Up: Mining in the Hipparcos data
Before proceeding with the new solutions it is worth recalling the
various categories of astrometric solutions published in the
Hipparcos Catalogue. The Hipparcos Catalogue is the primary
result of the observations and reductions of the satellite data
acquired over 37 months between November 1989 and March 1993. The
Catalogue comprises 118218 entries with median astrometric
positions of the order of 1 milliarcsec and specific results
(separation, magnitude difference) for double and multiple
systems. For single stars the standard astrometric model yields
the five astrometric parameters (position, parallax and the two
components of the proper motion) together with their full
covariance matrix.
Many catalogue entries were known to be, or found to be,
components of a double or multiple system. The final astrometric
solution for these complex entries, given either for the photocenter of the
system or for the
brighter component, is fully independent of the relative
position of the two components. In cases of detected duplicity the
observation model had to be extended to account for resolved
systems or for systems exhibiting a significant motion on the sky
over the mission. One major difficulty in the double star
processing was the consequence of the large variety of categories
of systems and the difficulty to solve simultaneously for the
relative and absolute astrometry.
It was clear when the time came to publish the results that
several hundred problem stars were left and their solution could
not be given with the same level of reliability as for the bulk of
the Catalogue. The main problems were primarily linked to the double and
multiple
star solutions and much more time would have been
required to clear the remaining difficulties for a subset no larger than a
thousand stars. The main sources of problems were:
- Due to the presence of a periodic modulation grid, the
relative position between the two components of a double system
may be wrong by an integral number of grid periods, of about 1.2
arcsec. Should this happen, for particular elementary observations and
provided it remained undetected, no
reliable astrometric solution can
be obtained in the subsequent processing.
- The double star processing may provide numerous spurious
solutions and in some instances the final choice was based on
the fact that a solution was found in close agreement with the
ground based configuration. If the latter was not correct,
because of a misidentification or because of the relative motion
since the measurement epoch, the error was propagated in the
Hipparcos solution.
- Finally the data processing by FAST and NDAC may have led
to different solutions for the parameters of a binary and a
choice was made at the end. Either one solution was retained or
the entry was classified as suspect binary and processed as
single. In such a case the final fit was usually of poor quality
and published with a warning or with large standard errors.
Up: Mining in the Hipparcos data
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