Supernova 1987A was discovered on February 24, 1987 in the nearby, irregular galaxy, the Large Magellanic Cloud, which is located in the southern sky. SN 1987A is the first supernova to reach naked eye visibility after the one studied by Kepler in 1604 AD and is undoubtedly the supernova event best studied ever by the astronomers. Actually, despite the fact that SN 1987A has been more than hundred times fainter than its illustrious predecessors in the last millennium, it has been observed in such a detail and with such an accuracy that we can define this event as a first under many aspects (e.g. neutrino flux, identification of its progenitor, gamma ray flux) and in any case as the best of all. Reviews of both early and more recent observations and their implications can be found in Arnett et al. (1989) and Gilmozzi and Panagia (1999), respectively.
SN 1987A early evolution has been highly unusual and completely at variance with the wisest expectations. It brightened much faster than any other known supernova: in about one day it jumped from 12th up to 5th magnitude at optical wavelengths, corresponding to an increase of about a factor of thousand in luminosity. However, equally soon its rise leveled off and took a much slower pace indicating that this supernova would have never reached those peaks in luminosity as the astronomers were expecting. Similarly, in the ultraviolet, the flux initially was very high, even higher than in the optical. But since the very first observation, made with the International Ultraviolet Explorer (IUE in short) satellite less than fourteen hours after the discovery, the ultraviolet flux declined very quickly, by almost a factor of ten per day for several days. It looked as if it was going to be a quite disappointing event and, for sure, quite peculiar, thus not suited to provide any useful information about the other more common types of supernova explosions. But, fortunately, this proved not to be true and soon it became apparent that SN 1987A is the most valuable mean to test our ideas and theories about the explosion of supernovae.
And even particle emission was directly measured from Earth: on February 23, around 7:36 Greenwich time, the neutrino telescope ("Kamiokande II", a big cylindrical "tub" of water, 16 m in diameter and 17 m in height, containing about 3300 m3 of water, located in the Kamioka mine in Japan, about 1000 m underground) recorded the arrival of 9 neutrinos within an interval of 2 seconds and 3 more 9 to 13 seconds after the first one. Simultaneously, the same event was revealed by the IMB detector (located in the Morton-Thiokol salt mine near Faiport, Ohio) and by the "Baksan" neutrino telescope (located in the North Caucasus Mountains, under Mount Andyrchi) which recorded 8 and 5 neutrinos, respectively, within few seconds from each other. This makes a total of 25 neutrinos from an explosion that allegedly produces 10 billions of billions of billions of billions of billions of billions of them! But a little more than two dozens neutrinos was more than enough to verify and confirm the theoretical predictions made for the core collapse of a massive star (e.g., Arnett et al. 1989 and references therein). This process was believed to be the cause of the explosion of massive stars at the end of their lives, and SN 1987A provided the experimental proof that the theoretical model was sound and correct, promoting it from a nice theory to the description of the truth.
From both the presence of hydrogen in the ejected matter and the conspicuous flux of neutrinos, it was clear that the star which had exploded was quite massive, about twenty times more than our Sun. And all of the disappointing peculiarities were due to the fact that just before the explosion the supernova progenitor was a blue supergiant star instead of being a red supergiant as common wisdom was predicting. There is no doubt about this explanation because SN 1987A is exactly at the same position as that of a well known blue supergiant, Sk -69° 202. And the IUE indicated that such a star was not shining any more after the explosion: the blue supergiant had gone BANG (Gilmozzi et al. 1987, Kirshner et al. 1987).
On the other hand, common wisdom cannot be wrong and it was not quite wrong, after all. At later times, in late May 1987, the IUE revealed the presence of emission lines of nitrogen, oxygen, carbon and helium in the ultraviolet spectrum. They kept increasing in intensity with time and proved to be quite narrow, indicating that the emitting matter was moving at much lower speeds (less than a factor of hundred slower) than the supernova ejecta. The chemical abundances and the slow motion were clear sign that that was matter ejected by a red supergiant in the form of a gentle wind. But there was no such a star in sight just before the explosion. Therefore, the same star that exploded, had also been a red supergiant, less than hundred thousand years before the explosion itself: a short time in the history of the star but quite enough to make all the difference.
3.3. Explosive Nucleosynthesis
The optical flux reached a maximum around mid-May, 1987, and declined at a quick pace until the end of June, 1987, when rather abruptly it slowed down, setting at a much more gentle decline of about 1% a day (Pun et al. 1995). Such a decay has been followed since then quite regularly: a perfect constant decay with a characteristic time of 114 days, just the same as that of the radioactive isotope of cobalt, 56Co, while transforming into iron. This is the best evidence for the occurrence of nucleosynthesis during the very explosion: 56Co is in fact the result of 56Ni and this latter can be formed at the high temperatures which occur after the core collapse of a massive star. So now, not only are we sure that such a process is operating in a supernova explosion, just as theorists predicted, but we can also determine the amount of nickel produced in the explosion, slightly less than 8/100 of a solar mass or, approximately, 1% of the mass of the stellar core before the explosion. And the hard X-ray emission detected since July 1987 and the subsequent detection of gamma-ray emission confirm the reality of this process and provide more details about its exact occurrence (e.g., Arnett et al. 1989 and references therein).
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Figure 2. The field of view centered on SN 1987A, as viewed by the HST-WFC2, was observed with HST-STIS and the resulting ultraviolet spectrum is shown in the bottom panels. |
The Hubble Space Telescope was not in operation when the supernova exploded, but it did not miss its opportunity in due time and its first images, taken with the ESA-FOC in August 23 and 24, 1990, revealed the inner circumstellar ring in all its "glory" and detail (cf. Jakobsen et al. 1991), showing that, despite spherical aberration, HST was not a complete disaster, after all.
Since those early times, Hubble has kept an attentive eye on SN 1987A, obtaining both imaging and spectrographic observations (e.g., Fig. 2) at least once a year, accumulating valuable data and revealing quite a number of interesting results (see Gilmozzi & Panagia 1999), such as:
- The sequence of images obtained over more than 8 years has allowed us to measure the expansion of the supernova material directly: this the first time it has ever been possible and has permitted to identify the correct models to understand the explosion phenomenon (Pun et al. 1999, in preparation).
- The origin and the nature of the beautiful circumstellar rings are still partly a mistery. They have been measured to expand rather slowly, about 10-20 km s-1 , i.e. 100-2000 times slower than the SN ejecta, and to be highly N rich: both these aspects indicate that the rings were expelled from the progenitor star when it was a red supergiant, about 20,000 years before the explosion (Panagia et al. 1996). However, one would have expected such a star to eject material in a more regular fashion, just pushing away material gently in all directions rather than puffing rings like a pipe smoker. Another puzzle is that the star was observed to be a "blue" supergiant in the years before the explosion, and not a red supergiant anymore. This forces one to admit that the star had a rather fast evolution, which was not predicted by "standard" stellar evolution theory, and still is hard to understand fully.
- The highest velocity material expelled in SN 1987A explosion has been detected for the first time by the Space Telescope Imaging Spectrograph (STIS) (e.g., Sonneborn et al. 1998). The spectrograph has found the first direct evidence for material from SN 1987A colliding with its inner circumstellar ring. The fastest debris, moving at 15,000 km s-1 are now colliding with the slower moving gas of the inner circumstellar ring (Fig. 2).
In less than a decade the full force of the supernova fast material will hit the inner ring, heating and exciting its gas and producing a new series of cosmic fireworks that will offer a spectacular view for several years. This is going the "beginning of the end" because in about another century most, if not all, the material in the rings will be swept away and disappear, loosing their identities and merging into the interstellar medium of the Large Magellanic Cloud. This is not a complete loss, however, because by studying this destructive process, we will be able to probe the ring material with a detail and an accuracy which are not possible with current observations.