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Description
Open clusters are recognised as excellent tracers of the Galactic thin disc properties. At variance with intermediate-age and old open clusters, for which a conspicuous number of studies is now available, until a few years ago clusters younger than 150Myr had been mostly overlooked in terms of their chemical composition (with few exceptions). On the other hand, previous investigations seem to indicate an anomalous behaviour of young clusters, which include (but is not limited to) slightly sub-solar iron (Fe) abundances and extreme, unexpected barium (Ba) enhancements. In a series of papers, we plan to expand our comprehension of this topic and investigate whether these chemical peculiarities are instead related to abundance analysis techniques. We present here a new determination of the atmospheric parameters for 23 dwarf stars observed by the Gaia-ESO survey in five young open clusters (age less than 150Myr) and one star forming region (NGC 2264). We exploit a new method based on titanium (Ti) lines to derive spectroscopic surface gravity and, most important, microturbulence parameter. A combination of Ti and Fe lines have been used to obtain effective temperatures. We also infer abundances of FeI, FeII, TiI, TiII, NaI, MgI, AlI, SiI, CaI, CrI and NiI. Our findings are in fair agreement with Gaia-ESO iDR5 results for effective temperatures and surface gravities, but suggest that for very young stars the microturbulence parameter is over-estimated when Fe lines are employed. This impacts the derived chemical composition, causing the metal content of very young clusters to be under-estimated. Our clusters display a metallicity [Fe/H] between +0.04 and +0.12; thus they are not more metal-poor than the Sun. Although based on a relatively small sample size, our explorative study suggests that we may not need to call for ad hoc explanations to reconcile the chemical composition of young open clusters with Galactic chemical evolution models.
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