The Gaia ESO Survey and Gaia satellite join their forces to determine clusters age. Published by A&A the study “The Gaia-ESO Survey: open clusters in Gaia-DR1. A way forward to stellar age calibration” of S. Randich (INAF-Firenze)

Stellar clusters are important targets for the study of stellar evolution, since their members constitute a sample of stars, sometime even richer than few thousands, with similar age but covering a wide range of masses. The most widely used method to determine clusters age consists in comparing magnitudes and colors of their stars with those predicted by stellar evolutionary models. This method, however, suffers strong uncertainties, due, for instance, to the precision of the photometric measures, the reliability of cluster members identification, and the accuracy of used stellar models. Another important method consists in calculating Lithium abundance in cluster members. Lithium is in fact quickly consumed in stellar interior and thus its presence is a test for stellar youth. Besides, the efficiency at which Lithium has been depleted in stars with given mass is a function of stellar age. However, also this method is model dependent.

 

The determination of clusters age is thus a challenge. In the past years, however, several projects will  improve the accuracy of how the age of stellar clusters is obtained. Two of these projects are the Gaia mission and the Gaia-ESO Survey. Gaia is a European Space Agency mission which will provide precise astrometric measurements for more than one billion stars, allowing the determination of their proper motion and distance with unprecedented precision. The Gaia-ESO Survey aims at completing the information provided by Gaia for stellar clusters, providing accurate membership and estimate of stellar parameters and radial velocity by means of high resolution optical spectroscopy with FLAMES of the European Southern Observatory.

 

In the study “The Gaia-ESO Survey: open clusters in Gaia-DR1. A way forward to stellar age calibration” of s. Randich (INAF – Osservatorio Astrofisico di Arcetri), recently published by Astronomy & Astrophysics with the collaboration of INAF-OAPA astronomers, data from Gaia and the Gaia-ESO Survey are analyzed together in order to derive membership and age of 8 open clusters. As a result, for the first time it has been possible to define a consistent age scale where these open clusters have been included. This is an important step for the study of stellar evolution and this method will be exploited for a larger number of clusters during Gaia mission.

 

The figure (link) shows the membership diagnostics used for the open cluster NGC 2547.