Particle physicists have detected neutrinos Sol, directly revealing for the first time that the CNO fusion cycle (carbon-nitrogen-oxygen) it’s working on our star.
The CNO cycle is the dominant energy source that powers stars heavier than the Sun, but until now it had never been directly detected 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 occurs 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 since the 1930s is that the CNO cycle will be dominant in heavy stars.
The 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 operates at just one percent, reinforces our confidence that we understand how the stars work.“.
Beyond this, CNO neutrinos can help solve an important open question in stellar physics, he adds. That is to say, how the central metallicity of the sun, which can only be determined by the CNO neutrino rate of the nucleus, is related to metallicity in other parts of a star. Traditional models have encountered a difficulty: spectroscopic surface metallicity measurements do not agree with subsurface metallicity measurements inferred from a different method, helioseismology observations.
Pocar says that neutrinos are really the only direct probe science has for the cores 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, yet 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 is located deep in the Apennines in central Italy at the Laboratori Nazionali del Gran Sasso del IN FN. It detects neutrinos as flashes of light produced when neutrinos collide with electrons in 300 tons of ultrapure 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.
Until their latest detections, the Borexino collaboration had successfully measured the components of the solar ‘proton-proton’ neutrino fluxes, helping to refine the oscillation parameters of the neutrinos’ ‘taste’ and, most impressively, even measured the first step of the cycle: very low energy ‘pp’ neutrinos, remember Pocar.
Its researchers dreamed of broadening 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 revealed 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 were successful. “Beyond revealing CNO neutrinos, which is the subject of the article by Nature This week, now there is even potential to help solve the metallicity problem “, dice Pocar.
Before the discovery of the CNO neutrino, the lab had scheduled Borexino to end its operations in late 2020. But because the data used in the analysis for the paper by Nature froze, scientists continued to collect data as core purity continued to improve, making a new result focused on metallicity a real possibility, says Pocar. Data collection could be extended to 2021 as the necessary logistics and permits, while ongoing, are non-trivial and time-consuming. “Every extra day helps,” he says.
(With information from Europa Press)