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

TL;DR

This study investigates the Galactic metallicity gradient using open clusters, analyzing the impact of radial migration over time, and finds a nearly constant gradient with age-dependent migration rates, aligning well with models.

Contribution

It provides a detailed analysis of metallicity gradients and radial migration effects in open clusters, incorporating Gaia data and new photometric metallicity estimates.

Findings

The metallicity gradient is approximately -0.06 dex/kpc across different ages.

Mean migration rate is about 1 kpc/Gyr for clusters up to 2 Gyr old.

Older clusters show lower migration rates, with significant epicyclic excursions contributing to 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 $-$0.06 dex kpc$^{-1}$. Furthermore, using the derived gradients and an analysis of the individual objects, we estimate a mean migration rate of 1 kpc Gyr$^{-1}$ 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$^{-1}$. 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.