Gianluca Imbriani

The Chemical Composition of White Dwarfs as a Test of Convective Efficiency during Core Helium Burning

Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate 12C(α,γ)16O reaction rate are able to account for a high central oxygen abundance. The rate of the 12C(α,γ)16O used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the 12C(α,γ)16O reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase.

The 12C(α, γ)16O Reaction Rate and the Evolution of Stars in the Mass Range 0.8 ≤ M/M☉ ≤ 25

We discuss the influence of the 12C(α, γ)16O reaction rate on the central He burning of stars in the mass range 0.8-25 M☉, as well as its effects on the explosive yields of a 25 M☉ star of solar chemical composition. We find that the central He burning is only marginally affected by a change in this cross section within the currently accepted uncertainty range. The only (important) quantity that varies significantly is the amount of C left by the He burning. Since the 12C(α, γ)16O is efficient in a convective core, we have also analyzed the influence of the convective mixing in determining the final C abundance left by the central He burning. Our main finding is that the adopted mixing scheme does not influence the final C abundance provided the outer border of the convective core remains essentially fixed (in mass) when the central He abundance drops below 0.1 dex by mass fraction; vice versa, even a slight shift (in mass) of the border of the convective core during the last part of the central He burning could appreciably alter the final C abundance. Hence, we stress that it is wiser to discuss the advanced evolutionary phases as a function of the C abundance left by the He burning rather than as a function of the efficiency of the 12C(α, γ)16O reaction rate. Only a better knowledge of this cross section and/or the physics of the convective motions could help in removing the degeneracy between these two components. We also prolonged the evolution of the two 25 M☉ stellar models up to the core collapse and computed the final explosive yields. Our main results are that the intermediate-light elements, Ne, Na, Mg, and Al (which are produced in the C convective shell), scale directly with the C abundance left by the He burning because they depend directly on the amount of available fuel (i.e., C and/or Ne). All the elements whose final yields are produced by any of the four explosive burnings (complete explosive Si burning, incomplete explosive Si burning, explosive O burning, and explosive Ne burning) scale inversely with the C abundance left by the He burning because the mass-radius relation in the deep interior of a star steepens as the C abundance reduces. We confirm previous findings according to which a low C abundance (0.2 dex by mass fraction) is required to obtain yields with a scaled solar distribution.

Fluorine Abundances in Galactic Asymptotic Giant Branch Stars

An analysis of the fluorine abundance in Galactic asymptotic giant branch (AGB) carbon stars (24 N-type, 5 SC-type, and 5 J-type) is presented. This study uses the state-of-the-art carbon-rich atmosphere models and improved atomic and molecular line lists in the 2.3 μm region. Significantly lower F abundances are obtained in comparison to previous studies in the literature. This difference is mainly due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low-mass AGB stars, solving the long-standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed.

Evolution and Nucleosynthesis of Primordial Low-Mass Stars

We discuss in detail the evolutionary properties of low-mass stars (M ≤ 1 M☉) having metallicity lower than Z = 10-6 from the pre-main sequence up to (almost) the end of the early asymptotic giant branch phase. We also discuss the possibility that the large [C/Fe], [N/Fe] observed on the surface of the most iron-poor star currently known, HE 0107-5240, could be attributed to the autopollution induced by the penetration of the He convective shell into the H-rich mantle during the He core flash of a low-mass, very low metallicity star. On the basis of a quite detailed analysis, we conclude that the autopollution scenario cannot be responsible for the observed chemical composition of HE 0107-5240.

Activation Measurement of the ^3He(alpha, gamma)^7Be Cross Section at Low Energy

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity, and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148, and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S factor to solar energies.

He3(alpha,gamma)Be7 cross section at low energies

The flux of 7Be and 8B neutrinos from the Sun and the production of 7Li via primordial nucleosynthesis depend on the rate of the 3He(alpha,gamma)7Be reaction. In extension of a previous study showing cross section data at 127 - 167 keV center of mass energy, the present work reports on a measurement of the 3He(alpha,gamma)7Be cross section at 106 keV performed at Italys Gran Sasso underground laboratory by the activation method. This energy is closer to the solar Gamow energy than ever reached before. The result is sigma = 0.567 +- 0.029(stat) +- 0.016(syst) nbarn. The data are compared with previous activation studies at high energy, and a recommended S(0) value for all 3He(alpha,gamma)7Be activation studies, including the present work, is given.

Underground experiments and their impact on stellar modelling

Low-energy studies of thermonuclear reactions in a laboratory at the earth’s surface are hampered predominantly by background induced by cosmic rays in the detectors, leading typically to more than 10 events per hour in common detectors. Conventional passive or active shielding around the detectors can only partially reduce the problem. The best solution so far was to install an accelerator facility in a deep underground laboratory, in a similar way to solar neutrino detectors. In the case of National Laboratory of Gran Sasso the 1400m of rock above the laboratory halls leads to a reduction of the muon flux, the most penetrating component of the cosmic-rays, by a factor 106 with respect to the Earth’s surface. Several cross sections belonging to hydrogen burning have been studied in the framework of this unique project, called LUNA (Laboratory for Underground Nuclear Astrophysics), that was initiated almost 20 years ago. The most important results achieved and their impact on stellar evolution and nucleosynthesis modeling will be reviewed.

Precision study of ground state capture in the 14N(p,g)15O reaction, Phys. Rev. C 78,022802 R , 2008

Previous extrapolations for the ground state contribution disagreed by a factor 2, corresponding to 15% uncertainty in the total astrophysical S-factor. At the Laboratory for Underground Nuclear Astrophysics (LUNA) 400kV accelerator placed deep underground in the Gran Sasso facility in Italy, a new experiment on ground state capture has been carried out at 317.8, 334.4, and 353.3 keV centerof-mass energy. Systematic corrections have been reduced considerably with respect to previous studies by using a Clover detector and by adopting a relative analysis. The previous discrepancy has been resolved, and ground state capture no longer dominates the uncertainty of the total Sfactor.

Measurement of 25Mg(p,γ)26Al resonance strengths via gamma spectrometryJournal of physics G : nuclear and particle physics. - ISSN 0954-3899. - 35:1(2008 Jan). - p. 014013-014013.

The COMPTEL instrument performed the first mapping of the 1.809 MeV photons in the Galaxy, triggering considerable interest in determining the sources of interstellar 26 Al. The predicted 26 Al is too low compared to the observation; for a better understanding more accurate rates for the 25 Mg(p, γ) 26 Al reaction are required. The 25 Mg(p, γ) 26 Al reaction has been investigated at the resonances at 11 E r = 745,418,374, and 304 keV at Ruhr-Universitat -Bochum using a Tandem accelerator and a 4π NaI detector. In addition, the resonance at E r = 189 keV has been measured deep underground at Laboratori Nazionali del Gran Sasso, exploiting the strong suppression of a cosmic background. This low resonance has been studied with the 400 kV LUNA accelerator and an HPGe detector. The preliminary results of the resonance strengths will be reported.