Known issues with the Gaia DR2 data
This page lists the known issues in our second data release. Tips on how to better make use of the Gaia Archive can be found here.
Astrometry: 2- versus 5-parameter solutions
Gaia DR2 provides astrometry for 1.69 billion objects. The vast majority of them, 1.33 billion sources, have 5-parameter solutions, i.e., in addition to right ascension and declination, also the proper motion (in right ascension and declination) and the parallax are given. The remaining 361 million objects have 2-parameter solutions (right ascension and declination only). The criterion for receiving either a 2- or a 5-parameter solution has been based on the quality of the astrometric and photometric data, as set out in Equations (11) and (12) in Lindegren et al. (2018).
The decision between these 2- and 5-parameter solutions was based on preliminary (non-published) photometry used early in the Gaia DR2 data processing. During the final stages of the photometric data processing, the so called 'bronze' sources (see Riello et al. (2018) and Evans et al. (2018) for explanation) received updated photometry, which has been published as part of Gaia DR2. However, the astrometric selection between 2- and 5-parameter solutions was not re-done using these new data. As a consequence, the published photometry and the astrometric selection criteria between 2- and 5-parameter solutions are inconsistent in some 9 million cases.
Of these approximately 9 million cases, there are some 7 million sources in Gaia Data Release 2 with a 2-parameter solution while the criteria for a 5-parameter solution are met. In addition, there are some 2 million sources with a 5-parameter solution while a 2-parameter solution would be expected based on the documented criteria and published data.
Astrometry: Considerations for the use of DR2 astrometry
An overview of Gaia DR2 astrometry was presented by Lindegren et al. at the IAU GA Division A meeting in Vienna on 27 August 2018. Topics covered include random and systematic errors, quality indicators, and spurious parallaxes.
An extended version of the presentation, containing detailed tips and recommendations for the best uses of the DR2 astrometry, is given here. Among other things it contains recipes for estimating the systematic uncertainty of the mean parallax or proper motion of a cluster, and for calculating the re-normalised unit weight error (RUWE).
The recipes use auxiliary data currently not available in the Gaia Archive: a table of spatial covariances (DR2_spatialCov_V1.zip), and tables for calculating the RUWE (DR2_RUWE_V1.zip). Each zipped directory comes with a readme.txt file.
When using the data given in "DR2_spatialCov_V1.zip" we ask you to follow our Gaia Data Release 2 credit and citation guidelines and cite the "Gaia Data Release 2: the astrometric solution" paper. When using the data given in "DR2_RUWE_V1.zip" we ask you to cite the technical note GAIA-C3-TN-LU-LL-124-01.
While we expect to find Gaia counterparts for most of the Hipparcos2 sources (van Leeuwen F., 2007, A&A 474, 653), with the exception of the brightest ones, the cross-match results include only about 2/3 of them. This means that according to the adopted cross-match algorithm only ∼2/3 of Hipparcos2 objects have a Gaia counterpart compatible within position errors (i.e. have at least one good neighbour). This issue is discussed in Section 6.1 of Marrese et al. 2018.
We provide in the file "Hipparcos2GaiaDR2coneSearch.zip" the results of a 1 arcsec cone search described in the paper. The table contains three columns: the Gaia and Hipparcos2 identifiers and the angular distance (in arcsec) for each nearest neighbour.
When using the data given in "Hipparcos2GaiaDR2coneSearch.zip” we ask you to follow our Gaia Data Release 2 credit and citation guidelines and cite the "Gaia Data Release 2: Cross-match with external catalogues - Algorithms and results” paper.