eso8711 — Fréttatilkynning
Astronomers and Physicists Meet at ESO at the First Full-Scale International Conference on Supernova 1987A
8. júlí 1987
The first full-scale, international meeting about the bright Supernova 1987A in the Large Magellanic Cloud (LMC) was held at the European Southern Observatory in Garching near Munich on July 6 - 8, 1987. ESO was a natural meeting place in view of the many different observational studies of SN 1987A which have been carried out at the ESO La Silla observatory.
After three days of detailed discussions and long working sessions, the two hundred participants from all over the world concluded that much new and exciting knowledge has been gained from 4 1/2 months of intensive observational and theoretical studies of this unique object. Nobody doubted, however, that a major effort is still required to solve the many outstanding questions and in addition to providing an up-to-date review of current supernova research, the meeting also served to initiate future collaboration in the most urgent problem areas.
It was a historical occasion: for the first time satellite-, rocket-, balloon- and ground-based astronomical observations of an object outside the solar system were supplemented by the measurement of antineutrinos in several huge particle detectors, deep underground. Astronomers and physicists met on common ground to the mutual benefit, and it was obvious that both parties were pleased to learn from each other.
Summarizing the main results of this conference this afternoon, Professor Sidney van den Bergh of the Dominion Astrophysical Observatory, Victoria, Canada, was impressed by the width and accuracy of the presented data, and he added: “It has been tremendously exciting for all of us and we have experienced a textbook example of how the observational and organizational problems, associated with such a sudden event, can be overcome when all involved scientists join their forces within an open and extensive, international collaboration. We can be proud of what was achieved around SN 1987A and the efficient way in which it was done should serve as an enlightening example to people in other fields of human endeavour."
What have we learned so far from this supernova?
In general, there are four fields of astrophysics which are directly related to the interpretation of a supernova explosion:
- stellar evolution theories which explain how a star reaches a stage of instability,
- collapse physics which deals with the implosion of the inner parts of the star at which moment great quantities of neutrinos are emitted, and the creation of an extremely compact object (neutron star or black hole) at the centre,
- explosion physics that trace the collision of the rapidly expanding, inner layers with the outer layers and the ejection of a shell of material into surrounding space, and
- nucleosynthesis, the creation of heavy elements during the brief moment of extreme physical conditions which do not exist anywhere else in the Universe.
In all of these fields, some problems have now been solved, thanks to the availability of accurate, observational data from SN 1987A and one of the speakers commented that the theoretical interpretations have "taken a giant step forward". Nevertheless, there are still many unexplained features, which it is hoped to solve by continued interaction between observers and theoreticians.
There was general agreement that it was the star Sanduleak -69 202 which exploded. Stellar model calculations have shown that this star with a mass of 15 - 20 times that of the Sun may previously have developed into a red supergiant star, but also that shortly before the explosion, it would have lost a significant amount of mass and became a blue supergiant. The relatively low metallicity in the LMC may have contributed to this. It was reported that there is a relative overabundance of the elements helium and nitrogen in similar blue stars in the LMC. This points towards an advanced evolutionary state where carbon burning is taking place.
Supernova 1987A can be classified as of Type II, because its electromagnetic spectrum now (130 days after the explosion) rather closely ressembles a typical Type II spectrum; because of its very blue colour, immediately after the explosion and because of the overall shape of its lightcurve. However, it is not a typical Type II, since the brightness increased much slower than a normal Type II; since the colour very rapidly changed from blue to deep red; since the initial expansion velocity of the shell was extremely high, more than 30.000 km/s in some UV spectral lines, and also because the light maximum in mid-May is at least 1.5 magnitude fainter than what a normal Type II supernova would have reached.
From statistics in other galaxies, it is estimated that the supernova-rate in the LMC is about 1 per 500 years. We have therefore been very lucky to observe SN 1987A.
From polarimetric measurements and also recent IUE (International Ultraviolet Explorer, a joint NASA-ESA satellite) spectral data in the ultraviolet, it appears that the expanding envelope is now breaking into smaller fragments. This raises the hope that it shall soon become possible to learn what was left over at the centre of the explosion. From a comparison with the Crab Nebula and its associated pulsar (the remnants of a supernova explosion in the year 1054), as well as with other pulsars, it is estimated that a possible pulsar in SN 1987A may have a rotation period of about 10 milliseconds, but the apparent magnitude may not be brighter than 17. It is therefore necessary to wait until SN 1987A fades significantly, before optical observations of the pulsar may become feasible. It is quite likely that such a pulsar will manifest itself earlier in other wavelengths, like X-rays or radio.
High spatial resolution optical and infrared observations have given the first direct images of the expanding envelope and also a "mystery spot", an unidentified point-like object, at a distance of about 20 light-days from the supernova. It appears to be moving away from the supernova, but the nature of this object is still unknown. It must be related to the supernova since it is only 10 times fainter, i.e. at magnitude 5 or 6. If it had been there before the explosion, it would have been 100 times brighter than any other object in the LMC and would therefore have been discovered long ago.
An interesting prediction made at the meeting was that a very strong electro-magnetic pulse, released at the moment of the explosion, would have deposited enough energy (1 erg cm-2) in the so-called E-layer in the Earth's atmosphere that its effects should be observable. Atmospheric physicists have therefore begun to study the data recorded on February 23, 1987.
The undisputed highlight of the meeting was the presentation of detections of antineutrinos from SN 1987A which were made with particle detectors, located in the Mount Blanc tunnel between France and Italy, and also in Japan, USA and USSR. Never before has it been possible to observe directly the core collapse during a supernova explosion and it may be a long time before another supernova explodes sufficiently close to us to permit similar measurements.
It was not yet possible to decide definitively which of these events refer to the core collapse, but the exact coincidence in time between Japanese and US detections at 7:36 UT is taken as evidence in favour of these. The USSR detection is some 20 seconds later, but this might be due to a timing problem. But then, what does the Mont Blanc detection at 02:52 UT signify? Is it possible that there are other, dramatic events which produce neutrinos before the core collapse? Or does, after all, the Mount Blanc event correspond to the initial core collapse and the others to the transformation of a short-lived neutron star into a black hole?
Upper limits for the mass of the neutrino of the order of 15 to 20 eV were reported from an analysis of the arrival times of the individual particles; a zero mass can not be excluded.
Although the spectrum of SN 1987A is complicated, new computations of synthetic supernova spectra show reasonable agreement with what is actually observed. It has been possible to identify many atomic species, some of them highly ionized. Following an intensive exchange of experience, theoretical groups in several places announced that they will soon improve their computer programmes and that a fuller understanding of the violent events in the expanding envelope is within reach.
Up to 40 individual interstellar and intergalactic clouds have now been observed along the line of sight to the supernova and according to one group of astronomers, these unique observations indicate the existence of a "bridge of matter" between the LMC and the Milky Way Galaxy. It is also possible that some of the narrow absorption lines seen in the SN spectrum belong to the expanding shell of matter or even to the matter that may have been expelled during a phase of rapid mass loss, soon before the supernova explosion.
Scientists with access to X-ray and γ-ray detectors on satellites and balloons are on stand-by, waiting for the moment when the envelope becomes transparent. So far, attempts to observe the supernova in these wavelength regions have failed. Radio-observations were made during the first few days after the explosion, but then the signal faded. A new, possible radio-detection in Brazil in late June has not yet been confirmed, but Very-Long-Baseline-Interferometry (VLBI) observations will start in Australia as soon as the supernova again begins to radiate strongly at radio wavelengths. According to theory, this may happen any moment. These radio measurements, when compared with optical observations, will allow an accurate determination of the distance to the supernova. This distance, in turn, will be of great importance in correctly estimating the overall cosmical distance scale.
The ESO Conference has undoubtedly brought us a long way towards the unraveling of the secrets of supernova explosions, but as one of the participants said: “We leave this meeting in a great air of inspiration, but I also know that there is a lot of hard work to be done during the next many years!"
SN 1987A is still visible with the unaided eye. The magnitude is now 4.5.
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