Attila Csoto

Stellar Production Rates of Carbon and Its Abundance in the Universe

The bulk of the carbon in our universe is produced in the triple-alpha process in helium-burning red giant stars. We calculated the change of the triple-alpha reaction rate in a microscopic 12-nucleon model of the (12)C nucleus and looked for the effects of minimal variations of the strengths of the underlying interactions. Stellar model calculations were performed with the alternative reaction rates. Here, we show that outside a narrow window of 0.5 and 4% of the values of the strong and Coulomb forces, respectively, the stellar production of carbon or oxygen is reduced by factors of 30 to 1000.

Three-body resonances in He-6, Li-6, and Be-6, and the soft dipole mode problem of neutron halo nuclei

Using the complex scaling method, the low-lying three-body resonances of [sup 6]He, [sup 6]Li, and [sup 6]Be are investigated in a parameter-free microscopic three-cluster model. In [sup 6]He a 2[sup +], in [sup 6]Li a 2[sup +] and a 1[sup +], and in [sup 6]Be the 0[sup +] ground state and a 2[sup +] excited state are found. The other experimentally known 2[sup +] state of [sup 6]Li cannot be localized by our present method. We have found no indication for the existence of the predicted 1[sup [minus]] soft dipole state in [sup 6]He. We argue that the sequential decay mode of [sup 6]He through the resonant states of its two-body subsystem can lead to peaks in the excitation function. This process can explain the experimental results in the case of [sup 11]Li, too. We propose an experimental analysis, which can decide between the soft dipole mode and the sequential decay mode.

Study of the 3He(4He,γ)7Be and 3H(4He,γ)7Li Reactions in an Extended Two-Cluster Model

Abstract. The 3He(4He, γ)7Be and 3H(4He, γ)7Li reactions are studied in an extended two-cluster model which contains α + h/t and 6Li + p/n clusterizations. We show that the inclusion of the 6Li + p/n channels can significantly change the zero-energy reaction cross sections, S(0), and other properties of the 7Be and 7Li nuclei, like the quadrupole moments Q. However, the results agree with the known correlation trend between S(0) and Q. Moreover, we demonstrate that the value of the zero-energy derivatives of the astrophysical S-factors are more uncertain than currently believed.

Sensitivity of the C and O production on the 3α rate

We investigate the dependence of the carbon and oxygen production in stars on the 3α rate by varying the energy of the 0+2-state of 12C and determine the resulting yields for a selection of low-mass, intermediate-mass and massive stars. The yields are obtained using modern stellar evolution codes that follow the entire evolution of massive stars, including the supernova explosion, and consider in detail the 3rd dredge-up process during the thermally pulsating asymptotic giant branch of low-mass and intermediate-mass stars. Our results show that the C and O production in massive stars depends strongly on the initial mass, and that it is crucial to follow the entire evolution. A rather strong C production during the He-shell flashes compared to quiescent He burning leads to a lower sensitivity of the C and O production in low-mass and intermediate-mass stars on the 3α-rate than predicted in our previous work. In particular, the C production of intermediate-mass stars seems to have a maximum close to the actual value of the 0+2 energy level of 12C.

^7Be(p,γ)^8B cross section and the properties of ^7Be

We study the nonresonant part of the {sup 7}Be({ital p},{gamma}){sup 8}B reaction using a three-cluster resonating group model that is variationally converged and virtually complete in {sup 4}He+{sup 3}He+{ital p} model space. The importance of using adequate nucleon-nucleon interaction is demonstrated. We find that the low-energy astrophysical {ital S} factor is linearly correlated with the quadrupole moment of {sup 7}Be. A range of parameters is found where the most important {sup 8}B, {sup 7}Be, and {sup 7}Li properties are reproduced simultaneously; the corresponding {ital S} factor at {ital E}{sub c.m.}=20 keV is 24.6--26.1 eV b.We study the nonresonant part of the ^7Be(p,γ)^8B reaction using a three-cluster resonating group model that is variationally converged and virtually complete in ^4He+^3He+p model space. The importance of using adequate nucleon-nucleon interaction is demonstrated. We find that the low-energy astrophysical S factor is linearly correlated with the quadrupole moment of ^7Be. A range of parameters is found where the most important ^8B, ^7Be, and ^7Li properties are reproduced simultaneously; the corresponding S factor at E_(c.m.)=20 keV is 24.6–26.1 eV b.

S-matrix and R-matrix determination of the low-energy {sup {bold 5}}He and {sup {bold 5}}Li resonance parameters

We study the low-energy 3/2{sup {minus}} and 1/2{sup {minus}} states of {sup 5}He and {sup 5}Li in a microscopic cluster model. The scattering phase shifts of Bond ({alpha}+n) and of Schwandt ({alpha}+p), respectively, are well reproduced. We determine the resonance parameters by localizing the poles of the analytically continued S matrix at complex energies. Our results differ from conventional R-matrix resonance parameters, that were extracted from experimental data using the definition of a resonance based on the positions and widths of reaction cross section peaks. However, they nicely agree with the results of an extended R-matrix method that works at complex energies. {copyright} {ital 1997} {ital The American Physical Society}

Coulomb displacement energy and the low-energy astrophysical S17 factor for the 7Be(p,γ)8B reaction

Abstract The relationship between the Coulomb displacement energy for the A = 8, J = 2 + , T = 1 state and the low-energy astrophysical S 17 factor for the 7 Be(p,γ) 8 B reaction is discussed. The displacement energy is interpreted in a particle-hole model. The dependence of the particle displacement energy on the potential well geometry is investigated and is used to relate the particle displacement energy to the rms radius and the asymptotic normalization of the valence proton wave function in 8 B. The asymptotic normalization is used to calculate the astrophysical S 17 factor for the 7 Be(p,γ) reaction. The relationship to the 7 Li(n,γ) reaction, the 8 B quadrupole moment, radial density, and break-up momentum distribution are also discussed.

Proton skin of 8B in a microscopic model

Abstract The experimentally predicted existence of a proton skin in 8 B is confirmed by dynamical microscopic cluster model calculations. The thickness of the proton skin in 8 B is 0.5 fm while in 8 Li a 0.4 fm thick neutron skin has been found. It is shown that the large quadrupole moment of 8 B partly comes from the very distortable 7 Be core.

Three-alpha structures in 12C

Abstract We search for three-alpha resonances in 12 C by using the complex scaling method in a microscopic cluster model. All experimentally known low-lying natural-parity states of 12 are localized. For the first time we unambiguously show in a microscopic model that the 0 2 + state in 12 C, which plays an important role in stellar nucleosynthesis, is a genuine three-alpha resonance.

Fine tuning the basic forces of nature through the triple alpha process in red giant stars

We show that the synthesis of carbon and oxygen through the triple-alpha process in red giant stars is extremely sensitive to the fine details of the nucleon-nucleon (N-N) interaction. A +/-0.5% change in the strength of the N-N force would reduce either the carbon or oxygen abundance by as much as a factor of 30-1000. This result may be used to constrain some fundamental parameters of the Standard Model.