The Chemical Evolution of the Galaxy: The Two‐Infall Model

Abstract

In this paper we present a new chemical evolution model for the Galaxy which assumes two main infall episodes for the formation of halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin) but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk.The formation of the thin-disk is much longer than that of the thick disk, implying that the infalling gas forming the thin-disk comes not only from the thick disk but mainly from the intergalactic medium.The timescale for the formation of the thin-disk is assumed to be a function of the galactocentric distance, leading to an inside-out picture for the Galaxy building. The model takes into account the most up to date nucleosynthesis prescriptions and adopts a threshold in the star formation process which naturally produces a hiatus in the star formation rate at the end of the thick disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighbourhood and the whole disk. Among these constraints, the tightest one is the metallicity distribution of the G-dwarf stars for which new data are now available. Our model fits very well these new data.The model predicts also the evolution of the gas mass, the star formation rate, the supernova rates and the abundances of 16 chemical elements as functions of time and galactocentric distance. We show that in order to reproduce most of these constraints a timescale ≤ 1 Gyr for the (halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar vicinity are required.-3 -We predict that the radial abundance gradients in the inner regions of the disk (R < R ⊙ ) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner ones steepen in time during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process is discussed.

A simple and general method for solving detailed chemical evolution with delayed production of iron and other chemical elements

“…We have assumed the Galaxy disc to assemble by means of a single gas accretion episode, with typical time scale τ = 7 Gyr. In any case, our method can be easily extended also for a two-infall model (Chiappini et al 1997).…”

On the Cosmic Origins of Carbon and Nitrogen

“…They note that theoretical chemical evolution models (Carigi 1994) in which carbon comes from intermediate stars apparently predict too shallow a C/O relation to fit their observed galaxy abundance gradients. Other recent discussions of C/O gradients across our own or other galaxies, or of C/O ratios in low metallicity galaxies, have been given by Götz and Köppen (1992), Prantzos, Vangioni-Flam & Chauveau (1994), Kunth et al (1995), Garnett et al (1995), Carigi et al (1995), Mollá, Ferrini, and Díaz (1997) and Chiappini, Matteucci & Gratton (1997).…”

Extragalactic planetary nebulae: Tracers of the chemical evolution of nearby galaxies

“…Classical chemical evolutionary models predict different temporal behaviors depending on assumptions over the gas inflow and outflow rates, the star and cloud formation efficiencies, and other set-up physical conditions. Models might predict a steepening of metallicity gradients with time (e.g., Chiappini et al 1997Chiappini et al , 2001), or, conversely, a flattening of such gradients (e.g., Mollá et al 1997Mollá et al , 2005Magrini et al 2007b). The main differences between the two groups of models are the efficiency of the enrichment processes in the inner and outer regions of the disk, and the nature of the material (primordial or pre-enriched) falling from the halo onto the disk.…”