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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