4 Astrometric results

The observed positions, referred to the ACT catalogue, are presented in Table 1. The same data referred to the Tycho and TAC catalogues are contained in Table 2. The stars are relatively well spread in right ascension, while occupying an equatorial belt.

  

Table 1: Astrometric results referred to the ACT catalogue. N is the number of independent observations and $N_{\rm r}$ the number of reference stars

  

Table 2: Astrometric results referred to the Tycho and TAC catalogues. The number of observations, mean epoch, hours and minutes of right ascension and degrees and minutes of declination follow those on Table 1. $N_{\rm r}$ is the number of reference stars. The TAC position for UX Com could not be determined

  

Table 3: Proper Motions derived from the ACT referred positions. The derived proper motions (Cols. 2 to 5) are based on the radio star positions obtained in this work and those from the AC2000 catalogue. For comparison, proper motions from the ACT are also given (Cols. 6 to 9, with the upper asterisk), as well as additional resources of optical positional data for the stars (Col. 10). All values are in mas/yr. First epoch positions for Gl 402 and HM Sge were extracted from Hipparcos and CMC4 (on Hipparcos system), respectively, for these stars are absent on the AC2000 and because those positions give minimum proper motion errors

There is a fine agreement between the Tycho and ACT referred positions, which merely derives from the fact that the ACT second epoch positions are taken from Tycho and both the Tycho and CCD observations were made at a relatively short time interval from each other. The offsets (arithmetic mean and its standard error), in the sense ACT minus Tycho, for the observed radio stars positions are: $-47 \pm 35$ mas (in right ascension) and $+15 \pm 19$ mas (in declination).

Three independent reductions are compared in Fig. 4, which displays the differences with respect to the average among the three results, for the 16 radio stars. The TAC referred to positions were transported to the Hipparcos reference frame by means of the procedures given by Feissel & Mignard (1998). It is verified that the ACT and Tycho referred results cluster together, while the results referred to the TAC stand noticeably apart from the other two. This is in line with the Hipparcos and FK5 reference frames differences (Argyle et al. 1996) in the concerned declination zone. For the small sample displayed in the figure, the offsets (as before), in the sense ACT minus TAC, are: +98 $\pm$ 32 mas (in right ascension) and +113 $\pm$ 35 mas (in declination).

To confirm such possible systematic trend and its magnitude, we have extended the comparison to all the 415 common reference stars in the observed fields. In this case, the offsets for ACT minus TAC referred positions are: $\Delta\alpha$ cos$\delta$ = +123 $\pm$ 6 mas ($\sigma$ = 122 mas) and $\Delta\delta$ = +108 $\pm$ 7 mas ($\sigma$ = 146 mas), with $\sigma$ being the corresponding standard deviations. For comparison, the corresponding differences for the 510 common ACT and Tycho observed reference stars are found smaller: $\Delta\alpha$ cos$\delta$ = -34 $\pm$ 12 mas ($\sigma$ = 260 mas); $\Delta\delta$ = -10 $\pm$ 9 mas ($\sigma$ = 208 mas). Despite this, the standard deviations are larger than those found for the ACT minus TAC differences. These results, in line with what is seen on the catalogue offsets histograms, point to the high quality of the TAC 1.0 catalogue, although they are affected by zonal warps originating from the FK5 frame.

  

Figure 4: Comparison of position differences: The filled symbols represent the mean positional difference between the sample of 16 radio stars for the three catalogues, respectively. Individual position differences are represented by the plain symbols. The abbreviation TYC, in the figure, is associated with the Tycho catalogue

Proper motions for the program stars were derived exclusively from the positions referred to the ACT and from the AC2000 (Urban et al. 1998b), which furnish first epoch positions. First epoch positions for Gl 402 and HM Sge were extracted from Hipparcos and CMC4 (on Hipparcos system), respectively, for these stars are absent on the AC2000 and because those positions give minimum proper motion errors. For comparison, intermediate epochs data were collected from various sources: the catalogues Hipparcos, Tycho, TAC, AGK3U (Bucciarelli et al. 1992), PPM (Röser & Bastian 1993), ACRS (Corbin & Urban 1991), Bordeaux (Réquième & Mazurier 1991) and Carlsberg (Morrison 1985 through 1994). The data referred to the FK5 system were reduced to the Hipparcos according to the procedures of Feissel & Mignard (1998).

The results are given in Table 3 and Fig. 5. The graph for HM Sge is suggestive of the occurrence of multiplicity, which could explain the relatively large declination errors for this star. No first epoch position was found for FL Vir and for HBC 652.

  

Figure 5: Proper motions for the program radio stars, as derived from Valinhos observations and AC2000 positions. Intermediary positions from various catalogues are plotted for comparison. (See Table 3). First and third columns give right ascension proper motions, while the second and fourth columns give the declination ones. All values are in arcsec/yr. The last four plots (Gl 402, DT Vir, Gl 555 and EQ Peg) are in a different scale from the others, since they refer to high proper motion stars