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.
Highlights
The Hertzsprung-Russell diagram (HRD) is one of the most important tools in stellar studies
A major issue for the globular cluster data is the uncertainties on the parallaxes that result from the systematics, which is in most cases about one order of magnitude larger than the standard uncertainties on the mean parallax determinations for the globular clusters
They are in the same region of the HRD as the hot subdwarfs, which creates a clump at MG = 4 and GBP − GRP = −0.5 that is well visible in Figs. 1 and 5. These stars are nicely characterised in terms of variability, including binary-induced variability, in Gaia Collaboration (2018b). These hot subdwarfs are considered to be red giants that lost their outer hydrogen layers before the core began to fuse helium, which might be due to the interaction with a low-mass companion, other processes might be at play (e.g. Heber 2009)
Summary
The Hertzsprung-Russell diagram (HRD) is one of the most important tools in stellar studies. Clusters provide empirical HRDs for a range of ages and metal contents and are widely used in stellar evolution studies To be conclusive, they need homogeneous photometry for inter-comparisons and astrometry for good memberships. With its global census of the whole sky, homogeneous astrometry, and photometry of unprecedented accuracy, Gaia DR2 is setting a new major step in stellar, galactic, and extragalactic studies. It provides position, trigonometric parallax, and proper motion as well as three broad-band magnitudes (G, GBP, and GRP) for more than a billion objects brighter than G ∼ 20, plus radial velocity for sources brighter than GRVS ∼ 12 mag and photometry for variable stars (Gaia Collaboration 2018a).
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