H. Oberhummer

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.

{sup 144}Sm-{alpha} optical potential at astrophysically relevant energies derived from {sup 144}Sm({alpha},{alpha}){sup 144}Sm elastic scattering

For the determination of the {sup 144}Sm-{alpha} optical potential we measured the angular distribution of {sup 144}Sm({alpha},{alpha}){sup 144}Sm scattering at the energy E{sub lab}=20 MeV with high accuracy. Using the known systematics of {alpha}-nucleus optical potentials we are able to derive the {sup 144}Sm-{alpha} optical potential at the astrophysically relevant energy E{sub c.m.}=9.5 MeV with very limited uncertainties. {copyright} {ital 1997} {ital The American Physical Society}

Dependence of direct neutron capture on nuclear-structure models

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. We calculate direct neutron capture on the even-even isotopes Sn124-145 and Pb208-238 with energy levels, masses, and nuclear density distributions taken from different nuclear-structure models. The utilized structure models are a Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a macroscopic-microscopic model based on the finite-range droplet model and a folded-Yukawa single-particle potential. Due to the differences in the resulting neutron separation and level energies, the investigated models yield capture cross sections sometimes differing by orders of magnitude. This may also lead to differences in the predicted astrophysical r-process paths.

Alpha scattering and capture reactions in the A =7 system at low energies

Differential cross sections for [sup 3]He-[alpha] scattering were measured in the energy range up to 3 MeV. These data together with other available experimental results for [sup 3]He+[alpha] and [sup 3]H+[alpha] scattering were analyzed in the framework of the optical model using double-folded potentials. The optical potentials obtained were used to calculate the astrophysical [ital S] factors of the capture reactions [sup 3]He([alpha],[gamma])[sup 7]Be and [sup 3]H([alpha],[gamma])[sup 7]Li, and the branching ratios for the transitions into the two final [sup 7]Be and [sup 7]Li bound states, respectively. For [sup 3]He([alpha],[gamma])[sup 7]Be excellent agreement between calculated and experimental data is obtained. For [sup 3]H([alpha],[gamma])[sup 7]Li an [ital S](0) value has been found which is a factor of about 1.5 larger than the adopted value. For both capture reactions a similar branching ratio of [ital R]=[sigma]([gamma][sub 1])/[sigma]([gamma][sub 0])[approx]0.43 has been obtained.

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.

Reaction rate for two-neutron capture by4He

Recent investigations suggest that the neutrino-heated hot bubble between the nascent neutron star and the overlying stellar mantle of a type-II supernova may be the site of the r-process. In the preceding α-process building up the elements toA≈100, the4He(2n,γ)6He- and6He(α,n)9Be-reactions bridging the instability gap atA=5 andA=8 could be of relevance. We suggest a mechanism for4He(2n,γ)6He and calculate the reaction rate within the α+n+n approach. The value obtained is about a factor 1.6 smaller than the one obtained recently in the simpler direct-capture model, but is at least three orders of magnitude enhanced compared to the previously adopted value. Our calculation confirms the result of the direct-capture calculation that under representative conditions in the α-process the reaction path proceeding through6He is negligible compared to4He(αn,γ)9Be.

Reaction rates for Neutron Capture Reactions to C-, N- and O-isotopes to the neutron rich side of stability

The reaction rates of neutron capture reactions on light nuclei are important for reliably simulating nucleosynthesis in a variety of stellar scenarios. Neutron capture reaction rates on neutron-rich C-, N-, and O-isotopes are calculated in the framework of a hybrid compound and direct capture model. The results are tabulated and compared with the results of previous calculations as well as with experimental results.

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.

Bridging the mass gaps at A=5 and A=8 in nucleosynthesis

In nucleosynthesis three possible paths are known to bridge the mass gaps at A=5 and A=8. The primary path producing the bulk of the carbon in our Universe proceeds via the triple-alpha process He4(2alpha,gamma)C12. This process takes place in helium-burning of red giant stars. We show that outside a narrow window of about 0.5% of the strength or range of the strong force, the stellar production of carbon or oxygen through the triple-alpha process is reduced by factors of 30 to 1000. Outside this small window the creation of carbon or oxygen and therefore also carbon-based life in the universe is strongly disfavored. The anthropically allowed strengths of the strong force also give severe constraints for the sum of the light quark masses as well as the Higgs vacuum expectation value and mass parameter at the 1% level.