The fusion of H into He in the solar interior has to be accompanied by the emission of about 1038 neutrinos per second. Thus at the earth surface one expects a flux of the order of 1011 neutrinos cm-2 sec-1. The successful detection of these solar neutrinos in several experiments over the world has raised the attention to the Sun as a neutrino source, originating in the same time (see e.g. Bahcall 1989, Bahcall et al. 1995) the long debated problem of the discrepancy between experimental data and theoretical predictions.
All stars that are burning and/or have been burning H in the Universe can be regarded as cosmic sources of both photons and neutrinos, thus simultaneously contributing to the background of those particles. Clearly this stellar neutrino flux is expected to be much smaller than the solar one; however the amount and the energy of stellar neutrinos contributing to the neutrino background appears as an interesting question to be addressed in the frame of the present theoretical knowledge of stellar evolutionary structures. A first step in such a direction has been recently presented by Hartmann et al. (1994), assuming all stars in their initial (Main Sequence, MS) phase. A quick inspection of the stellar evolutionary scenario reveals that such an assumption has to be regarded only as a first approximation to the problem. As a matter of fact, we know, e.g.,that low mass main sequence stars burn H into He through the proton-proton chain, thus emitting a mixture of neutrinos from pp, 7 Be and 8 B reactions. In the meantime, we know that in the same structures the largest amount of He is produced at higher temperatures in the Red Giant stage, where H burning is dominated by CNO reactions. Accordingly, one can easily predict that all along the life of a low mass star CNO neutrinos will eventually dominate the yield, in spite of the MS behaviour.
Moreover, the burning of H into He is not the only way used by stars to produce neutrinos. In the advanced phases of stellar evolution, neutrinos can be produced not as a by-product of thermo-nuclear reactions (thermo-nuclear neutrinos) but directly at expenses of the thermal energy of stellar matter. According to current physics, cooling neutrinos can originate from several processes, as plasmon decay (plasma-neutrinos) or photon-electron collisions (photoneutrinos). Their energy however is lower by about two orders of magnitude than the energy of thermo-nuclear neutrinos; their detection is thus even more difficult than the detection of neutrinos from nuclear burning.
In this paper, we will discuss all these neutrino sources following the evolution of a suitable sample of stellar structures all along the major phases of H and He burning. The investigation will be completed by discussing the relevant contribution given by neutrinos from cooling White Dwarf (WD) structures. In the next section we will discuss the properties of thermo-nuclear neutrino sources. Sect. 3 will be devoted to cooling neutrinos both during the nuclear life of a star and in the final WD structures. An evaluation of the expected galactic and extragalactic neutrino background will close the paper.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998