Novae the Beryllium and Lithium factory
Paolo Molaro, astronomer at the National Institute for Astrophysics and head of a team of researchers, has discovered a high abundance of Beryllium-7 in the material expelled from Nova Sagittarii no. 2, a star discovered by amateur astronomer John Seach in 2015 in the constellation Sagittarius, one of the brightest celestial objects in its class in recent years, among the few visible to the naked eye.
Beryllium is a chemical element in the periodic table of the elements, used mainly as a reinforcing agent in alloys (copper-beryllium), carcinogenic to humans. Beryllium-7, resulting from the fusion of helium 3 and helium 4, is important in nuclear physics. It is an unstable isotope which, with the capture of an electron and the emission of a neutrino, transforms into lithium 7 with a half-life of 53.2 days.
Lithium 7 is a primordial isotope, produced during the Big Bang. It is used as a constituent of lithium fluoride for molten salt nuclear reactors.
The researchers, therefore, believe that Novae can be regarded as a real Lithium factory present in the Milky Way, solving a problem that had puzzled astronomers for decades.
Novae occur when hydrogen accumulates on the surface of a white dwarf which causes a huge nuclear explosion, causing the star to become much brighter than usual. Hydrogen comes from a companion star of the white dwarf, which is compressed and heated by gravity and triggers the thermonuclear reactions which blow off the outer layers of the white dwarf, producing the phenomenon of the “Nova”, with a brightness up to 100,000 times higher than its normal values.
To look for signs of the presence of Beryllium-7 in the material expelled by the expanding star, the researchers used the UVES spectrograph installed on ESO’s Very Large Telescope. “With a series of observations between 57 and 82 days after the maximum of the Nova – explained Massimo Della Valle, Director of INAF- Astronomical Observatory of Capodimonte, who participated in the study – it was possible for the first time to follow the evolution of the absorption in correspondence to the Beryllium-7 transition and find the “signature” of the isotope. Moreover, it was also possible to calculate precisely the Beryllium abundance. And the Beryllium-7 abundance was really huge”.
“The abundance calculated is awesome” said Molaro, lead author of the article published in the journal Monthly Notices of the Royal Astronomical Society on the amount of Lithium that can be obtained from Beryllium-7 decay. “We are about 10 million times above the value of Lithium found in our Sun. Such a high abundance suggests the Nova as a possible source of Lithium in the Galaxy. Indeed, the amount is so high that Novae are probably the source of all Lithium except for that formed during the Big Bang – about 25% – and an almost negligible fraction produced by fragmentation processes of cosmic rays in the interstellar medium. And of course this is also true for the Lithium contained in the batteries of our mobile phones”.
The team that conducted the study, led by Paolo Molaro, is composed of Luca Izzo (post-doc researcher at the Institute of Astrophysics of Andalucia IAA-CSIC in Granada, Spain), Elena Mason (INAF- Astronomical Observatory of Trieste), Piercarlo Bonifacio (GEPI, Observatory of Paris Meudon) and Massimo Della Valle (INAF- Astronomical Observatory of Capodimonte, Naples).