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Neutron cross section measurements are often made relative to a neutron cross section standard. Thus, the accuracy of the neutron standards determines the best possible accuracy of the neutron measurements. The 235U(n,f) cross section is widely used as reference, while it is considered a standard at thermal point and between 0.15 to 200 MeV. For this reason, additional cross section data for the 235U(n,f) reaction are useful in order to improve the accuracy and to extend the energy range of the standard. In this work, preliminary results of the measurement of the 235U(n,f) cross-section relative to the standard 10B(n,a) reaction are presented. The high accuracy measurement was performed at the experimental area EAR-1 of the n_TOF facility at CERN, aiming at covering the energy range from the thermal region up to approximately 100 keV. The samples were produced at JRC-Geel in Belgium, while the experimental setup was based on Micromegas detectors.

Type Ia supernovae are used as distance indicators to measure the expansion rate of the Universe and to constrain the nature of dark energy. Current and upcoming surveys will allow to extend supernova Hubble diagrams to higher redshifts and to improve further their statistics. It is accepted that Type Ia supernovae are thermonuclear explosions of carbon-oxygen white dwarfs in binary systems. However, the identification of their progenitors, the evolutionary path leading to the explosion and the explosion mechanism itself have not been identified yet. This is critical, as we need to understand the potential evolution of their luminosity with cosmic time and, thus, with their stellar progenitors. We will review the current situation, considering observational hints. We will focus on our recent models, that follow the evolution of carbon-oxygen white dwarfs accreting mass up to thermonuclear runaway, and on their dependence with the initial metallicity of the white dwarf progenitors.

Noticeable improvements were recently introduced in the modelling of n-capture nucleosynthesis in the advanced evolutionary stages of giant stars (Asymptotic Giant Branch, or AGB, stars). Two such improvements are closely linked together and concern the introduction of non-parameterized, physical models for extended mixing processes and the adoption of accurate reaction rates for H- and He-burning reactions, including the one for the main neutron source 13C(α,n)16O. These improvements profited of a longstanding collaboration between stellar physicists and C. Spitaleri’s team and of his seminal work both as a leader in the Nuclear Astrophysics scenario and as a talent-scout in the recruitment of young researchers in the field. We present an example of the innovative results that can be obtained thanks to the novelties introduced, by estimating the contributions from a nearby AGB star to the synthesis of short-lived (t1/2 ≤ 10 Myr) radioactive nuclei which were alive in early Solar System condensates. We find that the scenario indicating an AGB star as the source of such radioactivities, discussed for many years by researchers in this field, appears now to be no longer viable, when the mentioned improvements of AGB models and nuclear parameters are considered.

The precise progenitor system of type Ia supernovae (SNe Ia), whether it is a white dwarf (WD) close to the Chandrasekhar limit or substantially less massive, has been a matter of debate for decades. Recent research by our group on the accretion and simmering phases preceding the explosion of a massive WD has shown that the central density at thermal runaway lies in the range 3.6 − 6.3 × 109 g cm−3 for reasonable choices of accretion rate on to the WD and progenitor metallicity. In this work, we have computed one-dimensional simulations of the explosion of such WDs, with special emphasis on the chemical composition of the ejecta, which in all cases is extremely rich in neutronized isotopes of chromium (54Cr) and titanium (50Ti). We show that, in order to reconcile such a nucleosynthesis with the isotopic abundances of the Solar system, Chandrasekhar-mass WDs can account for at most 26 per cent of normal-luminosity SNe Ia, or at most 20 per cent of all SNe Ia. [ABSTRACT FROM AUTHOR]

The luminosity constraint is a very precise relationship linking the power released by the Sun as photons and the solar neutrino fluxes. Such a relation, which is a direct consequence of the physical processes controlling the production and the transport of energy in the solar interior, is of great importance for the studies of solar neutrinos and has a special role for the search of neutrinos from the CNO cycle, whose first detection with a 5σ significance has been recently announced by the Borexino collaboration. Here we revise the luminosity constraint, discussing and validating its underlying hypotheses, in the light of latest solar neutrino and luminosity measurements. We generalize the current formulation of the luminosity constraint relation so that it can be easily used in future analysis of solar neutrino data, and we provide a specific application showing the link between CNO and pp neutrino fluxes. [ABSTRACT FROM AUTHOR]

Metal-poor stars with detailed information available about their chemical inventory pose powerful empirical benchmarks for nuclear astrophysics. Here we present our spectroscopic chemical abundance investigation of the metal-poor ([Fe=H] = -1:60 ± 0:03 dex), r-process-enriched ([Eu=Fe] = 0:73 ± 0:10 dex) halo star HD 20, using novel and archival high-resolution data at outstanding signalto- noise ratios (up to ~1000Å-1). By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with the spectroscopic analysis of iron lines under non-local thermodynamic equilibrium conditions, we derived a set of highly accurate and precise stellar parameters. These allowed us to delineate a reliable chemical pattern that is comprised of solid detections of 48 elements, including 28 neutron-capture elements. Hence, we establish HD 20 among the few benchmark stars that have nearly complete patterns and low systematic dependencies on the stellar parameters. Our light-element (Z ≤ 30) abundances are representative of other, similarly metal-poor stars in the Galactic halo that exhibit contributions from core-collapse supernovae of type II. In the realm of the neutron-capture elements, our comparison to the scaled solar r-pattern shows that the lighter neutron-capture elements (Z . 60) are poorly matched. In particular, we find imprints of the weak r-process acting at low metallicities. Nonetheless, by comparing our detailed abundances to the observed metal-poor star BD +17 3248, we find a persistent residual pattern involving mainly the elements Sr, Y, Zr, Ba, and La. These are indicative of enrichment contributions from the s-process and we show that mixing with material from predicted yields of massive, rotating AGB stars at low metallicity improves the fit considerably. Based on a solar ratio of heavy- to light-s elements - which is at odds with model predictions for the i-process - and a missing clear residual pattern with respect to other stars with claimed contributions from this process, we refute (strong) contributions from such astrophysical sites providing intermediate neutron densities. Finally, nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of 11:0 ± 3:8 Gyr. [ABSTRACT FROM AUTHOR]

Novae are the observational manifestations of thermonuclear runaways on the surface of accreting white dwarfs (WDs). Although novae are an ubiquitous phenomenon, their properties at low metallicity are not well understood. Using the publicly-available stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), we model the evolution of accreting carbon–oxygen WDs and consider models that accrete matter with metallicity Z  = 0.02 or 10−4. We consider both models without mixing and with matter enriched by CO-elements assuming that mixing occurs in the process of accretion (with mixing fraction 0.25). We present and contrast ignition mass, ejected mass, recurrence period, and maximum luminosity of novae for different WD masses and accretion rates for these metallicities and mixing cases. We find that models with Z  = 0.02 have ignition masses and recurrence periods smaller than models with low Z , while the ejected mass and maximum luminosity are larger. Retention efficiency during novae outbursts decreases with increasing metallicity. In our implementation, inclusion of mixing at the H/He interface reduces accreted mass, ejected mass, and recurrence period as compared to the no-mixing case, while the maximum luminosity becomes larger. Retention efficiency is significantly reduced, becoming negative in most of our models. For ease of use, we provide a tabular summary of our results. [ABSTRACT FROM AUTHOR]

We investigate the evolutionary properties of AM CVn stars with massive white dwarf (WD) donors and accretors. As a representative of them we consider a binary initially composed by a 0.30 M⊙ He WD and a 1.02 M⊙ CO WD. We evaluate the time-dependent mass transfer rate from the donor and compute the evolution of the accretor, accounting for the effects of mass exchange on the evolution of orbital parameters. We model the thermal response of the accreting CO WD with the funs evolutionary code coupled to a full nuclear network, from H to Bi, including more than 700 isotopes linked by about 1000 nuclear processes. We find that accretors in these systems evolve through the stages of steady He-burning and mild and strong He-flashes and become at the end CO WDs capped by a massive (∼0.1 M⊙) He-rich buffer. During He-flashes (both mild and strong) the temperature in the He-shell increases above 3 × 108 K, so that the |$\mathrm {^{22} Ne}(\alpha ,n)\mathrm {^{25} Mg}\,$| reaction becomes efficient and n -rich isotopes can be produced. During the Roche lobe overflow episodes triggered by strong non-dynamical He-flashes matter enriched in α-elements and n -rich isotopes is ejected, polluting the interstellar medium. Our results strongly suggest that massive AM CVn systems with WD donors do not experience a final very strong dynamical He-flash driving an explosive event like SN.Ia. Though the ejected matter is highly enriched in heavy isotopes, the relative contribution of massive AM CVn systems to the Galactic chemical evolution is, most probably, negligible due to their expected paucity. [ABSTRACT FROM AUTHOR]

We investigate the evolutionary properties of AM CVn stars with massive white dwarf (WD) donors and accretors. As a representative of them we consider a binary initially composed by a 0.30 M⊙ He WD and a 1.02 M⊙ CO WD. We evaluate the time-dependent mass transfer rate from the donor and compute the evolution of the accretor, accounting for the effects of mass exchange on the evolution of orbital parameters. We model the thermal response of the accreting CO WD with the funs evolutionary code coupled to a full nuclear network, from H to Bi, including more than 700 isotopes linked by about 1000 nuclear processes. We find that accretors in these systems evolve through the stages of steady He-burning and mild and strong He-flashes and become at the end CO WDs capped by a massive (∼0.1 M⊙) He-rich buffer. During He-flashes (both mild and strong) the temperature in the He-shell increases above 3 × 108 K, so that the |$\mathrm {^{22} Ne}(\alpha ,n)\mathrm {^{25} Mg}\,$| reaction becomes efficient and n -rich isotopes can be produced. During the Roche lobe overflow episodes triggered by strong non-dynamical He-flashes matter enriched in α-elements and n -rich isotopes is ejected, polluting the interstellar medium. Our results strongly suggest that massive AM CVn systems with WD donors do not experience a final very strong dynamical He-flash driving an explosive event like SN.Ia. Though the ejected matter is highly enriched in heavy isotopes, the relative contribution of massive AM CVn systems to the Galactic chemical evolution is, most probably, negligible due to their expected paucity. [ABSTRACT FROM AUTHOR]

We investigate the thermal response of CO WDs accreting He-rich matter directly from their companions in binary systems. Our results suggest that the He-channel cannot provide progenitors for the majority of “normal” SNe Ia. [ABSTRACT FROM PUBLISHER]

Little is known about the first stars, but hints on this stellar population can be derived from the peculiar chemical composition of the most metal-poor objects in the Milky Way and in resolved stellar populations of nearby galaxies. In this paper, we review the evolution and nucleosynthesis of metal-poor and extremely metal-poor (EMP) stars with low and intermediate masses. In particular, new models of 6 M ⊙ with three different levels of metallicity, namely Z = 10 − 4 , 10 − 6 and 10 − 10 , are presented. In addition, we illustrate the results obtained for a 2 M ⊙ , Z = 10 − 5 model. All these models have been computed by means of the latest version of the FuNS code. We adopted a fully coupled scheme of solutions for the complete set of differential equations describing the evolution of the physical structure and the chemical abundances, as modified by nuclear processes and convective mixing. The scarcity of CNO in the material from which these stars formed significantly affects their evolution, their final fate and their contribution to the chemical pollution of the ISM in primordial galaxies. We show the potential of these models for the interpretation of the composition of EMP stars, with particular emphasis on CEMP stars. [ABSTRACT FROM AUTHOR]

In this paper I will review the physical properties of Type Ia Supernovae and, basing on the observational evidence and on stellar evolution theory, I will derive a possible self-consistent scenario for both the progenitor systems and the explosion mechanism. [ABSTRACT FROM AUTHOR]

Novae are the observational manifestations of thermonuclear runaways on the surface of accreting white dwarfs (WDs). Although novae are an ubiquitous phenomenon, their properties at low metallicity are not well understood. Using the publicly-available stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa), we model the evolution of accreting carbon–oxygen WDs and consider models that accrete matter with metallicity Z  = 0.02 or 10−4. We consider both models without mixing and with matter enriched by CO-elements assuming that mixing occurs in the process of accretion (with mixing fraction 0.25). We present and contrast ignition mass, ejected mass, recurrence period, and maximum luminosity of novae for different WD masses and accretion rates for these metallicities and mixing cases. We find that models with Z  = 0.02 have ignition masses and recurrence periods smaller than models with low Z , while the ejected mass and maximum luminosity are larger. Retention efficiency during novae outbursts decreases with increasing metallicity. In our implementation, inclusion of mixing at the H/He interface reduces accreted mass, ejected mass, and recurrence period as compared to the no-mixing case, while the maximum luminosity becomes larger. Retention efficiency is significantly reduced, becoming negative in most of our models. For ease of use, we provide a tabular summary of our results. [ABSTRACT FROM AUTHOR]