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Astron. Astrophys. 335, L65-L68 (1998)

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2. The Tycho Reference Catalogue

2.1. Construction of the catalogue

The construction of the TRC shall here be described only briefly, the details to be given elsewhere (Kuzmin et al. 1998).

In order to ensure a safe identification of stars between the AC and TYC, stars with uncertain magnitudes and positions in TYC were rejected as well as stars with large significant proper motions, leaving a subset of [FORMULA] stars for the further processing.

The [FORMULA] Astrographic Catalogue plates were reduced onto the Hipparcos system with the method of zone-specific a-posteriori corrections (Kuzmin et al. 1997b) by means of the ACRS and HIP. A preliminary investigation of ACRS (Kuzmin et al. 1997b) using HIP revealed significant systematic errors, which had to be corrected before it could be used in the plate reductions. The residuals after a first reduction with a linear plate model were analyzed for significant plate-scale systematic terms. In a second step these terms were also solved for, again using the ACRS, and averaged for each zone. In the third step all the systematic effects were applied as known corrections, and reduction with the linear reduction model was performed again, but now using Hipparcos as the reference catalogue. 46 AC plates for which Hipparcos did not provide the necessary minimum number of reference stars were reduced onto the corrected ACRS.

Preliminary proper motion residuals, TRC-HIP, were analyzed in each zone for magnitude equation and the three step reduction was repeated if necessary. Finally, the resulting proper motions were again compared with the proper motions in the Hipparcos Catalogue and a residual magnitude equation, now as a function of declination rather than just zone, of up to about 0.5 mas/yr/mag, were found by a robust method. This correction was applied to obtain the final TRC.

Colour-dependent terms were also investigated, but were found to be negligible compared with the magnitude equation.

In summary, the ACRS was brought to the Hipparcos system and used together with HIP to bring AC as close as possible to HIP. In contrast, the AC reduction for the ACT relied strongly on the original ACRS and only made the transition to HIP as a last correction.

The matching of the resulting positions for each plate with stars in TYC did not use any proper motion, but assumed an upper limit of 180 mas/yr to define the size of the search window. Cases of doubt were resolved, if possible, by means of the magnitudes given in the two catalogues. Thus, no kinematic bias was introduced, as could happen e.g. if the closest candidate had been chosen. The price to pay is that high proper motion stars are excluded from TRC, but they will often be HIP stars and already have good proper motions. This strategy is very different from the one used for the ACT, which used TYC proper motions and relied on astrometry rather than photometry.

The proper motions were constructed from one TYC position and typically 2-3 AC positions using a weighted least squares fit. Unexpectedly poor fits led to the rejection of [FORMULA] stars the most likely reason being erroneous identification at one of the AC epochs.

Due to the specifics of the TRC proper motion derivation, based on two close AC epochs and TYC, internal proper motion error estimates provided by the least-squares adjustment account mostly for the random error of proper motions. Based on a comparison with Hipparcos, 1.6 mas/yr was added quadratically to account for residual systematic errors and thus derive an estimate of the external errors.

2.2. Content of the catalogue

The TRC content is precisely described in connection with its distribution through the CDS. Briefly, the catalogue contains for each star: the TYC identifier, position and proper motion referred to the International Celestial Reference System, ICRS, (Feissel & Mignard 1998) along with the respective standard errors. Furthermore, the TYC magnitudes [FORMULA]) and various flags are given.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998