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Previously Undiscovered Nuclear Reactions Help Heat Neutron Stars

December 2, 2013
Image Caption: A neutron star is the dense, collapsed core of a massive star that exploded as a supernova. The neutron star contains about a Sun's worth of mass packed in a sphere the size of a large city. Credit: NASA/Dana Berry

redOrbit Staff & Wire Reports – Your Universe Online

New research appearing in the latest edition of the journal Nature is casting doubts on currently accepted theories has to how the surface of a neutron star heats itself up.

Neutron stars are super-dense stars that form once a large star explodes, causing the core to collapse upon itself. Scientists had long believed that nuclear reactions within the crust contributed to the heating of the star’s surface, but Michigan State University physics and astronomy professor Hendrik Schatz and his colleagues have completed research that has them reconsidering that hypothesis.

Schatz and his team conducted a series of theoretical calculations which have identified previously unknown layers where nuclear reactions inside the crust lead to rapid neutrino cooling. Neutrinos are elementary particles that are formed through radioactive decay and which pass quickly through matter, the researchers explained.

“These cooling layers are pretty shallow beneath the surface. If heat from deeper within the star comes up, it hits this layer and never makes it to the surface,” he said in a statement Sunday, adding that the findings have produced more questions than answers. “This completely changes the way we think about the question of the star’s hot surface.”

On a sub-atomic level, the study authors discovered that the process is greatly influenced by the shape of the reacting nuclei. As co-author Sanjib Gupta of the Indian Institute of Technology (IIT) Ropar in India explained, “Many nuclei are round, and that suppresses the neutrino cooling. In this case, the nuclei are predicted by theorists to be ‘deformed,’ more football-shaped.”

The research, which was supported in part by the Joint Institute for Nuclear Astrophysics (JINA), also involved researchers from the Los Alamos National Lab, the University of Notre Dame, the Universidade de Sao Paulo in Brazil, the University of Washington, the Oak Ridge National Laboratory and the University of Tennessee.

In 2007, Schatz was part of a research team which initially revealed that the surfaces of some neutron stars were significantly hotter than previously expected.

“This is the first model that goes into some reasonable detail about the nuclear physics that occur in the crusts of accreting neutron stars,” the MSU professor said. His team discovered that the reactions release 10 times more heat than previous models had indicated, and their findings supported the efforts of theorists who explain why superbursts, which are ultra-violent explosions, should occur every three to four years, not every decade as theoretical astrophysicists had predicted.


Source: redOrbit Staff & Wire Reports - Your Universe Online



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