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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] 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.
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