<i>Gaia</i> Data Release 2

Abstract

Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.

Keywords

Affiliated Institutions

• Laboratoire d'Astrophysique de Bordeaux FR
• Scuola Internazionale Superiore di Studi Avanzati IT
• Sorbonne Université FR
• University College London GB
• Institut de Mécanique Céleste et de Calcul des Éphémérides FR
• Technische Universität Dresden DE
• European Space Astronomy Centre ES
• Université de Montpellier FR
• University of Edinburgh GB
• Centre National de la Recherche Scientifique FR
• University of Leicester GB
• Royal Observatory of Belgium BE
• Osservatorio Astrofisico di Catania IT
• Observatoire de la Côte d’Azur FR
• Osservatorio Astronomico di Padova IT
• Observatoire astronomique de Strasbourg FR
• Uppsala University SE
• Universitat de Barcelona ES
• Université de Strasbourg FR
• Université Paris Sciences et Lettres FR
• Max Planck Institute for Astronomy DE
• Fund for Scientific Research BE
• KU Leuven BE
• Université de Lille FR
• Centre National d'Études Spatiales FR
• University of Liège BE
• Systèmes de Référence Temps-Espace FR
• Université Libre de Bruxelles BE
• University of Cambridge GB
• Large Synoptic Survey Telescope Corporation US
• Galaxies, Etoiles, Physique et Instrumentation FR
• University of Geneva CH
• Lund University SE
• European Space Research and Technology Centre NL
• Leiden University NL
• University of Lisbon PT
• Université Côte d'Azur FR
• University of Vienna AT
• Université Grenoble Alpes FR
• Universidad Pablo de Olavide ES
• Osservatorio di Astrofisica e Scienza dello Spazio
• Universidade de Vigo ES
• Observatoire de Paris FR
• Université de Bordeaux FR
• Osservatorio Astrofisico di Torino IT
• Royal Observatory GB
• Arcetri Astrophysical Observatory IT
• National Observatory of Athens GR
• Heidelberg University DE
• Laboratoire Univers et Particules de Montpellier FR

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