Gaia 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.

The tale of the tail – disentangling the high transverse velocity stars in Gaia DR2

“…The sample was extracted from the GDR2 catalogue using the same ADQL query as described in Gaia Collaboration et al (2018b), which we reproduce in Appendix A. The query selects all stars within 1 kpc that have G < 17 and with transverse velocity v t = 4.74/ µ 2 α * + µ 2 δ > 200 kms/s.…”

“…Gaia Collaboration et al (2018b) have demonstrated the amount of detail it's possible to obtain from the analysis of the HR diagrams drawn from Gaia data (Gaia Collaboration et al 2016, 2018a. For instance, for those interested in the stellar halo and thick disc, their figure 21 is especially relevant (reproduced here in Section 4).…”

Li abundances for solar twins in the open cluster M67

“…The V, J, H, K S magnitudes and the B − V colour (Geller et al 2015;Pace et al 2012;Cutri et al 2003) were used with the photometric calibration from Casagrande et al (2010) to estimate the effective temperatures (T eff ). We adopted the M67 reddening E(B −V) = 0.037 from the Gaia Collaboration et al (2018). The reddening ratios used to correct the other colours, k = E(colour)/E(B−V), were taken from Ramírez & Meléndez (2005).…”

“…set (Yi et al 2001;Kim et al 2002). We adopt the photometric T eff previously mentioned, [Fe/H] from our spectroscopic analysis, the V magnitude from Geller et al (2015), the E(B − V) reddening from the Gaia Collaboration et al (2018), and parallax from the Gaia Data Release 2 (DR2) archive with the zero point offset correction (we added 0.03 mas) suggested by Lindegren et al (2018). The stellar parameters presented in Table 2 taken both by photometry and spectroscopy show the consistency between the two different methods used in this work and are compatible within 1σ.…”

Zodiacal exoplanets in time – X. The orbit and atmosphere of the young ‘neptune desert’-dwelling planet K2-100b

“…1). The star is a member of the nearby (189 pc, Gaia Collaboration et al 2018) young open cluster M-44, the Beehive or Praesepe. The age of the cluster is not known precisely; it is variously estimated as 625 Myr (Perryman et al 1998), 850 Myr (Brandt & Huang 2015), and 590 Myr (Gossage et al 2018;Schröder et al 2019), with an uncertainty of about 100 Myr.…”