The Galactic metallicity gradient shown by open clusters in the light of radial migration

Abstract

During the last years and decades several individual studies and large-scale spectroscopic surveys significantly improved our knowledge of the Galactic metallicity distribution based on open clusters. The availability of Gaia data provided a further step forward in our knowledge. However, still some open issues remain, for example the influence of radial migration on the interpretation of the observed gradients. We used spectroscopic metallicities from individual studies and from the APOGEE survey to compile a sample of 136 open clusters, with a membership verification based on Gaia DR2. Additionally, we present photometric metallicity estimates of 14 open clusters in a somewhat outer Galactic region. Eight age groups allow us to study the evolution of the metallicity gradient in detail, showing within the errors an almost constant gradient of about −0.06 dex/kpc. Furthermore, using the derived gradients and an analysis of the individual objects, we estimate a mean migration rate of 1 kpc/Gyr for objects up to about 2 Gyr. Here, the change of the guiding radius is clearly the main contributor. For older and dynamically hotter objects up to 6 Gyr we infer a lower migration rate of up to 0.5 kpc/Gyr. The influence of epicyclic excursions increases with age and contributes already about 1 kpc to the total migration distance after 6 Gyr. A comparison of our results with available models shows good agreement. However, there is still a lack of a suitable coverage of older objects, future studies are still needed to provide a better sampling in this respect.

Mapping the Galactic Metallicity Gradient with Open Clusters: The State-of-the-Art and Future Challenges

“…Open clusters have been used to track the radial metallicity gradient in our Galaxy since the late 1970s [39]. The results presented in subsequent works tend to converge on a bi-modal gradient, with a change of slope between the Galactocentric radii (R GC ) 10 and 16 kpc see, e.g., [21,[40][41][42][43][44][45][46][47][48][49]. There is a general consensus for a steeper gradient in the inner disc (R gal ∼12-13 kpc), with a much flatter one in the outer regions.…”

The Gaia-ESO survey: Age-chemical-clock relations spatially resolved in the Galactic disc

“…The relation between the abundances of neutron-capture elements and stellar ages has been widely investigated (e.g. D 'Orazi et al 2009;Maiorca et al 2011Maiorca et al , 2012Yong et al 2012;Mishenina et al 2013b,a;Jacobson & Friel 2013;Battistini & Bensby 2016;Marsakov et al 2016;Nissen 2016;Reddy & Lambert 2017;Delgado Mena et al 2017;Spina et al 2018;Magrini et al 2018;Tautvaišienė et al 2021;Casamiquela et al 2021;Baratella et al 2021;Zinn et al 2021;Sales-Silva et al 2021). However, most of the previous works were limited to the solar neighbourhood region, using both solar twins or star clusters.…”

“…What weight migration holds in shaping the spatial distribution of more massive populations such as clusters, with respect to single stars, is not yet settled (see, e.g. Anders et al 2017;Chen & Zhao 2020;Zhang et al 2021;Netopil et al 2021).…”

“…Among them, NGC6791, which is both old and metal rich, is known to have a high probability of being a migrator (cf. Jílková et al 2012;Netopil et al 2021).…”

“…Bisterzo et al 2014;Mishenina et al 2015;Trippella et al 2016;Magrini et al 2018;Spina et al 2018;Busso et al 2021) and to use their abundances to estimate the ages of stars, often using neutron capture s-process elements in combination with other elements with opposite behaviours, such as α elements -that we indicate as chemical clocks -and thus maximising the dependence of the relationship with age (see, e.g. Tucci Maia et al 2016a;Nissen 2016;Feltzing et al 2017;Fuhrmann et al 2017;Slumstrup et al 2017;Titarenko et al 2019). Once the existence of a relationship between age and chemical clocks was established (see, e.g.…”