arXiv subjects

D. Galli

Publications and source records attributed to D. Galli.

63 records · Page 4Linked to original sources

arXiv1999

Inhomogeneous Chemical Evolution of the Galactic Halo

We describe the basic features of a Monte Carlo model specifically designed to follow the inhomogenous chemical evolution of the Galactic halo, taking into account the effects of local enrichment and mixing of the halo gas, and with particular emphasis on elements like Eu produced by r-process nucleosynthesis. We compare our results with spectroscopic data for the chemical composition of metal-poor halo stars and globular clusters like M13, M5, M92, M4, and we infer some constraints on the star formation history of the halo and the rate and mass spectrum of supernovae during the first epoch of Galaxy evolution.

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arXiv1999

Scale-free Equilibria of Magnetized Polytropic Clouds

We investigate the equilibrium properties of self-gravitating magnetized clouds with polytropic equations of state with negative index n. In particular, we consider scale-free isopedic configurations that have constant dimensionless spherical mass-to-flux ratio lambda_r and that may constitute ``pivotal'' states for subsequent dynamical collapse to form groups or clusters of stars. For given Gamma = 1 + 1/n, equilibria with smaller values of lambda_r are more flattened, ranging from spherical configurations with lambda_r = infinity to completely flattened states for lambda_r=1. For a given amount of support provided by the magnetic field as measured by the dimensionless parameter H_0, equilibria with smaller values of Gamma are more flattened. However, logatropic (defined by Gamma = 0) disks do not exist. The only possible scale-free isopedic equilibria with logatropic equation of state are spherical uniformly magnetized clouds.

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arXiv1999

Galactic chemical evolution of heavy elements: from Barium to Europium

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable to reproduce a large set of Galactic (local and non local) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, it shows a sharp production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution given by the Galactic chemical evolution model at the epoch of the solar system formation. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars) showing that the observed trends can be understood in the light of the present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

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arXiv1998

Galactic chemical evolution of Ba-peak elements

The chemical evolution of the Galaxy is followed for the elements affected by neutron capture, in particular for those in the atomic number range 56 to 63 (Ba, La, Ce, Pr, Nd, Sm and Eu). Neutrons by the major 13^C source, released in radiative conditions in the interpulse periods of TP-AGB stars, give rise to an efficient s-processing, making low-mass AGB the major contributors to the chemical evolution of heavy elements. The s-process scenario, characterized is analyzed using AGB stellar evolutionary calculations with the FRANEC Code (FRASCATI Raphson-Newton Evolutionary Code), and are applied over a wide range of stellar masses and metallicities. The presence of r-process elements in low metallicity stars is indicative of a prompt enrichment of the Galaxy by early generation of stars, and low mass SNII appear to be good candidates for primary production of r-nuclei. The chemical evolution model used here is organized over three-zone, halo, thick and thin disk.

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arXiv1998

The Chemistry of the Early Universe

The process of molecule formation in the primordial gas is considered in the framework of Friedmann cosmological models from redshift z=1e4 to z=0. First, a comprehensive analysis of 87 gas phase reaction rates (both collisional and radiative) relevant in the physical environment of the expanding universe is presented and critically discussed. On this basis, calculations are carried out of the abundance of 21 molecular species as function of redshift evaluating consistently the molecular heating and cooling due to H2, HD and LiH molecules. One of the major improvements of this work is the use of a better treatment of H recombination that leads to a reduction of a factor 2-3 in the abundance of electrons and H+ at freeze-out, with respect to previous studies. We find that in the standard model, the residual fractional ionization at z=1 is 3.02e-4, and the main molecular species fractional abundances H2/H=1.1e-6, HD/H2=1.1e-3, HeH+/H=6.2e-13, LiH+/H=9.4e-18, and LiH/LiH+=7.6e-3. Finally, we provide accurate cooling functions of H2, HD and LiH in a wide range of density and temperature that can be conveniently used in a variety of cosmological applications.

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arXiv1997

Measurements of the 12C/13C ratio in Planetary Nebulae and implications for stellar evolution

We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebuale (PNe) by means of mm-wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition; the other group consists of 23 nebulae with rich molecular envelopes. We have determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have ratios well below the value predicted by standard evolutionary models (about 20), indicating that some extra-mixing process has occurred in these stars. We briefly discuss the implications of our results for standard and nonstandard stellar nucleosynthesis.

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arXiv1997

Lithium hydride in the early universe and in protogalactic clouds

We examine the processes of formation and destruction of LiH molecules in the primordial gas at temperatures T \leq 5000 K in the framework of standard Friedmann cosmological models and we compute the optical depth of the Universe due to Thomson scattering of cosmic background photons on LiH. With the help of a simple model of evolution of a spherical density perturbation we follow the linear growth and the subsequent collapse of protogalactic clouds of various masses, evaluating consistently their physical characteristics and chemical composition. We determine the expected level of anisotropy of the cosmic background radiation due to the presence of LiH molecules in high-redshift protogalactic clouds. We conclude that LiH spectral features generated in the primordial gas are hardly detectable with current techniques and instrumentation.

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arXiv1996

Post-T Tauri stars: a false problem

We consider the problem of the apparent lack of old T Tauri stars in low-mass star forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetime and ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. We conclude that the probability of finding a large population of old stars in a star forming region is intrinsically very small and that the post-T Tauri problem is by and large not existent.

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arXiv1996

3He in Planetary Nebulae: A Challenge to Stellar Evolution Models

The discrepancy between the observed abundances of 3He in the ISM and those predicted by stellar and galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (PNe) with the corresponding predictions of stellar evolution theory. The determination of the mass of the PNe progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of 3He in the envelope of all low-mass stars is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the galactic evolution of 3He with the help of a series of models with standard and non-standard nucleosynthesis prescriptions. The results are found to be consistent with the observed galactic abundances only if the vast majority of low-mass stars follows non-standard prescriptions. This implies that either the sample of PNe nebulae under exam is highly biased, or the solution to the 3He problem lies elsewhere.

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