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Particle physicists have detected neutrinos from the sun, revealing that directly for the first time the CNO merger cycle (carbon-nitrogen-oxygen) is working on our star.
The CNO cycle is the dominant energy source that powers the stars heavier than the Sun, but until now it had never been detected directly in any starexplains co-author Andrea Pocar of the University of Massachusetts Amherst in a statement. The results are published in Nature.
For much of his life, stars get energy by fusing hydrogen into helium. In stars like our sun or lighter, this happens mainly through strings ‘proton–proton’. However, many stars are heavier and hotter than our sun and include elements heavier than helium in their composition, a quality known as metallicity. The prediction from the 1930s is this the CNO cycle will be dominant in heavy stars.
Neutrinos emitted as part of these processes provide a spectral signature that allows scientists to distinguish those in the “proton-proton chain” from those in the “CNO cycle.” Pocar points out: “Confirmation that CNO burns in our sun, where it only works at 1%, strengthens our confidence that we know how stars work.“.
In addition to this, CNO neutrinos can help solve an important open question in stellar physics, he adds. That means, as the central metallicity of the sun, which can only be determined by the velocity of the CNO neutrinos of the nucleus, is related to the metallicity in other parts of a star. Traditional models have encountered a difficulty: the spectroscopic measurements of the surface metallicity do not agree with the measurements of the underground metallicity deduced from a different method, the helioseismological observations.
Pocar says neutrinos are indeed the only direct probe science has for the nuclei of stars, including the Sun, but they are extremely difficult to measure. Up to 420 billion of them hit every square inch of the earth’s surface per second, but virtually all of them pass without interacting. Scientists can only detect them using very large detectors with exceptionally low background radiation levels.
The detector Borexino it is found in the deep Apennines of central Italy at the Gran Sasso National Laboratories of IN FN. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultra-pure organic scintillator. Its great depth, size and purity make Borexino a unique detector for this type of science, one of a kind for low background radiation, says Pocar. The project was started in the early 1990s.
Up until the latest detections, the Borexino collaboration had successfully measured the components of “proton-proton” solar neutrino fluxes, helping to refine the neutrino “taste” oscillation parameters and, most impressive, even measured first phase of the cycle. : very low energy ‘pp’ neutrinos, remembers Pocar.
Its researchers dreamed of expanding the reach of science to search for CNO neutrinos as well, in a narrow spectral region with a particularly low background, but that prize seemed out of reach. However, research groups at Princeton, Virginia Tech, and UMass Amherst believed that CNO neutrinos could still be detected using the additional purification steps and methods they had developed to achieve the exquisite detector stability required.
Over the years and thanks to a sequence of movements to identify and stabilize the antecedents, the American scientists and the entire collaboration have been successful. “In addition to revealing CNO neutrinos, the subject of the article by Nature This week, now there is also the potential to help solve the metallicity problem “, says Pocar.
Prior to the discovery of the CNO neutrino, the lab had scheduled Borexino to finish its operations at the end of 2020. But since the data used in the analysis for the article Nature frozen, scientists continued to collect data as core purity continued to improve, making a new metallicity-centric result a real possibility, says Pocar. Data collection could be extended until 2021, as the necessary logistics and permits, although ongoing, are not trivial and time consuming. “Each extra day helps,” he says.
(With information from Europa Press)
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