Poster presented at the International Conference of the Astronomische Gesellschaft ,
Heidelberg, 14.-19. September 1998
Abstract published in : Astronomische Gesellschaft Abstract Series 14 ,  112

Revised VLA positions in relation to the Hipparcos Catalogue

R.Hering, H.Lenhardt and H.G.Walter
Astronomisches Rechen-Institut, Mönchhofstrasse 12 -14, D-69120 Heidelberg, Germany

 

Since the derivation of the B1950.0 radio star positions by means of the VLA (Florkowski et al. 1985), revised coordinates of the respective extragalactic calibrators have been determined referring to the International Celestial Reference Frame (ICRF). The revision of the calibrator coordinates propagates to the radio star positions in some intricate way due to the procedures introduced for coordinate transformation to the standard epoch J2000.0 . Note that the corrected radio star position cannot be found from comparing the revised J2000 position of the calibrator with the calibrator position that results from the formal transformation of the B1950 position to J2000 since, in general, the observation epoch of the B1950 position is missing.
 
We have adapted the 20 VLA positions given in Florkowski et al. (1985) to the conventions of the J2000 coordinate system. Compared with the former coordinates obtained from standard transformation methods (Walter et al. 1997), we find offsets for nearly 50% of the stars that are larger than 20 milliarcseconds, which is on average the mean error of the radio star positions. Plots are presented of the coordinate differences between these revised positions and the Hipparcos positions at the epoch of the radio observations together with the corresponding error bars, and discussed. Finally, by determining rotation angles, we study to what extent the revised VLA positions have potential to influence the tie of the Hipparcos frame to the ICRF.

 

References

ESA , 1997, ESA SP-1200, The Hipparcos and Tycho Catalogues
Florkowski D.R., Johnston K.J., Wade C.M., de Vegt C., 1985,  AJ 90, 2381
Walter,H.G., Hering,R., de Vegt,C., 1997,  Astron.Astrophys.Suppl.Ser. 122, 529

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Created: 14 Jul 1999

e-mail :  R.Hering,  ARI
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