The Tycho-Gaia astrometric solution. How to get 2.5 million parallaxes with less than one year of Gaia data

TL;DR

The paper presents a method to derive accurate stellar positions, parallaxes, and proper motions for 2.5 million stars using less than a year's Gaia data combined with Tycho-2 and Hipparcos information.

Contribution

It introduces the TGAS method, adapting Gaia's solution to incorporate Tycho-2 data, enabling early and precise astrometric measurements with limited Gaia observations.

Findings

TGAS can deliver sub-milliarcsecond accuracy for 2.5 million stars.

The method allows earlier execution of astrometric solutions compared to previous projects.

Hipparcos parallaxes can serve as a consistency check in the TGAS/HTPM solutions.

Abstract

Context. The first release of astrometric data from Gaia will contain the mean stellar positions and magnitudes from the first year of observations, and proper motions from the combination of Gaia data with Hipparcos prior information (HTPM). Aims. We study the potential of using the positions from the Tycho-2 Catalogue as additional information for a joint solution with early Gaia data. We call this the Tycho-Gaia astrometric solution (TGAS). Methods. We adapt Gaia's Astrometric Global Iterative Solution (AGIS) to incorporate Tycho information, and use simulated Gaia observations to demonstrate the feasibility of TGAS and to estimate its performance. Results. Using six to twelve months of Gaia data, TGAS could deliver positions, parallaxes and annual proper motions for the 2.5 million Tycho-2 stars, with sub-milliarcsecond accuracy. TGAS overcomes some of the limitations of the HTPM project and allows its execution half a year earlier. Furthermore, if the parallaxes from Hipparcos are not incorporated in the solution, they can be used as a consistency check of the TGAS/HTPM solution.