J. Astrophys. Astr.
(2019) 40:33
https://doi.org/10.1007/s12036-019-9600-7
© Indian Academy of Sciences
CCD UBV(RI) K C photometry of NGC 2323 and NGC 2539 open clusters
İNCI AKKAYA ORALHAN1,∗ , RAÚL MICHEL2 , WILLIAM J. SCHUSTER2 ,
YÜKSEL KARATAŞ3 , YONCA KARSLI1 and CARLOS CHAVARRÍA2
1 Department
of Astronomy and Space Science, Science Faculty, Erciyes University, TR-38039, Kayseri,
Turkey.
2 Observatorio Astronomico Nacional, Universidad Nacional Autonoma de México, Apartado Postal 106,
CP 22800 Ensenada, BC, Mexico.
3 Department of Astronomy and Space Sciences, Science Faculty, Istanbul University, 34119 Istanbul, Turkey.
∗ Corresponding author. E-mail: iakkaya@erciyes.edu.tr
MS received 22 March 2019; accepted 12 June 2019
Abstract. The open clusters NGC 2323 and NGC 2539 have been analysed using CCD UBV(RI ) K C photometric data, observed at the San Pedro Mártir Observatory. Cluster memberships have been determined with the
proper motion and parallax measures from the GaiaDR2 astrometric data release. Photometric metal and heavy
element abundances have been obtained as ([M/H ], Z ) = (−0.10, 0.012) (NGC 2323) and (−0.31, 0.007)
(NGC 2539) from the δ(U –B) technique in the two-colour diagrams, which are used to select the appropriate
PARSEC isochrones. The estimated reddening of NGC 2323 is E(B − V ) = 0.23 ± 0.04 using 11 early
type stars. For NGC 2539, we find E(B − V ) = 0.02 ± 0.06. For (B − V ) colour, distance moduli and
distances for NGC 2323 and NGC 2539 are derived as (V0 − MV , d (pc)) = (10.00 ± 0.10, 1000 ± 50) and
(V0 − MV , d(pc)) = (10.00 ± 0.04, 1000 ± 20), respectively. The median GaiaDR2 distance d = 1000 ± 140
pc (̟ = 0.998 ± 0.136 mas) for the likely members of NGC 2323 is in concordance with its four-colour photometric distances 910–1000 pc. For NGC 2539, its GaiaDR2 distance d = 1330 ± 250 pc (̟ = 0.751 ± 0.139
mas) is close to its four-colour photometric distances, 1000 pc. Fitting the PARSEC isochrones to the colour
magnitude diagrams (CMDs) gives an old age, 890 ± 110 Myr, for NGC 2539. Whereas NGC 2323 has an
intermediate age, 200 ± 50 Myr. One red clump/red giant candidate (BD-12 2380) in the CMDs of NGC 2539
has been confirmed as a member in terms of the distances d I = 950 ± 50 pc and dV = 910 ± 90 pc of V I
filters within the uncertainties, as compared to the distance 1000 ± 20 pc of NGC 2539. This giant’s GaiaDR2
distance (d = 1200 ± 70 pc) is not close to these photometric distances.
Keywords. Galaxy evolution: open clusters and associations—individual: Hertzsprung–Russell and colour
magnitude diagrams.
1. Introduction
CCD UBV(R I ) K C photometric data which include the
U filter for open clusters (OCs) are quite valuable for
determining the interstellar reddening, E(B − V ) and
the photometric metallicity and heavy-element abundance ([M/H ], Z ) on colour–colour diagram (CCD).
The two parameters are of vital importance for deriving the distance modulus, (V0 − MV ) and the distance,
d (kpc) and age, A (Gyr) from the colour magnitude
diagrams (CMDs).
In this study, we have analysed the deep CCD
UBV(R I ) K C photometry of the OCs, NGC 2323 and
0123456789().: V,-vol
NGC 2539 from the Sierra San Pedro Mártir National
Astronomical Observatory (SPMO) open cluster survey (cf. Schuster et al. 2007; Tapia et al. 2010; Akkaya
et al. 2010, 2015). Both OCs are uniformly and homogeneously analysed with regards to the instrumentation,
observing techniques, and reduction and calibration
methods. The two OCs which are located in the 3rd
quadrand have no spectroscopic metal abundances in
the literature. Their basic parameters from the literature (Kharchenko et al. 2013) are listed in Table 1.
NGC 2323 has been studied by Sharma et al. (2006)
using CCD UBVI photometry and by Claria et al.
(1998) from photoelectric UBV photometry. For NGC
33
Page 2 of 18
J. Astrophys. Astr.
Table 1. The central equatorial (J2000) and galactic coordinates plus the average μα and μδ values together with their
standard deviations of Kharchenko et al. (2013). Our findings
from the GaiaDR2 data are given in parentheses.
Cluster
α2000 (h m s)
δ2000 (◦ ′ ′′ )
ℓ (◦ )
b(◦ )
μα (mas yr−1 )
μδ (mas yr−1 )
NGC 2323
NGC 2539
07 02 43.1
(07 02 51.6)
−08 22 12
(−08 20 42)
221.66
–1.33
+0.50 ± 0.17
(−0.816 ± 0.248)
−1.75 ± 0.17
(−0.653 ± 0.210)
08 10 40.8
(08 10 42)
−12 50 24
(−12 50 31)
233.73
11.11
−2.28 ± 0.19
(−2.347 ± 0.181)
−1.91 ± 0.19
(−0.569 ± 0.205)
2539, Choo et al. (2003) used CCD UBVI photometry to search for variable stars. Joshi and Sagar (1986)
studied its photoelectric UBV photometry. Claria &
Lapasset (1986) used CCD DDO and CMT1 T2 photometries for this OC. Many published works of these
OCs (Table 9 in this paper) are of the brighter stars,
which are based on old photographic and photoelectric
photometries.
Additionally, we use the GaiaDR2 astrometric data
(proper motion components and parallaxes) (Gaia Collaboration, Brown et al. 2018) for determining the
probable members for NGC 2323 and NGC 2539. The
membership determinations of previous works have
been based on the proper motions (PPMXL) of Roeser
et al. (2010) in combination with 2MASS JHKs values of Skrutskie et al. (2006). Cantat-Gaudin et al.
(2018) stated that the proper motion uncertainties of
UCAC4 fall in the range 1–10 mas yr−1 (Roeser et al.
2010; Zacharias et al. 2013), whereas the ones of
Tycho-Gaia Astrometric Solution (TGAS) (Gaia Collaboration, Brown et al. 2016) have the range of 0.5–2.6
mas yr−1 .
Because of the above-mentioned scientific grounds,
we combine the recent GaiaDR2 astrometric data with
deep CCD UBV(R I ) K C photometry of both OCs, to
derive their reddenings, metal abundances, distance
moduli and distances and ages for four-colour indices,
(B−V ), (V −I ), (R−I ) and (G B P −G R P ). This kind of
photometric data are also valuable for classifying earlytype stars, blue stragglers, and RC/RG candidates in the
CMDs, and thus possible candidates are proposed for
spectroscopic observations.
This paper is organized as follows: Section 2
describes the observation and reduction techniques. The
(2019) 40:33
technique for determining cluster membership is presented in Section 3. Section 4 describes the general
procedures concerning the derivation of astrophysical
parameters and in Section 5, the results for these astrophysical parameters are presented. Discussions and
conclusions are given in Section 6.
2. Observations and reductions
Observations were carried out at SPMO during photometric nights, from June 7–10, 2013 UT using the
0.84-m (f/15) Ritchey–Chretien telescope equipped
with the Mexman filter wheel and the ESOPO CCD
detector, a 2048 × 4608 13.5-µm square-pixel e2v
CCD42-90 with a gain of 1.7e− /ADU and a readout
noise of 3.8e− at 2 × 2 binning. The combination of the
telescope and the detector ensures an unvignetted fieldof-view of 7.4 × 9.3 arc min2 . Each OC was observed
through Johnson’s UBV and Kron–Cousin’s R I (personel communication of A. Landolt) filters with short
and long exposure times in order to properly record both
the brighter and fainter stars of the fields under study.
Standard fields (Landolt 2009) were also observed near
the meridian and at 1.14–1.52 air masses to properly
determine the atmospheric extinction coefficients and
the equations to transform the instrumental data to the
standard system. Flat fields were taken at the beginning and/or the end of the night, whereas biases were
recorded between the cluster observations. For more
details, see the works of Tapia et al. (2010) and Akkaya
et al. (2010). The log of the observations is shown in
Table 2. It contains the object names, coordinates at the
centres of the observed fields, local Julian date of the
observations, air-mass range during the observations,
and exposure times in each band. The second part of
Table 2 contains data of the observed standard fields.
The data reduction was carried out by R. Michel1
using the IRAF2 package and together with some homemade auxiliary Fortran programs and Awk scripts.
The supervised-automatic procedure, implemented by
C. Chavarria, can be condensed as follows: All the
images were bias subtracted and flat-field corrected
(CCDRED); cosmic rays were then removed with the
L. A. Cosmic3 script (van Dokkum 2001).
1 Data may be requested from R. Michel.
2 IRAF is distributed by the National Optical Observatories, operated by the Association of Universities for Research in Astronomy
Inc., under co-operative agreement with the National Science Foundation.
3 http://www.astro.yale.edu/dokkum/lacosmic.
4, 15
10
80
50
15, 16
6, 20
25
120
45
25, 27
10, 30
60
220
65
45, 50
1.352–1.382
1.414–1.423
1.135–1.136
1.174–1.175
1.198–1.523
NGC 2323
NGC 2539
PG0918+029
RU 149
RU 152
07:02:52.4
08:10:43.4
09:21:32.6
07:24:15.5
07:29:58.4
−08:21:01.2
−12:51:41.9
+02:47:42.9
−00:32:58.0
−02:06:11.2
2452669
2452669
2452669
2452669
2452669
60, 300
300
300
300
2 × 300
Filter I
Exp. time (s)
Filter R
Exp. time (s)
Filter V
Exp. time (s)
Filter B
Exp. time (s)
Filter U
Exp. time (s)
Air mass
range
LJD
(days)
DEC
(2000)
RA
(2000)
Cluster
Table 2. The observed open clusters and the standard-star Landolt fields.
4, 15
8
80
50
15, 18
(2019) 40:33
J. Astrophys. Astr.
Page 3 of 18
33
The images of each set of observations were aligned
and trimmed (IMALIGN), generating a template image
of each cluster field (IMSUM). By using these template images, in addition to the reference images from
the ESO Digital Sky Survey,4 and the equatorial coordinates from the 2MASS All-Sky Catalog of Point
Sources (Cutri et al. 2003), an astrometric solution is
obtained (CCMAP and CCTRAN) providing a means
to transform pixel coordinates to equatorial coordinates
of each detected star.
For each image, the average sky level and its standard deviation are iteratively calculated with the help of
the IMSTAT routine. The average FWHM of the stars
in each image is also found by means of DAOFIND,
FITPSF, and filtering routines. With this information,
the bright, unsaturated and isolated stars are identified
and used iteratively to build the point spread function (PSF) for each image. Consequently, instrumental
magnitudes are calculated using the ALLSTAR routine
(Stetson 1987).
The aperture photometry (PHOT) of the standard
stars was calculated using a fixed aperture radius
of 2(FWHM + 3σ ), where FWHM was the average FWHM of the run, and σ , its standard deviation.
The pixel coordinates of each measured star are then
transformed to their corresponding equatorial coordinates. The standard magnitudes were taken from
the catalogue by Landolt (2009) and supplemented
with the secondary photometric standards by Cutri
et al. (2013). As a result, the transformation coefficients were found (FITPARAMS) using Equations
(1)–(10):
u = U + zUU B + kUU B X U + cUU B (U − B)
+ pUU B X U (U − B),
u = U + zUU V + kUU V X U + cUU V (U − V )
+ pUU V X U (U − V ),
b = B + z BU B + k BU B X B + c BU B (U − B)
+ p BU B X B (U − B),
b = B + z B BV + k B BV X B + c B BV (B − V )
+ p B BV X B (B − V ),
v = V + z V BV + k V BV X V + cV BV (B − V )
+ pV BV X V (B − V ),
v = V + z V V R + k V V R X V + cV V R (V − R)
+ pV V R X V (V − R),
4 http://archive.eso.org/dss/dss.
(1)
(2)
(3)
(4)
(5)
(6)
Page 4 of 18
c
rms
291
301
291
355
355
427
427
388
368
388
0.022
0.021
0.023
0.021
0.020
0.021
0.022
0.021
0.024
0.025
0.010 ± 0.015
0.001 ± 0.002
0.047 ± 0.015
0.008 ± 0.007
0.012 ± 0.007
0.014 ± 0.013
0.029 ± 0.014
−0.009 ± 0.010
−0.010 ± 0.003
−0.009 ± 0.011
0.031 ± 0.039
0.014 ± 0.012
0.014 ± 0.043
0.033 ± 0.016
0.012 ± 0.015
−0.015 ± 0.023
0.013 ± 0.025
−0.015 ± 0.025
0.036 ± 0.014
0.025 ± 0.029
−0.144 ± 0.050
−0.072 ± 0.016
−0.104 ± 0.055
−0.111 ± 0.021
0.033 ± 0.020
0.118 ± 0.031
−0.002 ± 0.032
0.081 ± 0.033
−0.084 ± 0.019
−0.141 ± 0.038
50
56
50
56
56
66
66
56
52
56
Points
c
p
Second pass
(10)
0.018
0.020
0.013
0.014
0.014
0.017
0.016
0.010
0.018
0.022
q
(9)
(2019) 40:33
k
0.417 ± 0.016
0.428 ± 0.017
0.206 ± 0.013
0.194 ± 0.013
0.118 ± 0.013
0.131 ± 0.015
0.077 ± 0.015
0.091 ± 0.011
0.004 ± 0.018
0.021 ± 0.020
z
4.188 ± 0.022
4.198 ± 0.024
2.527 ± 0.018
2.571 ± 0.019
2.189 ± 0.019
2.170 ± 0.021
2.254 ± 0.021
2.226 ± 0.014
2.547 ± 0.027
2.532 ± 0.028
Index
UU B
UU V
BU B
B BV
V BV
VVR
RV R
RRI
IV I
I RI
−0.082 ± 0.009
−0.049 ± 0.006
−0.032 ± 0.007
−0.052 ± 0.010
0.067 ± 0.010
0.122 ± 0.022
0.036 ± 0.021
0.065 ± 0.010
−0.047 ± 0.015
−0.116 ± 0.017
Points
(8)
where u, b, v, r and i are the magnitudes in the instrumental system, U , B, V , R and I are the magnitudes in
the standard system, and X is the air mass during the
measurement. During the first iteration, only the firstorder extinction measurements (for the stars observed
over an extended range of air mass) were used to calculate the zero point (z), first-order extinction (k) and
colour coefficients (c); the second-order extinction ( p)
was zeroed. For the second iteration, all the data were
used, having zero points and first-order extinction values fixed to the values found in the previous iteration,
while the colour-dependent coefficients ( p and c) were
re-calculated. p corrects the shift in λeff due to the convolution of the spectral-energy distribution of the stars
with the wavelength dependence of k. These values are
given in Table 3.
For the case of this observing run, the number of valid
measurements used in the transformations is shown
in the first three rows of Table 4 together with the
colour and air-mass range near the zenith of the standard fields. Table 4 includes the local Julian date of the
observations, the number of measurements, the interval of the colour (B–V ), the air-mass range and the
Landolt’s field name. The resulting coefficients found
using Equations (1)–(10), the rms deviations of the
fits, and the number of stars used in the calculations
are given in three rows at the bottom of Table 4. The
new images that are contained only the PSF stars were
generated with the help of the GROUP, NSTAR and
SUBSTAR routines, and the aperture corrections were
calculated with PHOT and MKAPFILE of IRAF. The
mean value of the aperture correction is about −0.2
magnitude.
Figure 1 presents the finding charts5 of NGC 2323
18.3′ × 17.23 ′ ) and NGC 2539 (15.0′ × 14.33 ′ ). The
blue rectangles indicate the analysed regions of these
OCs, the field-of-view of the SPMO detector, 7.4′ ×
9.3′ .
5 Obtained from https://www.aavso.org (AAVSO).
rms
(7)
First pass
r = R + z RV R + k RV R X R + c RV R (V − R)
+ p RV R X R (V − R),
r = R + z R R I + k R R I X R + c R R I (R − I )
+ p R R I X R (R − I ),
i = I + z I V I + k I V I X I + c I V I (V − I )
+ p I V I X I (V − I ),
i = I + z I R I + k I R I X I + c I R I (R − I )
+ p I R I X I (R − I ),
J. Astrophys. Astr.
Table 3. Coefficients of the transformation equations.
33
(2019) 40:33
J. Astrophys. Astr.
Page 5 of 18
33
Table 4. Observed standard stars (first three rows) and the subset of extinction
stars (last three rows).
HJD
Meas. and (B − V ) and air mass
2452668
596; −0.284–1.605; 1.19–1.34
2452669
606; −0.284–1.727; 1.14–1.50
All
1202; −0.284–1.727; 1.14–1.50
2452668
2452669
184; −0.034–1.170; 1.19–1.34
96; 0.043–1.528; 1.20–1.50
All
Landolt fields
PG1047+003 0035 CL13
RU149 0538 CL13
G3061911464112566400RM
PG0918+029 0068 CL13
PG1047+003 0035 CL13
RU149 0525 CL13
RU152 0561 CL13
RU149 0557 CL13
G3061911464112566400RM
RU152 0586 CL13
96; 0.043–1.528; 1.20–1.50
The photometric errors in V and colours (R–I ), (V –
I ), (B–V ), (U –B) of NGC 2323 (left panel in Figure 2)
and NGC 2539 (right panel in Figure 2) are presented.
Their mean errors are listed in Table 5. Our inspection
of Figure 2 and Table 5 indicates that stars brighter than
V ≈ 17m .00 have errors smaller than ≈0m .05 in both
magnitudes and colours. For V > 17m .00, the large
errors in (U –B) dominate. The stars with V < 17m .00
have been considered for our analyses.
A comparison of the present data with those in the
literature for stars in common is shown in Figure 3 for
NGC 2323 (left panels) and NGC 2539 (right panels).
For comparisons, we have considered only photoelectric photometry from the literature. For this purpose,
a finding chart is shown in Figure 4 for NGC 2323.
The size of the filled dots for our CCD UBV photometry is proportional to the magnitudes of stars:
V = 9m .0 − 15m .0 (large dots) and 15m .0 − 19m .0
(small dots). The blue squares show stars with the
photoelectric photometry of Claria et al. (1998) for
V < 13m .0. From the left panels of Figure 3 (for
10 stars in common), our magnitudes and colours,
V , (U –B) and (B–V ) do not show any systematic
trends with the photolectric data of Claria et al. (1998).
The three stars with up to V = 0m .50 are seen in
panel (a), and the two stars with large color deviations in (U − B) and (B − V ) at V = 10m .65
and V = 12m .87, in panels (b) and (c) of Figure 3.
The magnitudes and colours of Claria et al. (1998) are
somewhat fainter and redder than our CCD UBV photometry.
For NGC 2539, the comparison with the photoelectric data of Lapasset et al. (2000) is done for 53
stars in common in the right panels of Figure 3. For
V = (13m .0, 14m .0), the differences up to 0m .40
in (B − V ) and (U − B) for a few stars indicate
that our photometry gives bluer colours than the ones
of Lapasset et al. (2000); these 4–5 deviating stars
are not easily explained, not by cosmic rays or cosmetic effects in the CCD data, but perhaps by stellar
variability or typographical errors of Lapasset et al.
(2000).
3. Cluster membership
For the separation of the likely cluster members of
NGC 2323 and NGC 2539, firstly, the CCD
UBV(R I ) K C photometric data of NGC 2323 (170 stars)
and NGC 2539 (155 stars) have been matched with the
GaiaDR2 astrometric data (proper motions and parallaxes) from SIMBAD-VizieR.6 GaiaDR2 astrometric
and photometric data of the field stars in a region of
R = 20 arcmin centered on our target OCs have also
been considered to determine the cluster’s radii. The
μα versus μδ (vector point diagram, VPD) of the two
OCs are plotted in the top panels of Figure 5 for both the
background/foreground field stars (small grey dots) and
our OCs (filled dots). The proper motion radii (shown
with blue circles) of 0.6 mas yr−1 around the centres
of their VPDs define the membership criteria. These
proper motion radii have been empirically fitted until
the likely members inside these radii provide good single stellar sequence on G, (G B P −G R P ) and V, (B−V )
6 http://vizier.u-strasbg.fr/viz-bin/VizieR.
33
Page 6 of 18
J. Astrophys. Astr.
(2019) 40:33
of NGC 2323 (74 probable members) and NGC 2539
(67 probable members). The big red pluses indicate the
median values of the proper motion components of the
two OCs.
For the likely members of our OCs, the uncertainties
of proper motions and the parallaxes are less than 0.30
mas yr−1 and 0.15 mas, respectively. These limits nearly
remain within the uncertainties of the proper motion
components of σμα < 0.28 mas yr−1 and σμδ < 0.24
mas yr−1 , and the uncertainty σ̟ < 0.16 mas for G <
18 mag, which are reported by the Gaia Collaboration,
Lindegren et al. (2018) (see their Table B.1).
The median proper motion components plus their
equatorial coordinates, found from GaiaDR2 astrometric data for the likely members of our OCs are listed
in parentheses in Table 1. Having applied these proper
motion criteria to our OCs, the numbers of the likely
cluster members are now appropriate for determining
the astrophysical parameters in CC and CMDs.
4. Astrophysical parameters
4.1 Reddenings and photometric metal abundances
Figure 1. The images from https://www.aavso.org
(AAVSO) of NGC 2323 (18.3 ′ × 17.23 ′ ) and NGC 2539
(15.0 ′ × 14.33 ′ ) are shown from top to bottom, respectively.
The blue rectangles indicate the field-of-view of the SPMO
detector, 7.4′ × 9.3′ .
plots (bottom panels of Figure 5). Therefore, the chosen
radii are a good compromise for the likely cluster members. These proper motion radii have been constructed
via the mathematical equations, x = x0 + r cos(θ ) and
y = y0 + r sin(θ ). Here, (x0 , y0 ) are the median
values
2
−1
2
of (μα , μδ ) (mas yr ), the radius r = σμα + σμδ
mas yr−1 , and θ = 0◦ to 360◦ . Note that there almost
seems to be a clear concentration of stars inside the chosen proper motion radii. The inset plots (Figure 5) show
the likely cluster members inside the proper motion radii
The two-colour (U − B), (B−V ) diagrams of the probable members of NGC 2323 and NGC 2539 are displayed
in Figures 6 and 7. Note that there are 56 (NGC 2323)
and 42 stars (NGC 2539) which have (U − B) colour. It
appears that NGC 2323 contains early-type stars, which
are mostly members of young open clusters. By using
11 early type stars with (U − B) < 0 (Figure 6), the
mean interstellar reddening is estimated as E(B −V ) =
0.23 ± 0.04 from the Q-technique of early-type stars.
The reddened intrinsic-colour sequence of the SchmidtKaler (1982) (blue curve) for this E(B − V ) = 0.23
is fitted on the CC of NGC 2323. For this, we adopt
the following relations of Johnson & Morgan (1953):
Q = (U − B)−0.72(B − V ), and (B − V )0 = 0.332Q.
Here E(U − B) = 0.72E(B − V ) + 0.05E(B − V )2 ,
E(B − V ) = (B − V ) − (B − V )0 and E(U − B) =
(U − B) − (U − B)0 . The interstellar reddening,
E(B − V ) of NGC 2539 is derived from the displacement of the intrinsic-colour sequences of dwarfs
and red giants from Schmidt-Kaler (1982) in the CC
diagram (Figure 7), until the best-fit to the cluster members has been achieved: along the (U − B) axis by
0.72E(B − V )+0.05E(B − V )2 and along the (B − V )
axis by E(B − V ).
The F-type stars on the CC plots of these two OCs
show ultraviolet excess, δ(U − B) above the Hyades
main sequence (MS) (green dashed curve), which are
J. Astrophys. Astr.
(2019) 40:33
Page 7 of 18
33
Figure 2. Photometric errors for the V magnitude and several broad-band colours plotted against the V magnitudes of
NGC 2323 (left panels) and NGC 2539 (right panels).
quite valuable for determining the photometric metal
abundance, [M/H ]. Here, δ(U − B) = (U − B)Hyades −
(U − B)0 . By shifting the Hyades main-sequence
according to the E(B − V ) (column 2 of Table 6),
a fit is applied to the F-type stars, the same as used
to fit the RC/RG stars to the SK82 giant colours. The
best-fit of iso-metallicity curves as representative of the
mean metal abundances of the two OCs are shown as
red solid lines in the CC diagrams (Figures 6 and 7).
The average (B − V )0 , (U − B)Hyades , (U − B)0
colours (columns 3–5 of Table 6) have been fixed as
mean values from the distribution of the F-type stars
in each cluster. By using these average values, the
ultraviolet excesses, δ(U − B) have been measured
and normalized to (B − V )0 = 0.6 via the data of
Table 1A given by Sandage (1969). We use [M/H ] =
2 by
+0.13(±0.04)−4.84(±0.60)δ0.6 −7.93(±2.24)δ0.6
Karataş & Schuster (2006) to estimate the photometric metallicity values ([M/H ]) of the two OCs. With
the equation Z = Z ⊙ · 10[M/H ] , their [M/H ] values are converted into the heavy-element abundance
mass fraction Z . The solar metal content is adopted as
Z ⊙ = 0.0152. The mean values δ(U − B), δ(U − B)0.6 ,
[M/H ] and Z of the two OCs are listed in colunms 6–9
of Table 6.
The E(B − V ) values from extinction maps given
by Schlegel et al. (1998) (SFD) (based on the IRAS
100-micrometer surface brightness converted to extinction) have been obtained from NASA Extragalactic
Database (NED) as E(B − V )SFD,∞ = 0.713 and 0.032
for NGC 2323 and NGC 2539, respectively. Taking
into consideration their distances d (kpc) (column 3 of
33
Page 8 of 18
J. Astrophys. Astr.
(2019) 40:33
Table 5. The mean photometric errors of V , (R–I ), (V –I ),
(B–V ) and (U –B) for NGC 2323 and NGC 2539 in terms of
the V mag.
V
σV
σ R−I
σV −I
σ B−V
σU −B
NGC 2323
9–10
10–11
11–12
12–13
13–14
14–15
15–16
16–17
17–18
18–19
0.002
0.001
0.001
0.002
0.002
0.002
0.003
0.007
0.024
0.064
0.004
0.003
0.004
0.004
0.004
0.006
0.006
0.009
0.020
0.050
0.004
0.003
0.004
0.004
0.004
0.005
0.006
0.011
0.030
0.074
0.003
0.002
0.002
0.003
0.005
0.005
0.012
0.025
–
–
0.003
0.001
0.002
0.003
0.007
0.009
0.018
0.041
–
–
NGC 2539
10–11
11–12
12–13
13–14
14–15
15–16
16–17
17–18
18–19
0.003
0.002
0.003
0.004
0.005
0.007
0.013
0.022
0.040
0.006
0.007
0.006
0.009
0.010
0.014
0.024
0.041
0.067
0.005
0.006
0.006
0.008
0.009
0.012
0.021
0.037
0.065
0.004
0.004
0.005
0.007
0.009
0.014
0.025
0.051
0.085
0.004
0.004
0.005
0.008
0.012
0.021
0.039
0.077
0.110
Figure 4. The finding chart is presented for NGC 2323
only. The size of filled dots for our CCD UBV photometry is proportional to the magnitudes of stars: V = 9–15
mag for large dots, and 15–19 mag for small dots. The blue
squares show stars with photoelectric photometry of Claria
et al. (1998) for V < 13.00. α and δ are in units of arcmin,
and are measured from the cluster center of NGC 2323 in
WEBDA.
Figure 3. For NGC 2323 (left panels) and NGC 2539 (right panels), comparisons of the present CCD photometry with data
from the literature: Claria et al. (1998) and Lapasset et al. (2000), respectively. The difference (present photometry minus
literature values) as a function of the V mag is shown.
J. Astrophys. Astr.
(2019) 40:33
Page 9 of 18
33
Figure 5. The μα versus μδ for NGC 2323 (left, top panel) and NGC 2539 (right, top panel). Small dots represent the
GaiaDR2 astrometric and photometric data for a 20 arcmin region centered on the two OCs. Blue circles denote the radii of
0.6 mas yr−1 . The likely members inside the blue circles are also indicated in the inset of the top panels. The big red pluses
indicate the median proper motions of the OCs. The bottom panels show the G, G B P − G R P of GaiaDR2 stars for a 20 arcmin
field region which is centered on the OCs and the likely cluster members (filled dots). V, (B − V ) plots are also displayed for
all cluster stars: 170 small dots, NGC 2323, and 155 small dots, NGC 2539, together with their likely members (filled dots)
inside the blue circles.
Tables 7 and 8) and the galactic latitudes (column 5 of
Table 1), the final reddening, E(B − V )SFD for a given
star is reduced compared to the total reddening E(B −
V )(ℓ, b)∞ by a factor {1 − exp[−d sin |b|/H ]}, given
by Bahcall and Soneira (1980), where b, d and H are
the galactic latitude, distance and scale-height. These
reduced E(B − V )SFD values are 0.13 (NGC 2323) and
0.02 (NGC 2539). Here, we adopted H =125 pc (Bonifacio et al. 2000).
4.2 Distance moduli, distances and ages
For the determination of distances and ages of the two
OCs, we have used the PARSEC isochrones of Bressan et al. (2012) for Y values which correspond to Z .
Here, Y = 0.2485 + 1.78Z . The PARSEC isochrones
of Bressan et al. (2012) are over-plotted in four CMDs:
V, (B − V ), V, (V − I ), V, (R − I ), G, (G B P − G R P )
(Figures 8–11). The E(V − I ), E(R − I ) and E(G B P −
G R P ) colour excesses are converted from the relations
E(V − I ) = 1.25E(B −V ), E(R− I ) = 0.69E(B −V )
(Dean et al. 1978; Mathis 1990; Straiz̧ys 1995) and
E(B − V ) = 0.775E(G B P − G R P ) (Bragaglia et al.
2018). A visual extinction of A V = 3.1 × E(B − V )
is applied to the absolute visual magnitudes of the
isochrones.
The PARSEC isochrones are first shifted both vertically and horizontally on the CMDs according to the
interstellar reddening values of E(B − V ), E(V − I ),
E(R − I ) and E(G B P − G R P ). Then the PARSEC
isochrones have been shifted vertically to obtain the
best-fit to the observed main sequence as well as the
RC sequence. This vertical shift gives the (true) distance modulus, D M = (V0 − MV ). The distance moduli
33
Page 10 of 18
Figure 6. The reddened (U –B, B–V ) diagram for the
likely members of NGC 2323. The blue dashed line shows the
relation of Schmidt-Kaler (1982) for both the main sequence
(upper part) and the red giants (lower part). The green
dashed line denotes the Hyades main-sequence. The fitted
iso-metallicity line is indicated by the solid red curve. The
grey dots indicate the non-members. The reddening vector is
the dashed red arrow, and the big circles indicate the seven
definite members (see Table 10).
J. Astrophys. Astr.
(2019) 40:33
For the determination of the ages (A, log(A)) of
these OCs, the isochrones of PARSEC, selected according to their Z values, have been shifted both vertically
and horizontally in the CMD’s with the expression
MV + 3.1E(B − V ) + D M, for the vertical displacement and C0 (λ1 − λ2 ) + E(λ1 − λ2 ), for the
horizontal displacement, where λ denotes the wavelengths of the various passbands. Here C0 means the
de-reddened colour index. Then the age of the isochrone
is varied until a satisfactory fit to the data has been
obtained through the observed main-sequence (MS),
turn-off (TO), sub-giant (SG) and RG/RC sequences.
The derived ages are given in columns 4–5 of Tables 7
and 8.
To appreciate the uncertainties of the distance moduli
and ages, additional PARSEC isochrones have usually
been over-plotted in each CMD. The best-fit is shown by
red solid line, whereas, the uncertainties are represented
by gray solid lines. The photometric uncertainties of
colour indices are indicated in the bottom left of CC
and CMD plots.
For comparisons with literature, we have taken into
consideration mostly those physical parameters given
by our (B–V ) colour indices, because the astrophysical parameters of these OCs are mostly given, and best
represented, in terms of the CMD: V , (B −V ) (Table 9).
5. Results
5.1 NGC 2323
Figure 7. The reddened (U –B, B–V ) diagram for
NGC 2539. The six definite members are shown as circles. The meanings of the symbols are the same as in
Figure 6.
(V0 − MV ) and distances d (kpc), for the two OCs are
presented in columns 2–3 of Tables 7 and 8.
From 11 early type stars with (U − B) < 0.0 in Figure 6,
the reddening E(B − V ) = 0.23 ± 0.04 of NGC 2323
is determined. As seen from Table 9, this reddening value is quite consistent with the literature values,
E(B − V ) = 0.20–0.28, within the uncertainties. Our
reddening value and the literature values are larger than
E(B − V )SFD = 0.13. NGC 2323’s δ(U − B)0.6
gives the photometric abundance of ([M/H ], Z ) =
(−0.10, 0.012) from the CC diagram (Figure 6).
From the CMDs, for the colour indices (B–V ), (V –
I ), (R–I ) and (G B P –G R P ) (Figures 8, 9), the values
for the distance moduli, distances and ages together
with their uncertainties for NGC 2323 are presented
in Table 7. The 200 Myr PARSEC isochrone with
Z = 0.012 fits well the main sequence for all colour
indices. No evolved stars are seen in the CMDs of
NGC 2323. The PARSEC isochrones fit well the evolutionary sequences, as is seen from the CMDs. We
derived the distance modulus and distance as (V0 − MV ,
d(kpc)) = (10.00 ± 0.10, 1.00 ± 0.05), by fitting the
J. Astrophys. Astr.
(2019) 40:33
Page 11 of 18
33
Table 6. The reddenings (column 2) and the mean values of (U − B)Hyadas of the Hyades reference line, (U –B)0 values
for (B–V )0 (columns 3–5) as set by the iso-abundance lines for F-type stars of the two OCs. The δ(U –B), δ0.6 , [M/H ] and
Z are given in columns 6–9 together with their uncertainties.
Cluster
NGC 2323
NGC 2539
E(B–V )
(B–V )0
(U –B) H
(U –B)0
δ(U –B)
δ0.6
[M/H ]
Z
0.23 ± 0.04
0.02 ± 0.06
0.48
0.51
+0.01
0.06
−0.03
−0.03
0.04
0.03
0.044 ± 0.03
0.031 ± 0.03
−0.10 ± 0.11
−0.31 ± 0.12
0.012 ± 0.003
0.007 ± 0.003
Table 7. The derived fundamental astrophysical parameters of NGC 2323
four-colour indices. Its reddening, metal and heavy element abundances
are as follows: E(B–V ) = 0.23 ± 0.04, [M/H ] = −0.10 ± 0.11 and
Z = 0.012 ± 0.003.
Colour
(B–V )
(V –I )
(R–I )
(G B P –G R P )
(V0 –MV )
d (kpc)
log(A)
A (Gyr)
10.00 ± 0.10
09.90 ± 0.15
09.90 ± 0.15
09.80 ± 0.10
1.00 ± 0.05
0.96 ± 0.07
0.96 ± 0.07
0.91 ± 0.04
8.30 ± 0.10
8.30 ± 0.15
8.30 ± 0.15
8.35 ± 0.10
0.20 ± 0.05
0.20 ± 0.08
0.20 ± 0.08
0.20 ± 0.06
Table 8. The derived fundamental astrophysical parameters of NGC 2539
four-colour indices. Its reddening, metal and heavy element abundances
are as follows: E(B − V ) = 0.02 ± 0.06, [M/H ] = −0.31 ± 0.12 and
Z = 0.007 ± 0.003.
Colour
(B–V )
(V –I )
(R–I )
(G B P –G R P )
(V0 –MV )
d (kpc)
log(A)
A (Gyr)
10.00 ± 0.04
10.00 ± 0.10
10.00 ± 0.10
10.10 ± 0.10
1.00 ± 0.02
1.00 ± 0.05
1.00 ± 0.05
1.00 ± 0.05
8.95 ± 0.05
8.95 ± 0.10
8.95 ± 0.10
8.95 ± 0.10
0.89 ± 0.11
0.89 ± 0.23
0.89 ± 0.23
0.89 ± 0.23
PARSEC isochrones to the (V, (B − V )) diagram (Figure 9(a)). The differences with literature are at a level
of (V0 − MV ) = 0–0.46 mag and d = 0–0.11 kpc.
Our age value is somewhat older than the age range of
100–140 Myr in the literature (Table 9). This is to be
expected since we used Z = 0.012 instead of the solar
metal abundance. The distance modulus and distance
of this OC are in agreement with the literature values
(Table 9). Note that some authors of the literature do
not give their uncertainties.
Our CCD UBV (R I ) K C photometric data of NGC
2323 contain seven definite early-type members,
according to the SIMBAD database. The GaiaDR2 parallaxes/distances plus UBV photometry of these seven
definite members are listed in the top rows of Table 10.
These are also marked as big circles in the CC and
CMDs (Figures 6, 8 and 9). Our photometric distances
of (B − V ), (V − I ), (R − I ) and (G B P − G R P ) provide
good agreement with the Gaia DR2 distances (column 7,
Table 10) within their uncertainties.
5.2 NGC 2539
A reddening value E(B − V ) = 0.02 ± 0.06 for
NGC 2539 (Figure 7) is derived. Within the uncertainties, this value is in good coherence with the ones of the
literature (Table 9). The reduced E(B − V )SFD = 0.02
value for this OC is in good agreement with our reddening value. NGC 2539’s δ(U − B)0.6 gives the photometric abundance of ([M/H ], Z ) = (−0.31, 0.007)
from the CC diagram (Figure 7).
The CMDs for the colour indices (B − V ), (V − I ),
(R − I ), (G B P − G R P ) of NGC 2539 are presented in
panels (a), (b) of Figures 10 and 11. The isochrone with
Z = 0.007 fits well the main sequence for all colour
indices. The derived fundamental parameters together
33
Page 12 of 18
Table 9. Comparison of our sample of OCs with the literature. The comparison is given here for our (B–V ) results.
Cluster
E(B − V )
(V0 –MV )
NGC 2323 0.23 ± 0.04 10.00 ± 0.10
0.23 ± 0.06 10.00 ± 0.15
–
–
0.23
–
0.25
9.85
0.28
0.20
0.22
Age (Myr) log A
1.00 ± 0.05
1.00 ± 0.07
–
0.895
1.00
200 ± 50
140 ± 20
115 ± 20
100
140
8.30
8.15
8.06
8.00
8.15
1.107 ± 0.07
0.95
1.00 ± 0.08
–
100
130
8.30
8.00
–
9.86
0.94
100 ± 20
–
–
–
–
0.997 ± 0.057
–
10.00 ± 0.04 1.00 ± 0.02 890 ± 110
10.20 ± 0.10 1.10 ± 0.05
630
10.42
1.21
630
10.10 ± 0.30 1.05 ± 0.15
540
09.80 ± 0.50 0.91 ± 0.21
640 ± 80
0.10 ± 0.05 10.50 ± 0.50 1.29 ± 0.29
–
–
0.754 ± 0.064
–
–
8.00
–
–
8.95
8.80
8.80
8.73
8.81
–
–
Z
Isochrone
0.012
Solar
Solar
–
0.019
Bressan et al. (2012)
Yi et al. (2001);
Bressan et al. (2012)
–
Girardi et al. (2000)
Photometry
CCD UBVRI
UBV; CFHT
UBV; CFHT
–
Various broadband
photometries
0.019 Girardi et al. (2002) 2MASS JHK s
0.020 Bertelli et al. (1994) CCD UBV
0.020 Ventura et al. (1998) BV with CFHT12Mosaic
camera
0.020 Bertelli et al. (1994) Photoelectric UBV
–
–
Stromgren
–
–
GaiaDR2 astrometry
0.007 Bressan et al. (2012) CCD UBVRI
0.019 Girardi et al. (2000) CCD UBVI
0.020 Schaller et al. (1992) Photoelectric UBV
0.030 Hejlesen (1980)
Photoelectric UBV
CCD DDO, CMT1 T2 ,
0.016 Mermilliod (1981)
UBV
–
–
Photoelectric UBV
–
–
GaiaDR2 astrometry
References
This work
Cummings et al. (2016)
Cummings et al. (2016)
Paunzen et al. (2014)
Frolov et al. (2012)
Bukowiecki et al. (2011)
Sharma et al. (2006)
Kalirai et al. (2003)
Claria et al. (1998)
Schneider (1987)
Cantat-Gaudin et al. (2018)
This work
Choo et al. (2003)
Lapasset et al. (2000)
Joshi and Sagar (1986)
Claria & Lapasset (1986)
Pesch (1961)
Cantat-Gaudin et al. (2018)
J. Astrophys. Astr.
0.25 ± 0.05
0.257
–
NGC 2539 0.02 ± 0.06
0.06 ± 0.03
0.06
0.08 ± 0.02
0.08 ± 0.02
10.46
10.50
10.00 ± 0.17
d (kpc)
(2019) 40:33
J. Astrophys. Astr.
(2019) 40:33
Figure 8. For NGC 2323, CMDs of (V, R − I ) (a) and
(G, G B P − G R P ) (b). The red curves show the PARSEC
isochrones interpolated to Z = +0.012. The solid grey
isochrones have been drawn to provide a means for appreciating the uncertainties of the ages. The filled grey dots indicate
the members and non-members, respectively.
with their uncertainties from these CMDs have been
given in Table 8. For the (B − V ) colour, our values
(V0 − MV ) = 10.00 ± 0.04 and d(kpc) = 1.00 ± 0.02
are in good concordance with the ones of the literature
within the uncertainties (Table 9). The differences with
literature for NGC 2539 are at a level of (V0 − MV ) =
0.10 − 0.50 mag and d = 0.05 − 0.29 kpc. Our age
value 0.89 ± 0.11 Gyr (890 Myr) for NGC 2539 seems
to be slightly older than the age range of 540–640 Myr
of the literature (Table 9).
Page 13 of 18
33
Figure 9. For NGC 2323, the CMDs of (V, B − V ) (a) and
(V, V − I ) (b) are shown. The symbols are the same as in
Figure 8.
No RC/RG candidates are detected in the CMDs of
NGC 2323 because of its younger age (A = 0.20 Gyr).
Note the possible RC/RG candidates in the CMDs of
NGC 2539 (square dots in Figures 10, 11). One of
them is named BD-12 2380, according to the SIMBAD
database. One can expect to detect this possible RC/RG
star in the central part of NGC 2539, depending on its
age (A = 0.89 Gyr). For this giant candidate, from V
(B − V ) and V (V − I ) CM diagrams (square dot, panels (a)–(b) of Figure 10), we have utilised the distance
criteria for its membership as an initial test. In the case
of the possible RG candidates, the RG candidates will
33
Page 14 of 18
Figure 10. For NGC 2539, CMDs of (V, B − V ) (a) and
(V, V –I ) (b). Red curves show the PARSEC isochrones interpolated to Z = +0.007. The solid grey isochrones have
been drawn to provide a means for appreciating the uncertainties of the ages. The big open circles mark six definite
members. Note that the two giant members (square dots)
with (V, B − V ) = (10.462, 0.642) (BD-12 2380) and
(V, B − V ) = (11.144, 0.982).
not necessarily give the right distances to these clusters
since their apparent magnitudes will depend on their
position along the RG branch.
For the case of a possible RC candidate, its distances have been estimated from the mean absolute
magnitudes of MV = +0.60 ± 0.10 (Twarog et al.
1997) and M I = −0.22 ± 0.03 (Groenewegen 2008),
and listed in Table 11 together with its equatorial
J. Astrophys. Astr.
(2019) 40:33
Figure 11. For NGC 2539, CMDs of (V, R–I ) (a) and
(G, G B P –G R P ) (b). The meanings of the symbols are the
same as in Figure 10.
coordinates plus its magnitudes of I and V . For the
estimates, the total absorption relations of A V =
3.1E(B − V ) and A I = 1.98E(B − V ) (Gim et al.
1998) are included, with the E(B − V ) = 0.02 value of
NGC 2539. The magnitudes of I and V in columns 3–4
of Table 11 are used for estimating the d I and dV distances. These distances are estimated as d I = 0.95 ±
0.05 kpc and dV = 0.91 ± 0.09 kpc. The giant candidates within the 1-σ agreement of the uncertainties
of d I and dV from the main-sequence fitting (Table 8)
have been assigned as members, ‘M’, otherwise as nonmembers ‘NM’ (columns 7–8 of Table 11). The giant
(2019) 40:33
J. Astrophys. Astr.
Page 15 of 18
33
Table 10. Our UBV photometry and Gaia DR2 parallaxes for the definite members of NGC 2323 (seven) and NGC 2539
(six), according to the SIMBAD database. The star designations are given in columns 1–2 and V , (U –B) and (B–V ) are
given in columns 3–5. GaiaDR2 parallaxes (mas) and their converted distances (pc) together with the uncertainties are given
in columns 6 and 7.
Star-ID
Star no.
V
(U − B)
(B − V )
̟ ± σ̟
d ± σd
NGC 2323
228
163
256
402
359
70
105
HD 52965
BD-08 1703
BD-08 1700
BD-08 1695
BD-08 1696
BD-08 1708
HD 52980B
9.13
9.30
9.85
9.88
9.90
9.90
11.20
−0.34
−0.22
−0.31
−0.18
−0.38
−0.32
−0.18
0.09
0.13
0.10
0.11
0.02
0.14
0.20
1.004 ± 0.042
1.170 ± 0.045
0.914 ± 0.047
0.967 ± 0.046
1.008 ± 0.047
0.951 ± 0.060
1.023 ± 0.041
996 ± 42
855 ± 33
1094 ± 56
1034 ± 49
992 ± 46
1052 ± 66
978 ± 39
NGC 2539
37
168
234
122
98
79
BD-12 2380
BD-12 2373
V0691Pup
V0693Pup
V0694Pup
V0695Pup
10.46
11.28
12.39
13.07
14.18
15.72
0.31
0.06
0.05
0.03
0.01
0.30
0.64
0.16
0.25
0.30
0.55
0.82
0.833 ± 0.047
0.713 ± 0.063
0.796 ± 0.048
0.782 ± 0.038
0.687 ± 0.034
0.730 ± 0.050
1200 ± 70
1403 ± 120
1256 ± 80
1279 ± 60
1456 ± 70
1370 ± 90
Table 11. The first two rows show the membership test according to the distance criterion for the possible red clump
(RC) member of NGC 2539. RA, DEC (columns 1–2), V I photometry (columns 3–4), distances for I and V magnitudes
(columns 5–6) and test distances (columns 7–8) are given. ‘M’ in columns 7–8 means member. The morphological age
indices (MAI) of NGC 2539 are given at the bottom. ‘TO’ and ‘RC’ mean turn-off and red clump stars, respectively. The
morphological age values, log A and A (Gyr) are listed in columns 7–8.
RA
122.71
122.64
VTO
11.40
DEC
I
V
d I (kpc)
dV (kpc)
d I (test)
dV (test)
–12.88
–12.86
9.71
10.20
10.462
11.144
0.95 ± 0.05
0.96 ± 0.06
0.91 ± 0.09
1.23 ± 0.14
M
M
M
NM
VRC
(B − V )TO
(B − V )RC
δV
δ1
log A
A (Gyr)
10.70
0.08
0.89
0.73
0.81
9.04 ± 0.05
1.09 ± 0.27
star, BD-12 2380 which fulfils both the d I and dV
distances within its uncertainties is a definite member of
NGC 2539. Its GaiaDR2 distance, d = 1.20 ± 0.07 kpc
(̟ = 0.833 ± 0.047) mas is close to the photometric
distances of Tables 8 and 11.
The other giant candidate does not seem to be a RC
candidate due to its location (V, B − V ) =(11.144,
0.982) in the CMDs (square dot in Figures 10, 11). Its
GaiaDR2 distance d = 1370 ± 80 pc (̟ = 0.732 ±
0.041 mas) is not close to the photometric distance, 1000
pc of Table 8.
From TO and RC/RG sequences on the CMDs of
NGC 2539, for BD-12 2380 RC/RG candiate, we have
determined its morphological age index (MAI). For this,
δV and δ1 indices given by Phelps et al. (1994) are
determined within its CMDs. Here, δV is the magnitude difference between the TO and RC stars, δV =
VTO − VRC , and δ1 is the difference in the colour indices
between the bluest point on the main sequence at the
luminosity of the TO and the colour at the base of the
RG branch, one magnitude brighter than the TO luminosity δ1 = (B − V )TO − (B–V )RG . We measured
the values of δ1 from its RC candidate, and then
converted into δV by means of the equation of δV =
3.77–3.75δ1 of Phelps et al. (1994). The photometric
values obtained are listed at the bottom of Table 11.
With the aid of its photometric metal abundance, its
morphological age is determined from the equation,
33
Page 16 of 18
log A = 0.04δV 2 + 0.34δV + 0.07[Fe/H] + 8.76 of
Salaris et al. (2004). The estimated MAI age is listed in
the bottom line of Table 11 (columns 7–8) together with
its uncertainty. The age difference between the MAI and
isochrone techniques is log A = 0.09 (A = 0.20
Gyr). These small differences indicate a good consistency between the techniques.
The GaiaDR2 parallaxes/distances plus UBV photometry of the six definite members which belong to
NGC 2539, according to SIMBAD database, are listed
in the bottom rows of Table 10. The differences fall in
the range of d = 200–456 pc between the 1000 pc
value of all colours (Table 8) and the ones of GaiaDR2
(column 7; Table 10). In a sense, the distances of these
six definite members are not close to the photometric
distances of NGC 2539 of the four-colour indices within
the uncertainties.
6. Discussions and conclusions
The reddenings of E(B − V ) = 0.23 ± 0.04 of
NGC 2323 and E(B − V ) = 0.02 ± 0.06 of NGC 2539
are quite consistent with the ones in the literature
(Table 9). The reddening differences with the literature
are at a level of 0.00–0.05 for NGC 2323 and 0.04–0.08
for NGC 2539. For the (B − V ) colour, the distance
moduli and distances of NGC 2323 and NGC 2539 are
(V0 − MV , d(pc)) = (10.00 ± 0.10, 1000 ± 50) and
(V0 − MV , d(pc)) = (10.00±0.04, 1000±20), respectively. The differences with the literature, derived using
the (B − V ) colour are at a level of (V0 − MV ) =
0.00–0.46 mag and d(kpc) = 0.00 − 0.11 for NGC
2323. These differences for NGC 2539 are at a level of
(V0 − MV ) = 0.10–0.50 mag and d(kpc) = 0.05–
0.29. In a sense, our detections are in good concordance
with the values in the literature within the uncertainties
(Table 9). For the four-colour indices (Tables 7 and 8),
the values of (V0 − MV , d (kpc)) are 9.80–10.00 and
0.91–1.00 kpc for NGC 2323, and 10.00 and 1.00 kpc
for NGC 2539, respectively, which are in agreement
with each other and the literature values (Table 11).
The median GaiaDR2 parallax (̟ = 0.998 ± 0.136
mas) of NGC 2323 (74 likely members) provides d =
1000±140 pc. This value is also similar to the photometric ones (d = 910−1000 pc) (Table 7), and also in good
agreement with the distances of the literature (Table 9).
For NGC 2539 (N = 67 likely members), the median
GaiaDR2 parallax ̟ = 0.751 ± 0.139 mas measures
d = 1330 ± 250 pc, which is reasonably close to our
1000 pc value (Table 8) within the uncertainties. For
NGC 2539, the literature values are not as close to each
J. Astrophys. Astr.
(2019) 40:33
other: ̟ = 0.91 mas (d = 1.10 kpc) and ̟ = 0.78
mas (d = 1.29 kpc). Our median parallaxes are similar to the values 0.997 ± 0.057 mas (NGC 2323) and
0.754 ± 0.064 mas (NGC 2539) of Cantat-Gaudin et al.
(2018) (Table 9).
Our age value (200 ± 50 Myr) of NGC 2323 is somewhat older than the age range of 100–140 Myr in the
literature (Table 9). This occurs because we estimated
Z = 0.012 from the ultraviolet excess (Figure 6),
instead of assuming the solar metal abundance. Likewise, our estimated age 890 ± 110 Myr for NGC 2539
is somewhat older than the age range 540–640 Myr of
the literature (Table 9). The works of Joshi and Sagar
(1986), Claria & Lapasset (1986), Choo et al. (2003)
and Lapasset et al. (2000) use different isochrones
(column 8 of Table 9), and the differences in ages
with respect to our values result from their usage of
solar/different Z abundances with these isochrones.
This work uses the Z = 0.007 abundance, which has
been obtained from the δ(U − B)0.6 value of Figure 7. The morphological age of NGC 2539 (the MAI
method) of the one probable RC star (Table 11) has been
determined as 1.09 ± 0.27 Gyr. This MAI age is quite
compatible with its isochrone age (0.89 Gyr; Table 8).
The giant star, BD-12 2380 appears to be a definite
member of NGC 2539 (square dot in Figures 10–11)
since the distances of d I = 950 ± 50 pc and dV =
910 ± 90 pc for V I filters (Table 9) are in harmony
with the photometric distance 1000 ± 20 pc (Table 8)
within the uncertainties. However, its GaiaDR2 distance
d = 1200±70 pc (̟ = 0.833±0.047) mas is not close
to the photometric distance of NGC 2539. The other
giant candidate’s GaiaDR2 distance is d = 1370 ± 80
pc (̟ = 0.732±0.041 mas), but is not close to the photometric distance of NGC 2539. In a sense, this may be a
field giant. For further confirmation of its membership,
the spectroscopic observations are required.
Acknowledgements
The observations of this publication were made at the
National Astronomical Observatory, San Pedro Mártir, Baja California, México, and the authors wish to
thank the staff of the Observatory for their assistance
during these observations. They thank H. Cakmak for
useful help on the manuscript. This research made use
of the WEBDA open-cluster database of J.-C. Mermilliod and also the SIMBAD database. This work has
been supported by the CONACyT (México) projects
33940, 45014 and 49434, and PAPIIT-UNAM (México)
projects IN111500 and IN103014. This paper has made
J. Astrophys. Astr.
(2019) 40:33
use of the results from the European Space Agency
(ESA) space mission Gaia, the data from which were
processed by the Gaia Data Processing and Analysis
Consortium (DPAC). Funding for the DPAC has been
provided by the national institutions, in particular, the
institutions participating in the Gaia Multilateral Agreement. The Gaia mission website is http://www.cosmos.
esa.int/gaia.
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