The evolution of the Galactic metallicity gradient from high-resolution spectroscopy of open clusters

Abstract

Open clusters offer a unique possibility to study the time evolution of the radial metallicity gradients of several elements in our Galaxy, because they span large intervals in age and Galactocentric distance, and both quantities can be more accurately derived than for field stars. We re-address the issue of the Galactic metallicity gradient and its time evolution by comparing the empirical gradients traced by a sample of 45 open clusters with a chemical evolution model of the Galaxy. At variance with previous similar studies, we have collected from the literature only abundances derived from high--resolution spectra. The clusters have distances $7<RGC<22$ kpc and ages from $\sim 30$ Myr to 11 Gyr. We also consider the $\alpha$-elements Si, Ca, Ti, and the iron-peak elements Cr and Ni. The data for iron-peak and $\alpha$-elements indicate a steep metallicity gradient for R_GC<12$kpcandaplateauatlargerradii.Thetimeevolutionofthemetallicitydistributionischaracterizedbyauniformincreaseofthemetallicityatallradii,preservingtheshapeofthegradient,withmarginalevidenceforaflatteningofthegradientwithtimeintheradialrange7-12kpc.Ourmodelisabletoreproducethemainfeaturesofthemetallicitygradientanditsevolutionwithaninfalllawexponentiallydecreasingwithradiusandwithacollapsetimescaleoftheorderof8Gyratthesolarradius.Thisresultsinarapidcollapseintheinnerregions,i.e.$R_{\rm GC}\lesssim 12$ kpc (that we associate with an early phase of disk formation from the collapse of the halo) and in a slow inflow of material per unit area in the outer regions at a constant rate with time.

16 pages, 18 figures, A&A accepted

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