Mon. Not. R. Astron. Soc. 362, 649–656 (2005)
doi:10.1111/j.1365-2966.2005.09340.x
The intermediate-age open clusters Ruprecht 61, Czernik 32, NGC 2225
and 2262
G. Carraro,1,2? † G. Baume,3 R. A. Vázquez,3 A. Moitinho4 and D. Geisler5
1 Departamento
de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile
Department, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
3 Facultad de Ciencias Astronómicas y Geofı́sicas de la UNLP, IALP-CONICET, Paseo del Bosque s/n, La Plata, Argentina
4 CAAUL, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
5 Universidad de Concepción, Departamento de Fisica, Casilla 160-C, Concepción, Chile
2 Astronomy
Accepted 2005 June 21. Received 2005 June 21; in original form 2005 March 28
ABSTRACT
We present the first BVI CCD photometry to V = 22.0 of four fields centred on the region of
the southern Galactic star clusters Ruprecht 61, Czernik 32, NGC 2225 and 2262 and of four
displaced control fields. These clusters have never been studied before, and we provide for the
first time estimates of their fundamental parameters, specifically radial extent, age, distance
and reddening. We find that the four clusters are all of intermediate age (around 1 Gyr), close
to the Sun and possess lower than solar metal abundance.
Key words: open clusters and associations: general – open clusters and associations: individual: Ruprecht 61 – open clusters and associations: individual: Czernik 32 – open clusters and
associations: individual: NGC 2225 – open clusters and associations: individual: NGC 2262.
1 I N T RO D U C T I O N
This paper continues a series dedicated to the study of open clusters
in the third Galactic Quadrant, and aims at addressing fundamental
questions like the structure of the spiral arms in this quadrant, and the
precise definition of the Galactic disc radial abundance gradient outside the solar circle. A more detailed illustration of the motivations
of this project are given in Moitinho (2001) and Baume et al. (2004).
Here we concentrate on four intermediate-age clusters (about the age
of the Hyades – 600 Myr – or older) Ruprecht 61 (VdB-Hagen 32),
Czernik 32 (VdB-Hagen 11), NGC 2225 and 2262, for which we
provide the first photometric data and try to clarify their nature and
to derive estimates of their fundamental parameters.
In Table 1 we list the cluster coordinates, which we redetermined
from Digital Sky Survey (DSS) maps on a visual inspection basis.
The layout of the paper is as follows. Section 2 illustrates the
observation and reduction strategies. An analysis of the geometrical
structure and star counts in the field of the clusters is presented in
Section 3, whereas a discussion of the colour–magnitude diagrams
(CMDs) is performed in Section 4. Section 5 deals with the determination of the reddening, distance, metallicity and age of the clusters
and, finally, Section 6 summarizes our findings.
2 O B S E RVAT I O N S A N D DATA R E D U C T I O N
CCD BVI observations were carried out with the CCD camera onboard the 1.0-m telescope at Cerro Tololo Interamerican Observa? On leave from Dipartimento di Astronomia, Università di Padova, Vicolo
Osservatorio 2, I-35122, Padova, Italy.
†E-mail: gcarraro@das.uchile.cl
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2005 RAS
tory (CTIO, Chile), on the nights of 2004 December 13 and 15. With
a pixel size of 0.469 arcsec, and a CCD size of 512 × 512 pixel,
each pointing samples a 4 × 4.1 arcmin field on the sky.
The details of the observations are listed in Table 2 where the
observed fields are reported together with the exposure times, the
average seeing values and the range of air-masses during the observations. Figs 1–4 show I = 600 s images obtained in the area
of Ruprecht 61, Czernik 32, NGC 2225 and 2262, respectively. Together with the clusters, we observed three control fields 15-arcmin
apart from the nominal cluster centres to deal with field star contamination. Exposure of 600 s in V and I were secured for these
fields.
The data have been reduced with the IRAF1 packages CCDRED,
DAOPHOT, ALLSTAR and PHOTCAL using the point spread function
(PSF) method (Stetson 1987). The two nights turned out to be photometric and very stable, and therefore we derived calibration equations for all the 130 standard stars observed during the two nights
in the Landolt (1992) fields SA 95-41, PG 0231+051, Rubin 149,
Rubin 152, T phe and SA 98-670 (see Table 2 for details).
The adopted calibration equations are of the form
b
v
v
i
=
=
=
=
B + b1 + b2 X + b3 (B − V )
V + v1 + v2 X + v3 (B − V )
V + v1,i + v2,i X + v3,i (V − I )
I + i 1 + i 2 X + i 3 (V − I ),
1 IRAF is distributed by National Optical Astronomy Observatories (NOAO),
which are operated by AURA under cooperative agreement with the National
Science Foundation.
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G. Carraro et al.
Table 1. Basic parameters of the clusters under investigation. Coordinates
are for J2000.0 equinox.
Name
Ruprecht 61
Czernik 32
NGC 2225
NGC 2262
RA
(hh:mm:ss)
08:25:14
07:50:30
06:26:37
06:39:39
(◦
Dec.
0 00 )
−34:08:31
−29:50:36
−09:38:07
+01:08:30
l
(◦ )
b
(◦ )
253.48
245.86
218.78
210.57
+2.08
−1.74
−9.86
−2.10
Table 2. Journal of observations of Ruprecht 61, Pismis 7 and Czernik 32
and standard star fields (2004 December 13 and 15).
Field
Filter
Exposure time
(s)
Seeing
(arcsec)
Airmass
Ruprecht 61
B
V
I
120,1200
30 600
30 600
1.2
1.3
1.2
1.00–1.10
1.00–1.10
1.00–1.10
NGC 2262
B
V
I
120,1200
30 600
30 600
1.2
1.3
1.2
1.15–1.30
1.15–1.30
1.15–1.30
NGC 2225
B
V
I
120 1200
30 600
30 600
1.2
1.3
1.2
1.15–1.30
1.15–1.30
1.15–1.30
Czernik 32
B
V
I
120,1200
30 600
30 600
1.2
1.3
1.2
1.25–1.40
1.25–1.40
1.25–1.40
SA 98-670
B
V
I
3 × 120
3 × 40
3 × 20
1.2
1.4
1.4
1.24–1.26
1.24–1.26
1.24–1.26
PG 0231+051
B
V
I
3 × 120
3 × 40
3 × 20
1.2
1.5
1.5
1.20–2.04
1.20–2.04
1.20–2.04
T Phe
B
V
I
3 × 120
3 × 40
3 × 20
1.2
1.3
1.3
1.04–1.34
1.04–1.34
1.04–1.34
Rubin 152
B
V
I
3 × 120
3 × 40
3 × 20
1.3
1.2
1.2
1.33–1.80
1.33–1.80
1.33–1.80
Rubin 149
B
V
I
3 × 120
3 × 40
3 × 20
1.1
1.2
1.2
1.21–1.96
1.21–1.96
1.21–1.96
SA 95-41
B
V
I
3 × 120
3 × 40
3 × 20
1.2
1.2
1.1
1.05–1.48
1.05–1.48
1.05–1.48
where BVI are standard magnitudes, bvi are the instrumental ones
and X is the airmass; all the coefficient values are reported in Table
3. The standard stars in these fields provide the colour coverage
−0.6 6 (B − V ) 6 2.2. The final rms of the calibration are 0.031,
0.024 and 0.023 for the B, V and I filters, respectively. We generally
used the third equation to calibrate the V magnitude in order to
get the same magnitude depth both in the cluster and in the field.
Photometric errors have been estimated following closely Patat &
Carraro (2001, appendix A), which the reader is referred to for all
Figure 1. Deep I image (600 s) of the open cluster Ruprecht 61. North is
up, east on the left, and the covered area is 4.1 × 4.1 arcsec.
Figure 2. Deep I image (600 s) of the open cluster Czernik 32. North is up,
east on the left, and the covered area is 4.0 × 4.1 arcsec.
the details. It turns out that the global photometric errors amount to
0.03, 0.05 and 0.20 at V = 12, 16 and 21.5 mag, respectively.
The final photometric catalogues for Ruprecht 61, Czernik 32,
NGC 2225 and 2262 (coordinates, B, V and I magnitudes and errors)
consist of 1166, 1047, 869 and 925 stars, respectively, and are made
available in electronic form at the WEBDA2 site maintained by J.-C.
Mermilliod.
3 S TA R C O U N T S A N D S I Z E S O F C L U S T E R S
As we will show in this Section, our photometry covers entirely the
area of each cluster allowing us to perform star counts to obtain
improved estimates of the size of the clusters. In fact these clusters are generally very faint, poorly populated and compact (see
Figs 1–4) and therefore could well fit within the CCD area.
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http://obswww.unige.ch/webda/navigation.html
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2005 RAS, MNRAS 362, 649–656
Ruprecht 61, Czernik 32, NGC 2225 and 2262
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Table 3. Coefficients of the calibration equations.
b 1 = 3.465 ± 0.009
v 1 = 3.244 ± 0.005
v 1,i = 3.244 ± 0.005
i 1 = 4.097 ± 0.005
b 2 = 0.25 ± 0.02
v 2 = 0.16 ± 0.02
v 2,i = 0.16 ± 0.02
i 2 = 0.08 ± 0.02
b 3 = − 0.145 ± 0.008
v 3 = 0.021 ± 0.005
v 3,i = 0.009 ± 0.005
i 3 = 0.006 ± 0.005
Figure 3. Deep I image (600 s) of the open cluster NGC 2225. North is up,
east on the left, and the covered area is 4.1 × 4.1 arcsec.
Figure 5. Star counts in the area of Ruprecht 61 as a function of radius and
magnitude. The dashed lines represent the level of the control field counts
estimated from the accompanying control field.
cluster seems to emerge from the background in the magnitude range
16 6 V 6 22. In this magnitude range the radius is not larger than
1.0 arcmin. We shall adopt the value of 1.0 arcmin as the radius
of Ruprecht 61 throughout this paper. This estimate is in perfect
agreement the value of 2.0 arcmin reported by Dias et al. (2002) for
the cluster diameter.
Figure 4. Deep I image (600 s) of the open cluster NGC 2262. North is up,
east on the left, and the covered area is 4.1 × 4.1 arcsec.
We derived the surface stellar density by performing star counts
in concentric rings around the nominal centres of the clusters (see
Table 1) and then dividing by their respective areas. Poisson errors have also been derived and normalized to the corresponding
area. The field star contribution has been derived from the control
field which we secured for each cluster, and the errors have been
computed in the same way as for the cluster field.
Ruprecht 61. The radial density profile for Ruprecht 61 is shown
in Fig. 5 as a function of the V magnitude. Clearly, the cluster does
not appear very concentrated, and it is deficient of bright stars. The
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Czernik 32. The radial density profile for Czernik 32 is shown in
Fig. 6 as a function of the V magnitude. The cluster shows a clear
lack of bright stars and it is mostly populated by stars of magnitude
in the range 18 6 V 6 22. In this magnitude range the radius is not
larger than 1.0 arcmin. In conclusion, we are going to adopt the value
of 1.0 arcmin as the Czernik 32 radius throughout this paper. This
estimate is in reasonable agreement with the value of 3.0 arcmin
reported by Dias et al. (2002) for the cluster diameter.
NGC 2225. The radial density profile for NGC 2225 is shown in
Fig. 7 as a function of the V magnitude. Also this cluster exhibits
a deficiency of bright stars ant it is mostly populated by stars of
magnitude in the range 16 6 V 6 22. In this magnitude range the
radius is not larger than 1.2 arcmin.
This estimate is in reasonable agreement with (a bit smaller than)
the value of 4.0 arcmin reported by Dias et al. (2002) for the cluster
diameter.
NGC 2262. The radial density profile for NGC 2262 is shown
in Fig. 8 as a function of the V magnitude. This cluster is composed mainly by stars of magnitude in the range 16 6 V 6 22,
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G. Carraro et al.
Figure 6. Star counts in the area of Czernik 32 as a function of radius and
magnitude. The dashed lines represent the level of the control field counts
estimated from the accompanying control field.
Figure 8. Star counts in the area of NGC 2262 as a function of radius and
magnitude. The dashed lines represent the level of the control field counts
estimated from the accompanying control field.
due to dynamical evolution and mass segregation tend to be normally underestimated. Larger and deep field coverage is necessary
to derive firmer estimates of the clusters radii.
4 T H E C O L O U R – M AG N I T U D E D I AG R A M S
In Fig. 9 we present the CMDs obtained for the observed fields of the
four clusters under investigation. All the stars observed in each field
have been plotted (not only those within the derived cluster radii).
In this figure, the open cluster Ruprecht 61 is shown together with
the corresponding control field in the lower panels, Czernik 32 and
NGC 2225 are presented in the middle panels, while finally NGC
2262 is presented in the upper panels. The control fields help us to
better interpret these CMDs, which are clearly affected by strong
foreground star contamination.
Figure 7. Star counts in the area of NGC 2225 as a function of radius and
magnitude. The dashed lines represent the level of the control field counts
estimated from the accompanying control field.
where the radius is around 1.0 arcmin. In conclusion, we are going to adopt the value of 1.0 arcmin as the radius of NGC 2262
throughout this paper. This estimate is much smaller than the
value of 5.0 arcmin reported by Dias et al. (2002) for the cluster
diameter.
The estimates we provide for the radius, although reasonable,
must be taken as preliminary. In fact, the size of the CCD is probably too small to derive conclusive estimates of the cluster sizes, that
Ruprecht 61. This cluster is presented in the lower panels of Fig. 9.
It exhibits a main sequence (MS) extending from V = 15–15.5,
where the turn-off point (TO) is located, down to V = 21.5. This
MS is significantly wide, wider than photometric error at a given
magnitude (see Section 2). We ascribe this to field star contamination, and to the presence of a sizeable binary star population, which
mainly enlarge the MS toward red colours. However, the reality of
this cluster seems to be secured by the shape of the MS with respect
to the control field MS, whose population sharply decreases at V
= 17. Also, the cluster MS is significantly bluer and more tilted
than the field MS, which derives from the superposition of stars of
different reddening located at all distances between the cluster and
the Sun. Another interesting evidence is the possible presence of a
clump of stars at V = 14.5, which does not have a clear counterpart in the field, and which implies a cluster of intermediate age.
In fact if we use the age calibration from Carraro & Chiosi (1994),
for a 1V (the magnitude difference between the red clump and the
TO) of 0.5 mag, we infer an age around 1 Gyr. This estimate does
not take into account the cluster metallicity, and therefore is simply
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2005 RAS, MNRAS 362, 649–656
Ruprecht 61, Czernik 32, NGC 2225 and 2262
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Control Fields
NGC 2262
NGC 2262
NGC 2225
NGC 2225
Cze 32
Cze 32
Rup 61
Rup 61
Figure 9. V versus (B − V ) (left panels) and V versus (V − I ) (middle panels) CMDs of Ruprecht 61, Czernik 32, NGC 2225 and 2262 and corresponding
control fields (right panels). We include all stars in each field.
Table 4. Fundamental parameters of the studied clusters. The coordinates system is centred on the Sun, with the X and Y axes lying on the Galactic plane and
Z perpendicular to the plane. X points in the direction of the Galactic rotation, being positive in the first and second Galactic quadrants; Y points toward the
Galactic anticentre; and Z indicates the north Galactic pole (Lynga 1982).
Name
Radius(0 )
E(B − V )
E(V − I )
(m − M)
d¯
X (kpc)
Y (kpc)
Z (kpc)
R GC (kpc)
Age (Gyr)
Metallicity
Ruprecht 61
Czernik 32
NGC 2225
NGC 2262
1.0
1.0
1.2
1.0
0.30 ± 0.1
0.85 ± 0.1
0.35 ± 0.1
0.55 ± 0.1
0.41 ± 0.1
1.08 ± 0.1
0.50 ± 0.1
0.72 ± 0.1
13.85 ± 0.2
15.70 ± 0.2
13.60 ± 0.2
14.50 ± 0.2
3.9
4.1
3.2
3.6
−3.8
−1.7
−1.9
−1.8
1.1
3.7
2.5
3.1
0.19
−0.12
−0.55
−0.13
9.4
10.8
11.2
11.7
1.3 ± 0.2
1.0 ± 0.3
1.3 ± 0.2
1.0 ± 0.2
0.008 ± 0.002
0.008 ± 0.002
0.008 ± 0.003
0.008 ± 0.004
a guess. In the following section we shall provide a more robust
estimate of the age through a detailed comparison with theoretical
isochrones.
Czernik 32. The open cluster Czernik 32 is presented in the lowermid panels of Fig. 9. The TO located at V ≈ 17, and a prominent
clump at V ≈ 16 with no counterpart in the field CMD are readily
seen, yielding an estimated age of around 1.0 Gyr. The overall morphology of the CMDs is in this case very different from the field
CMD leaving no doubt that Czernik 32 is a bona-fide intermediateage open cluster.
NGC 2225. The open cluster NGC 2225 is presented in the uppermid panels of Fig. 9. Again, the overall morphology of the cluster
CMDs, with a TO and an evident red clump, is very different from
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2005 RAS, MNRAS 362, 649–656
the field CMD. Indeed, the field sequence is much less populated and
stops at V ≈ 16.5, giving a first impression that this is a bona-fide
intermediate-age open cluster. The TO located at V ≈ 16, and red
clump at V ≈ 15, allow to estimate an age of around 1.0 billion years,
confirming the first impression that NGC 2225 is an intermediateage cluster.
NGC 2262. The open cluster NGC 2262 is presented in the upper
panels of Fig. 9. The CMD of the cluster reveals a TO is located at V
≈ 16, and a possible clump at V ≈ 16 as well, which provide a rough
estimate of around 0.5 Gyr for the age of NGC 2262. The overall
morphology of the CMDs is also in this case very different from
the field CMD confirming that this is a bona-fide intermediate-age
open cluster.
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Z = 0.004, age = 1.0 Gyr
Z = 0.008, age = 1.3 Gyr
Z = 0.019, age = 1.5 Gyr
Figure 10. Hunting for the best-fitting isochrone solution. The CMD of Ruprecht 61 is shown together with three different isochrones. The isochrones are for
the age of 1 Gyr and metallicity Z = 0.004 in the left panel, for the age of 1.3 Gyr and metallicity Z = 0.008 in the middle panel, and, finally, for the age of
1.5 Gyr and metallicity Z = 0.019 in the right panel.
Figure 11. Isochrone solution for Ruprecht 61. The isochrone is for the age
of 1.3 Gyr and metallicity Z = 0.008. The apparent distance modulus is (m −
M) = 13.85, and the reddening E(B − V ) = 0.30 and E(V − I ) = 0.41.
See text for more details. Only stars within the derived radius are shown.
5 D E R I V I N G F U N DA M E N TA L PA R A M E T E R S
OF CLUSTERS
In this section we are going to perform a detailed comparison of
the star distribution in the CMDs of the clusters with theoretical
Figure 12. Isochrone solution for Czernik 32. The isochrone is for the age
of 1 Gyr and metallicity Z = 0.008. The apparent distance modulus is (m −
M) = 15.7, and the reddening E(B − V ) = 0.85 and E(V − I ) = 1.08. See
text for more details. Only stars within the derived radius are shown.
isochrones. For this study, we adopt in this study the Padova library from Girardi et al. (2000). This comparison is clearly not
an easy exercise. In fact, the detailed shape and position of the
various features in the CMD (MS, TO and clump basically) depends mostly on age and metallicity, and then also on reddening and
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To further facilitate the fitting procedure, by increasing the contrast between the cluster and the field population, we shall consider only the stars which lie within the cluster radius as derived in
Section 3.
Finally, to derive the distances of the clusters from reddening and
apparent distance modulus, a reddening law must be specified. In
this study we shall adopt the normal reddening law Av = 3.1 ×
E(B − V ) in deriving the distances of the clusters.
In addition to finding the best fit, we also estimated the uncertainties in the basic parameters. These uncertainties simply reflect
the range of parameters that yields a reasonable fit to the clusters
CMDs. The errors are reported in Table 4, and an example of the
procedure is shown in Fig.10 for the case of Ruprecht 61. The best
fit for all the clusters, achieved simultaneously in the V versus (B −
V ) and in the V versus (V − I ) planes, are shown in Figs 11–14.
Figure 13. Isochrone solution for NGC 2225. The isochrone is for the age
of 1.3 Gyr and metallicity Z = 0.008. The apparent distance modulus is (m
− M) = 13.5, and the reddening E(B − V ) = 0.35 and E(V − I ) = 0.56.
See text for more details. Only stars within the derived radius are shown.
Ruprecht 61. The fitting procedure and the isochrone solution for
this cluster are shown in Figs 10 and 11. We obtained the best fit
for an age of 1.3 Gyr and a metallicity Z = 0.008 (see middle panel
of Figs 11 and 12). In fact, the shape of the TO in the left panel
of Fig. 11 (for the Z = 0.004 isochrone) is clearly different from
the underlying cluster sequence, and the same can be noticed for
the Z = 0.019 isochrone (right panel), where the red hook shows a
shape which does not fit very well the star distribution in the cluster.
In details, the red hook is too red and somewhat faint with respect
to the Z = 0.008 isochrone and the actual stars distribution. To get
a bluer and brighter red hook, one should use a younger isochrone,
which will, however, possess a too red clump and Red Giant Branch
(RGB), when fixed to the TO.
The inferred reddening and apparent distance modulus are
E(B − V ) = 0.30 [E(V − I ) = 0.41, right panel in Fig. 12] and
(m − M) = 13.85, respectively. As a consequence, the cluster possesses a heliocentric distance of 3.9 kpc, and is located at a Galactocentric distance of 9.4 kpc, assuming 8.5 kpc as the distance of the
Sun to the Galactic Centre.
Czernik 32. The isochrone solution for this cluster is displayed in
Fig. 12. We obtained the best fit for an age of 1 Gyr and a metallicity
Z = 0.008. The inferred reddening and apparent distance modulus
are E(B − V ) = 0.85 [E(V − I ) = 1.08] and (m − M) = 15.7,
respectively. These values situate the cluster at a heliocentric distance of 4.1 kpc, which corresponds to a Galactocentric distance of
10.8 kpc. The overall fit is also good in this case, the detailed shape
of the MS and TO are nicely reproduced, as well as the colour of
the clump.
Figure 14. Isochrone solution for Czernik 32. The isochrone is for the age
of 1 Gyr and metallicity Z = 0.008. The apparent distance modulus is (m −
M) = 14.5, and the reddening E(B − V ) = 0.55 and E(V − I ) = 0.72. See
text for more details. Only stars within the derived radius are shown.
distance. The complex interplay between the various parameters
is however well known, and we refer to Chiosi, Bertelli & Bressan (1992) and Carraro (2005) as nice examples of the underlying
technique.
Our basic strategy is to survey different age and metallicity isochrones in an attempt to provide the best fit of all the CMD features
both in the V versus (B − V ) and in the V versus (V − I ) CMD.
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NGC 2225. The isochrone solution for this cluster is presented in
Fig. 13. We obtained the best fit for an age of 1 Gyr and a metallicity
Z = 0.008, which reproduces the sharp cluster sequence extremely
well. The inferred reddening and apparent distance modulus are
E(B − V ) = 0.35 [E(V − I ) = 0.50] and (m − M) = 13.6,
respectively. Therefore the cluster has a heliocentric distance of
3.2 kpc, and is located at a Galactocentric distance of 11.2 kpc.
NGC 2262. The isochrone solution for this cluster is shown in
Fig. 14. We obtained the best fit for an age of 1 Gyr and a metallicity of Z = 0.008. The inferred reddening and apparent distance
modulus are E(B − V ) = 0.55 [E(V − I ) = 0.72] and (m − M) =
14.5, respectively, which put the cluster at a heliocentric distance of
3.6 kpc, or at a Galactocentric distance of 11.7 kpc.
6 CONCLUSIONS
We have presented the first CCD BVI photometric study of the star
clusters Ruprecht 61, Czernik 32, NGC 2225 and 2262. Through a
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star count analysis we have refined previous estimates of the coordinates and apparent radii of the cluster. A detailed comparison of the
CMDs of the clusters with theoretical isochrones has allowed us to
infer the basic parameters of the aggregates and their uncertainties,
which are summarized in Table 4.
In detail, the fundamental findings of this paper are as follows:
(i) the best-fitting reddening estimates support within the errors
a normal extinction law toward the four clusters;
(ii) all the clusters turn out to be of intermediate age, and not far
from the Sun toward the anti-centre direction; and
(iii) the photometric estimates of the metallicity are lower than
solar, as expected for clusters located between 9 and 11 kpc from
the Galactic Centre (Carraro, Ng & Portinari 1998).
AC K N OW L E D G M E N T S
The observations presented in this paper have been carried out at
Cerro Tololo Interamerican Observatory CTIO (Chile). CTIO is operated by the Association of Universities for Research in Astronomy,
Inc. (AURA), under a cooperative agreement with the National Science Foundation as part of the National Optical Astronomy Observatory (NOAO). The work of G.C. is supported by Fundación Andes.
D.G. gratefully acknowledges support from the Chilean Centro de
Astrofı́sica FONDAP No. 15010003. This work has been also de-
veloped in the framework of the Programa Cientı́fico-Tecnológico
Argentino-Italiano SECYT-MAE Código: IT/PA03 – UIII/077 –
perı́odo 2004–2005. A.M. acknowledges support from FCT (Portugal) through grant SFRH/BPD/19105/2004. This study made use of
Simbad and WEBDA data bases.
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2005 RAS, MNRAS 362, 649–656