The Gaia-ESO Survey: open clusters in Gaia-DR1 - a way forward to stellar age calibration

TL;DR

This paper combines Gaia-DR1 and Gaia-ESO Survey data to analyze open clusters, aiming to calibrate stellar ages by comparing observations with models and deriving consistent age estimates.

Contribution

It introduces a homogeneous methodology for analyzing open clusters using Gaia-DR1 and Gaia-ESO data, establishing a unified age scale and demonstrating the potential of combined datasets.

Findings

Parallaxes generally agree with Gaia Collaboration data, except for NGC 2516.

Cluster ages are consistent across models and with literature.

The approach effectively places clusters on a common age scale.

Abstract

We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members, and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration et al. (2017), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets.