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Observations of Radio Stars at the Valinhos CCD Meridian Circle ASTRONOMY & ASTROPHYSICS MAY I 1999, PAGE 531 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 136, 531–537 (1999) Observations of radio stars at the Valinhos CCD Meridian Circle P.A.A. Lopes1, A.H. Andrei1, S.P. Puliaev1,2, J.L. Penna1,R.Teixeira3, P. Benevides-Soares3, M. Assafin4,J.F.LeCampion5,andY.R´equi`eme5 1 Observat´orio Nacional/CNPq, Brasil 2 Pulkovo Observatory, Russia 3 Instituto Astronˆomico e Geof´ısico/USP, Brasil 4 Observat´orio do Valongo/UFRJ, Brasil 5 Observatoire de Bordeaux, France Received October 27, 1998; accepted February 18, 1999 Abstract. We present the astrometric results from the 1. Introduction optical observation of 16 stars from the list of Wendker (1995), with detected or suspected radio emission. The The determination of the position of radio emitting stars results are given on the Hipparcos/Tycho reference frame. in the Hipparcos reference frame (ESA 1997), as repre- The stars are evenly distributed in right ascension and the sented by the Tycho (ESA 1997) catalogue and the ACT declinations range from −15.6◦ to +28.6◦. This distribu- (Urban et al. 1998a) system of proper motions, enables, tion, also allowed to obtain positions relative to the Twin in principle, to study the relative orientation between that Astrographic Catalogue (TAC 1.0). frame and the International Celestial Reference Frame - The observations were carried out with the Valinhos ICRF (Kovalevski et al. 1997). Radio stars represent an CCD Meridian Circle, in Brazil, operating in drift scan- important and unique class of objects that is directly ob- ning mode. The average positional precision is at 40 mas servable by the primary instruments that define the ra- and generally below 2 mas/yr for proper motions, obtained dio and optical frames (Lindegren & Kovalevsky 1995; with the new reductions of the Astrographic Catalogue - Stone 1997). The advantage of using bright stars, whose AC2000, as first epoch. radio positions are directly tied to nearby ICRF sources, In the strips taken from the observation of the 16 radio however, is hampered by the errors of the stars’ proper stars, 573 Tycho stars, 545 ACT stars and 566 TAC stars motions (Andrei et al. 1995). Therefore the interest on were found. The analysis of the independent reductions the high-precision observations of radio emitting stars be- made relatively to those frames and the large number of longing to the Hipparcos reference frame is justified. They common stars enables to focus on the equatorial zone, as must be made on a continuing basis leading to refinement represented by the three catalogues. As a result, the qual- of the proper motions. ity of Valinhos observations with regard to ACT and TAC Densification of the Hipparcos reference frame, on the internal accuracies is verified1. other hand, is in itself desired, since many observing pro- grams require fainter and more numerous reference stars Key words: astrometry — reference systems — radio than those in the Tycho catalogue. The Twin Astrographic stars Catalog - TAC (Zacharias et al. 1996) represents an impor- tant step in this direction. In the present paper, its initial version (TAC 1.0) is used. A further version shall present an improved set of proper motions and is going to be ori- ented in the Hipparcos system. We point however that the epochs closeness between the considered catalogues and Send offprint requests to: P.A.A. Lopes, Observat´orio our observations decreases the errors due to faulty proper Nacional/CNPq, R. Gal. Jos´e Cristino 77, CEP20921-400, Rio motions. Thus we can extract from our data, a sample of de Janeiro, Brasil. e-mail: [email protected] 1 Table 1 of this paper, plus 16 additional tables with the internal accuracy for the TAC in the observed zone. observed positions for the ACT reference stars, are avail- At the IAG/USP CCD Meridian Circle, in Valinhos, h m s able in electronic form at the CDS via anonymous ftp to S˜ao Paulo (ϕ = –23◦ 000 0600, λ =+3 07 52.2 , cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u- h = 850 m), a program for the observation of 640 con- strasbg.fr/Abstract.html firmed or suspected radio stars, taken from the list of 532 P.A.A. Lopes et al.: CCD Meridian Circle observations of radio stars Wendker (1995), is being currently run. The selected stars 1.6 are contained in the declination zone −77◦ to +30◦ and in the magnitude range from 9 to 14. The instrument has an aperture of 0.19 m and focal length of 2.59 m. It is .4 equipped with a Thomson 7895A CCD camera, which has .8 a 512 × 512 pixels matrix. The pixels are squares of 19 µm side, that correspond to 1.5 arcsec on the sky. The observa- Frequency tions are proceeded in drift scanning mode, that is, leaving .2 the field parading across the matrix and integrating the resulting signal. The equivalent exposure time is 51s secδ. .6 All the observations have been made on fields of 13 arcmin 0 in declination by 30 minutes in right ascension, enabling 0 .02 .04 .06 .08 .1 .12 .14 an average of 36 Tycho stars and a few Hipparcos stars Accuracy in Declination (arcsec) in each field. The detection of objects is made at the level .6 of 3 sigma above the sky background. The center of the .4 objects are then found by a two-dimensional Gaussian fit. The reduction of the (x, y) so obtained is made through .4 a 3 constants model for each direction independently. A comprehensive account of the instrument capabilities and mode of operation is given by Viateau et al. (1999). .2 Frequency We present the results from the observation of 16 stars, .2 all within the TAC range of declinations. Of these stars, 8 belong to the TAC catalogue, 11 belong to the Tycho cat- alogue and 8 to the Hipparcos catalogue. Thus, 12 stars appear in at least one of the catalogues and 5 in all three. 00 0 .02 .2.4.6.81 .04 .06 .08 .1 .12 .14 In the observed fields 573 Tycho stars and 566 TAC stars Accuracy in Right Ascension (arcsec) are found and also have their positions re-measured rela- Fig. 1. Right ascension and declination frequency distribution tively to all three frames. for standard error. The reference stars with more than two Finally, positions from the new reduction of the used observations contribute to the plot; that is 409 ACT, 447 Astrographic Catalogue (Urban et al. 1998b) were TAC and 397 Tycho stars. ACT solution is displayed by the searched for the radio stars, leading to a new determina- dashed line, Tycho solution by the solid line and TAC solution tion of their proper motions. by the dotted line For the present sample of observations, the internal 2. Data reduction standard error for a single tour amounted to 000. 04 for both coordinates and for objects within the optimal magnitude Treatment of the data is accomplished in three stages. range interval, say V = 9 to 13. It increases steadily for First, the primary data is acquired and saved to image fainter magnitudes, attaining 000. 15 for V =15.5. files, usually of quite large sizes. Second, the images are located and identified, and the centering is done to pro- duce (x, y) files. Finally, the set of files covering the aimed 3. Data analysis object are reduced. The plate overlap concept (Eichhorn & Williams 1963) Although the internal accuracy of the measurements is is adopted for the least squares adjustment of a three not affected, to first order, by the number of reference constants model to both coordinates observed values. stars, the orientation of the obtained local frame would The model is simply a polynomial of first degree in x be. This was shown by the experimental solutions based and y. The solution is obtained by an iterative procedure on Hipparcos stars only. To obtain a sufficient number (Teixeira et al. 1992). The iteration step consists in the of those, it would be required to extend the observation conventional reduction of separated tours, followed by the too long, risking to be trapped by anomalous refraction. averaging of the residuals for the objects present in more Therefore, the reductions were made relatively to the three than one tour and subtraction from the observed values. densest, precise catalogues, ACT, Tycho and TAC. We The iteration proceeds at a fast pace and, as a rule, at- point that due to the inhomogeneity of TAC, one field (of tains a satisfactory point after three to five steps. Except the star UX Com) could not be reduced, because there for the origin, scale and alignment, the solution is, in prin- were only a few (3) reference stars on it. ciple, independent from the adopted reference catalogue, Figure 2 compares the observed minus catalogue off- as shown in Fig. 1. sets. All distributions display a normal characteristic. P.A.A. Lopes et al.: CCD Meridian Circle observations of radio stars 533 .6 .1 .6 .08 .1 .06 .5 Accuracy (") .04 .4 .02 Frequency .08 0 8 9 10 11 12 13 14 15 .4.2 Magnitude .1 .08 0 .06 -.4 -.2 0 .2 .4 .06 .3 Catalog Offsets in Declination (arcsec) Accuracy (") .04 .6 .02 .040 .2 20 40 60 80 100 Number of Reference Stars .4 .1 .08 Frequency .1 .02 .06 .2 Accuracy (") .04 .02 00 0 -.4 -.2 0 .2 .4 0 0 .02 .04 .06 .08 .1 .12 .14 0 2 4 5 6 8 10 10 12 14 15 16 Catalog Offsets in Right Ascension (arcsec) Number of Observations Fig.
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