interest. Perhaps it is present in some cases but at a lower level than in the "SN IIb" prototypes; this would set more constraints on the nature of the progenitors.

Wheeler et al (1994) pointed out a possible H absorption line in spectra of the prototypical SNe Ib 1983N and 1984L. Similarly, Branch (1972) noted that SN Ib 1954A may have exhibited H. The presence of weak H in SNe Ib is not unexpected, given the examples of SNe 1987K and 1993J; the entire hydrogen envelope need not be expelled prior to core collapse, regardless of whether the progenitor loses gas primarily through winds (very massive star, either isolated or in a wide binary) or via transfer to a companion.

Filippenko (1988, 1992) suggested that SNe Ic 1987M, 1988L, and 1991A had weak H emission; SN 1987M may also have exhibited H absorption (Filippenko et al 1990, Jeffery et al 1991). If hydrogen is indeed present in some SNe Ic, then SNe Ic cannot be the explosions of isolated type-WC Wolf-Rayet stars - i.e. those that have lost their entire H envelope and much of their He layer as well. [Note that Van Dyk et al (1996a) also argued against the WR progenitor model, based on a study of the association of SNe Ib/Ic with H II regions.] The binary scenario of Shigeyama et al (1990), on the other hand, may be consistent with a weak H line, since a helium layer (perhaps with some remaining hydrogen on it, or mixed with it) is present. However, there are other problems with this model - especially the absence of prominent He I lines (Baron 1992). Indeed, the spectral synthesis of Swartz et al (1993b) demonstrated that the progenitors of SNe Ic cannot even have much helium, let alone hydrogen, in their outer layers - if they did, then the signature of these elements would easily be visible at early times.