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

Context. The first release of astrometric data from Gala will contain the mean stellar positions and magnitudes from the first year of observations, and proper motions from the combination of Gala 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 Gala data. We call this the Tycho -Gain astrometric solution (TGAS). Methods. We adapt Gala's Astrornetric Global Iterative Solution (AGIS) to incorporate Tycho information, and use simulated Gala observations to demonstrate the feasibility of TGAS and to estimate its performance. Results. Using six to twelve months of Gala 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 FITPM 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 TOAS/IITPM results.