---
author:
- Daniel Michalik
- Lennart Lindegren
- David Hobbs
bibliography:
- 'TGAS.bib'
- 'agis.bib'
date: 'Accepted for publication in A&A, 24 Dec 2014.'
subtitle: 'How to get 2.5 million parallaxes with less than one year of [*Gaia*]{} data'
title: 'The [*Tycho*]{}–[*Gaia*]{} astrometric solution '
---

[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).]{} [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).]{} [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.]{} [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 