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Alain Coc

Publications and source records attributed to Alain Coc.

At least 19 recordsLinked to original sources
arXiv2022

Thermonuclear reaction rate of $^{29}$Si(p,$\gamma$)$^{30}$P

The thermonuclear rate of the $^{29}$Si(p,$\gamma$)$^{30}$P reaction impacts the $^{29}$Si abundance in classical novae. A reliable reaction rate is essential for testing the nova paternity of presolar stardust grains. At present, the fact that no classical nova grains have been unambiguously identified in primitive meteorites among thousands of grains studied is puzzling, considering that classical novae are expected to be prolific producers of dust grains. We investigated the $^{29}$Si $+$ $p$ reaction at center-of-mass energies of $200$ $-$ $420$~keV, and present improved values for resonance energies, level excitation energies, resonance strengths, and branching ratios. One new resonance was found at a center-of-mass energy of $303$ keV. For an expected resonance at $215$~keV, an experimental upper limit could be determined for the strength. We evaluated the level structure near the proton threshold, and present new reaction rates based on all the available experimental information. Our new reaction rates have much reduced uncertainties compared to previous results at temperatures of $T$ $\ge$ $140$~MK, which are most important for classical nova nucleosynthesis. Future experiments to improve the reaction rates at lower temperatures are discussed.

nucl-ex↗
arXiv2022

Hydrogen burning of $^{29}$Si and its impact on presolar stardust grains from classical novae

Presolar stardust grains found in primitive meteorites are believed to retain the isotopic composition of stellar outflows at the time of grain condensation. Therefore, laboratory measurements of their isotopic ratios represent sensitive probes for investigating open questions related to stellar evolution, stellar explosions, nucleosynthesis, mixing mechanisms, dust formation, and galactic chemical evolution. For a few selected presolar grains, classical novae have been discussed as a potential source. For SiC, silicate, and graphite presolar grains, the association is based on the observation of small $N(^{12}$C)/$N(^{13}$C) and $N(^{14}$N)/$N(^{15}$N) number abundance ratios compared to solar values, and abundance excesses in $^{30}$Si relative to $^{29}$Si, as previously predicted by models of classical novae. We report on a direct measurement of the $^{29}$Si(p,$\gamma$)$^{30}$P reaction, which strongly impacts simulated $\delta ^{29}$Si values from classical novae. Our new experimental $^{29}$Si(p,$\gamma$)$^{30}$P thermonuclear reaction rate differs from previous results by up to 50\% in the classical nova temperature range ($T$ $=$ $100$ $-$ $400$~MK), while the rate uncertainty is reduced by up to a factor of $3$. Using our new reaction rate in Monte Carlo reaction network and hydrodynamic simulations of classical novae, we estimate $\delta ^{29}$Si values with much reduced uncertainties. Our results establish $\delta ^{29}$Si values measured in presolar grains as a sensitive probe for assessing their classical nova paternity. We also demonstrate that $\delta ^{30}$Si values from nova simulations are presently not a useful diagnostic tool unless the large uncertainty of the $^{30}$P(p,$\gamma$)$^{31}$S reaction rate can be significantly reduced.

astro-ph.HE↗
arXiv2021

Bayesian Estimation of the D(p,$\gamma$)$^3$He Thermonuclear Reaction Rate

Big bang nucleosynthesis (BBN) is the standard model theory for the production of the light nuclides during the early stages of the universe, taking place for a period of about 20 minutes after the big bang. Deuterium production, in particular, is highly sensitive to the primordial baryon density and the number of neutrino species, and its abundance serves as a sensitive test for the conditions in the early universe. The comparison of observed deuterium abundances with predicted ones requires reliable knowledge of the relevant thermonuclear reaction rates, and their corresponding uncertainties. Recent observations reported the primordial deuterium abundance with percent accuracy, but some theoretical predictions based on BBN are at tension with the measured values because of uncertainties in the cross section of the deuterium-burning reactions. In this work, we analyze the S-factor of the D(p,$\gamma$)$^3$He reaction using a hierarchical Bayesian model. We take into account the results of eleven experiments, spanning the period of 1955--2021; more than any other study. We also present results for two different fitting functions, a two-parameter function based on microscopic nuclear theory and a four-parameter polynomial. Our recommended reaction rates have a 2.2\% uncertainty at $0.8$~GK, which is the temperature most important for deuterium BBN. Differences between our rates and previous results are discussed.

astro-ph.CO↗
arXiv2020

A new tension in the cosmological model from primordial deuterium?

Recent measurements of the D(p,$\gamma)^3$He, nuclear reaction cross-section and of the neutron lifetime, along with the reevaluation of the cosmological baryon abundance from cosmic microwave background (CMB) analysis, call for an update of abundance predictions for light elements produced during the big-bang nucleosynthesis (BBN). While considered as a pillar of the hot big-bang model in its early days, BBN constraining power mostly rests on deuterium abundance. We point out a new $\simeq1.8\sigma$-tension on the baryonic density, or equivalently on the D/H abundance, between the value inferred on one hand from the analysis of the primordial abundances of light elements and, on the other hand, from the combination of CMB and baryonic oscillation data. This draws the attention on this sector of the theory and gives us the opportunity to reevaluate the status of BBN in the context of precision cosmology. Finally, this paper presents an upgrade of the BBN code PRIMAT.

astro-ph.CO↗
arXiv2020

Thermonuclear reaction rates and primordial nucleosynthesis

Assuming the best numerical value for the cosmic baryonic density and the existence of three neutrino flavors, standard big bang nucleosynthesis is a parameter-free model. It is important to assess if the observed primordial abundances can be reproduced by simulations. Numerous studies have shown that the simulations overpredict the primordial $^7$Li abundance by a factor of $\approx$ $3$ compared to the observations. The discrepancy may be caused by unknown systematics in $^7$Li observations, poorly understood depletion of lithium in stars, errors in thermonuclear rates that take part in the lithium and beryllium synthesis, or physics beyond the standard model. Here, we focus on the likelihood of a nuclear physics solution. The status of the key nuclear reaction rates is summarized. Big bang nucleosynthesis simulations are performed with the most recent reaction rates and the uncertainties of the predicted abundances are established using a Monte Carlo technique. Correlations between abundances and reaction rates are investigated based on the metric of mutual information. The rates of four reactions impact the primordial $^7$Li abundance: $^3$He($\alpha$,$\gamma$)$^7$Be, d(p,$\gamma$)$^3$He, $^7$Be(d,p)2$\alpha$, and $^7$Be(n,p)$^7$Li. We employ a genetic algorithm to search for simultaneous rate changes in these four reactions that may account for all observed primordial abundances. When the search is performed for reaction rate ranges that are much wider than recently reported uncertainties, no acceptable solutions are found. Based on the currently available evidence, we conclude that it is highly unlikely for the cosmological lithium problem to have a nuclear physics solution.

astro-ph.CO↗
arXiv2019

Hierarchical Bayesian Thermonuclear Rate for the $^7$Be(n,p)$^7$Li Big Bang Nucleosynthesis Reaction

Big bang nucleosynthesis provides the earliest probe of standard model physics, at a time when the universe was less than a thousand seconds old. It determines the abundances of the lightest nuclides, which give rise to the subsequent history of the visible matter in the Universe. This work derives new $^7$Be(n,p)$^7$Li thermonuclear reaction rates based on all available experimental information. This reaction sensitively impacts the primordial abundances of $^{7}$Be and $^7$Li during big bang nucleosynthesis. We critically evaluate all available data and disregard experimental results that are questionable. For the nuclear model, we adopt an incoherent sum of single-level, two-channel R-matrix approximation expressions, which are implemented into a hierarchical Bayesian model, to analyze the remaining six data sets we deem most reliable. In the fitting of the data, we consistently model all known sources of uncertainty, including discrepant absolute normalizations of different data sets, and also take the variation of the neutron and proton channel radii into account, hence providing less biased estimates of the $^7$Be(n,p)$^7$Li thermonuclear rates. From the resulting posteriors, we extract R-matrix parameters ($E_r$, $\gamma^2_n$, $\gamma^2_p$) and derive excitation energies, partial and total widths. Our fit is sensitive to the contributions of the first three levels above the neutron threshold. Reaction rates were computed by integrating 10,000 samples of the reduced cross section. Our $^7$Be(n,p)$^7$Li thermonuclear rates have uncertainties between 1.5% and 2.0% at temperatures of $\leq$1 GK. We compare our rates to previous results and find that the $^7$Be(n,p)$^7$Li rates most commonly used in big bang simulations have too optimistic uncertainties.

astro-ph.CO↗
arXiv2019

Comment on "Measurement of d + 7Be Cross Sections for Big-Bang Nucleosynthesis"

In a recent Letter entitled "Measurement of d + 7Be Cross Sections for Big-Bang Nucleosynthesis", Rijal et al. describe the results of an experiment that put the d + 7Be reaction rate on firmer ground. The erratum of Rijal et al. claims that including this reaction rate in big bang nucleosynthesis brings a significant reduction of up to 8.1% of lithium production relative to calculations that exclude this reaction. In contrast, using several well tested BBN codes, we confirm earlier studies showing that the effect is less than 1%, so that this measurement cannot alleviate the cosmological lithium problem.

astro-ph.CO↗
arXiv2019

Precision Big Bang Nucleosynthesis with the New Code PRIMAT

Precision on primordial abundances, deduced from observations, have now reached the percent level for 4He and deuterium. Precision on big bang nucleosynthesis (BBN) predictions should, hence, reach the same level. The uncertainty on the 4He mass fraction is strongly affected by theoretical uncertainties on the weak reaction rates that interconvert neutrons with protons. All these corrections have been calculated in a self-consistent manner and implemented in a new, and public, Mathematica code PRIMAT, together with an extensive data base of reaction rates. Both can be obtained at http://www2.iap.fr/users/pitrou/primat.htm.

astro-ph.CO↗
arXiv2019

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.

astro-ph.HE↗
arXiv2019

Thermonuclear fusion rates for tritium + deuterium using Bayesian methods

The $^3$H(d,n)$^4$He reaction has a large low-energy cross section and will likely be utilized in future commercial fusion reactors. This reaction also takes place during big bang nucleosynthesis. Studies of both scenarios require accurate and precise fusion rates. To this end, we implement a one-level, two-channel R-matrix approximation into a Bayesian model. Our main goals are to predict reliable astrophysical S-factors and to estimate R-matrix parameters using the Bayesian approach. All relevant parameters are sampled in our study, including the channel radii, boundary condition parameters, and data set normalization factors. In addition, we take uncertainties in both measured bombarding energies and S-factors rigorously into account. Thermonuclear rates and reactivities of the $^3$H(d,n)$^4$He reaction are derived by numerically integrating the Bayesian S-factor samples. The present reaction rate uncertainties at temperatures between $1.0$ MK and $1.0$ GK are in the range of 0.2% to 0.6%. Our reaction rates differ from previous results by 2.9% near 1.0 GK. Our reactivities are smaller than previous results, with a maximum deviation of 2.9% near a thermal energy of $4$ keV. The present rate or reactivity uncertainties are more reliable compared to previous studies that did not include the channel radii, boundary condition parameters, and data set normalization factors in the fitting. Finally, we investigate previous claims of electron screening effects in the published $^3$H(d,n)$^4$He data. No such effects are evident and only an upper limit for the electron screening potential can be obtained.

nucl-th↗
arXiv2018

Astrophysical S-factors, thermonuclear rates, and electron screening potential for the $^3$He(d,p)$^{4}$He Big Bang reaction via a hierarchical Bayesian model

We developed a hierarchical Bayesian framework to estimate S-factors and thermonuclear rates for the $^3$He(d,p)$^{4}$He reaction, which impacts the primordial abundances of $^3$He and $^7$Li. The available data are evaluated and all direct measurements are taken into account in our analysis for which we can estimate separate uncertainties for systematic and statistical effects. For the nuclear reaction model, we adopt a single-level, two-channel approximation of R-matrix theory, suitably modified to take the effects of electron screening at lower energies into account. Apart from the usual resonance parameters (resonance location and reduced widths for the incoming and outgoing reaction channel), we include for the first time the channel radii and boundary condition in the fitting process. Our new analysis of the $^3$He(d,p)$^{4}$He S-factor data results in improved estimates for the thermonuclear rates. This work represents the first nuclear rate evaluation using the R-matrix theory embedded into a hierarchical Bayesian framework, properly accounting for all known sources of uncertainty. Therefore, it provides a test bed for future studies of more complex reactions.

astro-ph.IM↗
arXiv2018

Gamma-Ray Observations of Nova Sgr 2015 No. 2 with INTEGRAL

INTEGRAL observed the nova V5668 Sgr around the time of its optical maximum on March 21, 2015. Studies at UV wavelengths showed spectral lines of freshly produced Be-7. This could be measurable also in gamma-rays at 478 keV from the decay to Li-7. Novae are also expected to synthesise Na-22 which decays to Ne-22, emitting a 1275 keV photon. About one week before the optical maximum, a strong gamma-ray flash on time-scales of hours is expected from short-lived radioactive nuclei, such as N-13 and F-18. These beta-plus-unstable nuclei should yield emission up to 511 keV, but which has never been observed. The spectrometer SPI aboard INTEGRAL pointed towards V5668 by chance. We use these observations to search for possible gamma-ray emission of decaying Be-7, and to directly measure the synthesised mass during explosive burning. We also aim to constrain possible burst-like emission days to weeks before the optical maximum using the SPI anticoincidence shield (ACS). We extract spectral and temporal information to determine the fluxes of gamma-ray lines at 478 keV, 511 keV, and 1275 keV. A measured flux value directly converts into abundances produced by the nova. The SPI-ACS rates are analysed for burst-like emission using a nova model light-curve. For the obtained nova flash candidate events, we discuss possible origins. No significant excess for the expected gamma-ray lines is found. Our upper limits on the synthesised Be-7 and Na-22 mass depend on the uncertainties of the distance to the nova: The Be-7 mass is constrained to less than $4.8\times10^{-9}\,(d/kpc)^2$, and Na-22 to less than $2.4\times10^{-8}\,(d/kpc)^2$ solar masses. For the Be-7 mass estimate from UV studies, the distance to V5668 Sgr must be larger than 1.2 kpc. During three weeks before the optical maximum, we find 23 burst-like events in the ACS rate, of which six could possibly be associated with V5668 Sgr.

astro-ph.HE↗
arXiv2018

Precision big bang nucleosynthesis with improved Helium-4 predictions

Primordial nucleosynthesis is one of the three historical evidences for the big bang model, together with the expansion of the universe and the cosmic microwave background. Now that the number of neutrino families and the baryonic densities have been fixed by laboratory measurements or CMB observations, the model has no free parameter and its predictions are rigid. Departure from its predictions could provide hints or constraints on new physics or astrophysics in the early universe. Precision on primordial abundances deduced from observations have recently been drastically improved and reach the percent level for both deuterium and helium-4. Accordingly, the BBN predictions should reach the same level of precision. For most isotopes, the dominant sources of uncertainty come from those on the laboratory thermonuclear reactions. This article focuses on helium-4 whose predicted primordial abundance depends essentially on weak interactions which control the neutron-proton ratio. The rates of the various weak interaction processes depend on the experimentally measured neutron lifetime, but also includes numerous corrections that we thoroughly investigate here. They are the radiative, zero-temperature, corrections, finite nucleon mass corrections, finite temperature radiative corrections, weak-magnetism, and QED plasma effects, which are for the first time all included and calculated in a self consistent way, allowing to take into account the correlations between them, and verifying that all satisfy detailed balance. The helium-4 predicted mass fraction is $0.24709\pm0.00017$. In addition, we provide a Mathematica code (PRIMAT) that incorporates, not only these corrections but also a full network of reactions, using the best available thermonuclear reaction rates, allowing the predictions of primordial abundances up to the CNO region.

astro-ph.CO↗
arXiv2017

Effects of axions on Population III stars

Following the renewed interest for axions as a dark matter component, we revisit the effects of energy loss by axion emission on the evolution of the first generation of stars. These stars with zero metallicity are supposed to be massive, more compact, and hotter than subsequent generations. It is hence important to extend previous studies restricted to solar metallicity stars. Our analysis first compares the evolution of solar metallicity 8, 10 and 12 M$_\odot$ stars to previous work. We then calculate the evolution of 8 zero metallicity stars with and without axion losses and with masses ranging from 20 to 150 M$_\odot$. For the solar metallicity models, we confirm the disappearance of the blue loop phase for a value of the axion-photon coupling, $g_{a\gamma}=10^{-10} {\rm GeV}^{-1}$. Regarding the Pop. III stars, we show that for $g_{a\gamma}=10^{-10} {\rm GeV}^{-1}$, their evolution is not much affected by axion losses except within the range of masses 80$-$130 M$_\odot$. Such stars show significant differences both in their tracks within the $T_c$-$\rho_c$ diagram and in their central composition (in particular $^{20}$Ne and $^{24}$Mg). We discuss the origin of these modifications from the stellar physics point of view, and their potential observational signatures.

astro-ph.SR↗
arXiv2017

Primordial nucleosynthesis

Primordial nucleosynthesis, or big bang nucleosynthesis (BBN), is one of the three evidences for the big bang model, together with the expansion of the universe and the cosmic microwave background. There is a good global agreement over a range of nine orders of magnitude between abundances of 4He, D, 3He and 7Li deduced from observations, and calculated in primordial nucleosynthesis. However, there remains a yet-unexplained discrepancy of a factor 3, between the calculated and observed lithium primordial abundances, that has not been reduced, neither by recent nuclear physics experiments, nor by new observations. The precision in deuterium observations in cosmological clouds has recently improved dramatically, so that nuclear cross-sections involved in deuterium BBN needs to be known with similar precision. We will briefly discuss nuclear aspects related to the BBN of Li and D, BBN with nonstandard neutron sources, and finally, improved sensitivity studies using a Monte Carlo method that can be used in other sites of nucleosynthesis.

astro-ph.CO↗
arXiv2016

Primordial Nucleosynthesis

Primordial or big bang nucleosynthesis (BBN) is now a parameter free theory whose predictions are in good overall agreement with observations. However, the 7Li calculated abundance is significantly higher than the one deduced from spectroscopic observations. Most solutions to this lithium problem involve a source of extra neutrons that inevitably leads to an increase of the deuterium abundance. This seems now to be excluded by recent deuterium observations that have drastically reduced the uncertainty on D/H and also calls for improved precision on thermonuclear reaction rates.

astro-ph.CO↗
arXiv2016

Bayesian Estimation of Thermonuclear Reaction Rates

The problem of estimating non-resonant astrophysical S-factors and thermonuclear reaction rates, based on measured nuclear cross sections, is of major interest for nuclear energy generation, neutrino physics, and element synthesis. Many different methods have been applied in the past to this problem, almost all of them based on traditional statistics. Bayesian methods, on the other hand, are now in widespread use in the physical sciences. In astronomy, for example, Bayesian statistics is applied to the observation of extra-solar planets, gravitational waves, and type Ia supernovae. However, nuclear physics, in particular, has been slow to adopt Bayesian methods. We present astrophysical S-factors and reaction rates based on Bayesian statistics. We develop a framework that incorporates robust parameter estimation, systematic effects, and non-Gaussian uncertainties in a consistent manner. The method is applied to the d(p,$\gamma$)$^3$He, $^3$He($^3$He,2p)$^4$He, and $^3$He($\alpha$,$\gamma$)$^7$Be reactions, important for deuterium burning, solar neutrinos, and big bang nucleosynthesis.

astro-ph.SR↗