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I/212               Proper motions in NGC 3680  (Kozhurina-Platais+, 1995)

Catalogue of Proper Motions in the region of NGC 3680. Kozhurina-Platais V., Girard T.M., Platais I., van Altena W.F., Ianna P.A., Cannon R.D. <Bull. Inf. CDS 46, 9 (1995)> =1995BICDS..46....9K A proper motion study of the open cluster NGC 3680. Ozhurina-Platais V., Girard M., Platais I., van Altena W.F. <Astron. J. 109, 672 (1995)> =1995AJ....109..672K
ADC_Keywords: Clusters, open ; Positional data ; Proper motions ; Magnitudes, photographic ; Description (Abstract): A catalogue of positions, proper motions and photographic BV magnitudes for 2711 stars in and around the open cluster NGC 3680 is presented. The Catalogue is complete down to B=17.5 mag in a rectangular, one square degree area centered on the cluster. The limiting magnitude of the Catalogue is B=18.4 mag. The mean standard error of the proper motions for well-measured stars is ±0.35 mas/yr. The photographic B and V magnitudes are accurate to about ±0.1 mag. The mean error of the relative positions for all 2711 is ±0.025 arcsec at the mean epoch of the plate material, 1977.8. For each star, three different cluster membership estimators are presented. For convenience, cross-identifications to other studies are provided. Introduction: The open cluster NGC 3680 (C1123-429; l=286.8 deg, b=16.9 deg) fits the same age group as the well-studied cluster M67, although it is not a twin of that cluster. For instance, NGC 3680 contains a much smaller number of cluster members. Most likely it belongs to the group of so-called poorly-populated open clusters which has been studied by Platais (1994). Unfortunately, the lack of an astrometric study before now, impeded reaching definite conclusions as to the cluster's basic parameters and the luminosity function. Recently Kozhurina-Platais et. al (1995) have completed a comprehensive proper motion study of NGC 3680. Here we present an extended version of the proper motion catalogue for all 2711 stars measured in that study. The Catalogue should prove valuable for further studies of the cluster properties and also can be useful for galactic structure studies toward the galactic anti-rotation direction as demonstrated by Mendez et al. (1993). Data description: A detailed description of the astrometric and photometric data is available in Kozhurina-Platais et al. (1995). Only a brief summary of the reductions and attained accuracies is provided here. The astrometric reductions were based on twelve plates taken with the Yale-Columbia 26-in refractor (plate scale 18.85 "/mm). The earliest plate was taken in 1947, the latest - 1992. The largest plates cover 67' (E-W) by 52' (N-S) area. All of the plates were digitized with the Yale PDS microdensitometer equipped with laser encoders. Image centroids were obtained by fitting a two-dimensional bivariate Gaussian to the image density profiles. Proper motions were calculated using the basic principles of an iterative central-plate overlap technique supplemented with a very careful account of the magnitude equation and the magnitude/color corrections. Relative proper motions were calculated for 2711 stars, measured on at least three plates which spanned a minimum of ten years epoch difference. The standard error of the proper motion for bright (V<14.0 mag) stars, measured on all twelve plates, is ±0.35 mas/yr. The Catalogue of proper motions is complete down to B=17.5 mag, while the limiting magnitude is B=18.4 mag. The five best 26-in telescope plates were used to obtain the B magnitudes. V magnitudes were derived primarily from two visual Yale Southern Observatory 51-cm astrograph plates. The instrumental magnitudes were transformed into the standard BV magnitudes using photographic BV-photometry by Anthony-Twarog et al. (1991). The formal photometric error for the whole Catalogue is about ±0.1 mag in both B and V magnitude. Our BV photometry should be used with caution in a detailed analysis of the cluster's color-magnitude diagram since its primary purpose was for application of magnitude and color corrections made in the astrometric reductions. We are aware of several stars with totally erroneous V-photometry (for instance, Nos. 376, 2696) caused by the difficulty in separating blended images on the 51-cm astrograph visual plates. In addition to the proper motion, we also present each star's mean position. The mean error of the relative positions for all 2711 stars is ±0.45 micron or, in corresponding angular units, ±0.025 arcsec, and for the 524 stars brighter than V=14.5 mag, the internal error is ±0.18 micron (±0.010 arcsec) at the mean epoch of the plate material, 1977.8. Note that we had no control over possible magnitude equation in the positions. The mean positions were updated to epoch=2000.0 and then transformed into the HST Guide Star Catalog system of celestial coordinates using 219 reference stars. The unit weight mean error of this transformation is ±0.18 arcsec, which is dominated by the errors in the reference frame. Proper motion errors: In the Catalogue the single-coordinate proper-motion formal error is estimated from the scatter about the best-fit line for each star separately. Due to the small number of plates involved in the solution, this formal error may not always be a statistically reliable estimator of the true error. Here we would like to caution that the proper motion uncertainties given in Kozhurina-Platais et al. (1995) for the probable cluster members are calculated by median smoothing the proper-motion error distribution as a function of B magnitude. This procedure, however, was not applied to the stars with proper-motion errors exceeding three times the median-smoothed error estimate. The proper-motion membership probability calculation benefited significantly from the use of these median-smoothed proper-motion error estimates. Statistical properties of the formal errors, though, should be studied only from the error values provided in this Catalogue. Membership Probabilities: The probability of a given star being a cluster member is defined as the ratio of the frequency of cluster stars to all stars at the point within the proper motion distribution corresponding to that star's proper motion. To derive the probabilities, the standard technique (Sanders, 1971) of modeling the proper-motion distributions has been widely used. Following this tradition, we applied the modified Sanders method to the proper-motion data and derived the probabilities P_µ. Our best fit to the proper-motion distributions yields only 74 cluster members. With such a small fraction of cluster members, the associated membership probabilities may be sensitive to the shape of cluster's luminosity function and the spatial distribution. The latter can be rigorously taken into account by including an empirical spatial surface density distribution into the membership probability. The joint spatial/proper motion membership probabilities P_µ,r provide more realistic probability estimates but can be distorted by a varying cluster-to-field luminosity function ratio over the entire magnitude range. Therefore, we also provide a so-called local-sample membership estimator, P_s, which should not be biased by possible differences in the shape of the luminosity functions. Note that these probabilities are calculated only for stars measured on seven or more plates. For practical cluster member selection purposes, we recommend use of the P_µ,r probabilities with the caution not to use them to derive information regarding the cluster's spatial structure. A full discussion regarding the various membership probability estimates can be found in Kozhurina-Platais et al. (1995). File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file ngc3680 113 2711 The catalogue of stars in NGC 3680 xid 15 427 Cross-identifications with previous work
See also: J/AJ/113/1045 : BV photometry of NGC 3680 (Kozhurina-Platais+ 1997) J/A+AS/118/407 : NGC 3680 CCD photometry & velocities (Nordstroem+ 1996) J/A+A/322/460 : NGC 3680 photometry and radial velocities (Nordstrom+ 1997) Byte-by-byte Description of file: ngc3680
Bytes Format Units Label Explanations
2- 5 I4 --- No [1/2711]+ Sequential number of the star. 7- 11 F5.2 mag Vmag [7/18] Photographic BV-photometry. 13- 17 F5.2 mag B-V Photographic colour index 20- 21 I2 h RAh [11] Right Ascension (hours) Equinox=J2000, Epoch=J2000 23- 24 I2 min RAm [22/28] Right Ascension J2000 (minutes) 26- 30 F5.2 s RAs Right Ascension J2000 (seconds) 32 A1 --- DE- [-] Declination J2000 (sign) 33- 34 I2 deg DEd [42/43] Declination J2000 (degrees) 36- 37 I2 arcmin DEm Declination J2000 (minutes) 39- 42 F4.1 arcsec DEs Declination J2000 (seconds) 44- 45 I2 --- nPlates The number of plates used in the proper motion calculation. 47- 53 F7.2 mas/yr pmX Relative proper motions in X-coordinate. 55- 61 F7.2 mas/yr pmY Relative proper motions in Y-coordinate. 63- 67 F5.2 mas/yr e_pmX Formal error of pmX. 69- 73 F5.2 mas/yr e_pmY Formal error of pmY. 75- 77 I3 % Pr(pm) [0/100] Standard membership probability for the cluster NGC 3680. (2) 79- 81 I3 % Pr(pm,r) [0/100] Joint spatial/proper motion membership probability. (2) 83- 85 I3 % Pr(S) [0/100]?=-1 Local-sample membership probability; negative probabilities indicate a not computed probability. (2) 87- 94 F8.4 mm X Standard-plate coordinate (1) 96-103 F8.4 mm Y Standard-plate coordinate (1) 105-108 F4.1 um e_X Mean error on X (microns) 110-113 F4.1 um e_Y Mean error on Y (microns)
Note (1): Mean standard-plate coordinates derived from the iterative central-plate overlap solution (epoch=1977.8, scale=18.85 "/mm). The mean standard plate originated from the plate #Y36 with X=Y=0.0 placed close to the optical center. Note (2): see above section on Membership Probabilities
Byte-by-byte Description of file: xid
Bytes Format Units Label Explanations
2- 5 I4 --- No [1/2711]+= The present Catalogue number. 7- 10 A4 --- Ant-Twa Identification with Anthony-Twarog et al. (1991). 13- 15 A3 --- Eggen Identification with Eggen (1969).
References: [1] (1991AJ....102.1056A) Anthony-Twarog, B.J., Heim, E.A., Twarog, B.A., Caldwell, N.: 1991, Astron. J., vol. 102, 1056. [2] (1969ApJ...155..439E) Eggen, O.J.: 1969, Astrophys. J., vol. 155, 439. [3] (1995AJ....109..672K) Kozhurina-Platais, V., Girard, T.M., Platais, I., van Altena, W.F, Ianna, P.A., Cannon, R.D.: 1995, Astron. J. (in press) [4] Mendez, R.A., Kozhurina-Platais, V., Girard, T.M., van Altena, W.F.: 1993, in "Workshop on Databases for Galactic Structure", eds. A.G. Davis Philip, B. Hauck and A.R. Upgren, L. Davis Press, 223. [5] Platais, I.: 1994, in "Galactic and Solar System Optical Astrometry", eds. L.V. Morrison and G.F. Gilmore, Cambridge University Press, 211. [6] (1971A&A....14..226S) Sanders, W.L.: 1971, Astron. Astrophys., vol. 14, 226. E-mail (for the first author):
(End) Francois Ochsenbein [CDS] 08-Feb-1995
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