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Astron. Astrophys. 317, 689-693 (1997)

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2. Observations

Positions of 348 white dwarfs (WD), hot subdwarf B and hot subdwarf O (sdB and sdO) stars were measured with the Carlsberg Automatic Meridian Circle (CAMC) on La Palma during the period 1987 - 1993. The positions will be published elsewhere (CAMC Catalogues, 4 - 9). Of these stars, 100 sdB, 28 sdO, and 18 WDs could be found in the Astrographic Catalog and proper motions calculated. The typical error in the proper motions is quite small, 0.005 arcsec yr-1, and it is this high accuracy which makes the measurement of the kinematics of the subdwarf stars feasible. Our results are shown in Tables 4-9 for hot subdwarfs and Tables 10-11 for white dwarfs 1.

For 15 of our WDs proper motions exist in the literature (McCook&Sion 1987, and Gliese&Jahreiß 1991) and we compared these to our new measurements in Fig. 1. The agreement is very satisfactory. Note that our new data are much more accurate than the previously measured proper motions for these objects, which typically have errors of approximately 0.02 arcsec yr-1.

[FIGURE] Fig. 1. Comparison of white dwarf proper motions. The errors for the comparison proper motions was set at 0.02 "/year (Jahreiss, private communication, 1995).

It was also possible to calculate proper motions using the Guide Star Catalog (GSC) and the digitized version of the Palomar Observatory Sky Survey (POSS) for 23 stars. For those 17 targets that also have CAMC-AC derived proper motions we performed a comparison of the size of the proper motion and the position angle, and found satisfactory agreement.

For some of our objects the best available celestial coordinates were of low accuracy, so that it was uncertain if we had recovered the correct object with the CAMC, particularly in crowded fields. In all cases however, it was possible to check the measured object by working backward from the CAMC photometry and coordinates. First, we checked that the photometry was consistent with data in the literature, where available, and furthermore that the object was the singularly blue object in the field (using finding charts constructed from the APM two colour digitized POSS available via the Internet.) All objects were tested in this way and a small number of erroneous targets were weeded from our final lists.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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