The Gaia-ESO survey: Mapping the shape and evolution of the radial abundance gradients with open clusters

Abstract

Context. The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangling the scenarios of formation and evolution of the Galaxy. Aims. In this paper we used the sample of open clusters available in the final release of the Gaia-ESO survey to trace the Galactic radial abundance and abundance-to-iron ratio gradients, and their time evolution. Methods. We selected member stars in 62 open clusters, with ages from 0.1 to about 7 Gyr, located in the Galactic thin disc at galactocentric radii (R GC ) from about 6 to 21 kpc. We analysed the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and [El/Fe] combining elements belonging to four different nucleosynthesis channels, and their individual abundance and abundance ratio gradients. We also investigated the time evolution of the gradients dividing open clusters in three age bins.Results. The [Fe/H] gradient has a slope of −0.054 dex kpc −1 . It can be better approximated with a two-slope shape, steeper for R GC ≤11.2 kpc and flatter in the outer regions. We saw different behaviours for elements belonging to different channels. For the time evolution of the gradient, we found that the youngest clusters (age<1 Gyr) in the inner disc have lower metallicity than their older counterparts and that they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggest that the most likely one may be related to a bias introduced by the standard spectroscopic analysis producing lower metallicities in the analysis of low-gravity stars. Conclusions. To delineate the shape of the 'true' gradient, we should most likely limit our analysis to stars with low surface gravity log g>2.5 and microturbulent parameter ξ<1.8 km s −1 . Based on this reduced sample, we can conclude that the gradient has minimally evolved over the time-frame outlined by the open clusters, indicating a slow and stationary formation of the thin disc over the last 3 Gyr. We found a secondary role of cluster migration in shaping the gradient, with a more prominent role of migration for the oldest clusters.

A new and Homogeneous metallicity scale for Galactic classical Cepheids

“…The cluster ages of the GES sample changes by more than one order of magnitude, but their distribution is similar, within the errors. This is in agreement with what was found in Magrini et al (2023) for the radial gradient of several elements belonging to different nucleosynthesis channels, whereby a limited temporal evolution is observed in the age range covered by open clusters. The similarity of CCs and OCs iron radial gradients is soundly supported by the simi-larity of both zero-point and slope of the linear fits listed in Table 4.…”

“…We also performed a linear fit for Cepheids with Galactocentric distances smaller than 12.5 kpc. The current estimate agrees quite well with similar estimates available in the literature (Magrini et al 2023). The flattening in the outer disk fully supports preliminary results based on both CCs and OCs (Carraro et al 2007;Yong et al 2012;Donor et al 2020).…”

“…For the sample clusters, we adopted the [O/H] ratios from Magrini et al (2023). The abundance of O is computed using the forbidden [O i] line at 6300.3 Å, in the LTE approximation (see Tautvaišienė et al 2015, for details).…”

“…This comparison should be cautiously treated since it might be related to the different lines exploited for measuring oxygen abundances, and more in general to the details of the spectroscopic analysis (such as code, model atmospheres, and atmospheric parameters). Concerning OCs, by using data provided by Magrini et al (2023), we performed an independent fit following the same approach adopted for CCs. We found that the two gradients are, within the errors, in agreement with each other (−0.029 ± 0.006 vs. −0.040 ± 0.009 dex kpc −1 ), thus supporting a similarity of the oxygen radial gradients between CCs and OCs.…”

Beyond the two-infall model

“…It could be necessary, though, to assume a higher or smaller amount of accreted gas. In this respect, it also has to be mentioned that the identification of the young, metal-poor component could suffer from biases related to the adoption of the standard spectroscopic local thermodynamic equilibrium (LTE) analysis for those stars (see discussions in Magrini et al 2023). Furthermore, the parameterisation of young stars from high-resolution spectra (R ∼ 100 000) could also be affected by a combination of intrinsic factors, such as activity, fast rotation, and magnetic fields (Zhang et al 2021;Spina et al 2020Spina et al , 2022.…”