Abundance Gradients and the Formation of the Milky Way

Abstract

In this paper we adopt a chemical evolution model, which is an improved version of the Chiappini, Matteucci, & Gratton model, assuming two main accretion episodes for the formation of the Galaxy, the Ðrst forming the halo and bulge in a short timescale and the second one forming the thin disk, with a timescale that is an increasing function of the Galactocentric distance (being of the order of 7 Gyrs at the solar neighborhood). The present model takes into account in more detail than previously the halo density distribution and explores the e †ects of a threshold density in the star formation process during both the halo and disk phases. The model also includes the most recent nucleosynthesis prescriptions concerning supernovae of all types, novae, and single stars dying as white dwarfs. In the comparison between model predictions and available data, we have focused our attention on abundance gradients as well as gas, stellar, and star formation rate distributions along the disk, since this kind of model has already proven to be quite successful in reproducing the solar neighborhood characteristics. We suggest that the mechanism for the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially contribute to building up the outer disk. We also show that the existence of a threshold density for the star formation rate, both in the halo and disk phase, is necessary to reproduce the majority of observational data in the solar vicinity and in the whole disk. In particular, a threshold in the star formation implies the occurrence of a gap in the star formation at the halo-disk transition phase, in agreement with recent data. We conclude that a relatively short halo formation timescale (^0.8 Gyr), in agreement with recent estimates for the age di †erences among Galactic globular clusters, coupled with an "" inside-out ÏÏ formation of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents, at the moment, the most likely explanation for the formation of the Milky Way. This scenario allows us to predict abundance gradients and other radial properties of the Galactic disk in very good agreement with observations. Moreover, as a consequence of the adopted "" inside-out ÏÏ scenario for the disk, we predict that the abundance gradients along the Galactic disk must have increased with time and that the average S[a/Fe]T ratios in stars (halo plus disk) slightly decrease going from 4 to 10 kpcs from the Galactic center. We also show that the same ratios increase substantially toward the outermost disk regions and the expected scatter in the stellar ages decreases, because the outermost regions are dominated by halo stars. More observations at large Galact...

The prevalence of star formation as a function of Galactocentric radius

“…The known negative metallicity gradient in the Galactic disc, when traced by HII regions and OB stars, is in the region of 0.06 -0.07 dex kpc −1 within the approximate RGC range covered in the present study (Chiappini et al 2001;Lépine et al 2011), which translates to a reduction by a factor of 2 over 5 kpc, while the measured SFF slope of −0.026 kpc −1 produces only a 13% decline in 5 kpc. Reduced metallicity implies lower dust-to-gas ratio and reduced CO/H2 abundance, and so less efficient cooling and turbulent energy dissipation.…”

Are ancient dwarf satellites the building blocks of the Galactic halo?

“…The quantity X A i = σ i (t)/σ gas (t) is the abundance by mass of the element i in the infalling material, while t max = 1 Gyr is the time for the maximum infall on the thin disc, τ H = 0.8 Gyr is the timescale for the formation of the halo and thick disc and τ D (r) is the time-scale for the formation of the thin disc and is a function of the galactocentric distance (inside-out formation; Matteucci & François 1989;Chiappini, Matteucci & Romano 2001). In the 2IM model, the abundances X A i show primordial gas compositions and are constant in time.…”

Open clusters: their kinematics and metallicities

“…As the star formation activity migrated to the outer disk, the abundances got enhanced in that region as well, so that the gradients flatten out. In the model used by Chiappini et al (2001), two infall episodes are assumed to form halo and the disk. The disk is also formed in an "inside-out" formation scenario, in which the time-scale is a linear function of the galactocentric distance.…”