Supernova 1987A 10 years on: a review
by Dr Peter Meikle, Imperial College
[Dr Peter Meikle of Imperial College, London, opens the National Astronomy Meeting session on Supernovae and Pulsars on Thursday 10th April at the University of Southampton, with a look back at what has happened to Supernova 1987A since it went off on 23rd February 1987, and what astronomers have learned about it from 10 years of observations.]
Supernova 1987A is, by far, the most extensively studied supernova explosion in history. It has provided an astonishing wealth of discoveries about the physical processes which take place before, during and after a core-collapse-induced supernova.
We now believe that SN 1987A arose from the merger of a massive binary star system. The most dramatic evidence for this is the bright circumstellar ring, beautifully imaged by the Hubble Space Telescope (HST). Material that flowed out during the red supergiant phase was subsequently compressed by the blue supergiant wind to form the ring. This, together with the fainter outer loops, was then "lit-up" (ionized) by the ultraviolet flash as the shock of the explosion reached the surface of the star. While the progenitor of SN 1987A may have been unusual, it was not unique. A rather similar system has been recently discovered in our own Galaxy.
That there was a core-collapse in the star was firmly, and uniquely established by the detection of 19 neutrinos on 23 February 1987. It seems likely that the collapse resulted initially in a neutron star at the centre. However, there is still no convincing evidence for a pulsar at the centre. Indeed, the possibility that the core subsequently collapsed to form a black hole is still being considered. Understanding is growing as to how the core-collapse ultimately leads to the powerful ejection of most of the star. The key lies in the neutrinos, which emerge from the neutron star following the core-collapse. These particles can deposit a large amount of energy in the material just above the neutron star, producing instabilities and coupling the released gravitational energy to the star's outer layers. Observational evidence supporting this scenario has come from infrared spectra of SN1987A. However, it is still not understood what determines the fraction of neutrino energy that is actually deposited.
As the shock travelled through the star, explosive nucleosynthesis took place, creating new chemical elements. In particular, about 0.1 solar masses of nickel-56, which is radioactive, was formed. Several pieces of new evidence for this came from SN1987A. Of particular note was the demonstration, using infrared spectroscopy, of the presence of the decay product cobalt-56 and its decay, in turn, to iron-56. It is the decay of these and other radioactive elements which are responsible for maintaining the emission from the supernova. It is now generally accepted that the explosion and nucleosynthesis was not nicely spherically symmetric. What appears to have happened is that the nickel-56 was created in, perhaps 100, dense, localised pockets, or possibly even finger-like structures pointing radially outward. As the 56Ni decayed first to cobalt-56 and then to iron-56, these regions expanded relative to the rest of the ejecta. Extensive large-scale mixing occurred. Again, much of this is based on spectroscopic studies, especially at near- and far-IR wavelengths. (Asymmetric explosions could easily account for the high velocities many pulsars are observed to have.)
As the ejecta expanded, we saw not just atoms, but also the formation of substantial quantities of molecules and dust. Indeed, carbon monoxide played an important role in cooling large parts of the ejecta. The dust formed in dense, opaque fingers, which seem to be still opaque even after 10 years. As the supernova aged beyond about two years, two important nebular effects took place. These are known as the 'infrared-catastrophe' and the 'freeze-out', and were seen for the first time in SN1987A. The recognition of these processes is vital to our understanding of the way the light curves and spectra of supernovae develop long after the initial explosion.
At the moment, the emission from the ejecta of SN1987A is considerably fainter than from the circumstellar material which dominates the appearance of the SN 1987A system. Nevertheless, the HST has recently shown clearly that the ejecta have "split" into two blobs moving in opposite directions, confirming both the early indications and also the more indirect evidence for an asymmetric explosion. A particularly intriguing fact is that the line joining the two blobs lies exactly along the line to the unexplained and much-disputed Mystery Spot seen briefly a couple of months after the explosion in 1987.
As early as 1990, radio observations began to show the impact of the ejecta with the circumstellar material. The radio emission is steadily brightening, and the effects of the interaction have now also been detected in X-rays and the far-infrared. By about 2006, the ejecta will collide with the ring, with dramatic results. We should see the luminosity increase by a factor of 100 to 1000. This event will provide a beautiful "experiment" for a real-time study of shock interaction and particle acceleration.
Dr Peter Meikle, Dept. of Physics, Imperial College, London SW7 2AZ Phone (0)171-594-7551;
Dr Jacqueline Mitton, RAS Public Relations Officer