The stellar population of 70 star clusters revealed by Gaia/EDR3 and the Gaia-ESO Survey. The study: “The Gaia-ESO Survey: Membership probabilities for stars in 63 open and 7 globular clusters from 3D kinematics” of R. J. Jackson (Keele University) recently appeared on MNRAS
Our knowledge of stellar evolution relies on our capability of obtained precise measurements of stellar ages, a challenging task for isolated stars which, however, is typically easier for stars in clusters. In fact, clusters contain stars formed more or less simultaneously from a single molecular cloud. This means that they represent coeval samples of stars with different masses, which allow astronomers to determine their ages by comparing the observed colors and magnitudes with those predicted by stellar evolutionary models.
To this aim, it is mandatory to carefully identify the stars associated with star clusters. This can be a difficult task since stars clusters, mainly the young open clusters, typically lie in the Galactic plane, which means that a large number of field stars (both in the foreground and in the background) which do not belong to the clusters fall along the same line of sight. The difficult task of selecting the stars in clusters is typically achieved by adopting criteria based on properties shared by cluster members and not by unrelated stars. Some examples are: the intense magnetic activity typical of young stars, the presence of specific chemical elements, the peculiar spatial distribution around the cluster position, or the coherent dynamic. Attention must be payed when the determination of cluster members is achieved using criteria which may bias the scientific results. For instance, a selection based on chemical properties may bias the determination of the chemical evolution of stars.
The Gaia-ESO Survey is an observing campaign performed with the FLAMES spectrograph of the European Southern Observatory that covers all the components of the Milky Way, from the halo to the star-forming regions in the galactic plane, and it is designed in order to be complementary with the photometric, kinematic, and spectroscopic observations provided by the Gaia satellite of the European Space Agency. About 40% of the survey is focussed on star clusters, with a total of 85 open and 14 globular clusters included in the observations. In order to select the stars associated with these clusters, the team led by the astronomer R. J. Jackson (Astrophysics Group, Keele University) has analyzed the kinematic data provided by the Gaia-ESO Survey and the Early Third Data Release of Gaia. While the latter provides precise measurements of the motion of the stars in the sky, the proper motion, the former provides the radial component of stellar velocity. The combined analysis of these data allows thus to obtain the stellar kinematics in 3D, and to identify the stars with a coherent motion which is expected for stars in clusters. This study has allowed the authors to define lists of stars included in the Gaia-ESO Survey which are members of 70 star clusters, providing an important sample for studies of the evolution of star, their magnetic activity, dynamic, chemistry, and the properties of the Milky Way. The study is described in the paper: “The Gaia-ESO Survey: Membership probabilities for stars in 63 open and 7 globular clusters from 3D kinematics“, recently appeared on the journal The Monthly Notices of the Royal Astronomical Society. The astronomer L. Prisinzano of INAF – Astronomical Observatory of Palermo is among the coauthors of the study.
The figure (click here to visualize the entire image) shows a selection of diagrams of stars associated with the cluster NGC6705. a) The Hertzsprung-Russell diagram, with the background giant stars not associated with the cluster marked with blue cross; b) star motion in the right ascension and declination; c) a color-magnitude diagram built with the photometry provided by the Gaia satellite; d) a diagram which defines the probability for stars to belong to the cluster.
Mario Giuseppe Guarcello ( follow mguarce) ( youtube)
Subscribe the Youtube channel of the Astronomical Observatory of Palermo