185 WORK ON ASTROMETRY IN 1993{1996 AT SHANGHAI ASTRONOMICAL OBSERVATORY J.L. Zhao, J.J. Wang Shanghai Astronomical Observatory, CAS, 80 Nandan Road, Shanghai 200030, China avery go o d sample of 198 memb ers is obtained. The ABSTRACT accuracies of these prop er motions range from 0.2{ 5.0 mas per year, of which 60 per cent are b etter than 1.0 mas p er year, and the completeness is nearly In the last three years, a number of new results on down to B =15:5 mag. astrometry were obtained at Shanghai Astronomical Observatory. The studies of Praesep e and Pleiades High-precision p ositions and prop er motions of 441 gave high-precision p ositions and prop er motions of stars in the Pleiades astrometric standard region are numerous stars brighter than B = 16 mag in these determined byWang et al. 1996a. Based on a pre- regions, which should b e excellent extensions of the liminary reference catalogue, constructed by a com- Hipparcos system after they are linked with it. Rela- bination of the catalogues PPM and ACRS, and the tive prop er motions and memb ership probabilities of data given by other authors, 33 exp osures on 11 the op en clusters M67, M11, NGC2286, and Orion plates taken with the Sheshan 40 cm refractor in 86 were determined. A program of absolute prop er mo- years are reduced by the central overlapping tech- tions and space motions for some globular clusters nique, and high-precision p ositions and prop er mo- and Galactic eld RR Lyrae stars has started. This tions of 441 stars in the Pleiades astrometric standard program will b e based on the Hipparcos system after region are obtained. The standard errors of p ositions the Hipparcos Catalogue is available for us. Optical are less than 0.05 arcsec and the standard errors of observations of several radio sources were p erformed prop er motions for 90 p er cent of stars less than 1 mas with photographic plates and CCD. per year. Key words: op en clusters; globular clusters; reference system. 2. OTHER OPEN CLUSTERS A study of prop er motions in the region of the op en 1. PRAESEPE AND PLEIADES cluster M67 and memb ership of stars is p erformed by Zhao et al. 1993. Relative prop er motions and memb ership probabilities of 1064 stars in the op en Prop er motions of 296 AC stars in the central part of cluster M67 region are determined from measure- the Praesep e region are determined byWang & Jiang ments of 9 plate pairs scanned by PDS-1010MS at 1993. The measurements of six Paris AC plates and Purple Mountain Observatory. The plates were taken seven plates taken with the Zo-Se Sheshan 40 cm with the 40 cm refractor at Sheshan station of Shang- refractor are used to determine the prop er motions. hai Observatory. The average standard errors of Accuracies are higher than 0.005 arcsec p er year and prop er motions vary from 0.4 mas p er year for bright mostly arrive at ab out 0.001 arcsec p er year. stars in the inner part of the eld to some 1.5 mas per year for faint stars in the outer part of the eld. A high-precision study of prop er motions and mem- b ership of 924 stars in the central region of Praesep e Prop er motions of stars in the region of the op en clus- is p erformed byWang et al. 1995a. High-precision ter NGC 2286 and its memb ership study are deter- 2 mined by Tian et al. 1994. Relative prop er motions prop er motions for 296 stars in a 90 90 arcmin re- and memb ership probabilities of 2400 stars in the gion centred on BD+20:2170 are obtained from seven op en cluster NGC 2286 region are determined from plates taken with the 40 cm refractor f = 6895 mm measurements of 8 plates scanned by ASTROSCAN at Sheshan station of Shanghai Observatory and six at Leiden Observatory. The plates have the max- AC plates. In addition, high-precision prop er mo- imum epoch di erence of some 70 years and were tions for 257 stars in the same region are obtained taken with the 40 cm refractor at Sheshan station from the former seven plates and the stellar p osi- of Shanghai Observatory. The average standard er- tions in Russell's catalogue. Being combined with rors of prop er motions vary from 0.7 mas per year the data given by other authors, these prop er mo- for bright stars in the inner part of the eld to some tions give a sample of 924 stars in the region men- 1.6 mas p er year for faint stars in the outer part of tioned ab ove. With memb ership probabilities esti- the eld. mated by an improved maximum likeliho o d metho d, 186 Prop er motions of stars in the region of the Orion central overlapping metho d with the iteration pro- Nebula cluster C 0532-054 are determined by Tian cess, we obtain high-precision p ositions and prop er et al. 1996. Relative prop er motions and mem- motions of 22 Hipparcos stars around the radio source b ership probabilities for 333 stars within an area of IAU0316+413 and in the Pleiades astrometric stan- 1:6 1:8 degrees centred on the Orion Nebula M42 dard region. The mean errors of the prop er motions are determined using plates taken over a p erio d of of these stars in right ascension and declination are 83 years taken with the 40 cm refractor of Shanghai 0.51 mas p er year and 0.47 mas p er year, resp ectively. Observatory. The plates were measured with the AS- The external errors of p ositions and prop er motions TROSCAN automatic plate-measuring machine of of the stars are also discussed. Leiden Observatory. The average prop er motion ac- curacy obtained for stars in the photographic magni- With the CCD of the 60 cm re ector, optical p osi- tude range 7{14 mag is 0.3 mas p er year. Errors are tions of radio sources 0716+714 and 0839+187 have somewhat larger towards fainter and brighter mag- b een obtained by Wang & Tang 1996. The ref- nitude, but the ma jority lie well b elow 1 mas per erence stars are selected from Carlsb erg Automatic year. 64 per cent of the stars have b een measured Meridian Circle CAMC catalogue. The mean value successfully on 13 out of 18 plates. of internal accuracy for the p ositions of the sources is 0.19 arcsec. A comparison b etween our results and those in other optical and radio catalogues are also A study of the op en cluster M11 is p erformed bySu given. et al. 1996. 10 plate pairs with epoch di erences of 16{70 years of the op en cluster M11, whichwere taken with the 40 cm refractor of Shanghai Observa- tory,were measured on the PDS-1010 micro densito- REFERENCES meters and relative prop er motions of 872 stars in the cluster region are determined, of which accuracies of Su, C.G., Zhao, J.L., Tian, K.P., 1996, Annals of 85 p er cent are b etter than 0.1 arcsec p er century. Shanghai Observatory Academia Sinica, 17, 205 Tian, K.P., Zhao, J.L., van Leeuwen, F., 1994, A&AS, 105, 15 3. GLOBULAR CLUSTERS AND GALACTIC Tian, K.P., van Leeuwen, F., Zhao, J.L., et al., 1996, FIELD RR LYRAE STARS A&AS, 118, 503 Wang, J.J., 1997, Progress of Astronomy, 15, in press The results, progress, and op en questions in resp ect Wang, J.J., Chen, L., Zhao, J.H., et al., 1995a, of the determination of relative prop er motions and A&AS, 113, 419 estimation of memb ership probabilities, the detec- Wang, J.J., Chen, L., Zhao, J.H., et al., 1996a, Acta tion of internal motions, and the determination of Astronomica Sinica, 37, 69 absolute prop er motions and investigation of space motions of the globular clusters since the middle of Wang, J.J., Jiang, P.F., 1993, Annals of Shanghai 1970s are reviewed by Wang 1997. A program of Observatory Academia Sinica, 14, 75 astrometric and kinematic studies for some globular Wang, J.J., Mao, Y.Q., 1997, Annals of Shanghai clusters has started at Shanghai Observatory. Observatory Academia Sinica, 18, in press Wang, S.H., Tang, Z.H., 1996, Publications of Yun- High precision measurements of absolute prop er mo- nan Observatory, submitted tion for Galactic eld RR Lyrae stars on the refer- ence frame from the Lick NPM catalogue enables the Wang, S.H., Tang, Z.H., Jin, W.J., et al., 1996b, investigation of space motions for these stars based A&AS, submitted on more reliable data. A program has b een started Wang, S.H., Xu, T.Q., Lu, P.Z., 1995b, Acta Astro- to carry out such measurements from plates taken nomica Sinica, 36, 418 with the 40 cm refractor at Sheshan station, Shang- Zhao, J.L., Tian, K.P., He, Y.P., et al., 1993, A&AS, hai Observatory. The reduction technique used in the 100, 243 program is describ ed byWang & Mao 1997 and an example, SW Aqr, is given to compare with di erent techniques. 4. REFERENCE SYSTEM A discussion of the precession constant is p erformed byWang et al.