  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.
