Federico Ferrini

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 for reproducing a large set of Galactic (local and nonlocal) 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(α, n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(α, 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, there is a sharp production of the Ba-peak elements around Z Z☉/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 formation of the solar system. 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 present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

Evolution of Spiral Galaxies. VII. Time Evolution of the Radial Distributions of Abundances

This is an electronic version of an article published in The Astrophysical Journal. Molla, M., Ferrini, F. and A.I. Diaz. Evolution of spiral galaxies. VII. Time evolution of the radial distributions of abundances. The Astrophysical Journal 475 (1997): 519-533

Evolution of spiral galaxies. I - Halo-disk connection for the evolution of the solar neighborhood

This paper is the first of a series dealing with the evolution of spiral galaxies and in particular of the Milky Way. Here was present a complex model for the chemical evolution of the solar neighborhood, containing several improvements with respect to previous models. The solar neighborhood is subdivided into halo and disk regions and their individual but interconnected evolution is computed in detail. In particular, the evolution of abundances of 15 chemical species is predicted both tor the halo and the disk

Evolution of spiral galaxies. III. Application of the multiphase model to the galactic disk

This is an electronic version of an article published in The Astrophysical Journal. Ferrini, F., Molla, M., Pardi, M.C. and A.I. Diaz. Evolution of spiral galaxies. III. Application of the multiphase model to the galactic disk. The Astrophysical Journal 427 (1994): 745-758

Evolution of dust grains through a hot gaseous halo

The evolution of bare spherical dust grains in the halo of spiral galaxies is analyzed. Two different grain compounds, graphites and astronomical silicates, are considered. The detailed mass and luminosity distributions for the Milky Way and NGC 3198 (considered as representative of the Sb and Sc types, respectively) are used to evaluate the range of possible evolutionary tracks. Aside from radiative and gravitational forces, the effects of drag and sputtering from a gaseous halo are included. A simple isothermal and hydrostatic density structure, with temperatures in the range 3 × 10 5 -10 6 K, has been used for this gaseous halo

Galactic chemical evolution of Lithium: interplay between stellar sources

In this paper we study the evolution of 7Li in the Galaxy considering the contributions of various stellar sources: Type II supernovae, novae, red giant stars, and asymptotic giant branch (AGB) stars. We present new results for the production of 7Li in AGB stars via the hot bottom burning process, based on stellar evolutionary models. In the light of recent observations of dense circumstellar shells around evolved stars in the Galaxy and in the Magellanic Clouds, we also consider the impact of a very high mass-loss rate episode (superwind) before the evolution off the AGB phase on the 7Li enrichment in the interstellar medium. We compare the Galactic evolution of 7Li obtained with these new 7Li yields (complemented with a critical reanalysis of the role of supernovae, novae and giant stars) with a selected compilation of spectroscopic observations including halo and disk field stars as well as young stellar clusters. We conclude that even allowing for the large uncertainties in the theoretical calculation of mass-loss rates at the end of the AGB phase, the superwind phase has a significant effect on the 7Li enrichment of the Galaxy.

Evolution of spiral galaxies. VI. Radial distributions of abundances in external galaxies

This is an electronic version of an article published in The Astrophysical Journal. Molla, M., Ferrini, F. and A.I. Diaz. Evolution of spiral galaxies. VI. Radial distributions of abundances in external galaxies. The Astrophysical Journal 466 (1996): 668-685

Evolution of Li, Be, and B in the Galaxy

In this paper we study the production of Li, Be, and B nuclei by Galactic cosmic-ray spallation processes. We include three kinds of processes: (1) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes), (2) spallation by CNO cosmic-ray nuclei impinging on interstellar p and 4He (inverse processes), and (3) α-α fusion reactions. The latter dominate the production of 6,7Li. We calculate production rates for a closed box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multizone, multipopulation model that we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine-tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem but instead are the consequences of inaccurate treatments of Galactic evolution.

Photolevitation of diffuse clouds

Radiation pressure on dust grains can raise small dusty clouds above the main gaseous disk to high Galactic latitudes. This photolevitation effect drives neutral gas and dust into a soft Galactic fountain and can maintain a column density of the order of 10 to the 20th/sq cm above the main gaseous disk. This value is defined by dust opacity and corresponds to a normal dust-to-gas ratio with cosmic abundances. The maximum height reached by the photolevitated clouds depends on the radiation field and dust-to-gas ratios. Clouds located above luminous stellar clusters or near spiral arms with intense star formation can reach several hundred parsecs in height. 56 refs.

Oxygen and nitrogen abundances in Virgo and field spirals

The oxygen and nitrogen abundances in the H ii regions of the nine Virgo spirals of the sample from Skillman et al. (1996) and in nine field spiral galaxies are re-determined with the recently suggested P –method. We confirm that there is an abundance segregation in the sample of Virgo spirals in the sense that the H i deficient Virgo spirals near the core of the cluster have higher oxygen abundances in comparison to the spirals at the periphery of the Virgo cluster. At the same time both the Virgo periphery and core spirals have counterparts among field spirals. Some field spirals have H i to optical radius ratios, similar to that in H i deficient Virgo core spirals. We conclude that if there is a difference in the abundance properties of the Virgo and field spirals, this difference appears to be small and masked by the observational errors.