By Alejandro Clocchiatti, MAS Associate Researcher and Professor of the UC’s Institute of Astrophysics
Supernovae are one of the most powerful and striking phenomena of the universe. They correspond to explosive ends of some terminal states of stellar evolution when balance conditions, which had previously allowed stars to evolve as relatively stable entities for a long time, break. Supernovae play significant roles in the Universe evolution. Its explosions energize their mother galaxies’ interstellar environment and provide one of the few mechanisms to create chemical elements’ nuclei heavier and more complex than iron. Just as important as this, they also push outwards these new elements and those that the progenitor star had produced during its stable evolution, freeing them from this gravitational “well” in which they would have stayed buried forever if supernovae did not exist.
From the point of view of observational astronomy, properly interpreted supernovae explosions allow us to measure cosmologically relevant distances, which cover a substantial fraction of the Universe’s dimension. Thanks to them, we were able to finally understand that the Universe is expanding in an accelerating rate.
The observational and theoretical study of stellar evolution shows many stages where the star reaches a dead end. The study that I want to focus on is the one that looks at the fate of very massive stars, very “fat” ones, that start their lives with more than nine or ten times the mass of our Sun. Just like our star, these ones reach their first hydrostatic equilibrium point when the heat and pressure in their nucleus, generated by initial gravitational contraction, ignite the thermonuclear reaction that transmutes hydrogen into helium. Because they are so massive, unlike the Sun, they will consume their hydrogen quite fast (in millions, instead of billions of years.) Once its core is pure helium, the star begins to descend steps in the equilibrium point stairway, in which it shrinks, heats up and finds a new support point in the next possible exothermic nuclear reaction. The one that follows hydrogen is the one that transforms helium into carbon. But this stairway has a limit: when the star’s core becomes iron there are no more thermonuclear reactions possible to transfer energy. The only chance of equilibrium for this star is to continue producing heat in layers, like an onion, where all possible nuclear reactions take place. This configuration keeps leaving iron atoms in the core, which contracts slowly through the growing mass until temperature and pressure make impossible for matter to continue to exist in an atomic nuclei form.
This happens when the iron core reaches the mass of approximately one and a half times the Sun. Then, there is a moment –the process’ time scale is one or two seconds– where the core’s matter is transformed from atomic nuclei to neutrons. It goes, in an instant, from a sea of particles that groups protons and neutrons to be a sea of neutrons. That implies a huge increase in density: the core goes from being a ball with a mass of one and half Suns and a size similar to the one of the Earth to be a ball with less mass, but with a size comparable to the city of Santiago. This enormously dense object is known as neutron star and, if the explosion leaves it rotating at high speed, it will also be a pulsar. This process also generates a huge amount of neutrinos. This almost instantaneous transformation steals its gravitational support “floor” from the star, causing the fall of all the mass that was supported on the very core. (We should notice that the mass that falls is much larger than the core’s mass.)
This process is complex and not quite well understood yet, but somehow, in many cases, the previous sequence result in an explosion with ejection of many solar masses of matter processed by the star first and then the explosion. For decades, observational and theoretical astronomers have tried to come up with a detailed and consistent description of these explosions’ development, but we have never succeeded. Every time we believe we have reached an scenario in which we seemed to agree in everything, some idea appears, or some observation, or new model that makes us realize that we still cannot find some significant ingredient to get the great picture. What is frustrating about the situation is that this lack of detailed understanding prevent us from affectively linking the supernovae’s explosions with some of their most relevant consequences, such us the chemical enrichment of galaxies throughout cosmic evolution.
In this context, we present a work developed by Tomás Müller, PUC’s master student and MAS scholarship researcher, under the co-direction of José Luis Prieto, Professor of Universidad Diego Portales and MAS Young Researcher, and Alejandro Clocchiatti, Professor of Universidad Católica and MAS Associate Researcher (https://arxiv.org/abs/1702.00416, http://iopscience.iop.org/article/10.3847/1538-4357/aa72f1/meta;jsessionid=A70B3639B47422A570565BC853BEC51F.c3.iopscience.cld.iop.org ) in which we tried to use the classical observable data we have available for Type II Plateau Supernovae (those that show a constant period of luminosity) with some of the most strong properties of the explosion, such as the total energy and the mass of nickel atoms ejected. In order to interpret it we used an empirical and “holistic” model of the observations previously developed by Professor Prieto and O. Pejcha, researcher of the University of Princeton and also co-author of this paper.
The figure shows one of our results, a relatively noisy correlation between the “plateau” phase’s duration with the amount of nickel that the supernova tossed into the interstellar space. In this research we studied a sample of nineteen supernovae and analyzed how the observed nickel production statistics are contrasted with the ones predicted by some of the most detailed and complete explosion models available today. The results are encouraging: in the observational uncertainties and the interpretation that we reached thanks to Pejcha and Prieto’s model, the observational statistics is consistent with the one predicted by theory. Although we do not have a theoretical model that can lead us to an agreement yet, apparently, we are on the right track.
Main photo: Image of the “Crab Nebula,” the expanding remains of a supernova that exploded in our Milky Way in 1054. We know that it was a supernova of gravitational collapse since near the center there is a star of neutrons that rotates very quickly (a pulsar,) which is the remnant of the violent process of neutronization of the matter that destabilized the structure of the progenitor star and produced the explosion.