We have performed a new and homogeneous analysis of all the Li data available in the literature for main sequence stars (spectral-types from late F to K) in open clusters. In the present paper we focus on a detailed investigation of MS Li depletion and its time scales for stars in the 6350−5500 K effective temperature range. For the first time, we were able to constrain the age at which non-standard mixing processes, driving MS Li depletion, appear. We have also shown that MS Li depletion is not a continuous process and cannot be simply described by a t −α law. We confirm that depletion becomes ineffective beyond an age of 1−2 Gyr for the majority of the stars, leading to a Li plateau at old ages. We compared the empirical scenario of Li as a function of age with the predictions of three non-standard models. We found that models including only gravity waves as main mixing process are not able to fit the Li vs. age pattern and thus this kind of mixing can be excluded as the predominant mechanism responsible for Li depletion. On the other hand, models including slow mixing induced by rotation and angular momentum loss, and in particular those including also diffusive processes not related to rotation, can explain to some extent the empirical evidence. However, none of the currently proposed models can fit the plateau at old ages.
Context. 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. Aims. 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. Methods. At variance with previous similar studies, we have collected from the literature only abundances derived from highresolution spectra. The clusters have Galactocentric distances 7 < ∼ R GC < ∼ 22 kpc and ages from ∼30 Myr to 11 Gyr. We also consider the α-elements Si, Ca, Ti, and the iron-peak elements Cr and Ni. Cepheids trace instead the present-day Fe gradient in the inner parts of the disk. Results. The data for iron-peak and α-elements indicate a steep metallicity gradient for R GC < ∼ 12 kpc and a plateau at larger radii. The time evolution of the metallicity distribution is characterized by a uniform increase of the metallicity at all radii, preserving the shape of the gradient, with marginal evidence for a flattening of the gradient with time in the radial range 7−12 kpc. Our model is able to reproduce the main features of the metallicity gradient and its evolution with an infall law exponentially decreasing with radius and with a collapse time scale of the order of 8 Gyr at the solar radius. This results in a rapid collapse in the inner regions, i.e. R GC < ∼ 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 (that we associate with accretion from the intergalactic medium). An additional uniform inflow per unit disk area would help to better reproduce the metallicity plateau at large Galactocentric radii, but it is difficult to reconcile with the present-day radial behaviour of the star formation rate. Conclusions. Our results favour a scenario where the Galactic disk is formed inside-out by the rapid collapse of the halo and by a subsequent continuous accretion of intergalactic gas
Context. Galactic open clusters are since long recognized as one of the best tools for investigating the radial distribution of iron and other metals. Aims. We employed FLAMES at VLT to collect UVES spectra of bright giant stars in a large sample of open clusters, spanning a wide range of Galactocentric distances, ages, and metallicities. We present here the results for four clusters: Berkeley 20 and Berkeley 29, the two most distant clusters in the sample; Collinder 261, the oldest and the one with the minimum Galactocentric distance; Melotte 66. Methods. Equivalent width analysis was carried out using the spectral code MOOG and Kurucz model atmospheres to derive abundances of Fe, Al, Mg, Si, Ca, Ti, Cr, Ni, Ba; non-LTE Na abundances were derived by direct line-profile fitting. Results. We obtain subsolar metallicities for the two anticenter clusters Be 20 ([Fe/H] = −0.30, rms = 0.02) and Be 29 ([Fe/H] = −0.31, rms = 0.03), and for Mel 66 ([Fe/H] = −0.33, rms = 0.03), located in the third Galactic quadrant, while Cr 261, located toward the Galactic center, has higher metallicity ([Fe/H] = +0.13, rms = 0.05 dex). The α-elements Si, Ca and Ti, and the Fe-peak elements Cr and Ni are in general close to solar; the s-process element Ba is enhanced. Non-LTE computations of Na abundances indicate solar scaled values, suggesting that the enhancement in Na previously determined in giants in open clusters could be due to neglected non-LTE effects. Conclusions. Our results support the presence of a steep negative slope of the Fe radial gradient up to about 10-11 kpc from the Galactic center, while in the outer disk the [Fe/H] distribution seems flat. All the elemental ratios measured are in very good agreement with those found for disk stars of similar metallicity and no trend with Galactocentric distance seems to be present.
Context. The star-to-star scatter in lithium abundances observed among otherwise similar stars in the solar-age open cluster M 67 is one of the most puzzling results in the context of the so called "lithium problem". Among other explanations, the hypothesis has been proposed that the dispersion in Li is due to star-to-star differences in Fe or other element abundances which are predicted to affect Li depletion. Aims. The primary goal of this study is the determination of the metallicity ([Fe/H]), α-and Fe-peak abundances in a sample of Li-poor and Li-rich stars belonging to M 67, in order to test this hypothesis. By comparison with previous studies, the present investigation also allows us to check for intrinsic differences in the abundances of evolved and unevolved cluster stars and to draw more secure conclusions on the abundance pattern of this cluster. Methods. We have carried out an analysis of high resolution UVES/VLT spectra of eight unevolved and two slightly evolved cluster members using MOOG and measured equivalent widths. For all the stars we have determined [Fe/H] and element abundances for O, Na, Mg, Al, Si, Ca, Ti, Cr and Ni. Results. We find an average metallicity [Fe/H] = 0.03 ± 0.01, in very good agreement with previous determinations. All the [X/Fe] abundance ratios are very close to solar. The star-to-star scatter in [Fe/H] and [X/Fe] ratios for all elements, including oxygen, is lower than 0.05 dex, implying that the large dispersion in lithium among cluster stars is not due to differences in these element abundances. We also find that, when using a homogeneous scale, the abundance pattern of unevolved stars in our sample is very similar to that of evolved stars, suggesting that, at least in this cluster, RGB and clump stars have not undergone any chemical processing. Finally, our results show that M 67 has a chemical composition that is representative of the solar neighborhood.
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