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Astron. Astrophys. 360, 391-398 (2000)

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5. Conclusions

We compared the proper motions in four astrometric catalogues: the Hipparcos Catalogue (HIP), the Tycho Catalogue (TYC), the Tycho Reference Catalogue (TRC), and the Astrographic plus Tycho Catalogue Reference Catalogue (ACT). The first two resulted from the Hipparcos satellite mission while the last two were constructed using the almost-century old Astrographic Catalogue and the Tycho Catalogue. The HIP, TRC, and ACT give highly accurate (1, 3, and 3 mas yr-1 accuracy, respectively) proper motions, while the proper-motion errors in the TYC Catalogue are an order of magnitude larger. Note that ACT proper motions in Right Ascension do not include the [FORMULA] term. The aim of our comparison is to assess the quality of the proper motions and to detect any systematic difference between the catalogues. We created normalized proper-motion difference distributions for each possible pair of catalogues (Eq. (1)), for each of the 20 declination zones of the Astrographic Catalogue. These distributions should in principle be normal distributions with zero mean and unit variance.

For most pairs of catalogues the normalized proper-motion distribution does not show any dependence on zone. Only two zones show peculiarities. The Cape zone shows [FORMULA] standard deviations that are (a) larger than unity and (b) magnitude dependent, whereas all other zones have standard deviations smaller than, or close to, unity. The Vatican zone shows peculiar standard deviations in its [FORMULA] distributions. Its value of 1.5 is much larger than the value of 1.2 for the other zones.

We also find four zone-independent effects:

(1) A fraction of the ACT proper-motion errors is most likely underestimated by about 30%. This conclusion is based on the standard deviation of [FORMULA] which is systematically larger than unity ([FORMULA]1.4), and on the non-Gaussian shape of the [FORMULA] distribution. A independent comparison of the AT and the SPM catalogue confirms this result.

(2) The proper-motion errors of the faint Tycho stars are underestimated by almost 40%. The standard deviations of [FORMULA] and [FORMULA] for the faint stars ([FORMULA] mag) are dramatically different from those of the bright ([FORMULA] mag) and intermediate stars ([FORMULA] mag). The latter two regimes have standard deviations close to unity while the faint stars have standard deviations as large as 1.6. The standard errors given in the Tycho catalogue are formal errors. They are known to be underestimated for the faint stars, as stated by the authors of the Tycho Catalogue in ESA (1997: Vol. 1 p. xv and p. 142 and Vol. 4 Sect. 18.5).

(3) There are systematic differences between the proper motions in the TRC and the ACT. This conclusion is based on the distribution of the means of the [FORMULA] distributions of all zones. The proper motions in the TRC and ACT sometimes show systematic differences of more than 1 mas yr-1. These differences change from zone to zone and do not show any systematic behaviour. Our analysis cannot reveal the source of this discrepancy.

(4) As expected, the normalized proper-motion difference distributions [FORMULA] and [FORMULA] show that the TRC and ACT catalogues are correlated (standard deviations smaller than unity). This correlation is expected because both catalogues have been constructed in a similar fashion from the same material.

The Hipparcos, TRC, and ACT catalogs will remain the most complete and accurate astrometric catalogues for at least the next decade. In 10 to 20 years the results of several astrometric space missions, e.g., SIM (see, e.g., Shao 1998), FAME (see, e.g., Horner et al. 1999), and GAIA (see, e.g., Gilmore et al. 1998), will become available. These satellites will provide micro-arcsecond astrometry for over a billion stars.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000