New Astronomy 36 (2015) 70–79 Contents lists available at ScienceDirect New Astronomy journal homepage: www.elsevier.com/locate/newast A revisit to the regions of some van den Bergh open clusters using photometric and astrometric parameters R.B. Orellana a,b, M.S. De Biasi a,b,⇑, L.G. Paíz a, I.H. Bustos Fierro c, J.H. Calderón c a Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque s/n, 1900 La Plata, Argentina Instituto de Astrofísica de La Plata (CCT La Plata – CONICET, UNLP), Argentina c Observatorio Astronómico, Laprida 854, 5000 Córdoba, Argentina b h i g h l i g h t s  We 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 r t i c l e i n f o Article history: Received 2 June 2014 Received in revised form 10 October 2014 Accepted 15 October 2014 Available online 29 October 2014 Communicated by G.F. Gilmore Keywords: (Galaxy:) open clusters and associations: general (Galaxy:) open clusters and associations: individual (vdB80, vdB85, vdB130) Astrometry a b s t r a c t We 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  0 :00846; d ¼ 9 :66953  0 :01177; la cosd ¼ 2:13  0:47 mas=yr; ld ¼ 0:95  0:47 mas=yr; 120 , 15 members; vdB85: a ¼ 101 :71670  0 :00808; d ¼ 1 :34392  0 :01253; la cosd ¼ 0:89  0:43 mas=yr; ld ¼ 3:24  0:43 mas=yr; 80 , 9 members; vdB130: a ¼ 304 :44001 0 :01407; d ¼ 39 :32745  0 :00726; la cosd ¼ 4:14  0:25 mas=yr; ld ¼ 5:15  0:25 mas=yr; 90 , 9 members. 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. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In 1966 van den Bergh publishes a catalog of 158 reflection nebulae and remarks that several of them contain open clusters named as the reflection nebula to which they are associated. The knowledge of these embedded clusters let study the very initial phases of star formation (Bica et al., 2003). Some of these clusters have been investigated using photometric techniques in order to determine the member stars, consequently the cluster fundamental parameters, e.g. distances, masses, ages, ⇑ Corresponding author at: Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque s/n, 1900 La Plata, Argentina. E-mail addresses: rorellan@fcaglp.unlp.edu.ar (R.B. Orellana), debiasi@fcaglp. unlp.edu.ar (M.S. De Biasi), lpaiz@fcaglp.fcaglp.unlp.edu.ar (L.G. Paíz), ivanbf@oac. uncor.edu (I.H. Bustos Fierro), calderon@oac.uncor.edu (J.H. Calderón). http://dx.doi.org/10.1016/j.newast.2014.10.005 1384-1076/Ó 2014 Elsevier B.V. All rights reserved. metallicity, mean radial velocity can be derived. An accurate knowledge of their members is therefore crucial. Due to the fact that open cluster members share similar photometric and kinematic properties, it is worth to complete the study of the stars of the clusters field by employing astrometric methods. This analysis contributes to remove the contamination effect arised from a photometric membership determination, as Corti and Orellana (2013) demonstrate it in their investigations on the open cluster NGC 4755 and the stellar association CenOB1. In this work we present improvements of the membership probability of the stars in the regions of the open clusters vdB80, vdB85 and vdB130, whose photometric members are given in the literature. Open cluster vdB80: has equatorial coordinates a ¼ 6h 30m 50s and d ¼ 9 390 1800 ðl ¼ 219 :26; b ¼ 8 :93Þ and is placed in Monoceros. It was first identified by van den Bergh (1966) and R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 71 Fig. 1. Images of the optical environments of open clusters vdB80 (a), vdB85 (b), vdB130 (c). Right ascension and declination are given in the abscissa and ordinate, respectively. Orientation: north to the top and east to the left. contains the stars BD 9 1498 (HD 46060) and BD 9 1497. Racine (1968) performed photometry and spectroscopy for 80a (HD 46060) and photometry for 80b (BD 9 1497) y 80c. In 1972 Aveni and Hunter (1972) study this compact cluster and determined spectral type and photometric data for a 24-star sample. They found seven possible members considering their spectroscopic distances and estimated the cluster age in 106 yr. Ahumada et al. (2001) estimate its age in 4:5  1:5  106 yr by an integrated spectrum indicating that it is a very young cluster. A more extensive analysis using 2MASS data is done by Bonatto and Bica (2009). They identify the members up magnitude 16 from a colour-magnitude diagram where a poorly populated main sequence and many pre-MS stars are displayed. In addition, they determine the cluster age of 5  2 Myr, a distance of 2:1  0:3 kpc, a mass of 95  17 M  and the analysis of the stellar density profile gives the value of 0:46  0:080 for core and of 5:8  0:30 for the cluster. Open cluster vdB85: discovered in 1966 by van den Bergh, its is placed in Monoceros. Its equatorial coordinates are a ¼ 06h 46m 54s and d ¼ þ01 200 0000 ðl ¼ 211 :2280; b ¼ 00 :4014Þ and it includes the star BD + 1 1503 (HD289120). Racine (1968) makes photometric and spectroscopic observations of this star. The reflection nebula vdB-RN85 is also named as NGC 2282 in the literature. The first detailed near-infrared (JHK) imaging and photometry of the young open cluster associated to NGC 2282 is conducted by Horner et al. (1997). They investigate fundamental parameters of the cluster and the surrounding region, identify about 100 cluster members and locate the cluster at 1.7 kpc. Open cluster vdB130: is situated in Cygnus and their coordinates are a ¼ 20h 17m 42s and d ¼ þ39 210 0000 ðl ¼ 76 :9083; b ¼ þ02 :0721Þ. It is first identified by van den Bergh in 1966 including the star BD + 38 3993 (HD228789). In 1968, Racine identifies two stars as cluster members and carries out photometric and spectroscopic observations, being HD228789 one of these 72 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 stars. In 1974, he identifies 14 stars as open cluster members on the basis of their apparent connection with the nebular material and reports photometry and spectroscopy thereof. Moffat and Schmidt-Kaler (1976) review Racine’s data, rederived their intrinsic properties and find a distance of 1.4 kpc for the cluster. Using UCAC4 data, Dias et al. (2014) determine mean proper motion and membership probabilities from a small sample of 62 stars in a region of radius of 4 arcmin. They obtain la cos d ¼ 3:61  3:32 mas/yr and ld ¼ 4:58  3:37 mas/yr and find that 50 stars of the sample are cluster members. The images of the optical environments of the three open clusters are taken from the Digital Sky Survey (DSS)1 (Fig. 1). 2. The data The astrometric data used in this paper, stellar position and proper motion, come from the UCAC4 Catalogue with positions over 113 million stars and 105 million of them with proper motions. The catalogue is complete to about magnitude R = 16 and with at least about 40 stars per square deg anywhere on the sky. The average density of this catalogue is over 2000 stars per square deg. The UCAC4 catalogue presents very precise proper motions. They result from the combination with the re-analysis of additional 140 star catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. Errors in proper motions of the bright stars (to R  12) run from about 1–3 mas/yr benefited by the large epoch spans involved. For the fainter stars using SPM and NPM data, typical errors are 2–6 mas/yr. The astrometry provided in UCAC4 is on the Hipparcos system, i.e. the International Celestial Reference System (ICRS), as represented by the Tycho-2 catalog. Positions in UCAC4 are given at the standard epoch of Julian date 2000.0, thus the UCAC4 is a compiled catalog. Positional errors are about 15–20 mas for stars in the 10–14 mag range. These data are supplemented by 2MASS photometric data for about 110 million stars and 5-band (B, V, g, r, i) photometry from the APASS (AAVSO Photometric All-Sky Survey) for over 50 million stars. A detailed description of the construction of UCAC4 catalogue can be found in Zacharias et al. (2013). Data extraction has been performed using the Vizier.2 Table 1 Sizes of the grid and radius selected to evaluate the local density in the region of the clusters and the approximate cluster centre coordinate (a0 ; d0 ). vdB 80 vdB 85 vdB 130 Grid’s size s0 a0 [°] d0 [°] 00 :5  00 :5 00 :5  00 :5 00 :5  00 :5 20 :2 30 10 :5 97.71542 ± 0.01240 101.72167 ± 0.01667 304.43783 ± 0.01078 9.65500 ± 0.01222 1.32333 ± 0.01667 39.34550 ± 0.00833 is adopted as the centre of the cluster with coordinates (a0 ; d0 ). It represents an approximate value of the cluster centre because it is determined only considering the spatial over-density. The grid and radius sizes depend on the local cluster regions, as shown in Table 1. An example of the resulting projected stellar density is shown in Fig. 2. The centre of the over-density was adopted as the position ða0 ; d0 Þ of the grid-point with the highest projected spatial density. 3.2. Mean proper motion and membership determination An over-density in the sky as well as in the vector point diagram (VPD) indicates the existence of an open cluster. We adopt the mathematical model suggested by Vasilevskis et al. (1958) and the technique based upon the maximum likelihood principle developed by Sanders (1971) to detect these over-densities. The proper motion distribution consists in the overlapping of two bivariate normal frequency functions in an elliptical subregion of the VPD: Ui ðlxi ; lyi ; ri Þ ¼ /ci ðlxi ; lyi ; ri Þ þ /fi ðlxi ; lyi Þ; ð1Þ where /ci is a circular distribution for cluster stars, /fi is an elliptical distribution for field stars; lxi ; lyi are the ith star proper motion in x and y, respectively. These coordinate axes are coincident with the field distribution ones after rotating the VPD by an angle h (Vasilevskis et al., 1965). The model is improved using an exponential function qc to describe the areal stellar density for the cluster stars (Jones and Walker, 1988). qc ðri Þ ¼ q0 expðri =r0 Þ; ð2Þ 3. Astrometric analysis 3.1. Cluster centre coordinates We analyse the projected stellar density of each cluster in a region around each BD or CD star associated with the reflection nebulae catalogued by van den Bergh (1966). The size and magnitude limit of the selected region depend on local characteristics. We consider the faintest magnitude up to which an overdensity can be distinguished. Up to the 15 magnitude, we examine a region 0 0 0 0 sized of 15  15 for vdB85 and another of 24  24 for vdB130. A 0 0 region of 15  15 is chosen for vdB80 analysis considering all magnitudes. We evaluate the observed local density at the nodes of a grid of a given size by adding the stars within a circle whose radius s0 is weighted by a smoothing parameter following Stock and Abad’s rule (Stock and Abad, 1988). The node with the highest density 1 We thank the Leicester Database and Archive Service (LEDAS) <www.ledas.ac.uk/ DSSimage/aboutdss> and the Space Telescope Science Institute (STScI) for the use of their images. 2 <http://vizier.u.strasbg.fr/viz-bin/VizieR?-source=UCAC4>. Fig. 2. Projected density map of UCAC4 stars in the region of vdB80. The scale of grey represents the different values of the density. The point with the highest density is adopted as the centre of the cluster. 73 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 [a] [a] 30 −9.4 20 −9.5 µδ [mas/yr] 10 δ [°] −9.6 −9.7 0 −10 −9.8 −20 −9.9 −30 −30 98 97.9 97.8 97.7 97.6 97.5 97.4 α [°] [b] −20 −10 0 10 20 30 µαcos δ [mas/yr] [b] 40 1.5 30 1.45 20 1.4 µδ [mas/yr] 10 δ [°] 1.35 1.3 0 −10 1.25 −20 1.2 −30 1.15 −40 −40 1.1 101.9 101.85 101.8 101.75 101.7 101.65 101.6 101.55 101.5 α [°] [c] −30 −20 −10 0 10 20 30 40 µαcos δ [mas/yr] [c] 30 39.55 20 39.5 10 µδ [mas/yr] 39.45 δ [°] 39.4 39.35 0 −10 39.3 −20 39.25 −30 −30 39.2 −20 −10 0 10 20 30 µαcos δ [mas/yr] 39.15 304.7 304.6 304.5 304.4 304.3 304.2 α [°] Fig. 3. Stellar positions in the region of vdB80 (a), vdB85 (b) and vdB130 (c). The black circles represent the astrometric cluster members and crosses the rest of the stars in the ellipse. where q0 is the central cluster stellar density, r 0 is the characteristic radius and ri the distance from the cluster’s centre. The function for the areal stellar density for field stars qf is constant. Fig. 4. Vector point diagram in the region of vdB80 (a), vdB85 (b) and vdB130 (c), proper motions are in units of mas/yr. The black circles represent the astrometric cluster members and crosses the rest of the stars in the ellipse. The stellar radial density profile of the region is modelled by the overlapping of the functions mentioned above: qðri Þ ¼ qc ðri Þ þ qf : ð3Þ The circular and elliptical distributions take the following form (Orellana et al., 2010). 74 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 2 " ðlxi  lxc Þ þ ðlyi  lyc Þ q ðri Þ /ci ðlxi ; lyi ; ri Þ ¼ c 2  exp  2prc 2r2c 2 # ð4Þ and /fi ðlxi ; lyi Þ ¼ ðlxi  lxf Þ2 ðlyi  lyf Þ2 qf ;  exp   2prxf ryf 2r2xf 2r2yf " # ð5Þ where lxf ; lyf are the field mean proper motion, rxf ; ryf the elliptical dispersions for field stars, lxc ; lyc the cluster mean proper motion, and rc its circular dispersion. These parameters are obtained after applying the method of maximum likelihood to Eqs. (4) and (5). See Orellana et al. (2010) for a detailed explanation of the method. The probability for the ith star is calculated as Pci ðlxi ; lyi ; r i Þ ¼ /ci ðlxi ; lyi ; ri Þ : Ui ðlxi ; lyi ; r i Þ ð6Þ A cluster member is found when P ci = 0:5. 3.3. Results 3.3.1. Open cluster vdB80 We choose the stars in a circular region of radius 150 centred at a0 ¼ 97 :71542  0 :00833; d0 ¼ 9 :65500  0 :00833 containing 304 stars. The proper motion data is analysed in an elliptical subregion of the VPD containing 219 stars, where h ¼ 80 :26  1 :63. The parameters q0 ¼ 1:611  0:079 stars/ð0 Þ2 ; r 0 ¼ 10 :958  00 :117 and qf ¼ 0:272  0:011 stars/ð0 Þ2 adjust the function qðr i Þ to the radial stellar density profile. After Eqs. (4) and (5) are resolved, the cluster parameters are la cosdc ¼ 2:13  0:47 mas=yr; ldc ¼ 0:95  0:47 mas=yr; rc ¼ 1:84  0:33 mas=yr and the field parameters la cosdf ¼ 0:19 0:35 mas=yr; ldf ¼ 4:22  0:28 mas=yr; rla cosdf ¼ 4:98  0:25 mas=yr and rld f ¼ 4:00  0:20 mas=yr. 15 stars are found to be astrometric members of the cluster. Their coordinates lead to calculate the equatorial coordinates of the centre of the cluster ðac ¼ 97 :73938  0 :00846; dc ¼ 9 :66953  0 :01177Þ and a value of 60 for radius. The location Table A.4 vdB80 Astrometric members. No. UCAC4 a [°] a 3 4 5 7 8 13 14 16 17 18 21 22 38 47 64 402-013693 402-013689 402-013688 402-013686 402-013700 402-013704 402-013706 402-013694 402-013699 402-013707 402-013710 402-013702 402-013724 403-014263 402-013708 97.7124409 97.7093718 97.7075595 97.7012712 97.7283609 97.7339971 97.7418712 97.7137992 97.7217315 97.7499574 97.7777180 97.7307018 97.8101648 97.7916030 97.7600824 17 21 3 22 64 20 44 30 26 120 176 131 137 111 48 [mas] d [°] d la cosd [mas/yr] la cosd 9.6604706 9.6514173 9.6541095 9.6438942 9.6424945 9.6188395 9.6866170 9.7102220 9.7106406 9.7025853 9.6419756 9.7188525 9.6481231 9.5868687 9.7657906 17 9 3 23 45 20 42 30 26 110 187 137 141 111 40 2.7 0.6 1.8 0.4 1.9 3.0 0.8 3.6 2.0 5.8 2.4 0.4 1.1 4.4 2.0 3.2 4.2 1.6 1.7 4.7 4.1 4.6 4.3 4.5 6.1 7.5 5.5 5.9 5.2 4.5 d [°] d la cosd [mas/yr] la cosd 1.3158162 1.3282523 1.3242617 1.3378134 1.3012903 1.3131103 1.3797695 1.3923925 1.4025950 12 24 195 14 50 12 15 44 19 0.9 2.2 1.0 1.1 0.3 1.4 1.4 0.2 1.2 1.1 7.2 7.7 2.2 9.9 2.0 1.0 3.6 4.2 d [°] d la cosd [mas/yr] la cosd 39.3434017 39.3495545 39.3531487 39.3392048 39.3149856 39.3104470 39.3042850 39.3385159 39.2935173 18 11 18 49 31 26 34 12 23 4.4 4.3 4.0 3.9 4.4 3.5 3.2 4.8 4.7 0.9 0.8 3.3 4.4 1.9 6.0 4.8 0.9 1.3 [mas] [mas/yr] ld [mas/yr] l 0.3 0.1 0.9 0.8 2.0 3.5 4.1 0.2 0.4 1.7 2.7 2.6 2.2 1.2 1.1 1.5 1.9 1.2 1.6 4.9 4.5 5.0 4.6 4.9 6.2 8.0 5.9 6.1 5.4 4.8 ld [mas/yr] l 1.5 2.6 4.3 4.3 6.4 0.9 4.0 4.1 2.0 0.8 7.4 24.5 1.3 4.3 1.3 1.4 4.7 2.3 ld [mas/yr] l 5.4 4.9 3.9 7.0 5.3 5.1 5.8 4.5 4.9 1.1 0.5 4.0 7.0 3.7 2.8 4.5 0.6 1.7 d [mas/yr] Membership probability 0.98 0.96 0.97 0.94 0.93 0.83 0.65 0.90 0.89 0.71 0.80 0.68 0.60 0.54 0.50 Table A.5 vdB85 Astrometric members. No. UCAC4 a [°] a 7 10 13 20 23 51 54 72 108 457-022546 457-022567 457-022539 457-022575 457-022542 457-022500 457-022562 457-022533 458-023822 101.7151527 101.7346874 101.7066503 101.7395562 101.7106306 101.6670524 101.7276224 101.7020168 101.7469306 11 25 70 19 66 14 13 29 44 [mas] [mas] [mas/yr] d [mas/yr] Membership probability 0.96 0.97 0.94 0.97 0.76 0.52 0.88 0.68 0.51 Table A.6 vdB130 Astrometric members. No. UCAC4 a [°] a 1 3 4 6 22 26 38 54 99 647-086785 647-086776 647-086781 647-086775 647-086813 647-086802 647-086831 647-086871 647-086711 304.4318889 304.4258074 304.4286059 304.4248483 304.4574574 304.4475909 304.4740324 304.5125998 304.3572898 18 12 15 36 16 65 32 12 15 [mas] [mas] [mas/yr] d [mas/yr] Membership probability 1.00 1.00 0.98 0.91 0.98 0.97 0.84 0.84 0.63 75 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 Table 2 Centre coordinates, mean proper motion, number of members N and diameters of the clusters. 0 Cluster ac [°] dc [°] la cosd [mas/yr] ld [mas/yr] N Diameter [ ] vdB80 vdB85 vdB130 97.73938 ± 0.00846 101.71670 ± 0.00808 304.44001 ± 0.01407 9.66953 ± 0.01177 1.34392 ± 0.01253 39.32745 ± 0.00726 2.13 ± 0.47 0.89 ± 0.43 4.14 ± 0.25 0.95 ± 0.47 3.24 ± 0.43 5.15 ± 0.25 15 9 9 12 8 9 The cluster parameters take the values la cosdc ¼ 0:89  0:43 mas=yr; ldc ¼ 3:24  0:43 mas=yr; rc ¼ 1:28  0:29 mas=yr and the field parameters la cosdf ¼ 0:33  0:62 mas=yr; ldf ¼ 3:66 0:41 mas=yr; rla cosdf ¼ 6:80  0:43 mas=yr and rld f ¼ 4:52  0:29 mas=yr. We find 9 astrometric cluster members and then calculate the equatorial coordinates of the centre of the cluster ac ¼ 101 :71670  0 :00808; dc ¼ 1 :34392  0 :01253 and the value of 40 for the radius. The location of the members in the spatial distribution and in the VPD are shown with black circles in Figs. 3b and 4b, respectively. Table A.5 gives our numbering system, the UCAC4 number, the equatorial coordinates with their errors, the components of proper motion with their errors and the membership probability ðPci Þ. of the members in the spatial distribution and in the VPD are shown with black circles in Figs. 3a and 4a, respectively. Table A.4 gives our numbering system No., the UCAC4 number, the equatorial coordinates with their errors, the components of proper motion with their errors and the membership probability ðPci Þ. 3.3.2. Open cluster vdB85 We take 139 stars in a circular region of radius 90 :5 centred at a0 ¼ 101: 72167  0 :00833; d0 ¼ 1 :32333  0 :00833. The elliptical subregion of the VPD contains 127 stars and h ¼ 71 :08 1 :93. The parameters q0 ¼ 1:416  0:339 stars/ð0 Þ2 ; r0 ¼ 10 :171 0 0 :299 and qf ¼ 0:383  0:039 stars/ð0 Þ2 fit the function qðr i Þ to the radial stellar density profile. Table 3 Influence of observational errors: means of the mean proper motion and of the number of members N from the 25 simulated samples for each cluster. vdB80 vdB85 vdB130 la cosd [mas/yr] ld [mas/yr] N 1.93 ± 0.18 0.68 ± 0.23 4.30 ± 0.20 0.80 ± 0.25 2.04 ± 0.25 5.32 ± 0.21 15 ± 1 9±1 9±1 3.3.3. Open cluster vdB130 The model is applied in a circular region of radius 100 and centred at a0 ¼ 304: 43783  0 :00833; d0 ¼ 39 :34550  0 :00833 with 213 stars. The elliptical subregion of the VPD contains 192 stars and h ¼ 213 :25  3 :43. vdB 80 1.0 0.5 0 3 4 5 7 8 13 14 vdB 85 1.0 17 18 21 22 38 47 64 vdB 130 1.0 0.5 0.5 0 16 7 10 13 20 23 51 54 72 108 0 1 3 4 6 22 26 38 54 99 Fig. 5. Histogram showing the probability in which a cluster member previously determined keeps its membership condition throughout the 25 simulations for vdB80, vdB85 and vdB130. 76 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 The parameters q0 ¼ 3:645  0:190 stars/ð0 Þ2 ; r0 ¼ 00 :910  00 :040 and qf ¼ 0:656  0:009 stars/ð0 Þ2 adjust the function qðr i Þ to the radial stellar density profile. The cluster parameters take the values la cosdc ¼ 4:14  0:25 mas=yr; ldc ¼ 5:15  0:25 mas=yr; rc ¼ 0:75  0:18 mas=yr and the field parameters la cosdf ¼ 1:62 0:40 mas=yr; ldf ¼ 4:18  0:59 mas=yr; rla cosdf ¼ 5:38  0:28 mas=yr and rld f ¼ 7:99  0:42 mas=yr. It can be seen that these values are similar to the ones obtained by Dias et al. (2014). Nine stars are found to be the astrometric members of the cluster. Therefore, the equatorial coordinates of the centre of the cluster and radius take the values ac ¼ 304 :4401 0 :01407; dc ¼ 39 :327450 :00726 and 40 :5 respectively. The location of the members in the spatial distribution and in the VPD are shown with black circles in Figs. 3c and 4c, respectively. Table A.6 gives our numbering system, the UCAC4 number, the equatorial coordinates with their errors, the components of proper motion with their errors and the membership probability ðP ci Þ. In summary, the centre coordinates and components of the mean proper motion of each cluster are shown in Table 2. 8 9 10 J 11 13 14 3.4. Proper motion errors We analyze on the incidence of stellar proper motions errors in the determination of the cluster parameters and stellar membership. To do this, we follow the method proposed by Chen et al. (1997). For each cluster, we have generated 25 simulations by adding an increment ðDla cosd; Dld Þ to the real values ðla cosd; ld Þ of each star. The increases in the proper motion components have been randomly generated from a normal distribution with mean equal to ðla cosd; ld Þ and standard deviation equal to the individual error. Afterwards, we apply the procedure presented in subSection 3.2 to each simulated sample and obtain the cluster parameters and the astrometrical members. For each cluster, Table 3 shows the mean value of ðla cosd; ld Þ and of the number of members obtained from the 25 simulations. Comparing these results with those obtained in subSection 3.2 (see Table 2), we can see that the error does not significantly change the kinematic parameters of the studied clusters. We also analyse the effect of the proper motion errors on the determination of cluster members for the analysed clusters. Therefore, we count the number of times in which a cluster member obtained in subSection 3.3 keeps its membership condition throughout the 25 simulations. Fig. 5 shows these results by an histogram for each cluster, where the abscissa shows our numbering system for vdB80, vdB85 and vdB130 respectively. It is easy to see that approximately 90% of the members maintain their condition, except the case of vdB85 where the uncertainty in the identification of the members becomes greater. 4. Discussion We compare the cluster members of vdB80, vdB85 and vdB130 acquired by the astrometric technique with the ones obtained using photometry. 4.1. Open cluster vdB80 A notable infrared photometric analysis of vdB80 is made by Bonatto and Bica (2009) using 2MASS data in order to investigate its nature and derive its fundamental parameters. The Fig. 7 of this work shows the decontaminated J  ðJ  KsÞ colour-magnitude diagram with the photometric members up to J ¼ 16 mag. 12 15 16 0 0.2 0.4 0.6 0.8 1 J−Ks 1.2 1.4 1.6 1.8 Fig. 6. Reproduction of the colour-magnitude diagram of vdB80 from Bonatto and Bica (2009). The photometric members are plotted with crosses, the astrometric members with large circles and the non astrometric members with triangles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) We recreate the mentioned diagram in Fig. 6 and find that 23 stars of the sample are in the UCAC4 catalogue. Among these stars in common, we find that 8 stars are astrometric members and the rest 15 stars do not fulfil this condition due to their proper motions. These results are listed in Table B.7 and Fig. 6 shows Bonatto & Bica colour-magnitude diagram, where the photometric members are shown with crosses, the astrometric members with large circles and the non astrometric ones with triangles. Taking into account that cluster members must share similar kinematic features, the VPD of the photometric members (Fig. 7) explain the reason why 15 photometric members can not be considered members of the cluster. The photometric members are plotted with crosses and the eight astrometric members with large circles. The fact that NSV 2998 (UCAC4 402-013688) is an astrometric member supports the 5  2 Myr MS cluster age obtained by Bonatto & Bica. 4.2. Open cluster vdB85 The infrared observations acquired for the study of Horner et al. (1997) were taken with specific devices on the 1.3 meter and 4 meter at Kitt Peak National Observatory and on the 1.5 meter telescope at Cerro Tololo Inter-American Observatory. They detected approximately 1121 sources at J-band (mJ <17.5), 1127 sources at H-band (mH <16.5) and 779 sources at K-band (mK <15.5). Unfortunately, the only data available in the AAS CD-ROM Series, Vol. 8, 19973 are a list of 394 sources from the 1995 1.5 m telescope observations. These sources are located within 20000 of HD 289120 (UCAC4 457-022546). We identify 14 of them in the 3 <http://aas.org/archives/cdrom/volume8/>. 77 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 Table B.7 vdB80. Identification of 23 photometric members determined by Bonatto and Bica (2009) in the UCAC4 catalogue. UCAC4 J [mag] 402-013691 402-013693 402-013689 402-013688 402-013686 402-013700 402-013704 402-013710 402-013709 402-013675 402-013692 403-014257 402-013679 403-014240 402-013690 402-013721 402-013722 403-014249 402-013724 403-014247 402-013680 403-014223 403-014252 10.808 10.395 9.046 8.153 9.643 12.539 11.257 13.640 12.069 12.01 12.805 10.972 13.225 12.469 12.448 12.474 11.783 10.947 14.248 11.834 12.882 14.124 14.084 Table B.8 vdB85. Identification of 14 sources from the 1995 1.5 m telescope observations made by Horner et al. (1997) in the UCAC4 catalogue. J  Ks [mag] la cosd [mas/yr] ld Astrometric member Horner’s number UCAC4 [mas/yr] 0.491 0.156 0.309 0.048 0.265 0.701 1.177 0.845 1.165 0.491 0.495 1.532 0.752 0.561 1.334 1.194 0.475 0.332 0.749 0.577 0.506 0.836 0.835 5.40 2.70 0.60 1.80 0.40 1.90 3.00 2.40 2.60 8.90 4.80 3.80 2.30 2.4 2.50 14.0 3.20 7.40 1.10 79.9 7.30 4.2 1.60 6.70 0.30 0.10 0.90 0.80 2.00 3.50 2.70 5.60 3.50 9.00 4.20 3.40 13.0 2.40 8.1 9.80 7.80 2.20 49.0 5.50 31.5 8.20 No Yes Yes Yes Yes Yes Yes Yes No No No No No No No No No No Yes No No No No 177b 120 144 291 65 306 328f 309 339 83 137a 388 3 33 457-022546 457-022539 457-022542 457-022567 457-022523 457-022573 457-022580 457-022575 457-022583 457-022527 457-022541 457-022593 457-022500 457-022514 r [00 ] 1.09 43.26 55.61 80.86 98.18 111.3 113.32 117.58 120.93 122.78 144.34 174.23 174.84 191.96 la cosd [mas/yr] ld [mas/yr] Astrometric member 0.9 1.0 0.3 2.2 6.9 4.1 0.7 1.1 1.1 1.9 7.1 0.3 1.4 8.2 1.5 4.3 6.4 2.6 4.3 1.5 1.8 2.2 12.3 2.2 11.3 0.9 0.9 6.7 Yes Yes Yes Yes No No No Yes No No No No Yes No 0.3 0.4 0.7 0.6 0.5 J−H 20 0.4 10 0.3 0.2 µδ [mas/yr] 0 0.1 −0.2 −0.1 0 −10 0.1 0.2 0.5 H−K Fig. 8. vdB85. A part of the JHK colour-colour diagram of the photometric cluster members inside 10000 of the cluster centre. Astrometric members are symbolized by open circles and non astrometric member is symbolized by a triangle. −20 five sources in common are astrometric members and the other one do not fulfil this condition due to its proper motion, as seen in Table B.8. Astrometric members are shown with large circles and the non astrometric one with a triangle. −30 −40 −20 4.3. Open cluster vdB130 −10 0 10 µα cos δ [mas/yr] 20 Fig. 7. VPD of 22 stars in common in the region of vdB80. The photometric members are plotted with crosses and the eight astrometric members with large circles. Star UCAC4 403-014247 is not plotted due to its large proper motion. UCAC4 catalogue that are listed in Table B.8, where five of them are situated in a radius r ¼ 10000 . In Fig. 8, we reproduce a part of Fig. 4 rigth of their work where the JHK colour-colour diagram of the photometric cluster members inside 10000 of the cluster centre is plotted. We find that four of the Racine (1974) performs UBV photometry of 14 stars identified in Plate IV (p.989) of the Palomar Sky Survey that shows the cluster’s field. These stars are considered photometric cluster members due to their apparent connection with the nebular material. Even though all these members are in the UCAC4 catalogue, six stars are astrometric members and seven ones do not fulfill this condition due to their proper motions. Racine’s star No. 14 can not be evaluated by our astrometric analysis as it has no proper motion components.These results are shown in Table B.9. Due to the fact that open cluster members have to possess similar proper motion components, the VPD (Fig. 9) of the photometric members reveals that only six of them are confirm to be cluster 78 R.B. Orellana et al. / New Astronomy 36 (2015) 70–79 Table B.9 vdB130. Identification of 14 photometric members determined by Racine (1974) in the UCAC4 catalogue. Racine’s number UCAC4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 647-086816 647-086761 647-086871 647-086813 647-086841 647-086746 647-086750 647-086860 647-086849 647-086781 647-086776 647-086785 647-086775 647-086779 la cosd [mas/yr] ld [mas/yr] Astrometric member 23.9 5 4.8 4.4 7 53.1 32.4 6.1 4.7 4 4.3 4.4 3.9 – 27 2.5 4.5 5.3 2.9 34.5 8.5 4.3 9 3.9 4.9 5.4 7 – No No Yes Yes No No No No No Yes Yes Yes Yes – 5 µδ [mas/yr] 0 We revisit the regions vdB80, vdB85 and vdB130 to determine their astrometric parameters and the stellar membership. We apply a parametric model which analyses the proper motion distribution and the stellar density to find an open cluster. The astrometric data are obtained from UCAC4 catalogue. We report that vdB80 has la cosdc ¼ 2:13  0:47 mas=yr; ldc ¼ 0:95  0:47 mas=yr; rc ¼ 1:84  0:33 mas=yr, 15 astrometric members, ac ¼ 97: 73938  0 :00846; dc ¼ 9 :66953  0 :01177 and a value of 120 for the diameter. For vdB85, the astrometric parameters become la cosdc ¼ 0:89  0:43 mas=yr; ldc ¼ 3:24  0:43 mas=yr; rc ¼ 1:28  0:29 mas=yr, 9 astrometric members, ac ¼ 101 :71670  0 :00808; dc ¼ 1 :34392  0 :01253 and a value of 80 for the diameter. In the case of vdB130, the obtained values of these parameters are la cosdc ¼ 4:14  0:25 mas=yr; ldc ¼ 9 members, 5:15  0:25 mas=yr; rc ¼ 0:75  0:18 mas=yr, equatorial centre coordinates ac ¼ 304 :4401  0 :01407; dc ¼ 39 :32745  0 :00726 and a diameter of 90 . It is important to remark that the astrometric parameters of vdB80 and vdB85 are determined for the first time using UCAC4 catalogue data. We explore also the incidence of proper motion errors in the determination of the cluster parameters and of the stellar membership applying the model to 25 simulated samples of each cluster. We conclude that the observational errors do not significantly change the mean proper motion and the number of members of the studied clusters. Finally, the comparison of our results and the photometric ones given in the literature conduct to successful membership determinations. In the case of vdB80, a detailed comparison with the photometric membership determination given by Bonatto & Bica (2009) leads to confirm that eight stars are both astrometric and photometric members. For vdB130, the comparison with the photometric members given by Racine (1974) affirms that six stars are both astrometric and photometric members. Only four stars in a region of 10000 from the centre of vdB85 adopted by Horner et al. (1997) fulfill both astrometric and photometric membership. Acknowledgements −5 This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina, under Grant No. PIP 1359 and by Universidad Nacional de La Plata (UNLP) under Grant No. 11/G114. We thank Dr. Francisco Azpilicueta for the useful discussion on the observational errors. We thank the referee for many helpful comments that significantly improved the manuscript. −10 −10 −5 µα cos δ [mas/yr] 0 Fig. 9. VPD of 10 stars in common in the region of vdB130. The photometric members are plotted with crosses and the six astrometric members with large circles. Racine’s star numbers 1, 6 and 7 are not plotted due to their large proper motion. members. The photometric members are plotted with crosses and the six astrometric members with large circles. 5. Conclusion As open cluster members share similar photometric and kinematic properties, this work combines astrometric and photometric results in order to reduce the uncertainty introduced by each technique individually. Appendix A (See Tables A.4–A.6). Appendix B (See Tables B.7–B.9). References Ahumada, A.V., Clariá, J.J., Bica, E., Dutra, C.M., Torres, M.C., 2001. A&A 377, 845. Aveni, A.F., Hunter, J.H., 1972. AJ 77, 17. Bica, E., Dutra, C.M., Soares, J., Barbut, B., 2003. A&A 404, 223. Bonatto, C., Bica, E., 2009. MNRAS 397, 1915. Chen, B., Asiain, R., Figueras, F., Torra, J., 1997. A&A 318, 29. Corti, M.A., Orellana, R.B., 2013. A&A 553, A108. Dias, W.S., Monteiro, H., Caetano, T.C., Lépine, J.R.D., Assafin, M., Oliveira, A.F., 2014. A&A 564, A79. Horner, D.J., Lada, E.A., Lada, Ch.J., 1997. AJ 113, 1788. Jones, B.F., Walker, M.F., 1988. AJ 95, 1755. Moffat, A.F.J., Schmidt-Kaler, Th., 1976. A&A 48, 115. Orellana, R.B., De Biasi, M.S., Bustos Fierro, I.H., Calderón, J.H., 2010. 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