# Focus: Neutrinos Reveal Star’s Inner Secrets

Phys. Rev. Focus 7, 26
Neutrino observatories may be able to detect exotic matter in proto-neutron stars or witness the births of black holes.

Astronomers go to mountaintop observatories to get a good look at the sky, but the best view might be over a mile underground inside neutrino detectors. Neutrinos–neutral particles that regularly fly through the Earth undetected–may provide astronomers with information that light cannot. A paper in the 4 June PRL predicts that neutrinos from proto-neutron stars might reveal the existence of exotic quark matter inside these stars. It also shows that the presence of such matter could lead to the birth of a black hole, an event the authors believe could be detected from neutrino observations.

When a star explodes in a supernova, it leaves behind a dense core, called a proto-neutron star, which contains an equal number of protons and neutrons. The impulse from the supernova quickly converts the protons into neutrons–a process that releases many energetic neutrinos. Seconds after the explosion, the conversion is complete: The proto-neutron star has become a neutron star–a cold, dead star made primarily of neutrons. By the time the light and dust from the supernova dissipate, the proto-neutron star is gone.

“Observing the neutrinos is the only way we can observe the proto-neutron star,” says James Lattimer of the State University of New York (SUNY) at Stony Brook. In 1987, detectors around the world recorded nearly twenty neutrinos from a proto-neutron star in the heart of a supernova. Neutrinos like these, Lattimer says, could provide information about the life of such a star. In their paper, the SUNY team calculates the neutrino energy that would be detected for proto-neutron stars made of three proposed forms of “quark matter”: matter in which quarks become unglued from each other, matter filled with baryons containing strange quarks, and matter filled with kaon particles, which also contain strange quarks. In each case, the group describes a signature neutrino signal that could be detected at facilities like the new Sudbury Neutrino Observatory in Canada or Super-Kamiokande in Japan.

The team also shows that these detectors could watch black holes form. Their calculations reveal that the presence of quark matter could “soften” the proto-neutron star, making it easier to compress. In certain cases, the team believes that this softening could cause the proto-neutron star to collapse under its own gravity into a black hole. If that happened, detectors on earth would see a sudden cessation of neutrinos from the star. “You could catch the [black hole’s formation] during the time it’s happening, and that’s important,” says Madappa Prakash, the head of the SUNY group. “It’s like catching a thief in the act.”

The new neutrino detectors will have “unprecedented sensitivity,” says Adam Burrows of the University of Arizona in Tucson. Burrows believes that the SUNY team’s calculations will help these detectors analyze neutrino data from the next supernova. But until such an event occurs, he adds, it will be impossible to tell exactly what a proto-neutron star looks like.

–Geoff Brumfiel

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