OPTICAL SPECTRA OF SUPERNOVAE

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

4. TYPE Ib AND Ic SUPERNOVAE

Bertola (1964), Bertola et al (1965) noticed that the early-time spectra of some SNe I (specifically SNe 1962L and 1964L) lack the deep 6150-Å absorption trough. For two decades few, if any, new examples of such objects existed, and they were simply labeled as "peculiar SNe I" (SNe Ip). Interest in them was revitalized in the mid-1980s by the studies of several newly discovered SNe Ip by Uomoto & Kirshner (1985), Wheeler & Levreault (1985), Elias et al (1985), Panagia et al (1986b). Particularly influential (but unpublished) was an optical, UV, and IR investigation of SN 1983N done by Panagia et al (1986a); subsets of these data have been discussed by Panagia (1985), Gaskell et al (1986), and radio observations were presented by Sramek et al (1984).

As summarized by Porter & Filippenko (1987), SNe Ip seemed to constitute a distinct subclass, characterized by their (a) lack of the 6150-Å Si II absorption trough, (b) preference for galaxies having Hubble types Sbc or later, (c) proximity to H II regions, (d) rather low luminosity, typically 1.5-mag fainter than classical SNe I, (e) distinct IR light curves having no secondary maximum around one month past primary maximum, (f) reddish colors, and (g) emission of radio waves within a year past maximum. The subclass was coined "Type Ib" (Elias et al 1985) to distinguish it from normal SNe Ia. At least one of the earliest studies (Wheeler & Levreault 1985) concluded that the explosion mechanism might be more closely related to that of SNe II than to SNe Ia, but this was not yet certain because the spectroscopic appearance of SNe Ib near maximum seemed to resemble that of somewhat older SNe Ia (t approx 1 month).

The serendipitous discovery of SN 1985F (Filippenko & Sargent 1985, 1986) initially compounded the confusion. Its spectrum was dominated by very strong, broad emission lines of neutral and singly ionized species such as [O I] lambda lambda 6300, 6364, [Ca II] lambda lambda 7291, 7324, the Ca II near-IR triplet, Mg I] lambda 4571, and Na I D. The strength of the forbidden lines suggested that SN 1985F was an old SN, as did the exponential decline of the derived light curve (Filippenko et al 1986). This was later confirmed by Tsvetkov (1986), whose inspection of prediscovery plates showed that SN 1985F had reached maximum brightness at B = 12.1 mag (one of the brightest SNe in many years!) about 260 days prior to discovery. The complete absence of hydrogen led to a formal classification of SN I, although no known spectra of SNe I at any stage of development resembled that of SN 1985F. The dominance of intermediate-mass elements and other factors suggested that SN 1985F was the explosion of a massive star that had rid itself of hydrogen prior to exploding (Filippenko & Sargent 1986, Begelman & Sarazin 1986, Schaeffer et al 1987), somewhat like the progenitor long ago proposed for Cas A by Chevalier (1976). Was this yet another type of SN I, distinct from SNe Ia and SNe Ib?

An important "unification" occurred when Gaskell et al (1986) showed that a spectrum of the Type Ib SN 1983N, obtained eight months past maximum, was very similar to that of SN 1985F at the time of its discovery. Moreover, Kirshner (quoted in Chevalier 1986; see also Schlegel & Kirshner 1989) found that a late-time spectrum of the Type Ib SN 1984L also resembled that of SN 1985F. Thus, SN 1985F was probably a SN Ib discovered long after maximum, and, conversely, SNe Ib eventually turn into objects whose spectra really are vastly different from those of SNe Ia. Interestingly, Chugai (1986a) had, in fact, already suggested that SN 1985F might be a SN Ib discovered long after maximum. Note, however, that there are no early-time spectra of SN 1985F; thus, it may have been a SN IIb (Section 5.5) or perhaps even a SN Ic, although the latter is unlikely given the relatively slow decline of its light curve (Wheeler & Harkness 1990).

The link between SN 1985F and SNe Ib 1983N / 1984L, as well as the convincing discovery of He I lines in early-time spectra of the latter (Harkness et al 1987), provided substantial evidence that SNe Ib are a physically separate subclass of SNe I, probably driven by core collapse of an initially massive star (Wheeler & Levreault 1985). Detailed analysis of the nebular spectrum of SN 1985F (Fransson & Chevalier 1989) strongly supported this hypothesis; in their model, the progenitor had initial and final masses of 25 M and 8 M, respectively, and ~ 2 M of oxygen was ejected. Large departures from local thermodynamic equilibrium (LTE) were invoked by Harkness et al (1987) to produce the observed He I lines.

Gradually it became clear that SNe Ib constitute a heterogeneous subclass, with substantial variations in the observed He I strengths in spectra obtained around maximum brightness. Wheeler & Harkness (1986; see also Harkness et al 1987) suggested that SNe Ib should actually be divided into two separate categories: SNe Ib are those showing strong He I absorption lines (especially He I lambda 5876) in their early-time photospheric spectra, whereas SNe Ic are those in which He I is not easily discernible. However, they modeled SNe Ic in the same physical way as SNe Ib (Wheeler et al 1987) but with different relative concentrations of He and O in the envelope. Although this nomenclature has been adopted by most authors, use of two subtypes (Ib and Ic) might not be observationally warranted. Few objects have been studied in detail; it is possible that a continuum of helium strengths exists among SNe Ib and that He-rich objects are not fundamentally different from He-poor objects in terms of physical origin (Wheeler et al 1987). Clocchiatti & Wheeler (1997), on the other hand, have recently argued that SNe Ib and Ic show a roughly bimodal distribution of He I strengths and that their progenitors may have significantly different evolutionary phases.

The amount of ejected iron in SNe Ib/Ic is not yet clear. If radioactive Ni⁵⁶ powers the optical display, and if SNe Ia produce 0.6 M of this isotope (Woosley & Weaver 1986), then the corresponding mass for SNe Ib might be only ~ 0.15 M because they are roughly four times fainter than SNe Ia (Wheeler & Levreault 1985). Graham et al (1986) proposed the presence of [Fe II] lambda 1.644 µm emission in a late-time spectrum of SN Ib 1983N, and they calculated an ejected iron mass of 0.3 M, but Oliva (1987) suggested that the proper identification is Si I.