# Stong neutrino cooling by electron-capture and beta-decay cycles in the neutron star crust

### H. Schatz, S. Gupta, P. Moller, M. Beard, E.F. Brown, A.T. Deibel, L.R. Gasques, W.R. Hix, L. Keek, R. Lau, A.W. Steiner, M. Wiescher (2013)

Figure: A small region of table of nuclides4, labeled by the number of protons on the y-axis and the number of neutrons on the x-axis. The thick blue lines denote pairs of nuclei that create the strong neutrino luminosity. The thick black squares denote stable nuclei found on earth.

Summary:

Neutron stars1 are the final stage in the evolution of a star between 8 and 20 times the mass of the sun. They are extremely dense and compact: imagine an object with the mass of the sun (which has a radius of 430,000 miles) packed into the size of Chicago. When a neutron star has a normal star companion, the neutron stars' extreme gravity can steal matter from the companion - a process called accretion. Neutron stars are important because they have solid crusts made of neutron-rich nuclei2 - the very same nuclei that physicists are working to create on earth in places like the Facility for Rare Isotope Beams.

This new cooling comes from the combination of two separate processes. In the neutron star crust, a proton inside a nucleus captures an electron to form a neutron and a neutrino $$p + e \rightarrow n + \nu_e \, .$$ The neutrino $\nu_e$ leaves, taking energy away from the neutron star crust and causing it to cool. Following this reaction, the neutron can decay back into a proton (very similar to nuclear beta-decay3) $$n \rightarrow p + e + \bar{\nu}_e \, ,$$ which again creates a neutrino which cools the crust. These two nuclear reactions flip back and forth, creating two neutrinos in each cycle, continuing until the temperature is too small to allow the cycle to continue.

This new cooling process will require a fundamental revision of our current picture of how accreting neutron stars work and will also give us important insight into how nuclei work here on earth.

This project was a collaboration of a dozen researchers through JINA, the Joint Institute for Nuclear Astrophysics funded by the NSF.

See our article in Nature or the (free) version on arXiv.org.

Notes:

1See a diagram of stellar evolution from the Chandra X-ray observatory, their neutron star page, or the wikpedia entry on neutron stars.
2Atomic nucleus
3Nuclear beta decay
4See an interactive chart of the nuclides at Brookhaven National Lab or the wikipedia entry on the Table of nuclides.