Publications of the Astronomical Society of the Pacific Vol. 104 1992

Publications of the Astronomical Society of the Pacific

Vol. 104 1992 September No. 979

Publications of the Astronomical Society of the Pacific 104: 717-729, 1992 September The Progenitor of SN 1987A1

Philipp Podsiadlowski Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 OHA, England Electronic mail: [email protected]star Received 1992 May 4; accepted 1992 July 2

ABSTRACT. We review evolutionary models for the progenitor of SN 1987A and confront these models with available observational/theoretical constraints. For this purpose, we devise five tests a successful model has to fulfill. These include the three major anomalies of the supernova (the blue color of the progenitor, the ring surrounding it, and the progenitor's chemical anomalies), the characteristics of the supernova explosion, and general consistency with the theory of massive stars. We show that single-star models (with the possible exception of rapid-rotation models) fail at least two of these tests, while two binary models (accretion and merger models) are consistent with all available constraints. We conclude that it is most likely that the progenitor of SN 1987A had a binary companion, either at the time of the explosion or at least in the not-too-distant past. We discuss in detail how future observations and theoretical calculations are likely to settle this issue conclusively.

1. INTRODUCTION = 16,000( ± 1500) K, and radius i?=45( ± 15)/?Θ; see, e.g., Woosley 1988]. In most respects, it resembled a typi- Five years ago, SN 1987A in the Large Magellanic cal, not very evolved star in the LMC. The question of why Cloud (LMC) was the first naked-eye supernova since the progenitor was a blue supergiant rather than a red Kepler's supernova in 1604. It has confirmed many long- supergiant has remained the most persistent puzzle of this held beliefs about the final stages in the evolution of mas- supernova event and, even after 5 years, has not been fully sive stars. Most dramatically, the discovery of neutrinos resolved. The problem is not a lack of theoretical models to from the supernova (Hirata et al. 1987; Bionta et al. 1987) explain a blue progenitor—there are numerous ones and has proved more or less conclusively that Type II super- the large number of models is a direct measure of the mag- novae are triggered by the collapse of stellar cores (see, nitude of the problem—but to decide which one is the most e.g.. Burrows 1987). However, in many other respects, SN promising. 1987A did not comply with theoretical expectations and The purpose of this review is to reassess the general has, for the last 5 years, continued to surprise and puzzle issue of the progenitor and confront it with the wealth of observers and theoreticians alike. Some of these surprises observational information that has been accumulated over are probably a direct consequence of the higher quality and the last 5 years. In Sec. 2, we summarize the observational the larger variety of observations available for this super- and theoretical constraints for models of the progenitor nova than have ever been available before. Others may be and devise five tests a successful model has to fulfill. In unique to SN 1987A and there are a number of indications Sees. 3 and 4, we review the various single and binary that SN 1987A may have been a rather unusual and rare models for the progenitor, respectively, and rigorously ap- event (note that this may be in part due to the fact that ply these tests. In Sec 5, we discuss the results of this underluminous supernovae like SN 1987A are more diffi- procedure and its limitations, and in Sec. 6 we show how cult to detect). future observations and theoretical work will further help o 2 The progenitor, Sk — 69 202, was one of the major to constrain models of the progenitor. surprises of this supernova. It had been classified by Rous- seau et al. ( 1978) as a B3 I blue supergiant [with luminos- 5 2. CONSTRAINTS OF EVOLUTIONARY MODELS ity 1.1 ( ±0.3) X 10 L^, effective temperature Teñ FOR THE PROGENITOR OF SN 1987A The ultimate goal of a successful model for the progen- invited review paper. itor of SN 1987A has to be to explain the large variety of 2The system Sk — 69o202 is known to consist of at least three stars (Wal- born et al. 1987; Sonneborn et al. 1987), generally referred to as stars 1, observations that are available and to provide unambigu- 2, and 3. Throughout this paper, all references to Sk — 69o202 refer to ous predictions that can be tested by future observations. star 1 only, unless stated otherwise. While SN 1987A has provided many surprises, this in itself

717 © 1992. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 718 PODSIADLOWSKI does not imply that the supernova was unusual or anom- phreys 1984; Fitzpatrick and Garmany 1990) reveals that alous. Some of the surprises may just be a reflection of the stars as massive as —40 pass through an extended fact that we can observe this supernova in many more ways red-supergiant phase. If this observational fact is combined (often with instruments that have only recently become with the theoretical knowledge that, in very evolved stars, available) and in much greater detail than any other su- the evolution of the stellar envelope is essentially decou- pernova before. Some of the observations are based on pled from the evolution of the core, we may conclude that techniques which are still in their infancy, and, in a few the red-supergiant stage is the natural stage in which most cases, there is even some skepticism about the reality or the massive stars end their evolution. (This argument breaks interpretation of observations. Therefore, the problem of down for very massive stars which lose all of their enve- devising tests for models of the progenitor of SN 1987A lopes as a result of strong stellar winds and become Wolf- consists of finding constraints that, on one hand, are well Rayet stars; ) This argument also illustrates one of the dif- established and, on the other hand, are as restrictive as ficulties of many single-star models, since they—in effect— possible. As our main tests we have chosen the three major have to find an exception to this generic rule, which, in anomalies of the progenitor (its compactness, the ring of turn, often requires special circumstances. Of course, if ejected material surrounding it, and its chemical anoma- these circumstances are fulfilled in a particular stellar pop- lies), and the supernova explosion itself. We did not con- ulation (e.g., in a low-metallicity population), then blue sider many of the less well-established anomalies. For ex- supernova progenitors could be quite common in such a ample, we did not include the "mystery spot" (Nisenson et population and Supernovae like SN 1987A could provide al. 1987), because its very existence has remained contro- an important signature of such a population (Langer versial. We also did not consider the implications of the 1991b). asymmetric expansion of the supernova ejecta, as inferred from polarization measurements (Cropper et al. 1988; Méndez et al. 1988) and speckle interferometry (Papalio- 2.2. The Ring Around the Progenitor lios et al. 1988). While these asymmetries could be a direct Observations of the circumstellar nebula around SN consequence of a flattened envelope structure of the pro- 1987A with the NTT3 by Wampler et al. (1990) and the genitor (Chevalier and Soker 1989), they could also be HST3 by Jacobson et al. (1991) reveal that the circum- caused by an asymmetric supernova explosion (Chevalier stellar material around the supernova, first discovered with and Soker 1989; Yamada and Sato 1990). In the latter the IUE3 satellite (Fransson et al. 1989), has the morphol- case, they could be very important for understanding the ogy of a narrow ring rather than that of a spherical shell. basic supernova explosion mechanism, but might reveal The overabundance of this material in nitrogen relative to little about the structure of the progenitor. carbon and oxygen (Fransson et al. 1989) suggests that it In addition, a successful model has not only to be able to is mainly composed of material which has been processed explain the main features of this supernova, but it also has by nuclear reactions deep inside the progenitor and which to be consistent with all other, general observational and has subsequently been ejected. The ringlike geometry of theoretical constraints for massive stars. We would violate these ejecta implies an axisymmetric, but highly nonspher- one of the golden rules of theoretical physics, if we de- ical structure of the envelope of the progenitor and/or its signed a theory that could explain all the features of this winds. The origin of this nonsphericity provides a severe particular event, but could not describe the majority of the constraint for models of the progenitor. A plausible mech- stars in the universe. anism to provide the required asymmetry is the flattening of the progenitor's envelope caused by rapid rotation 2.1 The Compactness of the Progenitor (Chevalier and Soker 1989). However, Chevalier and Soker showed, using straightforward angular-momentum One of the major surprises of this supernova event was considerations, that a single star which was rapidly rotat- that the progenitor was a blue supergiant (see, e.g., Wal- ing on the main sequence would be a slow rotator in any born et al. 1987) rather than a red supergiant, the gener- subsequent supergiant phase and could not be significantly ally expected precursor for Type II supernovae (e.g., Falk flattened at the time of the supernova explosion. and Arnett 1977; Woosley and Weaver 1985). It has oc- A problem that may be related is the origin of the asym- casionally been argued that, on the basis of stellar evolu- metries (i.e., the deviations from spherical symmetry) ob- tion theory alone, it is not possible to decide whether mas- served in a large fraction of planetary nebulae (see, e.g, sive stars end their lives as blue or red supergiants. Indeed, Balick 1987). While a modest asymmetry may be sufficient it is true that plausible assumptions about metallicity, to explain the elliptical appearance of many planetary neb- opacities, or convection theory can give blue as well as red ulae, a rather large asymmetry is required to explain the presupernova models (see, e.g.. Lamb et al. 1976; Brunish less common planetary nebulae with strongly bipolar or and Truran 1982). However, the situation is less ambigu- butterfly morphology. The physical origin of these asym- ous when theoretical modeling is combined with observa- metries is not understood. (We note that many models tional constraints. Some of the best observational con- which are able to reproduce asymmetric planetary nebulae straints for the theory of massive stars are based on the distribution of massive stars in the Hertzsprung-Russell 3NTT: New Technology Telescope; HST: Hubble Space Telescope; IUE: (HR) diagram. The HR diagram for the LMC (Hum- International Ultraviolet Explorer.

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SN 1987A 719 invoke asymmetries in the outflow, but do not provide an far, a suitable mixing mechanism has not yet been unam- explanation for the postulated asymmetries; see, e.g., Kahn biguously identified. Weiss (1991) argued that the ob- and West 1985.) A plausible origin for the asymmetries is served N/C and N/O ratios are inconsistent with the he- binary motion/interaction (see, e.g., Paczyñski 1985; Iben lium enrichment for any one-phase mixing mechanism 1989; Morris 1990). Indeed, planetary nebulae with known (see, however, Saio et al. 1988). In addition, Mazzali et al. binary cores generally show large deviations from spherical (1992) infer that the barium enhancement is restricted to symmetry (Bond and Livio 1990). the innermost 89% of the mass of the progenitor. Rota- tional mixing (on the main sequence or during the final 2.3 Chemical Anomalies transition to the blue-supergiant phase) may be able to explain some of the observations. However, to date no fully There is substantial, observational evidence for anoma- self-consistent model exists which can explain all three lous chemical abundances in the hydrogen-rich envelope of chemical anomalies simultaneously (henceforth referred to the progenitor and in the presupernova ejecta (for an ear- as the nitrogen, helium, and barium anomalies). lier review, see Truran and Weiss 1990). A few weeks after the supernova explosion, the spectra revealed an overabundance of barium and other ^-process elements (Williams 1987; Danziger et al. 1988). These observations imply an 2.4 The Supernova Explosion apparent enhancement of these elements in the progeni- The supernova explosion itself provides further impor- tor's hydrogen-rich envelope (note that these elements are tant constraints for possible supernova progenitors. Com- only produced during helium burning and should therefore parisons of hydrodynamical calculations of the explosion not be found in the hydrogen-rich envelope). Detailed at- with the observed supernova light curve and detailed atmosphere calculations by Höflich (1988) and Mazzali mosphere studies are all consistent with an immediate su- et al. (1992) have confirmed the overabundances of s- pernova progenitor of radius R~3x\0n cm, core mass process elements and possibly helium in the outer enve- Mc~6 A/q, and envelope mass Menv^5 Mq (see, e.g., lope.4 Evidence for an enhanced helium abundance (He/ 5 Shigeyama et al. 1988; Höflich 1988; Woosley et al. 1988; H:=-0.2) is also found in the circumstellar material (Allen Arnett and Fu 1989). et al. 1989; Wang 1991). Höflich finds that only helium and ^-process elements are enriched, while other elements are consistent with scaled solar abundances. This strongly suggests that these apparent enhancements are caused by 2.5 Independent Observational/Theoretical Constraints real abundance effects and not by deficiencies in the atmo- 6 sphere calculations (non-LTE effects, etc.), since it would A successful model for the progenitor of this supernova be a coincidence if only those elements that are related by has to be consistent with the general theory of massive the nuclear process by which they were produced (5 pro- stars. Some of the best constraints of the evolution of mas- cessing during helium burning), but have unrelated atomic sive stars are based on the distribution of stars in the ob- and spectroscopic properties, showed these anomalies. The servational HR diagrams of massive stars. For example (as large nitrogen-to-carbon (N/C^8)5 and nitrogen-to- 5 already discussed above), the HR diagram of massive stars oxygen ratio (N/0~2) observed in the circumstellar ring in the LMC reveals that stars up to a mass M~40 (Fransson et al. 1989) have been used previously as an pass through a red-supergiant phase. An additional con- argument that the progenitor passed through a red- straint is the apparent main-sequence widening, i.e., the supergiant phase, in which CNO-processed material has apparent stretching of the main-sequence band up to been dredged up to the surface (see, e.g., Woosley 1988). spectral-type A (see Maeder and Meynet 1987). In order However, available stellar-evolution calculations show that to explain the latter, a number of authors have concluded the surface convection zone during a red-supergiant phase that it is necessary to include a substantial amount of semi- never penetrates deep enough to produce the large, ob- convection and/or convective overshooting during hydro- served nitrogen overabundance (Weiss et al. 1988). In- gen core burning, since these effects increase the main- stead, it seems that all of these observations can only be sequence lifetime of massive stars and extend their reconciled if the progenitor's envelope had been mixed evolution away from the zero-age main sequence (for dis- thoroughly with a significant fraction of material that un- cussions and references, see, e.g., Renzini 1987; Napi- derwent nuclear burning (hydrogen burning and at least wotzki et al. 1991; Zahn 1991 ). However, this issue has not partial helium burning) before the supernova explosion. So yet been resolved conclusively, since other factors, such as mass loss and opacities, also affect the main-sequence 4 We note that the claim by Mazzali et al. (1992) that their findings are width (De Grève 1992; Schaller et al. 1992). A completely consistent with standard stellar nucleosynthesis calculations (e.g., Prant- zos et al. 1988) without additional mixing is not correct, since this different explanation for the apparent main-sequence wid- conclusion is based on a comparison with stellar models that are inap- ening is that the region beyond the main sequence is pop- propriate for the progenitor of SN 1987A, i.e., models with very small ulated by stars on extended blue loops during helium core hydrogen-rich envelopes (see Sec. 2.4). burning, which gives the false appearance of a wider main 5 AI1 quoted abundance ratios are by number. sequence (for recent discussions, see Fitzpatrick and Gar- 6Local thermodynamic equilibrium. many 1990; Tuchman and Wheeler 1991; Langer 1991b).

Fig. 1— The evolutionary track in the HR diagram for a star of M =10 Fig. 2— The evolutionary track in the HR diagram for a star of mass Mq in the low-metallicity model (Z=0.005, no mass loss, no convective M=20 Mq in a restricted-convection model (Z=0.005, no mass loss, no overshooting; from Weiss 1989). The star bums helium as a blue su- convective overshooting; from Weiss 1989). The model uses the Ledoux pergiant and never becomes a red supergiant. criterion for convection (instead of the Schwarzschild criterion). The star spends part of its helium-core-buming phase as a red supergiant and part as a blue supergiant, and finally explodes as a blue supergiant. 3. SINGLE-STAR MODELS FOR THE PROGENITOR has no mechanism to produce the ring around the progen- 3.1 The Low-Metallicity Model itor and the chemical anomalies. We, therefore, have to In many ways the simplest model to explain a blue pro- conclude that low metallicity alone cannot be responsible genitor is the model in which the progenitor never becomes for a blue progenitor of S Ν 1987Α. a red supergiant and ends its life as a blue supergiant because of an unusually low metallicity (see Fig. 1; Arnett 3.2 Extreme-Mass-Loss Models 1987; Hillebrandt et al. 1987; Truran and Weiss 1987). The model is attractive, because it requires metallicity as Maeder (1987b) and Wood and Faulkner (1988) pro- the only free parameter. In addition, this parameter can— posed mass loss as the primary cause for a blue-supergiant in principle—be estimated independently by determining progenitor. In their models, the progenitor would have first the metallicity (or at least a plausible range for the metal- become a red supergiant, but returned to the blue- licity) of the parent population to which the progenitor supergiant region, after it had lost most of its hydrogen- belonged. Theoretical models require a metallicity of less rich envelope (typically not more than a few tenths of a than —1/3 ZQ (where ZQ~0.02 is the solar metallicity), solar mass remain in the hydrogen-rich envelope). These a value which may well be consistent with the metallicity models require not only very high mass-loss rates (typi- of young stars in the LMC (see, however, the discussion in cally 5-10 times larger than empirical rates; Chiosi and Sec. 6.2). Maeder 1986; Dupree 1986), but also extreme fine tuning The low-metallicity model has no difficulty in producing of those rates. For slightly different rates, the star would progenitors that resemble Sk—69o202 with stellar param- end its life either as a red supergiant or as a Wolf-Ray et eters consistent with the constraints imposed by the explo- star. sion. However, it is inconsistent with the distribution of This model can produce a progenitor star that resembles massive stars in the LMC in the HR diagram. Since the the observational characteristics of Sk — 69o202 and could model predicts that low-metallicity stars should never perhaps explain the chemical anomalies, since processed reach the Hayashi line, it cannot account for the observed material may have been already exposed to the surface. red supergiants in the LMC and Small Magellanic Cloud However, it cannot produce the observed supernova explo- (SMC) (Humphreys 1984; Fitzpatrick and Garmany sion, since the low mass in the hydrogen-rich envelope 1990). To make matters worse, by comparing the relative could not sustain the long rise of the light curve (see, e.g., numbers of red to blue supergiants in the Galaxy, the Woosley 1988). The model also appears to be inconsistent LMC, and the SMC, which is a sequence of decreasing with the theory of Wolf-Ray et stars, since it would require metallicity, one finds that there are relatively more red that single stars with initial masses as low as —20 MQ (the supergiants in low-metallicity systems than in high- maximum initial mass consistent with the luminosity of Sk metallicity systems (see, e.g., Humphreys 1984; Maeder — 69o202) can become Wolf-Rayet stars. This is signifi- 1984). This is exactly the opposite of what the low- cantly lower than typical estimates for the minimum initial metallicity model predicts. (This apparent paradox can mass of single stars that become Wolf-Rayet stars (see, probably be resolved when mass loss, and in particular, its e.g., Abbott and Conti 1987). Finally, the model also pro- metallicity dependence are taken into account; see, e.g., vides no mechanism to explain the ring around the super- Humphreys 1984; Maeder 1984). In addition, the model nova.

3.3 The Restricted-Convection Model

A very different evolutionary scenario was first suggested by Woosley et al. ( 1988; also see Langer et al. 1989; Weiss 1989), who showed that—under certain circumstances—it is possible that a star first becomes a red supergiant, but returns to the blue-supergiant region in the HR diagram near the end of its evolution (see Fig. 2). One of the most important assumptions in this model, which M = 20 M0 gives it its name, is the use of the Ledoux criterion for Ζ = 0.005 convective instability (Ledoux 1947) instead of the usually "Helium" preferred Schwarzschild criterion (Schwarzschild 1906; also see Dallaporta 1972). This suppresses semiconvection above the convective hydrogen-burning core and produces 1ο smaller hydrogen-exhausted cores with shallower H-He β Tel composition gradients in the hydrogen-rich envelope above the core. As a result, the progenitor spends most or all of Fig. 3— The evolutionary track in the HR diagram for a star of initial mass M =20 in the helium-enrichment model (Z=0.005, mass loss its helium-core-burning phase as a red supergiant, but be- according to De Jager et al. 1988; the model is similar to the model by comes a blue supergiant shortly before the supernova ex- Saio et al. 1988). The calculation uses the standard Schwarzschild crite- plosion. Whether the star returns to the blue also depends rion for convection. Near the end of helium core burning, the helium abundance in the envelope is (artificially) increased from 7=0.24 to on several other conditions; in the models by Woosley et al. 7=0.45. Due to the resulting opacity change, the star experiences a blue (1988), the metallicity and the helium abundance both loop and explodes as a blue supergiant. have to be low (Z^ 1/3 and 7;$0.23), and mass loss relatively unimportant. Convective overshooting from the plete blue loop, an additional, complete blue loop during core must also be unimportant, since even a moderate helium core burning. amount of convective overshooting would prevent the final Finally, all restricted-convection models have the addi- transition to the blue (see, e.g.. Langer 1991b). The reason tional problem that, in their red-supergiant phase, the pro- for the amount of fine tuning required in this model follows genitors never reach the true Hayashi line (Tuchman and from the discussion in Sec. 2.1, since the model attempts to Wheeler 1989); the red extremes in these models is be- reverse the general trend towards lower effective tempera- tween 4000 and 6000 K. This effective temperature range is tures, which characterizes the overall evolution of massive almost completely empty in the observational HR diagram. stars (see, e.g., the discussion in Maeder.1984). The pro- (This argument, however, assumes that effective tempera- genitor, in effect, undergoes a late blue loop (or Cepheid tures of red supergiants are well calibrated; see Böhm- loop) in the HR diagram, but explodes before it has time to Vitense 1981.) complete the loop and return to the red-supergiant branch. (For a detailed discussion of these loops, which often occur during helium core burning, and their sensitivity to the 3.4 The Helium-Enrichment Model input physics, see, e.g., Lauterborn et al. 1971.) Saio et al. (1988) proposed an evolutionary scheme in This model can explain a blue progenitor and the char- which the progenitor also undergoes a blue-red-blue evo- acteristics of the supernova explosion. It can also explain lution (see Fig. 3). They adopted the standard Schwarzs- the ratio of blue to red supergiants observed in the LMC. child criterion for convection and used relatively large The main problems of this model are that it cannot account mass-loss rates ( ~5 times larger than the empirical rates). for most of the chemical anomalies and that it provides no At the end of helium core burning (which mainly takes plausible mechanism for the formation of the ring around place in the blue-supergiant region), they artificially mix the progenitor. In addition, it may also have difficulties the hydrogen-rich envelope with a large fraction of the explaining some of the details of the distribution of stars in helium-rich interior. This produces a decrease of the opac- the HR diagram. Because stars have relatively small core ity in the radiative envelope and triggers a rapid contrac- masses at the end of core hydrogen burning, this model tion of the envelope. The star then ends its life as a blue cannot reproduce the apparent wide main-sequence band supergiant. The amount of mixing of helium into the en- (see the discussion in Sec. 2.5 and Tuchman and Wheeler velope depends on the assumed metallicity. Saio et al. find 1991). The region just beyond the real main sequence can that, for Z=0.005, the final helium mass fraction in the also not be repopulated by stars on a late blue loop, since envelope has to be 7—0.4 and increases with increasing the lifetime of this final blue-supergiant phase is too short metallicity. to be statistically significant ( ^0.1% of the main-sequence This model can explain a blue progenitor, the supernova lifetime). This problem is less serious in the models calcu- event itself, and perhaps (by design) the nitrogen and he- lated by Langer et al. (1989) and Weiss (1989; Langer lium anomalies (Saio et al. 1988). However, the model also et al. 1989 used a different treatment of semiconvection; has problems accounting for the distribution of stars in the Langer et al. 1983). These authors found that their pro- HR diagram. Since almost all of the helium-core-burning genitor models experienced, in addition to the final incom- phase is spent in the blue supergiant region and since the

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 722 PODSIADLOWSKI post-helium-core-burning lifetime is short compared to the supergiant (irrespective of the convection criterion?), pos- helium-core-burning lifetime (^He^post-He^ m0(iel sibly without ever passing through a red-supergiant phase; does not produce a sufficient number of red supergiants to its final location in the HR diagram depends on the helium be consistent with the observed HR diagram. However, the abundance in the envelope and the metallicity of the star. main problem of this scenario lies in the identification of a Preliminary models based on this general idea have been mixing mechanism that (spontaneously?) dredges up a presented by Weiss (1991) and Langer (1992). While substantial amount of helium-rich material. Since ordinary Weiss (1991), using the Schwarzschild criterion for con- turbulent convection cannot be effective (because of a steep vection, found only red-supergiant progenitors, Langer entropy gradient at the bottom of the hydrogen-rich enve- (1992), adopting a treatment similar to the Ledoux crite- lope), Saio et al. propose meridional circulation as a mix- rion, also obtained blue-supergiant models. ing mechanism. However, it is well known from the theory A rapid-rotation model could potentially avoid many of of meridional circulation (see, e.g., Mestel 1965; Tassoul the problems most other single-star models are confronted and Tassoul 1984) that even a small chemical gradient with. Just in those models, a blue progenitor could explain prevents the mixing of material across chemical gradients the observed supernova explosion. In addition, such a (via so-called μ-currents), as long as the star is not rotat- model may be able to reproduce many of the chemical ing close to breakup velocity. On the other hand, a star anomalies (although probably not the barium anomaly) that is not breaking up on the main sequence will not be and could perhaps provide a sufficient rotational asymme- close to breakup in a subsequent red-supergiant phase. This try to produce the supernova ring (in particular, if the star problem could be solved if there were a source of angular avoids a red-supergiant phase in which magnetic braking momentum, for example, in the form of a binary compan- would provide an efficient slowdown mechanism). In sum- ion which transfers mass and angular momentum to the mary, a rapid-rotation model appears to be the most prom- progenitor (see Sec. 4.2). This could then also provide a ising of the single-star models, although it will require mechanism to produce the supernova ring. more work before one will be able to draw any real conclusions. 3.5 Rotation Models 4. BINARY MODELS FOR THE PROGENITOR Rotational mixing (caused by meridional circulation or All of the models discussed in the previous section as- baroclinic instabilities) has been invoked by various au- sume single-star evolution. However, most stars actually thors (Weiss et al. 1988; Ramadurai and Wiita 1989; occur in binary or multiple systems (e.g., Abt and Levy Langer 1991a) in order to explain the large overabundance 1976, 1978; Kraicheva et al. 1978, 1979; Duquennoy and of nitrogen in the circumstellar material (Fransson et al. Mayor 1991). Indeed, it is more likely that any exploding, 1989). In these models, unlike the model by Saio et al. massive star has a companion at the time of the explosion ( 1988), rotational mixing occurs in the pre-main-sequence than that it is single. In many cases, the companion will be or early main-sequence phase, in which the star has not yet relatively distant and not affect the presupernova evolution built up a significant chemical gradient that would sup- of the progenitor (although it may still affect the explosion press rotational mixing. As long as the circulation time and the evolution of the remnant). However, in a substan- scale is much shorter than the nuclear time scale, rota- tial fraction of cases (20%-40% of stellar systems; Pod- tional mixing can keep the star (or at least a large fraction siadlowski et al. 1992; Tutukov et al. 1992), the companion of the star) homogeneous and prevent the buildup of a is close enough to interact with the progenitor and to com- chemical gradient. As the nuclear time scale shortens, this pletely alter its presupernova evolution and the resulting condition will ultimately break down and, after a chemical supernova explosion. gradient has been established, the subsequent evolution will resemble the evolution without rotation. The calcula- 4.1 Companion Models tions of Weiss et al. (1988) and Langer (1991a), which differ somewhat in their assumptions, are able to reproduce One class of binary models for the progenitor of SN the observed abundance ratios of the CNO elements, but 1987A is based on the idea that the star which exploded they produce at most only a moderate enhancement in was not Sk — 69o202, but instead a previously undetected helium. companion star. This requires that the more-evolved star In a more extreme version of this scenario, one can (the star that exploded) was more luminous than the less- imagine that very rapid rotation could keep the star chem- evolved star (Sk — 69o202). This is similar to the classical ically homogeneous for a significant fraction of its Algol paradox (Struve and Gould 1954; Crawford 1955) hydrogen-core-burning phase. In this case, the star's evo- and implies that Sk — 69o202 was originally the less mas- lution will initially follow the evolution of a homogeneous sive star of the system, but accreted a substantial amount star (see, e.g., Maeder 1987a). It may then be possible to of matter in a mass-transfer episode. Two models of this significantly increase the helium abundance in the star's type have been proposed. In the first model (Fabian et al. envelope before rotational mixing becomes too inefficient 1987), the progenitor had lost all of its hydrogen-rich en- to keep the star homogeneous. The further evolution of this velope in a case Β mass-transfer event and became a Wolf- star could be quite different from the case without rotation. Rayet star. Fabian et al. ( 1987) argued that the hydrogen In particular, such a star may end its evolution as a blue lines visible in the supernova spectrum (which observa-

blue supergiant instead of a red supergiant, as it would have been without the accretion phase (dotted curve). The primary (the mass donor), on the other hand, is most likely to have already completed its own evolution and to have become a neutron star (or black hole). Thus, this model predicts that, at the time of the explosion, the blue- supergiant progenitor still has a companion, which is either a compact star or, less likely, an ordinary star (Podsiad- lowski and Joss 1989; unless the system has been disrupted because of an asymmetric supernova explosion). Since the first supernova would have occurred —105-106 yr ago, the supernova remnant left from the first explosion would have been long dispersed. l0 T This model can explain a blue progenitor and appears to ® Cff be consistent with all stellar and binary constraints (for a discussion of the relevant aspects of binary evolution, see Fig. 4— The evolutionary track in the HR diagram for a star of mass Podsiadlowski et al. 1992). It is worth emphasizing that M=\5 Λ/q with and without accretion in the accretion model (Z=0.01, mass loss according to De Jager et al. 1988, moderate amount of convec- this model provides a generic mechanism to produce blue- tive overshooting). The solid and dashed curves show the evolutionary supergiant progenitors and does not require fine tuning of track of a star that accretes 5 during helium core burning. The dashed the input parameters. In particular, it is insensitive to the portion of the track represents the evolution from the beginning of the accretion phase to the supernova stage. The star explodes as a blue su- assumed metallicity, the treatment of convection, the de- pergiant. For comparison, the solid and dotted curves show the evolution tails of the mass-transfer process, or the amount of mass of a 15 star without accretion. In this case, the star explodes as a red that may be lost by the supernova progenitor in a stellar supergiant. wind. It requires only that at least several solar masses more be accreted by the progenitor than is subsequently tionally classified the event as a Type II supernova) were lost via a stellar wind, and that the accretion phase takes produced by hydrogen-rich material that was stripped place after the termination of the main-sequence phase of from the hydrogen-rich envelope of the companion the progenitor. The final color of the progenitor (which (Sk — 69o202). In the second model, Joss et al. (1988) reflects its compactness) depends on the amount of matter suggested that the progenitor lost most, but not all, of its that has been accreted. Podsiadlowski et al. (1992) esti- hydrogen-rich envelope during late case B/case C mass mated that ~5% of all massive stars should end their transfer and was a red supergiant with only a small evolution as blue supergiants because they accreted matter hydrogen-rich envelope at the time of the supernova explo- or because they merged with a companion (see Sec. 4.3). sion. These stars may also contribute to the apparent main- While both of these models are consistent with the consequence widening (see De Grève 1992). straints imposed by stellar and binary evolution, they are In an accretion model, the progenitor is expected to be inconsistent with the observed features of the SN 1987A rapidly rotating immediately after the accretion phase and explosion (see, e.g., Hsu et al. 1991). Furthermore, the possibly at the time of the supernova explosion. Indeed, an central prediction of these models, the survival of evolved star has to accrete only a few percent of its own Sk — 69o202, is now ruled out by the low luminosity of the mass to gain—in principle—enough angular momentum to observed bolometric light curve (Suntzeff et al. 1991). be spun up to breakup velocity (Packet 1981). This can provide plausible mechanisms for the origin of the circum- 4.2 Accretion Models stellar ring. For example, one possibility is that the ring is the product of an excretion-disk-like outflow (similar to The main feature of accretion models [Barkat and the slowly expanding disks in some models for Be stars; Wheeler 1989; Podsiadlowski and Joss 1989; De Loore and see, e.g., Poeckert 1982) that is swept up by a subsequent Vanbeveren 1992; also see Hellings 1983 who (almost) fast stellar wind (in the Be-star phase, the system would be anticipated this possibility] is that the progenitor ends its very similar to the binary radio pulsar PSR 1259 — 63; evolution as a blue supergiant, because it has accreted a Johnston et al. 1992). Another possibility is that it is the substantial amount of mass from its companion after its result of a discrete dynamical ejection event, possibly asso- hydrogen-core-burning phase. (If accretion takes place ciated with the restructuring of the progenitor's envelope when the progenitor is still on the main sequence, it would near the end of helium core burning, when the hydrogen- be rejuvenated as a main-sequence star and subsequently burning shell is being distinguished (which leads to a char- mimic the evolution of a more massive single star; see, e.g., acteristic loop in the HR diagram; see Fig. 4). Hellings 1983.) This evolution is illustrated in Fig. 4, The model may also be able to explain many of the which shows the evolutionary track in the HR diagram of chemical anomalies of the progenitor (in particular the a star of 15 that accretes 5 during the helium- nitrogen and the helium anomaly). Barkat and Wheeler core-burning phase (solid and dashed curves). Because of ( 1989) found that if the progenitor accretes during a red- the added mass, the immediate supernova progenitor is a supergiant phase, the convective envelope may penetrate

4.3 Merger Models

The merger models for SN 1987A (Hillebrandt and ★' 18.6 + 3.4 M0 Meyer 1989; Podsiadlowski et al. 1990; also see Chevalier and Soker 1989) are closely related to the accretion models. Instead of accreting part of the companion, the companion merges with the progenitor and is completely dis- solved in the latter's envelope. As a consequence, the progenitor becomes a blue supergiant, either because of the increased mass in the envelope (Podsiadlowski et al. 1990; see Fig. 5) or because of the dredge-up of helium (Hille- brandt and Meyer 1989), or both. The merger of two stars is generally believed to be one ^ Te, of the possible, relatively frequently occurring outcomes of dynamically unstable mass transfer (Paczyñski 1970; also Fig. 5— The evolutionary tracks in the HR diagram of a 16 star see the discussion in Podsiadlowski et al. 1992). During (solid curve) and two illustrative merger calculations (dashed and dot- the merging process, which may not last longer than sev- dashed curves; Ζ=0.02, no mass loss, moderate amount of convective eral times 103 yr (Taam et al. 1978; Meyer and Meyer- overshooting; from Podsiadlowski et al. 1990). The dashed curve shows the evolutionary track of a 16 star after it has completely merged with Hofmeister 1979), the system passes through a phase in a 3 Mq main-sequence star in a case Β common-envelope phase (the which the core of the original primary star and the second- common-envelope phase is not shown), and the dot-dashed curve shows ary star are imbedded in a common envelope, which the evolution of a 16 star which is still in the process of merging with mainly consists of material from the primary's envelope. a 6 Mq main-sequence star (case C scenario). In the latter case, the merger has not been completed by the time of the supernova explosion. The orbit of this immersed binary is slowly decaying due to drag forces and tidal friction. In the process, angular momentum is transferred from the orbital motion of the into the helium layer, which would lead to the dredge-up of spiraling-in binary to the envelope, and the envelope is helium and other processed material. (This behavior was systematically spun up. While the details of the common- not found in similar calculations by Podsiadlowski and envelope phase are poorly understood theoretically, it is Joss 1989; the origin of this discrepancy is not clear.) In likely that the frictional luminosity generated by the de- any case, the accreted matter will contain material that has been processed by CNO burning and is helium enriched caying orbit induces convective currents which may dredge (De Loore and Vanbeveren 1992). In addition, it is possi- up helium and possibly even some of the products of he- ble that the core of the progenitor will also be spun up to lium burning from the primary's core (Meyer and Meyer- nearly breakup velocity. Whether this occurs depends on Hofmeister 1979; Iben and Tutukov 1985), provided that how angular momentum is redistributed inside the accret- the primary is an evolved star. In addition, three- ing star and may be sensitive to the structure of its enve- dimensional simulations of the common-envelope phase lope (in particular, whether the envelope is radiative or (Livio and Soker 1988) suggest that the merger process is convective; core spin-up appears to be more likely if the accompanied by substantial mass loss and that most of the mass-gaining star has a convective envelope). If core spin- mass is lost in the equatorial plane. This will automatically up takes place, meridional currents may be able to over- lead to ring- or disklike structures surrounding the merged come the stabilizing effects of chemical gradients (Mestel system. Thus, the final outcome of the merger of two stars 1965; see, however, Tassoul and Tassoul 1984) and pro- is a rapidly rotating single blue supergiant, which may be duce large-scale mixing of the star, including the dredge-up thoroughly mixed and surrounded by a ring of ejected ma- of helium and perhaps ^-processed material (also see Sec. terial. Such a model could explain all of the constraints of 6.3). this supernova event.

Table 1 Summary of the tests devised in Sec. 2 and applied to the various single and binary models for the progenitor of SN 1987A (see Sees. 3 and 4). Blue Circumstellar Chemical Supernova Evolution of progenitor ring anomalies explosion massive stars Low-metallicity models yes no no yes no Extreme-mass-loss models yes no ? no no Restricted-convection models yes no no yes ? Helium-enrichment model yes no ? yes no Rapid-rotation model yes ? ? yes ? Companion models no ? no yes Accretion models yes yes ? yes yes Merger models yes yes ? yes yes

5. DISCUSSION 6. FUTURE WORK AND TESTS In Table 1, we summarize the results of applying the 6.1 The Presence of a Companion tests developed in Section 2 to the various single and binary The most straightforward confirmation of any model models discussed in the previous two sections. On face would be the detection of a companion star, as predicted by value, all single-star models (except for the rapid-rotation several of the binary models. Indeed, the fact that model for which, however, many important details have Sk — 69o202 did not reappear conclusively rules out any not yet been worked out), and the binary companion mod- model in which Sk — 69o202 was not the progenitor. els fail at least two of the five tests. Does this mean that In the accretion models, the most probable companion these models are conclusively ruled out? If not, how can is a neutron star, since it is likely that the star which trans- one avoid this conclusion? This, of course, depends on how ferred mass to the progenitor has already completed its much weight is given to the individual tests. It would seem own evolution (a normal star is also possible but unlikely; that both the extreme-mass-loss model and the binary com- see Podsiadlowski and Joss 1989). Since, in the supernova panion models are conclusively ruled out, since they can- explosion more than half the total mass was ejected, the not produce the features of the observed supernova event system is likely to become unbound (unless the supernova and since, in the latter model, the central prediction, the explosion was very asymmetric; then there is a small but reappearance of Sk — 69o202, has not materialized. On the non-negligible probability that the system remains bound; see, e.g.. Bailes 1989). This unbound old neutron star (or other hand, the strongest evidence against the other models black hole) will move with a velocity of the order of its arises from the observations of the ring around the super- -1 former orbital velocity, i;orb~ 50-100 kms , relative to nova and the chemical anomalies in the envelope of the the supernova remnant. It should become detectable either progenitor. as a radio pulsar with a characteristic age of ~ 106 yr or as The existence of a ring of material around the supernova an X-ray source powered by accretion of material from the is beyond doubt. It also appears implausible that a single supernova remnant. It will of course be important not to star could produce the extreme deviation from spherical confuse this "old" neutron star (or black hole) with any symmetry in its wind that could generate a ringlike struc- remnant the present supernova may have left, which is also ture (e.g., in an interacting-wind scenario). However, this likely to be a neutron star (or black hole). argument has not yet been quantified sufficiently and, until Confirmation of a merger model may be less straight- it has, other explanations cannot be completely ruled out forward, since, at the time of the supernova explosion, the (see Wang and Mazzali 1992 and Sec. 6.4). progenitor would probably have been a single blue su- Can the chemical anomalies be argued away? Are, e.g., pergiant without a surviving companion to provide a sim- non-LTE effects or other deficiencies in the atmosphere ple test of the model. However, if the merger occurred near calculations responsible for the apparent barium anomaly? the end of the progenitor's evolution (e.g., as a result of This is unlikely, as, since the first estimates by Williams late case C mass transfer), it is possible that the merger (1987), the atmosphere calculations (see, e.g., Höflich would not be completed by the time of the supernova ex- 1988) have continually improved and produced a very plosion, since the remaining evolutionary time scale for a massive star is of the same order as the spiral-in time. In consistent picture. In particular, they showed that only the this case, the progenitor could still have an immersed com- 5-process elements and helium are enriched (the latter be- panion at the time of the supernova explosion (Hillebrandt ing less certain), while other elements appear in scaled- and Meyer 1989; Podsiadlowski et al. 1990). solar abundances. An alternative is that these anomalies are primordial, i.e., reflect the star's initial abundances. 6.2 The Metallicity of the Progenitor This possibility can be tested, since the progenitor appears to be the member of an association of massive stars and The metallicity of the progenitor plays an essential role since these neighboring stars (in particular stars 2 and 3 of in many single-star models for SN 1987A. The typically the Sk — 69o202 system) should therefore show the same required metallicity is less than —1/3 ZQ. anomalies. (Note that the analysis by Mazzali et al. 1992, Unfortunately, the metallicity of massive stars in the which suggests that the barium anomaly is confined to the LMC is not well determined, and different metallicity indicators tend to yield different results. On the average, the innermost 89% of the star, may already rule out a primor- LMC is believed to be underabundant in heavy elements by dial origin for this anomaly. ) a factor of a few relative to the solar neighborhood. How- It is therefore most likely that the progenitor was a ever, the majority of stars in the LMC are old population- member of a binary and that the anomalies of this super- II stars, whereas star formation has been an ongoing, con- nova can be best explained by an accretion or a merger tinuous process in the LMC. Models for the chemical model. On the other hand, it is still too early to rigorously evolution of the LMC (see, e.g., Searle 1984) indicate that rule out some of the single-star models. In the following stars which formed in the recent burst of star formation in section, we shall therefore discuss how future work and the LMC, as Sk — 69o202, have metallicities only slightly future observations will be able to further constrain the less than solar. This appears to be consistent with most existing models and may lead ultimately to a conclusive metallicity determinations ./hich are based on stellar indi- resolution of this question. cators, whether they use cepheids (e.g., Z=0.02±0.0054;

Hams 1983), fits to the HR diagrams of the youngest open 6.3 Presupernova Mixing clusters (e.g., Z=0.018±0.003; Becker et al. 1984), or The chemical anomalies in the progenitor (see Sec. 2.3) individual abundances of typical metallicity tracers (like strongly suggest that substantial, nonstandard mixing has Ca) in supergiant atmospheres (e.g., Z=0.013±0.003; occurred in the progenitor (although not necessarily Smith 1980). On the other hand, abundance determina- straightforward one-phase mixing; Weiss 1991). At tions based on Η π regions (e.g., Dufour et al. 1982) yield present, no model is capable of explaining all three anom- significantly lower abundances for a number of alies (the nitrogen, helium, and barium anomalies) simul- intermediate-mass elements like C, O, and Ne (for a dis- taneously. Saio et al. (1988) showed that by artificially cussion of potential systematic problems with the determi- mixing most of the envelope after helium core burning, nation of abundances in H II regions, in particular their they could account for the nitrogen and the helium anom- absolute calibration, see Harris 1983). More recently, Rus- aly. In similar experiments, we have noticed that, in order sell and Bessell (1989) and Russell and Dopita (1990) to produce a surface helium abundance Γ~0.4 (as sug- have thoroughly reexamined the heavy-element abun- gested by the observations), one has to mix the envelope dances of gas and young stars in the LMC and compared completely down to a layer where the reaction 13C(a,« ) 160, one of the major neutron sources in stars, has the various abundance determination methods. They con- 13 cluded that the average metallicity of young stars in the completely destroyed C and released some neutrons for s LMC is —0.5 Zq and that there is no measurable zero- processing. Thus, some s-processed material will automatically be dredged up together with the helium. This, by point error between the abundance scales based on stellar itself, cannot explain the barium anomaly, since the indicators and those based on Η π regions. One major dis- amount of 13C available after CNO burning is too small to crepancy is the low carbon abundance inferred from the produce a noticeable enhancement in 5-process elements. Η π analysis. Russell and Dopita (1990) suggest that this However, if large-scale mixing of the envelope continues, could be explained by a larger efficiency of grain formation this could provide an ideal environment for efficient 5 pro- in Η π regions of the LMC than in Η II regions of the cessing (continued mixing could be expected in a scenario Galaxy. A metallicity as large as 0.5 Z^ appears to be where the mixing is caused by near-critical rotation after a problematic for all models which rely heavily on a low mass-transfer phase or after the merging of two stars). In metallicity for the progenitor. Of course, one may still ap- such a scenario, the layer in which the temperature is high peal to a range of metallicities for young stars in the LMC enough for 13C burning and concomitant s processing is 13 [Russell and Bessell ( 1989) find some evidence for this]. In continually resupplied with C by downward mixing of addition, different elements show different underabun- material which underwent CNO processing in a cooler dances relative to solar abundances. This raises the ques- layer further out, while the products of s processing are mixed upwards. In effect, the layers in which hydrogen tion of which elements contribute most significantly to the 13 total radiative opacity. It seems that the metallicity indi- burning (CNO burning) and partial helium burning ( C burning) take place are linked because of continued mix- cators that are based on cepheid variables or open-cluster ing, just as if they overlapped spatially. The amount of fits most directly measure an effective metallicity (at least 5-processed material produced in this case is entirely deter- as far as stellar opacities are concerned). Furthermore, mined by the rate at which CNO-processed material is preliminary results from various opacity projects suggest injected into the 13C-burning layer. Such a scheme could that the present opacity tables significantly underestimate also explain the otherwise very puzzling result by Mazzali the true opacity. Since the metallicity used in stellar evo- et al. (1992) that the material which shows the barium lution calculations is, to a large degree, a measure of the anomaly (unlike the nitrogen and helium anomaly) is con- opacity, this may suggest that the metallicity required in a fined to 89% of the enclosed mass of the star, since the successful single-star model has to be even lower than in- time scale for rotational mixing is longer in the outermost ferred from present calculations which use old opacity ta- layer and efficient, continued mixing does not have to per- bles. vade the whole envelope. The major attraction of this sce- Fortunately, it should be possible to resolve this issue nario is that it could potentially explain the three chemical with future observations and calculations. High-resolution anomalies of the progenitor by a single process. Detailed observations of stars 2 and 3 in the Sk — 69o202 system nucleosynthesis calculations should be able to confirm combined with modern atmosphere calculations should al- whether this mechanism works quantitatively and whether low not only a determination of the overall metallicity of it can produce the observed abundances. stars in the association to which the progenitor of SN 1987A belonged, but also of individual element abun- 6.4 The Presupernoya Nebula dances. These can then be used to generate custom-made The supernova is surrounded by a complex but clearly opacity tables, i.e., tables which use an abundance mixture structured nebula, which is now glowing in recombination most appropriate for the progenitor. In this way, at least radiation from the supernova's UV burst (Wampler et al. one of the uncertain parameters in a number of the single- 1990). The nebula consists of three main components. The star models can be removed. This will either strengthen or supernova ring (see Sec. 2.2) forms the inner part of the further weaken these models. nebula; two elliptical loops protrude from the northern and

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SN 1987A 111 the southern section of the ring, and the glowing portion of transverse pressure gradients). Possible sources for the the nebula is bounded by a structure that has the appear- asymmetry are rapid rotation and magnetic fields. If rota- ance of Napoleon's hat. The three structures are extremely tion is the source of the asymmetry and if the enhancement nonspherical, but highly axisymmetric and share a com- factor is as large as 5, a binary companion is almost cer- mon axis of symmetry. Since the whole nebula has been tainly required as a source of angular momentum (see the generated by the star or the stars in the neighborhood of discussion in Sec. 2.2). the supernova, it provides a direct imprint of the progenitor's late evolution. A successful model for the progenitor 6.5 The Asymmetries of the Ejecta has to be able to account for all the observed structures. In fact, the whole nebula may eventually provide one of the Speckle interferometry (Papaliolios et al. 1989) and po- most stringent tests for models of the progenitor. larization measurements (Cropper et al. 1988; Méndez A number of geometrical and dynamical models for the et al. 1988) suggest that the expansion of the supernova nebula have been proposed. Podsiadlowski (1991) sug- ejecta is highly asymmetric. A possible explanation for this gested that the ring constitutes a swept-up excretion disk, asymmetry is an asymmetric envelope structure of the pro- left over from an accretion or merger phase (see Sees. 4.2 genitor (Chevalier and Soker 1989; Höflich 1991; JeíFery and 4.3). Luo and McCray (1991), Wang and Mazzali 1991). Flattening of the progenitor's envelope caused by (1992), and Lundqvist (1992) adopted an asymmetric, rapid rotation could produce such an asymmetry (note, however, that the symmetry axis inferred from these ob- interacting wind model for the inner nebula [also see Soker servations does not coincide with the symmetry axis of the and Livio (1989), who anticipated this possibility for SN surrounding bipolar nebula!). As discussed earlier. Chev- 1987A]. In this model, the ring forms the waist of a bipolar alier and Soker ( 1989) showed that a single star which was (hour-glass) nebula and the elliptical loops might be the rapidly rotating on the main sequence would be slowly limb-brightened surfaces of the hour-glass bubble (Wang rotating in a later supergiant phase and could not be sig- and Mazzali 1992). Podsiadlowski et al. (1991) modeled nificantly flattened at the time of the explosion, and con- the outer "Napoleon's Hat" nebula as an elliptical double cluded that a binary companion is required to provide a cone. In this model, the elliptical loops form the truncated source of angular momentum. ends of the double cone (swept up by the energetic wind in An alternative explanation for the asymmetric expan- the progenitor's final blue-supergiant phase). sion of the ejecta is an asymmetric supernova explosion All of these models can be tested observationally in a (see Chevalier and Soker 1989; Yamada and Sato 1990). variety of ways. High-resolution images with the HST and An important question in such a model is whether these the NTT can resolve the nebula directly. Light-echo stud- asymmetries are damped out, when the supernova shock ies (see, e.g., Crotts and Kunkel 1991) can be used to map passes through the progenitor's envelope, which will tend out the structure of the whole nebula in time. Spectro- to make the shock more spherical (Chevalier and Soker scopic studies using IUE observations (see, e.g., Fransson 1989). According to the two-dimensional hydrodynamical et al. 1989; Lundqvist and Fransson 1991; Panagia et al. calculations by Yamada and Sato (1990), the asymmetry 1991; Dwek and Feiten 1992) may allow a reconstruction of a shock, generated in the core, is only weakly damped by of the density structure of the inner nebula (mainly of the a large, spherically symmetric, hydrogen-rich envelope. ring). All of these methods can be complemented by kine- They suggest that this asymmetry could be caused by a matic studies of the nebula (again mainly of the inner rotationally distorted core. This idea appears to be consis- nebula; see, e.g., Crotts and Heathcote 1991; Meikle et al. tent with scenarios in which rotation-induced meridional 1991). Eventually, when the supernova ejecta start to in- circulation is responsible for the dredge-up of helium- teract with the nebula (after the year 2000), this interac- processed material (we note, however, that these scenarios tion will allow a direct probing of the structure of the also predict a rotationally flattened envelope). nebula with radio waves to X-rays and will continue to provide useful data for the next few centuries. 6.6 Supernoya Surveys On the theoretical side, two main issues arise. One, what is the asymmetry required in the mass outflow from the Supernova surveys (see, for example, Perlmutter et al. progenitor that can produce the observed structures and, 1988) may provide additional, although less stringent tests two, what is the origin of this asymmetry. Most of the of progenitor models. For example, accretion and merger interacting wind models suggest that the mass-loss rate in models predict that of order 5% of all massive stars should the equatorial plane of the progenitor is enhanced by a explode as blue supergiants and that their occurrence rate factor ^ 5 relative to the mass loss in the polar direction is insensitive to the metallicity of the parent population (Soker and Livio 1989; Luo and McCray 1991; Lundqvist (note, however, that it is more difficult to detect these 1992). However, Wang and Mazzali (1992) claim that the supernovae than ordinary Type II supernovae). On the enhancement factor is only 1.2. This result is difficult to other hand, most single-star models predict that superno- understand. Since it is also at variance with the more de- vae of the SN 1987A variety should occur exclusively in tailed and hence more accurate hydrodynamical calcula- low-metallicity populations. SN 1990H closely resembled tions (Soker and Livio 1989; Lundqvist 1992), we suspect SN 1987A photometrically, as well as spectroscopically that this result is an artifact of their approximations (the (Filippenko 1992). SN 1990H may therefore be the first bubble approximation and, in particular, their neglect of cousin of SN 1987A and may also have had a blue-

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 728 PODSIADLOWSKI supergiant progenitor. A preliminary analysis of the metal- Bond, Η. R, and Livio, M. 1990, ApJ, 355, 568 licity of a Η Π region near SN 1990H (Filippenko 1991) Brunish, W. M., and Truran, J. W. 1982, ApJS, 49, 447 suggests that the metallicity of the progenitor may have Burrows, A. 1987, Phys. Today, 40, No. 9, 28 been subsolar (1/3-1/2 Zq). This would be consistent Chevalier, R. Α., and Soker, N. 1989, ApJ, 341, 867 with the metallicity required in many single-star models. Ghiosi, G, and Maeder, A. 1986, ARA&A, 24, 329 Grawford, J. A. 1955, ApJ, 121, 71 Cropper, M. et al. 1988, MNRAS, 231, 695 Crotts, A. P. S., and Kunkel, W. E. 1991, ApJ, 366, L73 7. CONCLUSIONS Crotts, A. P. S., and Heathcote, S. R. 1991; Nature, 350, 683 Dallaporta, N. 1972, in Colloquium on Supergiant Stars, ed. M. The main conclusion of this review is that, at present, Hack (Osservatorio Astronómico, Trieste), p. 250 the most likely explanation for the many unusual proper- Danziger, 1. J. et al. 1988, in Supernova 1987A in the Large ties of SN 1987A is that its progenitor had a binary com- Magellanic Cloud, ed. M. C. Kafatos and A. G. Michalitsi- panion, either at the time of the supernova explosion or at anos (Cambridge, Cambridge University Press), p. 37 least in the not-too-distant past. Both accretion and merger De Grève, J.-P. 1992, in IAU Symp., No. 151, Evolutionary Pro- models for the progenitor are consistent with the main cesses in Interacting Binary Stars, ed. Y. Kondo, R. F. Sistero, constraints imposed by observations of the supernova and and R. S. Polidan (Dordrecht, Kluwer), p. 41 its progenitor and the theory of massive stars, while single- De Grève, J.-P. 1992, private communication star models (with the exception of rapid-rotation models, De Jager, G., Nieuwenhiujzen, H., and van der Hucht, Κ. 1988, which also may be promising, but for which we cannot yet A&AS, 72, 259 draw firm conclusions) can account only for some, but not De Loore, G., and Vanbeveren, D. 1992, A&A, in press for all of the features of SN 1987A. This conclusion is not Dufour, R. J., Shields, G. Α., and Talbot, R. J., Jr. 1982, ApJ, yet incontrovertible, since many details have not yet been 252, 461 worked out in sufficient detail, and some revisions of these Dupree, A. K. 1986, ARA&A, 24, 377 arguments are possible, if not likely. We showed how fu- Duquennoy, Α., and Mayor, M. 1991, A&A, 248, 485 ture observations and calculations should be able to clarify Dwek, E., and Feiten, J. E. 1992, ApJ, 387, 551 Fabian, A. C., Rees, M. J., van den Heuvel, E. P. 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