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Astron. Astrophys. 355, L27-L30 (2000)

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1. The catalogue and its observational basis

The Tycho-2 Catalogue contains positions, proper motions and two-colour photometric data for the 2.5 million brightest stars in the sky. The sky is well covered to a magnitude about [FORMULA], and slightly deeper in some areas. The stellar density in Tycho-2 is shown in Fig. 1. Overviews of the catalogue content are given in Tables 1 and 2.

[FIGURE] Fig. 1. The Tycho-2 Catalogue: number of observed stars per square degree, in galactic coordinates centred at [FORMULA]. In some areas the density is above 400 stars deg-2, with a maximum of 450. The average density is 150 stars deg-2 at [FORMULA] and decreases to 25 stars deg-2 at [FORMULA]. Traces of the Hipparcos scanning pattern appear as stripes with lower star density.


[TABLE]

Table 1. Principal characteristics of the Tycho-2 Catalogue. By means of proper motions the positions are transferred to the year 2000.0, the epoch of the catalogue. The median values of internal standard errors are given.
Notes:
1) about all 3 axes
2) ratio of external to internal standard errors is [FORMULA] for positions and for proper motions. Systematic errors are [FORMULA] mas and [FORMULA] mas/yr
3) ratio of photometric external to internal standard errors at [FORMULA] is below 1.5



[TABLE]

Table 2. Number of stars and the precision of Tycho-2 within the given intervals of [FORMULA] magnitude.
Notes:
Explanation: `N' is the number of stars in the main Tycho-2 Catalogue. `[FORMULA] ' is the number of stars in the first supplement to Tycho-2. `Mean epoch' is the weighted mean epoch for the source catalogues from which the proper motion is derived; the median value for the N stars is given. `Position (Tycho)' refers to the position observed by the satellite. `Position (mean epoch)' refers to the mean position at the mean epoch.


Positions and magnitudes were derived from the Tycho star mapper observations on the Hipparcos satellite. The observational period was from 1989.85 to 1993.21.

The proper motions are derived for 96 per cent of the stars from the observed positions in Tycho-2, the Astrographic Catalogue and 143 other ground-based catalogues. A total of 7 700 851 ground-based positions were used. Virtually all of the 4% of the stars without proper motions are at the faint end. They are concentrated in certain zones of the Astrographic Catalogue as shown in Hog et al. (2000b), cited hereafter as H2000b. The mean position at the epoch J2000.0 is also given when the proper motion is available.

The Tycho-2 processing emphasized the optimal treatment of apparently single stars, which constituted the vast majority of targets observed. However, an effort was made to detect and measure double stars. This resulted in the measurement of [FORMULA] pairs with separations between 0.8 and 2.5 arcsec, while the lower limit of resolution in Tycho-1 was about 2 arcsec.

The Tycho-2 Catalogue supersedes in size and quality the Tycho-1 Catalogue itself with respect to photometry and astrometry of single and double stars. It also supersedes the astrometric reference catalogues ACT (Urban et al. 1998b) and TRC (Hog et al. 1998) both based on Tycho-1. The Tycho-2 is more complete than Tycho-1 at any given magnitude. Its construction and verification is described by Hog et al. (2000a), hereafter referred to as H2000a, and in H2000b.

1.1. Tycho observations

The observational capabilities and operational principles of the Hipparcos satellite have been presented in the literature, with the most complete and definitive treatment contained within the published Hipparcos and Tycho Catalogues (ESA 1997). Short descriptions are given by Perryman et al. (1997), Hog et al. (1997) and van Leeuwen et al. (1997).

The Tycho-1 Catalogue of one million stars was derived from photon counts obtained by scanning with the Hipparcos star mapper. The scanning was carried out simultaneously with the Hipparcos observations in the adjacent main field of view of the telescope. The Tycho-2 Catalogue of 2.5 times as many stars was obtained from the very same Tycho observations, but utilizing a more powerful data reduction technique. A photon superposition for the whole mission was applied as described in detail in H2000a, instead of detection of single transits. Also important was the use of calibration data from the first Tycho reduction, and the combination of several new catalogues, notably Hipparcos, Tycho-1, STARNET and GSC 1.2, into a new Tycho Input Catalogue (TIC2).

The key features of the Tycho observations were very similar to those of the Hipparcos main field observations and may be summarised as follows:

(a) the continuous scanning by the satellite, resulted in a catalogue of rather homogeneous limiting magnitude;

(b) the multiple epochs throughout the 3-year observation programme and simultaneous Hipparcos observations, resulted in a close connection to the Hipparcos astrometric reference frame represented by about [FORMULA] stars;

(c) the large number of observations per object, of order 130, and the various geometrical scan configurations provided accurate and homogeneous photometric information for each star, from which mean magnitudes in two passbands, [FORMULA] and [FORMULA], were derived for both single stars and double stars. The two Tycho passbands are close to Johnson B and V and approximate transformations are given in ESA97, Vol. 1, Sect. 1.3. It is, however, recommended to use [FORMULA] and [FORMULA] directly since the transformation is dependent especially on luminosity class and reddening which are usually unknown.

Further studies of variability and duplicity, including narrower double stars, are possible based on the original Tycho observations.

1.2. Ground-based observations

The proper motions in Tycho-2 were computed by combining the Tycho-2 positions with those from ground-based transit circle and photographic programmes. Note that this is somewhat different than the procedure used for ACT (Urban et al. 1998b) and TRC (Hog et al. 1998a), where the proper motions were computed from essentially two positions: from Tycho-1 at epoch 1991.25 and from the Astrographic Catalogue (AC) at epoch about 1905. The Tycho-2 proper motions were derived using 143 transit circle and photographic catalogues in addition to the AC and Tycho-2 positions. Including these extra catalogues, generally observed at epochs between the AC and Tycho-2, allows for an improved determination of proper motions, greater investigation of individual stars' errors, and a higher confidence of identification especially in the case of high proper motion stars.

Each of the source catalogues was put on the Hipparcos system individually. It was decided not to use the extant AC 2000 (Urban et al. 1998a), but to re-reduce the AC plate measures using a new reference catalogue based on the Hipparcos Catalogue and ground-based catalogues. Use of the photometry from Tycho-2 has minimized systematic errors due to stellar magnitude and colour. This is detailed in Urban et al. (in preparation). Weights for each of the positions were computed, and the proper motions were determined via weighted least-squares. Details of the computation of the proper motions are found in H2000a and H2000b, e.g. on the weighting of the source catalogues.

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Online publication: March 9, 2000
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