## Radial velocities for the HIPPARCOS-*Gaia
*Hundred-Thousand-Proper-Motion project^{⋆}

Research and Scientific Support Department in the Directorate of Science
and Robotic Exploration of the European Space Agency,
Postbus 299,
2200AG
Noordwijk,
The Netherlands

e-mail: jdbruijn@rssd.esa.int

Received:
14
March
2012

Accepted:
20
July
2012

*Context.* The Hundred-Thousand-Proper-Motion (HTPM) project will
determine the proper motions of ~113 500 stars using a ~23-year baseline. The proper
motions will be based on space-based measurements exclusively, with the Hipparcos
data, with epoch 1991.25, as first epoch and with the first intermediate-release
*Gaia *astrometry, with epoch ~2014.5, as second epoch. The expected
HTPM proper-motion standard errors are 30−190 *μ*as yr^{-1},
depending on stellar magnitude.

*Aims.* Depending on the astrometric characteristics of an object, in
particular its distance and velocity, its radial velocity can have a significant impact on
the determination of its proper motion. The impact of this perspective acceleration is
largest for fast-moving, nearby stars. Our goal is to determine, for each star in the
Hipparcos catalogue, the radial-velocity standard error that is required to
guarantee a negligible contribution of perspective acceleration to the HTPM proper-motion
precision.

*Methods.* We employ two evaluation criteria, both based on Monte-Carlo
simulations, with which we determine which stars need to be spectroscopically
(re-)measured. Both criteria take the Hipparcos measurement errors into account.
The first criterion, the Gaussian criterion, is applicable to nearby stars. For distant
stars, this criterion works but returns overly pessimistic results. We therefore use a
second criterion, the robust criterion, which is equivalent to the Gaussian criterion for
nearby stars but avoids biases for distant stars and/or objects without literature radial
velocity. The robust criterion is hence our prefered choice for all stars, regardless of
distance.

*Results.* For each star in the Hipparcos catalogue, we determine
the confidence level with which the available radial velocity and its standard error,
taken from the XHIP compilation catalogue, are acceptable. We find that for 97 stars, the
radial velocities available in the literature are insufficiently precise for a 68.27%
confidence level. If requiring this level to be 95.45%, or even 99.73%, the number of
stars increases to 247 or 382, respectively. We also identify 109 stars for which radial
velocities are currently unknown yet need to be acquired to meet the 68.27% confidence
level. For higher confidence levels (95.45% or 99.73%), the number of such stars increases
to 1071 or 6180, respectively.

*Conclusions.* To satisfy the radial-velocity requirements coming from our
study will be a daunting task consuming a significant amount of spectroscopic telescope
time. The required radial-velocity measurement precisions vary from source to source.
Typically, they are modest, below 25 km s^{-1}, but they can be as stringent as
0.04 km s^{-1} for individual objects like Barnard’s star. Fortunately, the follow-up spectroscopy is not time-critical
since the HTPM proper motions can be corrected a posteriori once (improved) radial
velocities become available.

Key words: techniques: radial velocities / astronomical databases: miscellaneous / catalogs / astrometry / parallaxes / proper motions

^{⋆}

The results data file is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/546/A61

*© ESO, 2012*