Gaia Data Release 2

Abstract

Context. Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. Aims. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these results performed within the astrometry task. Methods. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. Results. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G < 14 mag) sources, 0.1 mas at G = 17 mag, and 0.7 mas at G = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr −1 , respectively. The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr −1 . From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr −1 in proper motion are seen on small (< 1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.

The Milky Way bar/bulge in proper motions: a 3D view from VIRAC and Gaia

“…The standard deviation of the offsets, see figure titles, is of order 0.1 mas yr −1 and we observe a slight gradient across the map for <∆µ δ >. These demonstrate that there are systematic effects occurring in the proper motion correction which are likely due to a combination of (i) the known systematics in the Gaia proper motion reference frame (Lindegren et al 2018); and (ii) variations in <µ l > and <µ b > due to a varying distance distribution of reference sources because of variable extinction.…”

“…These are likely a combination of two effects. There are known systematics in the Gaia proper motion reference frame (Lindegren et al 2018), an example of which was observed in the LMC (Gaia Collaboration et al 2018b). Additionally there are possible variations in <µ l > and <µ b > on this scale due to variation in the average distance of the reference sources, causing a variation in the measured mean proper motions, caused by variable extinction.…”

“…The VIRAC catalogue presents the proper motions in right ascension (RA), µ α * , and declination (DEC), µ δ , relative to the mean proper motions in a VVV tile. To obtain the absolute proper motions each VVV tile is cross matched with the Gaia DR2 catalogue to make use of its exquisite absolute reference frame (Lindegren et al 2018). Only matches within 1.0 arcsec are considered.…”

SiO maser astrometry of the red transient V838 Monocerotis

“…The astrometric parameters and the distance and proper motion to the source are derived in Section 4. Over the course of the VLBA observations, the Gaia mission released its second catalog (DR2; Gaia Collaboration et al 2018;Lindegren et al 2018). We thus also compare our astrometry with that provided by Gaia.…”

CCD UBV(RI)KC photometry of the open clusters Juchert 9 and Berkeley 97

“…A global systematic offset of Gaia DR2 parallaxes is Δ = −0.029 mas in terms of an inertial reference frame, emphasized by Lindegren et al (2018). Recent values for the zero point shift of parallax are found as Δ = −0.045±0.009 mas (Yalyalieva et al 2018), Δ = −0.053 ± 0.003 mas (Zinn et al 2018), Δ = −0.046 ± 0.013 mas (Riess et al 2018), respectively.…”