The conversion of neutrons and other baryons to a quark-gluon plasma in blue supergiant stars could lead to supernovae explosions. That is the conclusion of David Blaschke of the University of Wroclaw and an international team of astrophysicists, who have calculated that such events could be observed via the distinct neutrino signals that they emit.

At the ends of their lives, blue supergiant stars, which can be over 50 times the mass of the Sun, may collapse to form remnants partly comprising a sea of free quarks and gluons. These events, which would emit two powerful neutrino pulses in rapid succession, could help to explain how such stars can undergo supernovae, when some conventional models suggest they should just form black holes.

When stars heavier than about nine solar masses have exhausted all the hydrogen in their cores, they continue fusing helium into carbon and oxygen. They then fuse a series of ever heavier elements, ending with the fusion of silicon into nickel, which decays to iron. At this point, the process stalls because no energy can be obtained by fusing iron nuclei. When the mass of the inert iron core exceeds the Chandrasekhar limit of 1.44 solar masses, it can no longer support itself against its own gravity. The nuclei are therefore crushed and a flood of neutrinos are emitted in a few milliseconds, heating the outer layers and until they are blown away in a giant supernova explosion.

What happens next is less predictable. The remnants of stars just above the nine-solar-mass limit form neutron stars. In stars above about 70 solar masses, however, this neutrino heating mechanism is not efficient enough to cause a sufficiently-strong explosion for the outer layers to escape the gravitational potential well of the core. The ejected material therefore falls back in and a black hole is formed. In between these limits, however, the details are unclear.

Quark-gluon plasma

In 2009, Irina Sagert of Goethe University in Frankfurt, Germany and colleagues suggested that, if the material ejected by the neutrino heating mechanism collapsed back onto the core, the rise in density could cause the baryons (mainly neutrons) at the centre of the core to be crushed. As the strong nuclear force becomes weaker at shorter distances, the quarks that make up the baryons would no longer be bound into individual baryons. Instead quarks would move around freely in a quark-gluon plasma.

The energy released by such a phase transition could re-ignite as a supernova, blowing off the outer layers and producing a neutron star stable against further collapse. However, the equation of state for the quark-gluon plasma used in the 2009 study predicted neutron stars heavier than about 1.6 solar masses should collapse into black holes. “Since then, one has observed two neutron stars with about 2 solar masses,” says nuclear astrophysicist Friedrich Thielemann of the University of Basel in Switzerland, one of Sagert’s collaborators, “That means this equation of state was not correct.”

Armed with a more sophisticated model of the quark-gluon plasma and greater computing power, researchers have now produced a more detailed model of a two-stage supernova in a star of 50 solar masses. Having initially undergone core collapse, it undergoes a phase transition into a proto-neutron star, producing a strong pulse of neutrinos. Within a few hundred milliseconds, it has expanded tenfold and begun to collapse again.

Shock front

As the shock front propagates inwards, the pressure rises at its centre, and almost immediately the centre begins to undergo a second phase transition to a quark-gluon plasma. This releases more energy and triggers a second, more powerful shock front propagating outwards. This stops the inward-propagating rebound from the first shock at a radius of 80 km just 1.2 s after the initial blast. The supernova then expands to several thousand kilometres in radius within a few milliseconds. The remnant is a neutron star of about two solar masses with a quark-gluon plasma core.

The optical signals produced by such supernovae could vary widely, say the researchers. However, such events would give one unambiguous signal: “From the first shock we have a neutrino signal and then, from the conversion to quark matter, we have an antineutrino signal,” explains team member Blaschke. Modern neutrino detectors such as Super-Kamiokande in Japan have millisecond time resolution. Therefore, if such a supernova occurred in our galaxy, the presence of two sharp signals barely a second apart – one of neutrinos and one of antineutrinos – should be detectable. “Any signals from extra-galactic events would be diluted so much that they are not detectable,” says Blaschke. The researchers are also investigating the potential to find evidence of quark matter in the gravitational wave signals from neutron star mergers.

Thielemann is cautiously impressed: “It’s a very nice prediction and it’s a possible scenario,” he says. “The equation of state is a little bit tuned to make this possible, but it’s within the uncertainties of present day experiments. One has to wait to see whether one can see such a supernova explosion with two neutrino bursts.”

The research is published in Nature Astronomy.