Astron. Astrophys. 333, 1107-1111 (1998)

4. Discussion

Here, we are aiming to test the consistency and coherence of our results in proper motions, as well as the applicability of this technique.

The positional results did not differ much from those presented in Teixeira (1990) and Teixeira et al. (1992). The precision for a single observation is shown in Table 2.

Table 2. Precions for a single observation

4.1. FK5 stars

Proper motion deviations relative to the FK5 stars were regarded as a diagnostic to our reduction method coherence and consistency, due to the good qualities of the proper motions contained in this catalogue.

In Figs. 1 to 4, each dot stands for as a deviation and to the FK5 stars. One can notice that the differences are close to and well distributed around the zero value. In addition, no relevant systematic trend is depicted when such differences are given as a function of the equatorial coordinates and .

 Fig. 1. Proper motion differences in the sense Bordeaux - FK5

 Fig. 2. Proper motion differences in the sense Bordeaux - FK5

 Fig. 3. Proper motion differences in the sense Bordeaux - FK5

 Fig. 4. Proper motion differences in the sense Bordeaux - FK5

The few points that tend to depart from the general trend deserved a closer investigation, and in all cases at least one out of two characteristics could be outlined: either a poor temporal distribution or a small number of observations when compared to other FK5 stars.

In Figs. 3 and 4, a larger dispersion is noticed for the low declination results. This fact is closely related to the large zenith distances that such observations were made, where atmospheric effects prevent precision from quality. It is worth remembering that Bordeaux observatory is close to latitude .

The mean square sum of the deviations and relative to the FK5 stars provide an estimate for the precision obtained from our reduction method for these parameters:

The above numbers should not be understood as representative to all objects, for the FK5 stars were the most intensively observed ones (about 30 observations per object), having a mean time lag of about six years.

Fig. 5 shows how the mean square sum of these deviations varies as a function of the time lag for the FK5 stars.

 Fig. 5. Estimate for the precision of the proper motions as a function of the time lag - FK5 stars

It is no surprise that the results become better as the time lag increases. Fig. 5 also shows that if larger lags (more than ten years) were available, one could infer that precisions of the order of those contained in catalogues like FK5, PPM and TYCHO, for example, would be possible mainly in right ascension. The difference between both curves reflects a better quality for right ascension observations, rather than those in declination. Such feature is particular to the Bordeaux observatory.

4.2. HIPPARCOS stars

Another way to verify the quality of our results was achieved by the direct comparison, as shown in Figs. 6 and 7 of the calculated proper motions of about one thousand and five hundred non-FK5 stars from our data set common to the HIPPARCOS main catalogue (ESA 1997). Although, in this case, the average time lag (3.4 years) as well as the average number of observations (11 per object) are not as favourable as they were for the FK5 stars, there is still a quite good agreement between the data, which clearly reinforces the quality of the global technique results.

 Fig. 6. HIPPARCOS vs Bordeaux proper motions in right ascension

 Fig. 7. HIPPARCOS vs Bordeaux proper motions in declination

The mean square sum of the differences Bordeaux-HIPPARCOS, for proper motions in both coordinates, is given by

Again, as in the case of the FK5 stars, Fig. 8 shows that results become better as time lag increases.

 Fig. 8. Estimate for the precision of the proper motions as a function of the time lag - HIPPARCOS stars

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998