arXiv subjects

Alain Coc

Publications and source records attributed to Alain Coc.

53 records · Page 3Linked to original sources

arXiv2008

Nucleosynthesis in novae: experimental progress in the determination of nuclear reaction rates

The sources of nuclear uncertainties in nova nucleosynthesis have been identified using hydrodynamical nova models. Experimental efforts have followed and significantly reduced those uncertainties. This is important for the evaluation of nova contribution to galactic chemical evolution, gamma--ray astronomy and possibly presolar grain studies. In particular, estimations of expected gamma-ray fluxes are essential for the planning of observations with existing or future satellites.

astro-ph↗

arXiv2007

Coupled Variations of Fundamental Couplings and Primordial Nucleosynthesis

The effect of variations of the fundamental nuclear parameters on big-bang nucleosynthesis are modeled and discussed in detail taking into account the interrelations between the fundamental parameters arising in unified theories. Considering only \he4, strong constraints on the variation of the neutron lifetime, neutron-proton mass difference are set. These constraints are then translated into constraints on the time variation of the Yukawa couplings and the fine structure constant. Furthermore, we show that a variation of the deuterium binding energy is able to reconcile the \li7 abundance deduced from the WMAP analysis with its spectroscopically determined value while maintaining concordance with D and \he4. The analysis is strongly based on the potential model of Flambaum and Shuryak that relates the binding energy of the deuteron with the nucleon and $σ$ and $ω$ mesons masses, however, we show that an alternative approach that consists of a pion mass dependence necessarily leads to equivalent conclusions.

astro-ph↗

arXiv2006

Big bang nucleosynthesis constraints on scalar-tensor theories of gravity

We investigate BBN in scalar-tensor theories of gravity with arbitrary matter couplings and self-interaction potentials. We first consider the case of a massless dilaton with a quadratic coupling to matter. We perform a full numerical integration of the evolution of the scalar field and compute the resulting light element abundances. We demonstrate in detail the importance of particle mass thresholds on the evolution of the scalar field in a radiation dominated universe. We also consider the simplest extension of this model including a cosmological constant in either the Jordan or Einstein frame.

astro-ph↗

arXiv2004

Compilation and R-matrix analysis of Big Bang nuclear reaction rates

We use the R-matrix theory to fit low-energy data on nuclear reactions involved in Big Bang nucleosynthesis. A special attention is paid to the rate uncertainties which are evaluated on statistical grounds. We provide S factors and reaction rates in tabular and graphical formats.

astro-ph↗

arXiv2004

Updated Big-Bang Nucleosynthesis compared to WMAP results

From the observations of the anisotropies of the Cosmic Microwave Background (CMB) radiation, the WMAP satellite has provided a determination of the baryonic density of the Universe, Ω_b.h^2, with an unprecedented precision. This imposes a careful reanalysis of the standard Big-Bang Nucleosynthesis (SBBN) calculations. We have updated our previous calculations using thermonuclear reaction rates provided by a new analysis of experimental nuclear data constrained by $R$-matrix theory. Combining these BBN results with the Ω_b.h^2 value from WMAP, we deduce the light element (4He, D, 3He and 7Li) primordial abundances and compare them with spectroscopic observations. There is a very good agreement with deuterium observed in cosmological clouds, which strengthens the confidence on the estimated baryonic density of the Universe. However, there is an important discrepancy between the deduced 7Li abundance and the one observed in halo stars of our Galaxy, supposed, until now, to represent the primordial abundance of this isotope. The origin of this discrepancy, observational, nuclear or more fundamental remains to be clarified. The possible role of the up to now neglected 7Be(d,p)2αand 7Be(d,α)5Li reactions is considered.

astro-ph↗

arXiv2003

A Re-examination of the $^{187}$Re Bound on the Variation of Fundamental Couplings

We reconsider the Re187 bound on the variation of the fine-structure constant. We combine the meteoritic measurement with several present-day lab measurements to firmly establish the quantitative upper limit to the time variation over the age of the solar system. We find that the relative change of the fine-structure constant between its present value and $α$ of \sim 4.6 Gyr ago is consistent with zero, Delta alpha/alpha = [-8\pm 8 (1σ)] x 10^{-7}. We also rederive this bound in models where all gauge and Yukawa couplings vary in an interdependent manner, as would be expected in unified theories.

astro-ph↗

arXiv2002

Constraints on the Variations of the Fundamental Couplings

We reconsider several current bounds on the variation of the fine-structure constant in models where all gauge and Yukawa couplings vary in an interdependent manner, as would be expected in unified theories. In particular, we re-examine the bounds established by the Oklo reactor from the resonant neutron capture cross-section of 149Sm. By imposing variations in Λ_{QCD} and the quark masses, as dictated by unified theories, the corresponding bound on the variation of the fine-structure constant can be improved by about 2 orders of magnitude in such theories. In addition, we consider possible bounds on variations due to their effect on long lived α- and β-decay isotopes, particularly 147Sm and 187Re. We obtain a strong constraint on Δα/ α, comparable to that of Oklo but extending to a higher redshift corresponding to the age of the solar system, from the radioactive life-time of 187Re derived from meteoritic studies. We also analyze the astrophysical consequences of perturbing the decay Q values on bound state β-decays operating in the s-process.

hep-ph↗

arXiv2001

Constraints on Omega_B from the nucleosynthesis of 7Li in the standard Big Bang

We update Standard Big Bang Nucleosynthesis (SBBN) calculations on the basis of recent nuclear physics compilations (NACRE in particular), experimental and theoretical works. By a Monte--Carlo technique, we calculate the uncertainties on the light element yields (4He, D, 3He, 7Li) related to nuclear reactions. The results are compared to observations that are thought to be representative of the corresponding primordial abundances. It is found that 7Li could lead to more stringent constraints on the baryonic density of the universe (Omega_B) than deuterium, because of much higher observation statistics and an easier extrapolation to primordial values. The confrontation of SBBN results with 7Li observations is of special interest since other independent approaches have also recently provided Omega_B.h^2 values: i) the anisotropies of the Cosmic Microwave Background by the BOOMERANG, CBI, DASI and MAXIMA experiments and ii) the Lyman-alpha forest at high redshift. Comparison between these results obtained by different methods provides a test of their consistency and could provide a better determination of the baryonic density in the universe. However, the agreement between Omega_B.h^2 values deduced from SBBN calculation and 7Li observation on the one hand and CMB observations on the other hand is only marginal.

astro-ph↗

arXiv2001

Synthesis of intermediate-mass elements in classical novae: from Si to Ca

Thermonuclear runaways driven by accretion into degenerate white dwarf cores are the source that power classical nova outbursts. In this paper, we identify the dominant nuclear paths involved in the synthesis of intermediate-mass elements, from Si to Ca, during such violent events. New evolutionary sequences of 1.35 Mo ONe novae have been computed, using updated nuclear reaction rates. The main nuclear activity in this region is powered by the leakage from the NeNa-MgAl region, where the activity is confined during the early stages of the explosion. We discuss the critical role played by 30P(p,gamma) in the synthesis of nuclear species beyond sulfur and point out the large uncertainty that affects its rate, which has dramatic consequences for studies of nova nucleosynthesis in the Si-Ca mass region.

astro-ph↗

arXiv2001

Big-bang nucleosynthesis with the NACRE compilation

We update the Big Bang Nucleosynthesis (BBN) calculations on the basis of the recent NACRE compilation of reaction rates. In particular, we calculate the uncertainties related to the nuclear reaction rates on the abundances of 7Li and compare our results with an other recent analysis.

astro-ph↗

arXiv2000

The Impact of the NACRE Compilation on the Big Bang Nucleosynthesis

We update the Big Bang Nucleosynthesis (BBN) calculations on the basis of the recent NACRE compilation of reaction rates. We estimate the uncertainties related to the nuclear reaction rates on the abundances of D, 3He, 4He, 6Li, 7Li, 9Be, 10B and 11B of cosmological and astrophysical interest. We use lithium as the main indicator of the baryon density of the Universe, rather than deuterium.

astro-ph↗

arXiv2000

On the 21Na(p,gamma)22Mg thermonuclear rate for 22Na production in novae

Classical novae are potential sources of gamma-rays, like the 1.275 MeV gamma emission following 22Na beta decay, that could be detected by appropriate instruments on board of future satellites like INTEGRAL. It has been shown that the production of 22Na by novae is affected by the uncertainty on the 21Na(p,gamma)22Mg rate and in particular by the unknown partial widths of the Ex = 5.714 MeV, J^pi = 2^+, 22Mg level. To reduce these uncertainties, we performed shell model calculations with the OXBASH code, compared the results with available spectroscopic data and calculated the missing partial widths. Finally, we discuss the influence of these results on the 21Na(p,gamma)22Mg reaction rate and 22Na synthesis.

astro-ph↗

arXiv2000

Big Bang Nucleosynthesis updated with the NACRE Compilation

We update the Big Bang Nucleosynthesis calculations on the basis of the recent NACRE compilation. The average values of the calculated abundances of light nuclei do not differ significantly from that obtained using the previous Fowler's compilation. ${^7}Li$ is slightly depressed at high baryon to photon ratio $η$. The main uncertainty concerns the $D(p,γ){^3}He$ reaction rate affecting the synthesis of ${^7}Li$ (via the ${^3}He(α,γ){^7}Be(eν){^7}Li$) at rather high baryonic density. On the left part of the lithium valley the uncertainty is strongly reduced due to the improvement of the measurement of the $T(α,γ)^{7}Li$ reaction rate. We use lithium-7 as the main baryometer, since, though much efforts have been devoted to the determination of Deuterium in absorbing clouds in the line of sight of remote quasars, the statistics is very poor compared to the long series of lithium measurements. Taking into account the lithium constraints, two possible baryonic density ranges emerge, $η_{10}= 1.4 - 1.9$ and $η_{10} = 3.3 - 5.1$. The Be and B abundances produced in the big bang are orders of magnitudes lower, and spallation of fast carbon and oxygen is probably their unique source, in the early Galaxy.

astro-ph↗

arXiv2000

Influence of new reaction rates on 18F production in novae

Gamma-ray emission from classical novae is dominated, during the first hours, by positron annihilation resulting from the beta decay of radioactive nuclei. The main contribution comes from the decay of 18F and hence is directly related to 18F formation during the outburst. A good knowledge of the nuclear reaction rates of production and destruction of 18F is required to study 18F synthesis in novae and the resulting gamma-ray emission. The rates relevant for the main mode of 18F destruction (i.e, through proton captures) have been the object of many recent experiments. However, subsequent analyses were focused on providing rates for X-ray burst nucleosynthesis not valid at nova temperatures (lower than 3.5 10^8 K). Accordingly, it is crucial to propose and discuss new reaction rates, incorporating all new experimental results, down to the domain of nova nucleosynthesis. We show that in this temperature regime, the 18F(p,gamma) and (p,alpha) reaction rates remain uncertain and deserve further experimental and theoretical efforts. Our hydrodynamic calculations including the new nuclear rates demonstrate that their impact on 18F synthesis in nova explosions is quite large and, consequently, the early gamma-ray emission from classical novae is also affected.

astro-ph↗

arXiv1999

Nuclear uncertainties in the NeNa-MgAl cycles and production of 22Na and 26Al during nova outbursts

Classical novae eject significant amounts of nuclear processed material into the interstellar medium. Among the isotopes synthesized during such explosions, two radioactive nuclei deserve a particular attention: 22Na and 26Al. In this paper, we investigate the nuclear paths leading to 22Na and 26Al production during nova outbursts by means of an implicit, hydrodynamic code that follows the course of the thermonuclear runaway from the onset of accretion up to the ejection stage. New evolutionary sequences of ONe novae have been computed, using updated nuclear reaction rates relevant to 22Na and 26Al production. Special attention is focused on the role played by nuclear uncertainties within the NeNa and MgAl cycles in the synthesis of such radioactive species. From the series of hydrodynamic models, which assume upper, recommended or lower estimates of the reaction rates, we derive limits on the production of both 22Na and 26Al. We outline a list of nuclear reactions which deserve new experimental investigations in order to reduce the wide dispersion introduced by nuclear uncertainties in the 22Na and 26Al yields.

astro-ph↗

arXiv1998

Nuclear uncertainties in the NeNa-MgAl cycles and synthesis of 22Na and 26Al in classical novae

Classical novae eject significant amounts of matter into the interstellar medium, as a result of thermonuclear runaways. Nucleosynthesis associated with nova outbursts includes products from explosive H burning, such as 17O, 15N and 13C, and also radioactive species like 7Be, 22Na and 26Al. In this paper we report on new hydrodynamic calculations of nova outbursts, from the onset of accretion to mass ejection. We stress the role played by the nuclear uncertainties associated with key reactions of the NeNa-MgAl cycles on the synthesis of 22Na and 26Al.

astro-ph↗

arXiv1997

New results on 26Al production in classical novae

The production of 26Al by explosions of classical novae has been computed by means of a hydrodynamic code that follows both the accretion and the explosion stages. A special emphasis has been put on the analysis of the influence of the initial abundances of the accreted envelope, as well as on the nuclear reaction rates involved. With the most recent values of chemical composition and reaction rates available, 26Al production is lowered with respect to previous computations. According to our results, the final contribution of novae to the galactic 26Al is at most 0.4 M_sun, which is a small part of the estimated 26Al in the Galaxy derived from COMPTEL observations of the 1809 keV emission.

astro-ph↗