A view of the Galactic halo using beryllium as a time scale

Beryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU Symposium, Volume 265, Chemical abundances in the Universe: connecting first stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure

Beryllium abundances and the formation of the halo and the thick disk

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 - Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C. Chiappini, eds

Generalized Polarizabilities in a Constituent Quark Model

We discuss low-energy virtual Compton scattering off the proton within the framework of a nonrelativistic constituent quark model. A simple interpretation of the spin-averaged generalized polarizabilities is given in terms of the induced electric polarization (and magnetization). Explicit predictions for the generalized polarizabilities obtained from a multipole expansion are presented for the Isgur-Karl model and are compared with results of various models.Comment: 10 pages, Latex, 2 figures, uses ws-p8-50x6-00.cls, talk given at NSTAR2001, Workshop on "The Physics of Excited Nucleons," Mainz, Germany, March 7-10, 200

Pion polarizabilities: No conflict between dispersion theory and ChPT

Recent attempts to determine the pion polarizability by dispersion relations yield values that disagree with the predictions of chiral perturbation theory. These dispersion relations are based on specific forms for the absorptive part of the Compton amplitudes. The analytic properties of these forms are examined, and the strong enhancement of intermediate-meson contributions is shown to be connected to non-analytic structuresComment: 9 pages, 4 figures; Proceedings of 6th International Workshop on Chiral Dynamics, 6-10 July 2009, Bern, Switzerlan

Depinning and dynamics of AC driven vortex lattices in random media

We study the different dynamical regimes of a vortex lattice driven by AC forces in the presence of random pinning via numerical simulations. The behaviour of the different observables is charaterized as a function of the applied force amplitude for different frequencies. We discuss the inconveniences of using the mean velocity to identify the depinnig transition and we show that instead, the mean quadratic displacement of the lattice is the relevant magnitude to characterize different AC regimes. We discuss how the results depend on the initial configuration and we identify new hysteretic effects which are absent in the DC driven systems.Comment: 6 pages, 4 figure

Dispersion analysis for generalized spin polarizabilities

We report on a dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.Comment: 5pages, 2 figures, to appear in the Proceedings of the Symposium on the Gerasimov-Drell-Hearn Sum Rule and the Spin Structure in the Nucleon Resonance Region (GDH2000), June 14-17 2000, Mainz, German

Dispersion relation formalism for virtual Compton scattering off the proton

We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into the $\Delta(1232)$-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the $\Delta(1232)$-resonance show an enhanced sensitivity to the generalized polarizabilities.Comment: 51 pages, 15 figure

Beryllium in turnoff stars of NGC6397: early Galaxy spallation, cosmochronology and cluster formation

We present the first detection of beryllium in two turnoff stars of the old, metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected and we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. The beryllium abundance is very similar to that of field stars of similar Fe content. We interpret the beryllium abundance observed as the result of primary spallation of cosmic rays acting on a Galactic scale, showing that beryllium can be used as a powerful cosmochronometer for the first stellar generations. With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the onset of star formation in the Galaxy, in excellent agreement with the age derived from main sequence fitting. From the same spectra we also find low O (noticeably different for the two stars) and high N abundances, suggesting that the original gas was enriched in CNO processed material. Our beryllium results, together with the N, O, and Li abundances, provide insights on the formation of this globular cluster, showing that any CNO processing of the gas must have occurred in the protocluster cloud before the formation of the stars we observe now. We encounter, however, difficulties in giving a fully consistent picture of the cluster formation, able to explain the complex overall abundance pattern.Comment: to appear in A&

Generalized polarizabilities and electroexcitation of the nucleon

Generalized nucleon polarizabilities for virtual photons can be defined in terms of electroproduction cross sections as function of the 4-momentum transfer $Q^2$. In particular, the sum of the generalized electric and magnetic polarizabilities $\Sigma=\alpha+\beta$ and the spin polarizability $\gamma$ can be expressed by virtual photon absorption cross sections integrated over the excitation energy. These quantities have been calculated within the framework of the recently developed unitary isobar model for pion photo- and electroproduction on the proton, which describes the available experimental data up to an excitation energy of about 1 GeV. Our results have been compared to the predictions of chiral perturbation theory.Comment: 21 pages, latex, 14 figure