Nucleosynthesis Of Light Elements Research Articles

The impact of overshoot on the i-process in AGB stars

The production of neutron-rich elements at neutron densities intermediate to those of the s - and r -processes, the so-called has been identified as possibly being responsible for the observed abundance pattern found in certain carbon-enhanced metal-poor (CEMP) stars. The production site may be low-metallicity stars on the asymptotic giant branch (AGB) where the physical processes during the thermal pulses are not well known. We investigate the impact of overshoot from various convective boundaries during the AGB phase on proton ingestion events (PIEs) and the neutron densities as a necessary precondition for the as well as on the structure and continued evolution of the models. We therefore analyzed models of a $ msun $, Z= 5e-5 star. A fiducial model without overshoot on the AGB (overshoot was applied during the pre-AGB evolution) serves as a reference. The same model was then run with various overshoot values and the resulting models were compared to one another. Light element nucleosynthesis is also discussed. Additionally, we introduce a new timescale argument to predict PIE occurrence to discriminate between a physical and a numerical reason for a nonoccurrence. A comparison to observations as well as previous studies was conducted before finally presenting the most promising choice of overshoot parameters for the occurrence of the in low-mass, low-metallicity models. The fiducial model reveals high neutron densities and a persistent split of the pulse-driven convection zone (PDCZ). Overshoot from the PDCZ results in either temporary or permanent remerging of the split PDCZ, influencing the star's structure and evolution. While both overshoot and non-overshoot models exhibit PIEs generating neutron densities suitable for the they lead to varied and $ N O $ ratios and notable Li enhancements. Comparison with previous studies and observations of CEMP-r/s stars suggests that while surface enhancements in our models may be exaggerated, abundance ratios align well. Though, for high values of overshoot from the PDCZ the agreement becomes worse.

Testing CCC+TL Cosmology with Observed Baryon Acoustic Oscillation Features

The primary purpose of this paper is to see how well a recently proposed new model fits (a) the position of the baryon acoustic oscillation (BAO) features observed in the large-scale distribution of galaxies and (b) the angular size measured for the sound horizon due to BAO imprinted in the cosmic microwave background (CMB) anisotropy. The new model is a hybrid model that combines the tired light (TL) theory with a variant of the ΛCDM model in which the cosmological constant is replaced with a covarying coupling constants’ (CCC) parameter α. This model, dubbed the CCC+TL model, can fit the Type Ia supernovae Pantheon+ data as accurately as the ΛCDM model, and also fit the angular size of cosmic dawn galaxies observed by the James Webb Space Telescope, which is in tension with the ΛCDM model. The results we obtained are 151.0 (±5.1) Mpc for the absolute BAO scale at the current epoch, and the angular size of the sound horizon θ sh = 0.°60, matching Planck’s observations at the surface of the last scattering when the baryon density is set to 100% of the matter density and ∣α∣ is increased by 5.6%. It remains to be seen if the new model is consistent with the CMB power spectrum, the Big Bang nucleosynthesis of light elements, and other critical observations.

Investigating black hole accretion disks as potential polluter sources for the formation of enriched stars in globular clusters

Accretion disks surrounding stellar mass black holes have been suggested as potential locations for the nucleosynthesis of light elements, which are our primary observational discriminant of multiple stellar populations within globular clusters. The population of enriched stars in globular clusters are enhanced in N Na and sometimes in Al and/or in K . In this study, our aim is to investigate the feasibility of initiating nucleosynthesis for these four elements in black hole accretion disks, considering various internal parameters such as the temperature of the gas and timescale of the accretion. To achieve this, we employed a 132-species reaction network. We used the slim disk model, suitable for the Super-Eddington mass accretion rate and for geometrically and optically thick disks. We explored the conditions related to the mass, mass accretion rate, viscosity, and radius of the black hole-accretion disk system that would allow for the creation of N Na Al and K before the gas is accreted onto the central object. This happens when the nucleosynthesis timescale is shorter than the viscous timescale. Our findings reveal that there is no region in the parameter space where the formation of Na can occur and only a very limited region where the formation of N Al and K is plausible. Specifically, this occurs for black holes with masses lower than 10 solar masses ($m<10 M_ with a preference toward even lower mass values ($m<1 M_ and extremely low viscosity parameters ($ $). Such values are highly unlikely based on current observations of stellar mass black holes. However, such low mass black holes could actually exist in the early universe, as so-called primordial black holes. In conclusion, our study suggests that the nucleosynthesis within black hole accretion disks of elements of interest for the multiple stellar populations, namely N Na Al and K is improbable, but not impossible, using the slim disk model. Future gravitational wave missions will help constrain the existence of tiny and light black holes.

Study of fusion cross-section and astrophysical S-factor for p+15N and α+12C at sub-barrier energy

Nuclear fusion reactions at low energies ([Formula: see text][Formula: see text]eV to few KeV) are crucial for the primordial nucleosynthesis of light elements. Nuclear fusion reactions in this energy window can be understood fruitfully in terms of quantum mechanical tunneling through the mutual Coulomb barrier of interacting nuclei. In this work, we studied the fusion cross-section, [Formula: see text] and astrophysical S-factor in the selective resonant tunneling model (SRTM) approach for the [Formula: see text]N and [Formula: see text]C nuclear fusion reactions. We used space varying complex Gaussian type nuclear potential here. The quantum mechanical findings of the fusion cross-section, [Formula: see text] and S-factor are in excellent agreement with the experimentally observed results found in the literature. The improved value of astrophysical S-factors for light nuclei may give a more clear picture of the elemental abundance in primordial nucleosynthesis.

Particle Acceleration by Supernova Shocks and Spallogenic Nucleosynthesis of Light Elements

In this review, we first reassess the supernova remnant paradigm for the origin of Galactic cosmic rays in the light of recent cosmic-ray data acquired by the Voyager 1 spacecraft. We then describe the theory of light-element nucleosynthesis by nuclear interaction of cosmic rays with the interstellar medium and outline the problem of explaining the measured beryllium abundances in old halo stars of low metallicity with the standard model of the Galactic cosmic-ray origin. We then discuss the various cosmic-ray models proposed in the literature to account for the measured evolution of the light elements in the Milky Way, and point out the difficulties that they all encounter. It seems to us that, among all possibilities, the superbubble model provides the most satisfactory explanation for these observations.

The Role of 14-MeV Neutrons in Light Element Nucleosynthesis

The 14-MeV neutron-induced nuclear reactions which might contribute to better understanding of the so-called “cosmological lithium problem” have been discussed. The big bang nucleosynthesis theory predicted 7Li overestimated value could be lowered by 2n-induced reactions on Li isotopes and/or 7Be destruction via resonance in the d + 7Be → 9B process. Proposals for 6Li + 2n and 7Li + 2n reactions’ cross section evaluations and measurements are discussed in some details. In addition, we discuss also a preliminary measurements of n–n coincidences from the 10B(n,2n) 9B reaction with two neutron detectors placed outside the cone of 14-MeV tagged neutron beam. From the time-of-flight measurements and known geometry, we tried to deduce the neutron energies and calculate the 9B missing mass spectrum. Improvements for better measurements are suggested.

Abundance of zinc in the red giants of Galactic globular cluster 47 Tucanae

Aims. We investigate possible relations between the abundances of zinc and the light elements sodium, magnesium, and potassium in the atmospheres of red giant branch (RGB) stars of the Galactic globular cluster 47 Tuc and study connections between the chemical composition and dynamical properties of the cluster RGB stars. Methods. The abundance of zinc was determined in 27 RGB stars of 47 Tuc using 1D local thermal equilibrium (LTE) synthetic line profile fitting to the high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian Telescope (AAT). Synthetic spectra used in the fitting procedure were computed with the SYNTHE code and 1D ATLAS9 stellar model atmospheres. Results. The average 1D LTE zinc-to-iron abundance ratio and its RMS variations due to star-to-star abundance spread determined in the sample of 27 RGB stars is 〈[Zn/Fe]〉1D LTE = 0.11 ± 0.09. We did not detect any statistically significant relations between the abundances of zinc and those of light elements. Neither did we find any significant correlation or anticorrelation between the zinc abundance in individual stars and their projected distance from the cluster center. Finally, no statistically significant relation between the absolute radial velocities of individual stars and the abundance of zinc in their atmospheres was detected. The obtained average [Zn/Fe]1DLTE ratio agrees well with those determined in this cluster in earlier studies and nearly coincides with that of Galactic field stars at this metallicity. All these results suggest that nucleosynthesis of zinc and light elements proceeded in separate, unrelated pathways in 47 Tuc.

The role of 14 MeV neutrons in light element nucleosynthesis

The 14 MeV neutron induced nuclear reactions which might contribute to better understanding of so-called “cosmological lithium problem” have been discussed. The BBN theory predicted 7Li overestimated value could be lowered by 2n induced reactions on Li-isotopes and/or 7Be destruction via resonance in d+7Be→9B process. Proposals for 6Li+2n and 7Li+2n reactions cross section evaluations and measurements are discussed in some details. We discuss also a preliminary measurements of n-n coincidences from the 10B(n,2n)9B reaction with two neutron detectors placed outside the cone of 14 MeV tagged neutron beam. From the time-of-flight measurements and known geometry we deduce the neutron energies and calculate the 9B missing mass spectrum.

Study of the nucleon radiative captures 8Li(n,γ)9Li, 9Be(p,γ)10B, 10Be(n,γ)11Be, 10B(p,γ)11C, and 16O(p,γ)17F at thermal and astrophysical energies

We have studied the neutron-capture reactions 8Li([Formula: see text]Li and its role in the primordial nucleosynthesis. The [Formula: see text] reaction has a significant astrophysical interest because it includes one of the variants of chain of primordial nucleosynthesis processes of the Universe and thermonuclear reactions in type II supernovae. Furthermore, we consider the [Formula: see text] reaction in the astrophysical energy range in the modified potential cluster model (MPCM) with splitting of orbital states according to Young tableaux and, in some cases, with forbidden states (FS). The reaction [Formula: see text] plays an important role in primordial and stellar nucleosynthesis of light elements in the [Formula: see text] shell. Hydrogen burning in second-generation stars occurs via the proton–proton (pp) chain and CNO cycle, with the [Formula: see text] reaction serving as an intermediate link between these cycles. Furthermore, the possibility of describing available experimental data for the total reaction cross-sections of neutron radiative capture on [Formula: see text]Be at thermal and astrophysical energies has been shown. This reaction is a part of one of the variants of the chain of primordial nucleosynthesis of the Universe due to which the elements with a mass of [Formula: see text] may be formed. The results in the field of study of thermonuclear proton-capture reaction on [Formula: see text] at ultralow, i.e., astrophysical energies will be presented further. The possibility of description of the experimental data for the astrophysical [Formula: see text]-factor of the proton radiative capture on [Formula: see text]O to the ground state (GS) of [Formula: see text]F was considered in the frame of the MPCM with FS and classification of the states according to Young tableaux. It was shown that on the basis of the [Formula: see text] transitions from the states of [Formula: see text]O scattering to the GS of [Formula: see text]F in the [Formula: see text]O channel generally succeed to explain the value of measured cross-sections at astrophysical energies.

Shell and explosive hydrogen burning

The nucleosynthesis of light elements, from helium up to silicon, mainly occurs in Red Giant and Asymptotic Giant Branch stars and Novae. The relative abundances of the synthesized nuclides critically depend on the rates of the nuclear processes involved, often through non-trivial reaction chains, combined with complex mixing mechanisms. In this review, we summarize the contributions made by LUNA experiments in furthering our understanding of nuclear reaction rates necessary for modeling nucleosynthesis in AGB stars and Novae explosions.

Reaction models in nuclear astrophysics

We present different reaction models commonly used in nuclear astrophysics, in particular for the nucleosynthesis of light elements. Pioneering works were performed within the potential model, where the internal structure of the colliding nuclei is completely ignored. Significant advances in microscopic cluster models provided the first microscopic description of the 3 He(α,γ ) 7 Be reaction more than thirty years ago. In this approach, the calculations are based on an effective nucleon-nucleon interaction, but the cluster approximation should be made to simplify the calculations. Nowadays, modern microscopic calculations are able to go beyond the cluster approximation, and aim at finding exact solutions of the Schrodinger equation with realistic nucleon-nucleon interactions. We discuss recent examples on the d + d reactions at low energies.

S-process production in rotating massive stars at solar and low metallicities

Rotation was shown to have a strong impact on the structure and light element nucleosynthesis in massive stars. In particular, models including rotation can reproduce the primary nitrogen observed in halo extremely metal-poor (EMP) stars. Additional exploratory models showed that rotation may enhance $s$-process production at low metallicity. Here we present a large grid of massive star models including rotation and a full $s$-process network to study the impact of rotation on the weak $s$-process. We explore the possibility of producing significant amounts of elements beyond the strontium peak, which is where the weak $s$-process usually stops. We used the Geneva stellar evolution code coupled to an enlarged reaction network with 737 nuclear species up to bismuth to calculate $15-40\,\text{M}_\odot$ models at four metallicities ($Z = 0.014,10^{-3}$, $10^{-5}$, and $10^{-7}$) from the main sequence up to the end of oxygen burning. We confirm that rotation-induced mixing between the convective H-shell and He-core enables an important production of primary $^{14}$N and $^{22}$Ne and $s$-process at low metallicity. At low metallicity, even though the production is still limited by the initial number of iron seeds, rotation enhances the $s$-process production, even for isotopes heavier than strontium, by increasing the neutron to seed ratio. The increase in this ratio is a direct consequence of the primary production of $^{22}$Ne. Despite nuclear uncertainties affecting the $s$-process production and stellar uncertainties affecting the rotation-induced mixing, our results show a robust production of $s$ process at low metallicity when rotation is taken into account. Considering models with a distribution of initial rotation rates enables to reproduce the observed large range of the [Sr/Ba] ratios in (carbon-enhanced and normal) EMP stars.

Spin Responses in Nuclei and Nuclear Weak Processes in Stars

New shell-model Hamiltonians which can successfuly describe spin-dependent transition rates in nuclei are applied to study nuclear weak processes in stars. New v-induced reaction cross sections in 12C and 56Fe evaluated by the new Hamiltonians are shown to reproduce well the experimental data. Nucleosynthesis of light elements in supernova explosions (SNe) as well as v oscillation effects are discussed with the new cross sections. Electron capture and β-decay rates in stellar environments are re-evaluated in fp- and sd-shell nuclei. Nucleosynthesis in Type-Ia SNe, rp-process and X-ray burst are discussed with the new reaction rates in Ni isotopes. Important roles of accurate e-capture and β-decay rates in sd-shell nuclei on the cooling of stars with 8-10 solar masses by nuclear URCA processes and the fate of the stars are demonstrated. β-decay half-lives of waiting-point nuclei at N =126 are evaluated by shell- model calculations, and r-process nucleosynthesis up to Th and U region in both core-collapse SNe and binary neutron star mergers are studied.

Non-standard s-process in low metallicity massive rotating stars

Context. Rotation is known to affect the nucleosynthesis of light elements in massive stars, mainly by rotation-induced mixing. In particular, rotation boosts the primary nitrogen production. Models of rotating stars are able to reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the first grid of stellar models for rotating massive stars at low Z, where a full s-process network is used to study the impact of rotation-induced mixing on the nucleosynthesis of heavy elements. Methods. We used the Geneva stellar evolution code that includes an enlarged reaction network with nuclear species up to bismuth to calculate 25 M$_\odot$ models at three different Z and with different initial rotation rates. Results. First, we confirm that rotation-induced mixing leads to a production of primary $^{22}$Ne, which is the main neutron source for the s process in massive stars. Therefore rotation boosts the s process in massive stars at all Z. Second, the neutron-to-seed ratio increases with decreasing Z in models including rotation, which leads to the complete consumption of all iron seeds at Z < 1e-3 by the end of core He-burning. Thus at low Z, the iron seeds are the main limitation for this boosted s process. Third, as Z decreases, the production of elements up to the Ba peak increases at the expense of the elements of the Sr peak. We studied the impact of the initial rotation rate and of the uncertain $^{17}$O$(\alpha,\gamma)$ rate (which strongly affects the neutron poison strength of $^{16}$O) on our results. This study shows that rotating models can produce significant amounts of elements up to Ba over a wide range of Z. Fourth, compared to the He-core, the primary $^{22}$Ne production in the He-shell is even higher (> 1% in mass fraction at all Z), which could open the door for an explosive neutron capture nucleosynthesis in the He-shell, with a primary neutron source.

Colloquium: The neutron lifetime

The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and is the simplest example of nuclear beta decay. It played a key role in the early Universe as it determined the ratio of neutrons to protons during the era of primordial light element nucleosynthesis. Neutron decay is physically related to important processes in solar physics and neutrino detection. The mean neutron lifetime has been the subject of more than 20 major experiments done, using a variety of methods, between 1950 and the present. The most precise recent measurements have stated accuracies approaching 0.1%, but are not in good agreement as they differ by as much as $5\ensuremath{\sigma}$ using quoted uncertainties. The history of neutron lifetime measurements is reviewed and the different methods used are described, giving important examples of each. The discrepancies and some systematic issues in the experiments that may be responsible are discussed, and it is shown by means of global averages that the neutron lifetime is likely to lie in the range of 880--884 s. Plans and prospects for future experiments are considered that will address these systematic issues and improve our knowledge of the neutron lifetime.

Spin-dependent nuclear weak processes and nucleosynthesis in stars

Spin dependent nuclear weak processes and nucleosynthesis in stars are investigated based on recent advances in shell model studies of stable and unstable exotic nuclei. Three topics on (1) neutrino–nucleus reactions in supernova explosions and nucleosynthesis of light elements as well as Mn, (2) electron capture reaction rates on Ni and Co isotopes at high densities and temperatures in the core-collapse process, and (3) new β -decay half-lives of N=126 isotones obtained by including both the Gamow–Teller and the first-forbidden transitions, and the effects on the element abundance in the r -process at the third peak region (A∼195), are studied with the use of new shell model Hamiltonians.

Constraining spacetime noncommutativity with primordial nucleosynthesis

We discuss a constraint on the scale $\Lambda_{\rm NC}$ of noncommutative (NC) gauge field theory arising from consideration of the big bang nucleosynthesis (BBN) of light elements. The propagation of neutrinos in the NC background described by an antisymmetric tensor $\theta^{\mu\nu}$ does result in a tree-level vector-like coupling to photons in a generation-independent manner, raising thus a possibility to have an appreciable contribution of three light right-handed (RH) fields to the energy density of the universe at nucleosynthesis time. Considering elastic scattering processes of the RH neutrinos off charged plasma constituents at a given cosmological epoch, we obtain for a conservative limit on an effective number of additional doublet neutrinos, $\Delta N_\nu =1$, a bound $\Lambda_{\rm NC} \stackrel{>}{\sim}$ 3 TeV. With a more stringent requirement, $\Delta N_\nu \lesssim 0.2$, the bound is considerably improved, $\Lambda_{\rm NC} \stackrel{>}{\sim} 10^3$ TeV. For our bounds the $\theta$-expansion of the NC action stays always meaningful, since the decoupling temperature of the RH species is perseveringly much less than the inferred bound for the scale of noncommutativity.

Supernova Neutrino Nucleosynthesis of Light Elements with Neutrino Oscillations

Light element synthesis in supernovae through neutrino-nucleus interactions, i.e., the v process, is affected by neutrino oscillations in the supernova environment. There is a resonance of 13-mixing in the O/C layer, which increases the rates of charged-current -process reactions in the outer He-rich layer. The yields of 7Li and 11B increase by about a factor of 1.9 and 1.3, respectively, for a normal mass hierarchy and an adiabatic 13-mixing resonance, compared to those without neutrino oscillations. In the case of an inverted mass hierarchy and a nonadiabatic 13-mixing resonance, the increase in the 7Li and 11B yields is much smaller. Observations of the 7Li/11B ratio in stars showing signs of supernova enrichment could thus provide a unique test of neutrino oscillations and constrain their parameters and the mass hierarchy.

Roles of Supernova Ejecta in Nucleosynthesis of the Light Elements Li, Be, and B

Explosions of Type Ic supernovae (SNe Ic) are investigated using a relativistic hydrodynamic code to study the role of their outermost layers of the ejecta in light-element nucleosynthesis through spallation reactions as a possible mechanism of the ‘‘primary’’ process. We have confirmed that the energy distribution of the outermost layers with a mass fraction of only 0.001% follows the empirical formula proposed by previous work when the explosion is furious. In such explosions, a significant fraction of the ejecta (>0.1% in mass) have energy greater than the threshold energy for spallation reactions. On the other hand, we find that the outermost layers of ejecta become more energetic than the empirical formula would predict when the explosion energy per unit ejecta mass is smaller than � 1:3;10 51 ergs M � 1 � . As a consequence, it is necessary to numerically calculate explosions to estimate light-element yields from SNe Ic. The usage of the empirical formula would overestimate the yields by a factor of k3 for energetic explosions, such as SN 1998bw, and underestimate the yields by a similar factor for less energetic explosions, such as SN 1994I. The yields of the light elements Li, Be, and B (LiBeB) from SNe Ic are estimated by solving the transfer equation of cosmic rays originated from ejecta of SNe Ic and compared with observations. The abundance ratios Be/O and B/O produced by each of our SNe Ic models are consistent with those of metal-poor stars. The total amounts of these elements estimated from observations indicate that energetic SNe Ic, such as SN 1998bw, could be candidates for a production site of Be and B in the Galactic halo only when the fraction of this type out of all the SNe was more than a factor of 100 higher than the value estimated from current observational data. This primary mechanism would predict that there are stars significantly deficient in light elements that were formed from the interstellar medium not affected by SNe Ic. Since this has no support from current observations, other primary mechanisms, such as the light-element formation in superbubbles, are needed for other types of SNe. The observed abundance pattern of all elements including heavy elements in metal-poor stars suggests that these two mechanisms should have supplied similar amounts of Be and B. Our calculations show that SNe Ic can not produce an appreciable amount of Li. Subject heading gs: cosmic rays — nuclear reactions, nucleosynthesis, abundances — relativity — shock waves — supernovae: general

Nucleosynthesis of Light Elements and Heavyr‐Process Elements through the ν‐Process in Supernova Explosions

We study the nucleosynthesis of the light elements 7Li and 11B and the r-process elements in Type II supernovae from the point of view of supernova neutrinos and Galactic chemical evolution. We investigate the influence of the luminosity and average energy (temperature) of supernova neutrinos on these two nucleosynthesis processes. Common models of the neutrino luminosity, which is parameterized by the total energy Eν and decay time τν, and neutrino temperature are adopted to understand both processes. We adopt the model of the supernova explosion of a 16.2 M☉ star, which corresponds to SN 1987A, and calculate the nucleosynthesis of the light elements by postprocessing. We find that the ejected masses of 7Li and 11B are roughly proportional to the total neutrino energy and are weakly dependent on the decay time of the neutrino luminosity. As for the r-process nucleosynthesis, we adopt the same models of the neutrino luminosity in the neutrino-driven wind models of a 1.4 M☉ neutron star. We find that the r-process nucleosynthesis is affected through the peak neutrino luminosity, which depends on Eν/τν. The observed r-process abundance pattern is better reproduced at a low peak neutrino luminosity. We also discuss the unresolved problem of the overproduction of 11B in the Galactic chemical evolution of the light elements. We first identify that the ejected mass of 11B is a factor of 2.5-5.5 overproduced in Type II supernovae when one adopts neutrino parameters similar to those in previous studies, i.e., Eν = 3.0 × 1053 ergs, τν = 3 s, and a neutrino temperature T = T = 8.0 MeV/k. We have to assume Eν ≤ 1.2 × 1053 ergs to avoid the overproduction of 11B, which is too small to accept in comparison to the 3.0 × 1053 ergs deduced from the observation of SN 1987A. We here propose to reduce the temperatures of νμ,τ and μ,τ to 6.0 MeV/k in a model with Eν ~ 3.0 × 1053 ergs and τν ~ 9 s. This modification of the neutrino temperature is shown to resolve the overproduction problem of 11B while still keeping a successful r-process abundance pattern.