Discovery of the Lithium production sites
According to the current cosmological model, only one fourth of the Lithium observed in the Universe formed during the Big Bang era. The origin of the remaining three quarters has been a mistery until now. A recent work of an inernational team led by Paolo Molaro of our observatory proposes Novae as the production sites of the unexplained Lithium.
Now, it is proved: some Novae, the percentage is not known precisely, produce large, very large quantities of beryllium-7.This evidence comes from a study of Nova Sagittarii N. 2 2015 - a Nova discovered by John Seach, Chatsworth Island, NSW, Australia, in March 15, 2015 - in the constellation Sagittarius (see figure), that reached a maximum brightness of visual magnitude of 4.3 on march 21, visible with naked eye, and remained relatively bright for several months like the so-called “slow nova”. This nova was the brightest since Nova Centauri 2013 that peaked at magnitude 3.3.
The study was conducted at the ESO VLT in Paranal, Chile, by a team led by Paolo Molaro from INAF - Osservatorio Astronomico di Trieste, and composed by Luca Izzo, from the Institute of Astrophysics of Andalucia, Elena Mason from INAF-OATs, Piercarlo Bonifacio, from Observatory of Paris and Massimo della Valle from INAF - Osservatorio di Napoli.
Classical novae are not all alike but they share a common underlying mechanism: a white-dwarf star accretes gas from a close companion star that's overflowing its gravitational boundary. The overflow produces a stream of gas onto the white dwarf where new material builds up in a new layer on the white dwarf's surface. As the material accretes this layer becomes dense and hot enough to ignite in a runaway hydrogen-fusion reaction, which quickly spreads around the whole white dwarf’s surface. The layer becomes an H-bomb in the form of a thin shell which contains only about 1/10,000 of a white dwarf mass. Yet, the brightness typically jumps by 10 magnitudes. In the following weeks the shell cools, thins out, and becomes transparent. The white dwarf eventually settles down to its previous state and the cycle begins anew till a new explosion.
The first Be-7 detection in a Nova observation was made by a Japanese group in Nova Delphini 2013, which peaked at magnitude +4.3, with the Subaru telescope in Hawaii and so when a new bright Nova exploded we immediately thought, to try a new Be-7 detection. We applied for Director Discretionary Time who approved the project.
With a series of high resolution observations taken with the UVES spectrograph on ESO's VLT taken between day 57 and 82 after the maximum of the Nova - explains Elena Mason- it was possible for the first time to follow the evolution of the absorption in correspondence to the Be-7 transition at 3130 Angstroms, a region extremely difficult for the ground observations, and find the spectroscopic signature of Be-7. Moreover, it was also possible to calculate precisely the Beryllium abundance. An the Be-7 abundance was really huge.
Be-7 is a radioactive element and decays with a half-life of 53.2 days in Li-7. Its discovery in the expanding shell of the Nova means that this isotope must have formed on the surface of the white dwarf that originates the Nova during or immediately before the explosion. This possibility was suggested in the mid-70s by Marcel Arnould and Norgaard but never proven. Indeed, the systematic absence of lithium-7 in the Nova spectra generated doubts about this hypothesis and only few, not even the authors, were believing in it.
But the real surprise is not its presence but the very high amount which was found in the expanding shell. The Be-7 decays into Li-7 and then a very high amount of Beryllium corresponds to a high amount of Lithium- tells Luca Izzo author of the only Lithium detection in the very early days of a Nova. Spectrum. The abundance calculated is awesome , we are talking of about 10 million times the solar value!
Such an 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" lithium factory by making the 75% of all Lithium with the rest made mainly in the Big Bang and an almost negligible fraction by fragmentation processes of cosmic rays in the interstellar medium.
Be-7 is born from the fusion of helium 3 and helium 4 in a nuclear reaction at temperatures of millions of degrees on the surface of the white dwarf. This is a reaction that takes place also in the interior of the Sun, and in the first three minutes of the Universe in the primordial nucleosynthesis. Now we know that Be-7 is made also on the surfaces of white dwarfs in the moments preceding the explosion of Nova.
The nucleus that is born, beryllium 7, is famous in nuclear physics to be studied by the group of Fermi in the forties for the formulation of the beta decay theory. The fact Beryllium 7 is one of the very few nuclei in which it is possible to slightly modify the half-life make of Be-7 a case study. Normally this nucleus captures one of the inner electrons of the electron shell. If all the electrons are removed the nucleus is no longer capable of capturing an electron and becomes stable.
Beryllium also has a huge cosmological importance. And in fact it is the absence of a stable nucleus formed by with 4 protons and 4 neutrons (the Be-8) that prevents the Big Bang to form heavier nuclei , as Gamow, the father of the idea of the Big Bang, had assumed. This is the precisely the reason why the primordial nucleosynthesis stops with the lithium nucleus.
But the lithium of the Big Bang is just about 25% of the lithium that we see today. For years astronomers have tried to figure out from where the remaining 75% was coming from. Giant stars or Supernovae were suggested but without being totally convincing . With the presently calculated values now just two Novae per year of the type of Nova Sagittarius N. 2 2015 suffice to produce all the lithium required. Since the Be-7 was found in both the three Novae in which it was tried is very likely that Be-7 synthesis is the rule in Novae and that this is the dominant Galactic lithium source.