SN 1987A, First Star of JWST observations

SN 1987A is a supernova that exploded in the Large Magellanic Cloud, a dwarf galaxy near the Milky Way about 164,500 light years (1.6 billion billion kilometers) away, making it the closest supernova observed since the so-called Kepler supernova in 1604 (which, for weather reasons, was only able to observe it well after its appearance – it was raining in Prague!), which took place in our Milky Way, shortly after the Tycho supernova Brahe, who was on site at the abbey of Herrevad, which allowed him to observe it in November 1572. The purpose of this article is not to return to these historical events, which are so fascinating, but to highlight the fact that it took almost 400 years for an earthling to be able to observe such an event.

Indeed, the Large Magellanic Cloud is only visible from the southern hemisphere. For the reader who is interested in it, it should be noted that the name of the Small and Large Magellanic Clouds took these names because contrary to what can be observed in the Northern hemisphere where the pole is marked by a star , nothing like it in the southern hemisphere. At the time when GPS did not exist, navigators used a sextant during the day (you had to see the horizon) but kept their course at night based on the stars. There were two clouds in the sky. We weren’t talking about a galaxy yet, we didn’t even know the concept! Magellan was the first to realize that to know where the south pole was, it was enough to construct an equilateral triangle of which two points would be the center of these clouds and the third the pole. It was remarkably ingenious.

After this diversion, let’s return to the supernova SN 1987A. The first observations of the phenomenon were made just hours after its glow reached Earth, on the night of February 23, 1987 by several amateur and professional astronomers from South America, Australia and New Zealand. Very early on, the first neutrinos were detected, which was a first, and spectacularly confirmed the current theories which predicted the formation of a neutron star. In addition, these detections augured that a new era of astrophysics was about to begin. It is clear that we will have to wait to detect more neutrinos emerging from astrophysical phenomena.

We expected this supernova to become very bright, but we must admit that our hopes were somewhat disappointed. Very quickly theorists realized that this was due to the fact that the collapsing star was a blue giant. We were taught at school that this kind of implosion (and not an explosion) could only come from the end of the life of a red supergiant. First anomaly, quickly explained by the same theorists. But also a first discovery. The evolution from blue super giant to supernova is explained by a loss of mass before its explosion, which can result in a transition from red supergiant to blue supergiant. This theory was confirmed by the presence of three gas rings around SN 1987A.

The second discovery, of great importance, was to observe that dust could have condensed in this extremely violent environment, 400 days after the implosion. The presence of molecules in the debris was revealed very quickly after the event (approximately 100 days later). This was followed, in August 1988, by the discovery of dust condensation thanks to observations carried out in the infrared at ESO (the European Southern Observatory, located in Chile). But later, from observations made with the Herschel infrared satellite in 2010, then confirmed by ESO’s ALMA submillimeter radio telescope in January 2014, the presence of an enormous quantity of cold dust in the debris (0, 25 solar mass!) not only surprised the astronomical community but also revived the debate on the origin of dust in the early Universe. But we still do not know the composition of this cold dust. Had one of the big questions in current astrophysics found an answer? Where did the first dust come from, since we knew that that released by cacochyme stars did not arrive until very late (the evolution of stars takes a certain time). The quantity of dust detected, however, seemed much lower than that which could answer the question: did this dust result from that detected in 1988, or did it have another origin?

The excellent angular resolution and the extreme sensitivity of the JWST instruments, in particular MIRI, make it the only observatory capable of observing the distribution of dust in the circumstellar medium around SN1987A and in the ejecta. On the other hand, the neutron star (or pulsar?) that formed at the time of the supernova implosion has still not been detected. Theoretical models predict that it could be with observations made in thermal infrared. If this is the case, only MIRI could offer us the luxury of this discovery!

The environment already disturbed by the passage of the shock wave is now affected by a reverse shock wave which approaches the external regions of the ejecta. While it is true that the study of SN 1987A generally allowed us to confirm, or even refine, the theory, unknowns remain: for example, what is the origin of the observed circumstellar structures? What can we learn about the interstellar medium before the imploding star has even formed? What is the mechanism responsible for the emission observed in thermal infrared, attributed to the presence of dust? Are those that had condensed in the ejecta shortly after the explosion now destroyed by this reverse shock wave? What remains at the heart of the explosion? Can we detect the neutron star, the pulsar, that resulted from the event? The answers strongly depend on the one we give to a fundamental question which, 30 years after the explosion, still remains highly debated: was the star which gave birth to SN 1987A part of a binary system?