Gaia Data Release 2. Summary of the contents and survey properties (1804.09365v2)

Abstract: We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the $G_\mathrm{BP}$ (330--680 nm) and $G_\mathrm{RP}$ (630--1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent.

• The paper presents a major upgrade by providing over 1.3 billion stellar positions, parallaxes, and proper motions.
• The paper employs advanced calibration and modeling techniques to refine satellite attitude and optical aberration corrections.
• The paper extends its scope by including radial velocities for 7 million stars and epoch data for 14,000 minor planets, broadening research applications.

Overview of Gaia Data Release 2

The second data release from the Gaia mission, referred to as Gaia Data Release 2 (DR2), represents a significant advancement in the field of astrometry and astrophysics. Building on the preliminary findings of the first data release (DR1), DR2 provides a substantial increase in the volume and quality of astrometric data, with more than 1.3 billion stellar positions, parallaxes, and proper motions available. In addition to these data, DR2 delivers comprehensive all-sky photometry, radial velocities for a substantial subset of stars, and various astrophysical parameters and variability information, marking an important milestone in the mission's objectives to fully map and understand the Milky Way.

Key Content and Improvements

DR2 incorporates measurements collected from the Gaia satellite during its first 22 months of operation. The data processing was performed by the Gaia Data Processing and Analysis Consortium (DPAC), with significant improvements made since DR1. These improvements include enhanced modeling of the satellite's attitude, better calibration of optical aberrations, and more accurate correction for the basic angle variations, culminating in an astrometric solution that sets new standards in precision.

One notable addition in DR2 is the inclusion of median radial velocities for approximately 7 million sources, representing a crucial step toward obtaining a three-dimensional kinematic mapping of the Milky Way. Furthermore, for the first time, DR2 presents epoch astrometry and photometry for 14,000 known minor solar system bodies, expanding Gaia's impact into planetary sciences. This release also offers stellar parameters, including effective temperature, extinction, and information on stellar radii and luminosities for millions of stars, derived from Gaia's photometric measurements.

Numerical Results and Catalog Coverage

The release includes 1.7 billion celestial positions, with full astrometric solutions available for 1.3 billion sources, and enhances the understanding of variable stars by expanding their catalog to 500,000 entries. With this database, DR2 redefines the celestial reference frame by providing a new optical reference based solely on extragalactic sources, including over half a million quasars, which directly align the reference frame with the ICRF.

In terms of completeness, DR2 achieves near-total coverage for stars between $G = 12$ and $G = 17$. However, challenges remain in regions of dense stellar populations, where incompleteness due to crowding occurs. Additionally, peculiarities such as duplicate identifiers still exist; users must cross-reference DR1 and DR2 carefully due to source identifier changes.

Implications and Future Work

The enhanced data provided by DR2 opens up manifold possibilities for research, from stellar dynamics and structure to the dynamics of solar system objects. By providing parallaxes and proper motions for over one billion stars, DR2 facilitates advanced research into the Galactic structure, enabling researchers to delineate the Milky Way's disk and halo with unprecedented clarity.

However, users should be mindful of the remaining limitations—systematic errors are still present, and uncertainties in radial velocities and photometric measurements require careful handling. Despite these limitations, DR2 marks a profound step forward, setting the stage for subsequent releases that will refine and expand Gaia's dataset, integrating more comprehensive temporal observations, and addressing systematic discrepancies.

The Gaia mission, as evidenced by DR2, continues to revolutionize our understanding of the cosmos, firmly establishing itself as a cornerstone for contemporary astrophysical research. As we anticipate future releases, the potential for Gaia to redefine our conception of our own galaxy and beyond becomes increasingly apparent. The scientific community eagerly awaits these subsequent releases, as they promise further insights and refinements of the stellar and astrophysical data offered by this invaluable mission.