At the end of their life, very massive stars explode as supernovae, which are among the most spectacular phenomena in the universe. Progressively, their core burns out its nuclear fuel and turns into iron. As soon as its mass becomes higher than 1.1 solar masses, the pressure contributed by degenerate electrons is no longer able to act against gravity and it suddenly collapses. In less than a second, the radius decreases from several hundreds of thousands to about fifty kilometers, while the density reaches a hundred millions of tons per cubic centimeter. Nuclear interaction becomes repulsive, inducing a bounce of infalling matter layers and giving birth to a shock wave. This wave propagates outwards, heated and pushed by the neutrinos that are produced in the core, before stalling from energy losses and from matter still falling to the center from outer layers.
In the core, the state of compressed matter – which strongly depends on its composition – determines whether this core shall become a neutron star or a black hole. The whole process is understood rather poorly and no simulation has yet been able to fully explain the collapse, bounce and post-bounce phases. The field is complex because it requires the knowledge of particle physics under extreme conditions, three-dimensional relativistic magnetohydrodynamics, coupled to relativistic gravity and neutrino radiative transfer.
The state of matter
One of the main unknowns are the equation of state and composition of hot and very high density matter. Most numerical simulations which have been performed until now were using the same standard content : free protons and neutrons, alpha [2] particles, electrons, positrons, photons and a representative heavy nucleus. However, it was known that the results could strongly depend on matter composition and that several other particles should certainly appear during the process.
An observation made in 2010 enabled to precisely determine the mass of the neutron star PSR J 1614-2230 to be 2 solar masses, within a few percents. This observation has put strong constraints on the equations of state and in particular on those with additional particles, because most of those which have been proposed predict a maximum mass for neutron stars lower than two solar masses. The team members have computed an equation of state containing pions and Lambda hyperons [3], compatible with this observation and they used two different compositions for matter : the standard one with pions, on the one hand, and hyperons on the other. They have considered two initial abundances for the progenitor star: one where heavy elements (metallicity) account for 1/10000th of solar abundance, representative for primordial stars, and the other with solar abundance. In both cases, the progenitor is a star of 40 solar masses when it entered the main sequence. For the numerical model, the researchers have used the code CoCoNuT (standing for Core-Collapse with “Nu” (=new) Technology) developed in collaboration with other institutes, which, for the moment operates in spherical symmetry, but can be used also in two or three dimensions.
A phase transition occurs in some models, leading to a brusque increase of density before the collapse to the black hole. The presence of hyperons induces, after the bounce, a longer and more intense peak neutrino luminosity (Figure 1). But, the most robust result is that the inclusion of additional particles diminishes the collapse time to the black hole.
What observation would be able to test this model? The phase transition could be detected by combining neutrinos and gravitational waves observations. The sudden stop in the neutrino signal, when the core enters the black hole horizon, could be a confirmation of the formation of a black hole. New observational predictions will be computed in collaboration with research teams in the Max-Planck Institute in Garching (Germany) and the University of Valencia (Spain).
Reference :
"Influence of pions and hyperons on stellar black hole formation"
Peres, Oertel and Novak, Phys. Rev. D 87, 043006 (2013)
http://arxiv.org/abs/1210.7435
Contact:
- B. Peres
Observatoire de Paris - LUTH - CNRS, Univ. Paris Diderot - M. Oertel
Observatoire de Paris - LUTH - CNRS, Univ. Paris Diderot - J. Novak
Observatoire de Paris - LUTH - CNRS, Univ. Paris Diderot
[1] An equation of state relates temperature and pressure, thus determining the microscopic behavior of matter.
[2] An alpha-particle is the nucleus of a helium atom.
[3] Hyperons are fermions made of three quarks, from which at least one is a strange quark. In the present case, the Lambda is the most abundant type of hyperons. Pions, or pi-mesons, are light particles which mediate strong interaction.