arXiv subjects
Publications and source records attributed to B. Ricci.
We extract information on the fluxes of Be and CNO neutrinos directly from solar neutrino experiments, with minimal assumptions about solar models. Next we compare these results with solar models, both standard and non standard ones. Finally we discuss the expectations for Borexino, both in the case of standard and non standard neutrinos.
We extract information on the fluxes of Be and CNO neutrinos directly from solar neutrino experiments, with minimal assumptions about solar models. Next we compare these results with solar models, both standard and non standard ones. Finally we discuss the expectations for Borexino, both in the case of standard and non standard neutrinos.
A variation of Newton's constant $G$ over cosmological time scales would modify the main-squence time of globular cluster (GC) stars. We have calculated the evolution of low-mass stars typical for GCs both for standard non-varying $G$ and under the assumption of a linear variation of $G$. The age of the isochrones resulting from the latter models then was chosen such that the isochrones mimicked the standard ones at the turnoff. Assuming that the true age of GCs is between $8$ and $20\,\rm Gyr$, and because their apparent age is between $14$ and $18\,\rm Gyr$, we find that today $-35{\times}10^{-12}\,{\rm yr}^{-1}\la\dot{G}/G\la 7{\times}10^{-12}\,{\rm yr}^{-1}$. The upper limit (gravity weaker in the past) is competitive with direct present-day bounds from celestial mechanics. Within independently determined $\dot{G}/G$ limits a time-varying $G$ as an explanation for the discrepancy between the cosmic expansion age and the apparent GC ages is conceivable.
We connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potential $U_{lab}$ in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented by $U_{lab}$. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600-700 eV, close to that found by experiments in the lab.
We quantitatively determine the effect and the uncertainty on solar neutrino production arising from the screening process. We present predictions for the solar neutrino fluxes and signals obtained with different screening models available in the literature and by using our stellar evolution code. We explain these numerical results in terms of simple laws relating the screening factors with the neutrino fluxes. Futhermore we explore a wider range of models for screening, obtained from the Mitler model by introducing and varying two phenomenological parameters, taking into account effects not included in the Mitler prescription. Screening implies, with respect to a no-screening case, a central temperat reduction of 0.5%, a 2% (8%) increase of Beryllium (Boron)-neutrino flux and a 2% (12%) increase of the Gallium (Chlorine) signal. We also find that uncertainties due to the screening effect ar at the level of 1% for the predicted Beryllium-neutrino flux and Gallium signal, not exceeding 3% for the Boron-neutrino flux and the Chlorine signal.
Recent solar neutrino results together with the assumption of a stationary Sun imply severe constraints on the individual components of the total neutrino flux : $Φ_{Be} \leq 0.7 \cdot 10^{9} cm^{-2} s^{-1}, Φ_{CNO} \leq 0.6 \cdot 10^{9} cm^{-2} s^{-1}$ and $64 \cdot 10^{9} cm^{-2} s^{-1} \leq Φ_{pp+pep} \leq 65 \cdot 10^{9} cm^{-2} s^{-1}$ (at 1$ σ$ level), the constraint on $ν_{Be}$ being in strong disagreement with $Φ_{Be}^{SSM} = 5 \cdot 10^{9} cm^{-2} s^{-1}$. We study a large variety of non-standard solar models with low inner temperatures, finding that the temperature profiles T(m) follow the homology relationship: T(m)=k$T(m)^{SSM}$, so that they are specified just by the central temperature $T_{c}$. There is no value of $T_{c}$ which can account for all the available experimental results and also if we restrict to consider just Gallium and Kamiokande results the fit is poor. Finally we discuss what can be learned from new generation experiments, planned for the detection of monochromatic solar neutrinos, about the properties of neutrinos and of the Sun.