GaiaData Release 2

Abstract

Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data? Aims. The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances by using Bayesian methods. In particular we also show that negative parallaxes, or parallaxes with relatively large uncertainties still contain valuable information. Finally, we provide examples that show more generally how to use astrometric data for parameter estimation, including the combination of proper motions and parallaxes and the handling of covariances in the uncertainties. Methods. The paper contains examples based on simulated Gaia data to illustrate the problems and the solutions proposed. Furthermore, the developments and methods proposed in the paper are linked to a set of tutorials included in the Gaia archive documentation that provide practical examples and a good starting point for the application of the recommendations to actual problems. In all cases the source code for the analysis methods is provided. Results. Our main recommendation is to always treat the derivation of (astro-)physical parameters from astrometric data, in particular when parallaxes are involved, as an inference problem which should preferably be handled with a full Bayesian approach. Conclusions. Gaia will provide fundamental data for many fields of astronomy. Further data releases will provide more data, and more precise data. Nevertheless, to fully use the potential it will always be necessary to pay careful attention to the statistical treatment of parallaxes and proper motions. The purpose of this paper is to help astronomers find the correct approach.

Revealing the tidal scars of the Small Magellanic Cloud

“…Restricting ourselves to the confirmed SMC members of our extended data set we end up with 5636 stars with radial velocity determinations and proper motions from Gaia DR2. Due to the distance to the SMC (∼60 kpc) and the faintness of the stars in our extended sample (G magnitude between 15 and 17 for 94% of the sample), the parallaxes from the Gaia DR2 astrometric solution do not give a meaningful constraint on the distance (Arenou et al 2018;Lindegren et al 2018;Luri et al 2018). Since we already selected the SMC members with the radial velocity information, there is no need to rely on distance measurements to remove foreground MW stars.…”

Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2

“…As noted by Luri et al (2018), using small relative parallax error quality cuts introduces artifical asymmetries, or biases, into a data set, because the parallax error is a function of position on the sky due to the Gaia scanning law's non-uniform sampling of different regions of the sky. This would irreparably bias the study of symmetries over an appreciable extent of the galaxy, as shown in Fig.…”

SiO maser astrometry of the red transient V838 Monocerotis

“…Here and throughout the paper we use the value of −0.029 mas derived by the Gaia-DR2 team (Lindegren et al 2018). 2 We have added quadratically systematic errors of 0.1 mas for parallax and 0.1 mas yr −1 for proper motions (Luri et al 2018). a radius of 2 .3 of V838 Mon, we find only one candidate in the Gaia DR2 catalog, star 3107789583219504000, which is 6 east of V838 Mon.…”