New Astronomy 36 (2015) 70–79
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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]
−10
−20
0
10
30
20
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
ld
[mas/yr]
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
5:15 0:25 mas=yr; rc ¼ 0:75 0:18 mas=yr,
9
members,
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).
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