Aims. Based on stellar positions and proper motions, we aim to re-analyse the region of the controversial open cluster Collinder 132. Methods. We have developed a model which analyses the proper motion distribution and the stellar density to find moving groups. The astrometric data were obtained from four Carte du Ciel (CdC) and one Astrogaphic Catalogue (AC) plates of the Córdoba Astronomical Observatory collection (Argentina). Results.We detected an open cluster from the field stars and calculated the mean proper motion and the membership probabilities of the region's stars. We report new coordinates of its centre α c = 108.• 347, δ c = −31.• 011, the components of mean proper motion μ α cos δ = −2.62 ± 0.44 mas/yr, μ δ = 4.79 ± 0.88 mas/yr. Thirteen stars are astrometric members giving a value of 20 for the cluster angular diameter. Six stars fulfil the astrometric and photometric criteria for being cluster members and locate the cluster at 360 pc from the Sun. We propose a simple model for the analysis of the proper motion distribution of an association. We report the components of the association mean proper motion μ α cos δ = −1.38± 0.14 mas/yr, μ δ = 2.26± 0.16 mas/yr. We found 149 astrometric members, 11 of which have reliable photometric data that locate them betweeen 417 and 660 pc from the Sun.
Due to the high quality of Gaia DR2 stellar parallaxes and proper motions, we can study the Cygnus OB2 region up to G = 17.5 mag in circular region centred at (l, b) = (79.8○, +0.8○) of radius 1○. We detect four over-densities in the Vector Point Diagram. We apply a parametric model of the proper motion distribution and determine their mean proper motions, identify their members and estimate their distances by analyzing the parallaxes of the most probable members. We confirm that one of these regions is Cygnus OB2, whose mean proper motion is (μαcosδ, μδ) = (-2.71 ± 0.02, -4.24 ±0.02) mas/yr. We identify 2767 astrometric members at an estimated distance of 1683 ± 5 pc. A detailed comparison of these members with 333 photometric and spectroscopic Cygnus OB2 members shows that 33 stars of the last group are not members, 16 of them are O-B stars. 300 members in common lead to estimate a distance to the association of 1669 ± 6 pc. One of the remaining over-densities is the open cluster UCB585 for which we find eight more members, located at ∼1460 pc. The others two groups count 179 and 188 proper motion members which are distributed throughout the region and situated a distance of ∼1280 pc. The estimated distances of these four groups suggest the existence of different substructures in the Cygnus OB2 region along the line-of-sight.
h i g h l i g h t sWe determine astrometric parameter and stellar members of vdB80, vdB85 and vdB130. vdB80 and vdB85 astrometric parameters are determined for the first time using UCAC4. We find that proper motion errors do not significantly affect astrometric results. Astrometric members are compared with the photometric ones given in the literature. a b s t r a c tWe present results of a study that combines photometry and astrometry for the open clusters vdB80, vdB85 and vdB130. We apply a model which analyses the proper motion distribution and the stellar density to find the kinematic parameters and stellar membership in the region of the mentioned open clusters. The astrometric data are obtained from UCAC4 catalogue. For each cluster, we report the centre coordinates, the components of mean proper motion, the angular diameter and the astrometric members. They are: vdB80: a ¼ 97 :73938 AE 0 :00846; d ¼ À9 :66953 AE 0 :01177; l a cosd ¼ À2:13 AE 0:47 mas=yr; l d ¼ À0:95 AE 0:47 mas=yr; 12 0 , 15 members; vdB85: a ¼ 101 :71670 AE 0 :00808; d ¼ 1 :34392 AE 0 :01253; l a cosd ¼ 0:89 AE 0:43 mas=yr; l d ¼ 3:24 AE 0:43 mas=yr; 8 0 , 9 members; vdB130: a ¼ 304 :44001AE 0 :01407; d ¼ 39 :32745 AE 0 :00726; l a cosd ¼ À4:14 AE 0:25 mas=yr; l d ¼ À5:15 AE 0:25 mas=yr; 9 0 , 9members. We analyse the incidence of the proper motion errors in the determination of the cluster parameters and of the stellar membership and find that they are not significantly changed. We finally compare the astrometric members with the photometric ones given in the literature.
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