Summary: This note sets forth the main technical aspects of the HTPM project, to release accurate proper motions of the Hipparcos stars few months after Gaia launch, from the combination of early Gaia astrometry with Hipparcos positions acquired 21 years before. The interest of the project in term of science and as a communication tool is recalled in the general presentation. Accurate expressions to obtain the proper motions from two positions are derived and shown to meet the accuracy requirements, even for the most difficult nearby and fast-moving stars. Results based on a realistic simulation built upon the Hipparcos Catalogue are presented and discussed. One finds that the proper motion in right-ascension can be ascertained with an accuracy (RMS of the true error) of 65 micro as/yr, while this is as low as 48 micro as/yr in declination. A simple error propagation model allows also to evaluate these uncertainties from only known quantities during the real exploitation. Provided the radial velocity of the 2000 nearest stars is known to within 1 to 2 km/s, the modelling error can be reduced below 10 micro as/yr for these stars. It is shown that the issue is not critical for stars with parallaxes below 20 mas and that the required radial velocities are very likely available in existing databases.


Bibtex entry for this abstract:

   author = {F.~Mignard},
   title={{T}he {H}undred {T}housand {P}roper {M}otions {P}roject},
   institution={Observatoire de la Cote d'Azur, Nice},
   type={Technical Note}