Distant Open Clusters Research Articles

Stellar clustering and the kinematics of stars around Collinder 121 using Gaia DR3

ABSTRACT We study the region around Collinder 121 (Cr 121) using newly available 6D data from the Gaia DR3 catalogue. Situated in the third quadrant, near the Galactic plane, Collinder 121 lies in the region of Canis Major centred around (l, b) ≈(236°, −10°). Previous studies have suggested that the stellar associations in this region comprise an OB association (CMa OB2) lying at about 740 pc with a more distant open cluster (Cr 121) at approximately 1170 pc. Despite these studies, the precise nature of Collinder 121 remains uncertain. This study investigates the region bounded by the box (l, b) = (225°–245°, 0.00° to −20.00°) to a depth of 700 pc from 500 to 1200 pc which fully encompasses the region discussed in the literature. Using Gaia DR3 data, we do not find associations at the distances given in the literature. Instead, using the hdbscan machine learning algorithm, we find a major association of OB stars centred around 803 pc. Within this association we find four smaller subgroups that may be indicative of a larger association and which are located at a mean distance of 827 pc. Proper motion studies find coherence between these four subgroups and show a distinctive east-to-west increase in the size of the velocity vectors which supports contemporary studies that show similar trends in OB populations in Cygnus and within the Carina spiral Arm. Therefore, we hypothesize that Cr 121 and CMa OB2 are the same cluster, consistent with the 1977 study by Hoogerwerf.

An analysis of the most distant cataloged open clusters

Context. Several studies have been presented in the last few years applying some kind of automatic processing of data to estimate the fundamental parameters of open clusters. These parameters are then employed in larger scale analyses, for example the structure of the Galaxy’s spiral arms. The distance is one of the most straightforward parameters to estimate, yet enormous differences can still be found among published data. This is particularly true for open clusters located more than a few kiloparsecs away. Aims. We cross-matched several published catalogs and selected the 25 most distant open clusters (> 9000 pc). We then performed a detailed analysis of their fundamental parameters, with emphasis on their distances, to determine the agreement between the catalogs and our estimates. Methods. Photometric and astrometric data from the Gaia EDR3 survey was employed. The data were processed with our own membership analysis code, pyUPMASK, and our package for the automatic estimation of fundamental cluster parameters, ASteCA. Results. We find differences in the estimated distances of up to several kiloparsecs between our results and those cataloged, even for the catalogs that show the best matches with ASteCA values. Large differences are also found for the age estimates. As a by-product of the analysis we find that vd Bergh-Hagen 176 could be the open cluster with the largest heliocentric distance cataloged to date. Conclusions. Caution is thus strongly recommended when using cataloged parameters of open clusters to infer large-scale properties of the Galaxy, particularly for those located more than a few kiloparsecs away.

Exploring open cluster properties with Gaia and LAMOST

Context. In Gaia DR2, an unprecedented high level of precision has been reached at sub-milliarcsecond for astrometry and millimagnitudes for photometry. Using cluster members identified with the astrometry and photometry in Gaia DR2, we can obtain a reliable determination of cluster properties. However, because of the shortcomings of Gaia spectroscopic observations in dealing with densely crowded cluster regions, the RVs and metallicity values for cluster member stars from Gaia DR2 are still lacking. It is necessary to combine the Gaia data with the data from large spectroscopic surveys, such as LAMOST, APOGEE, GALAH, and Gaia-ESO. Aims. In this study our aim is to improve the cluster properties by combining the LAMOST spectra. In particular, we provide the list of cluster members with spectroscopic parameters as an add-value catalog in LAMOST DR5, which can be used to perform a detailed study for a better understanding of the stellar properties, by using their spectra and fundamental properties from the host cluster. Methods. We cross-matched the spectroscopic catalog in LAMOST DR5 with the identified cluster members in Cantat-Gaudin et al. (2018, A&A, 618, A93). We then used members with spectroscopic parameters to derive statistical properties of open clusters. Results. We obtained a list of 8811 members with spectroscopic parameters and a catalog of 295 cluster properties. The provided cluster properties include astrometric parameters, spectroscopic parameters, derived kinematic and orbital parameters, and isochrone fitting results. In addition, we study the radial and vertical metallicity gradient and age-metallicity relation with the compiled open clusters as tracers, finding slopes of −0.053 ± 0.004 dex kpc−1, −0.252 ± 0.039 dex kpc−1, and 0.022 ± 0.008 dex Gyr−1, respectively. The slopes of the metallicity distribution relation for young clusters (0.1 Gyr < Age < 2 Gyr) and the age-metallicity relation for clusters within 6 Gyr are both consistent with the literature results. In order to fully study the chemical evolution history in the disk, more spectroscopic observations for old and distant open clusters are needed for further investigation.

Photometric studies of two W UMa type variables in the field of distant open cluster NGC 6866

We present photometric analysis of the two W UMa type binaries identified in the field of distant open star cluster NGC 6866. Although these systems, namely ID487 and ID494, were reported by Joshi et al., a detailed study of these stars has not been carried out before. The orbital periods of these stars are found to be 0.415110±0.000001 day and 0.366709±0.000004 day, respectively. Based on the photometric and infrared colors, we find their respective spectral types to be K0 and K3. The photometric light variations of both stars show the O'Connell effect which can be explained by employing a dark spot on the secondary components. The V and I band light curves are analyzed using the Wilson-Devinney (WD) code and relations given by Gazeas which yield radii and masses for the binary components of star ID487 of R1 = 1.24 ± 0.01 R⊙, R2 = 1.11 ± 0.02 R⊙, and M1 = 1.24 ± 0.02 M⊙, M2 = 0.96 ± 0.05 M⊙ and for star ID494 of R1 = 1.22±0.02R⊙, R2 = 0.81±0.01 R⊙, and M1 = 1.20±0.06 M⊙, M2 = 0.47±0.01 M⊙.

On the subject of the Ba overabundance in the open clusters stars

For eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583, we determined the yttrium and barium abundances using the UVES, VLT spectra (ESO, Chile). The stars of one young cluster (Ruprecht 7) demonstrate significant barium overabundance(∼0.55 dex) that can not be due to the determination error. We have considered the Ba abundance determination errors due to LTE approach, saturation of the lines, synthetic and observed barium line fitting, and the causes of the Ba overabundance associated with the Galactic disc enrichment or the origin of open clusters. Possible explanation for this overabundance can be the origin of n-capture elements enrichment of the clusters (galactic or extragalactic) or additional sources of the Ba production.

Barium and yttrium abundance in intermediate-age and old open clusters★

Barium is a neutron capture element, that, in open clusters, is frequently over-abundant with respect to the Iron. A clear explanation for this is still missing. Additionally, its gradient across the Galactic disk is poorly constrained. We measure the abundance of yttrium and barium using the synthetic spectrum method from UVES high-resolution spectra of eight distant open clusters, namely Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, Berkeley 75, NGC 6192, NGC 6404, and NGC 6583. The barium abundance was estimated using NLTE approximation. We confirm that Barium is indeed over-abundant in most clusters, especially young clusters. Finally, we investigated the trend of yttrium and barium abundances as a function of distance in the Galaxy and ages. Several scenarios for the barium over-abundance are then discussed.

Photometry and Classification of Stars in the Direction of Clusters NGC 7129 and NGC 7142 in Cepheus. I. Magnitudes, Color Indices and Spectral Types of 2140 Stars

Abstract In a 1.5 square degree area, centered at RA = 21:44:30, DEC = +65:58, we have determined magnitudes and color indices for 2140 stars in the Vilnius seven-color system down to V = 17 mag. This area includes a star-forming region with the open cluster NGC 7129 embedded in a dense dust cloud TGU 645 (LDN 1181), close to the Cepheus Flare, and a distant old open cluster NGC 7142 seen through a semi-transparent window. For most of the stars spectral and luminosity classes in the MK system, determined from the photometric data, are given.

ELEMENTAL ABUNDANCE RATIOS IN STARS OF THE OUTER GALACTIC DISK. IV. A NEW SAMPLE OF OPEN CLUSTERS

We present radial velocities and chemical abundances for nine stars in the old, distant open clusters Be 18, Be 21, Be 22, Be 32, and PWM 4. For Be 18 and PWM 4, these are the first chemical abundance measurements. Combining our data with literature results produces a compilation of some 68 chemical abundance measurements in 49 unique clusters. For this combined sample, we study the chemical abundances of open clusters as a function of distance, age, and metallicity. We confirm that the metallicity gradient in the outer disk is flatter than the gradient in the vicinity of the solar neighborhood. We also confirm that the open clusters in the outer disk are metal-poor with enhancements in the ratios [alpha/Fe] and perhaps [Eu/Fe]. All elements show negligible or small trends between [X/Fe] and distance (< 0.02 dex/kpc), but for some elements, there is a hint that the local (RGC < 13 kpc) and distant (RGC > 13 kpc) samples may have different trends with distance. There is no evidence for significant abundance trends versus age (< 0.04 dex/Gyr). We measure the linear relation between [X/Fe] and metallicity, [Fe/H], and find that the scatter about the mean trend is comparable to the measurement uncertainties. Comparison with solar neighborhood field giants shows that the open clusters share similar abundance ratios [X/Fe] at a given metallicity. While the flattening of the metallicity gradient and enhanced [alpha/Fe] ratios in the outer disk suggest a different chemical enrichment history to the solar neighborhood, we echo the sentiments expressed by Friel et al. that definitive conclusions await homogeneous analyses of larger samples of stars in larger numbers of clusters. Arguably, our understanding of the evolution of the outer disk from open clusters is currently limited by systematic abundance differences between various studies.

On the metallicity of open clusters

Metallicity is one of four free parameters typically considered when fitting isochrones to the cluster sequence. Unfortunately, this parameter is often ignored or assumed to be solar in most papers. Hence an unknown bias is introduced in the estimation of the other three cluster parameters (age, reddening and distance). Furthermore, studying the metallicity of open clusters allows us not only to derive the Galactic abundance gradient on a global scale, but also to trace the local solar environment in more detail. In a series of three papers, we investigate the current status of published metallicities for open clusters from widely different photometric and spectroscopic methods. A detailed comparison of the results allows us to establish more reliable photometric calibrations and corrections for isochrone fitting techniques. Well established databases such as WEBDA help us to perform a homogeneous analysis of available measurements for a significant number of open clusters. The literature was searched for [Fe/H] estimates on the basis of photometric calibrations in any available filter system. On the basis of results published by Tadross, we demonstrate the caveats of the calibration choice and its possible impact. In total, we find 406 individual metallicity values for 188 open clusters within 64 publications. The values were, finally, unweightedly averaged. Our final sample includes [Fe/H] values for 188 open clusters. Tracing the solar environment within 4000x4000 pc**2 we identify a patchy metallicity distribution as an extension to the Local Bubble that significantly influences the estimation of the Galactic metallicity gradient, even on a global scale. In addition, further investigations of more distant open clusters are clearly needed to obtain a more profound picture at Galactocentric distances beyond 10 000 pc.

The metallicity of the open cluster Tombaugh 2

We investigate the nature of the chemical composition of the outer disc open cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested to possess an intrinsic metal abundance dispersion. We aim to investigate such claims by high resolution spectra obtained for a number of stars in the Tombaugh 2 field, together with independent UBVIc photometry. The spectra, together with input atmospheric parameters and model atmospheres, are used to determine detailed chemical abundances for a variety of elements in 13 members having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with no evidence for an intrinsic abundance dispersion, in contrary to the recent results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced while Ni and Sc are solar. The r-process element Eu was found to be enhanced, giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the upper giant branch and maintains a low level for red clump stars. The mean metallicity we derive is in good agreement with that expected from the radial abundance gradient in the disc for a cluster at its Galactocentric distance. The surprising result found by Frinchaboy et al. (2008), that is the presence of 2 distinct abundance groups within the cluster, implying either a completely unique open cluster with an intrinsic metallicity spread, or a very unlikely superposition of a cold stellar stream and a very distant open cluster, is not supported by our new result.

Multi-Conjugate Adaptive Optics VLT imaging of the distant old open cluster FSR 1415

We employ the recently installed near-infrared Multi-Conjugate Adaptive Optics demonstrator (MAD) to determine the basic properties of a newly identified, old and distant, Galactic open cluster (FSR 1415). The MAD facility remarkably approaches the diffraction limit, reaching a resolution of 0.07 arcsec (in K), that is also uniform in a field of ∼1.8 arcmin in diameter. The MAD facility provides photometry that is 50 per cent complete at K ∼ 19. This corresponds to about 2.5 mag below the cluster main-sequence turn-off. This high-quality data set allows us to derive an accurate heliocentric distance of 8.6 kpc, a metallicity close to solar and an age of ∼2.5 Gyr. On the other hand, the deepness of the data allows us to reconstruct (completeness-corrected) mass functions (MFs) indicating a relatively massive cluster, with a flat core MF. The Very Large Telescope/MAD capabilities will therefore provide fundamental data for identifying/analysing other faint and distant open clusters in the Galaxy III and IV quadrants.

Investigating the age and structure of the infrared old open clusters LK 1, LK 10, FSR 1521 and FSR 1555

The combination of several mass-decreasing processes may critically affect the structure of open clusters (OCs), to the point that most dissolve into the field in a time-scale shorter than ≈1 Gyr. Therefore, as is observed, old OCs should be sparse within the Galaxy. These mass-decreasing processes can only be quantifiably examined given the derivation of the fundamental parameters, age, distance and mass, for a sample of old OCs. In this paper we focus on four candidate old star clusters, namely, LK 1, LK 10, FSR 1521 and FSR 1555. The first two clusters, discovered by Le Duigou & Knodlseder, are projected towards the Cygnus association, whilst the remaining two have been detected in the fourth quadrant as cluster candidates from stellar overdensities by Froebrich, Scholz & Raftery. To analyse the target clusters we construct near-infrared colour-magnitude diagrams (CMDs) and derive stellar radial density profiles (RDPs). The CMDs are constructed using Two Micron All-Sky Survey (2MASS) J, H and K s bands, and the intrinsic morphologies of the target OCs within these diagrams are revealed by applying a field-star decontamination algorithm. Fundamental parameters are estimated with Padova isochrones built for the 2MASS filters. We derive extinctions to the objects within the range 3.4 < A V ≤ 8.9, which makes them suitable for the near-infrared analysis, ages within 1.0-2.0 Gyr, and distances from the Sun within 1.4-4.5 kpc. These distances, in conjunction with the positions in the sky, place the present four OCs close to the solar circle (≤0.6 kpc). For LK 10 our photometry reaches a depth ≈3mag below the main-sequence turn off, from which we derive a relatively steep mass function slope (x = 2.4 ± 0.4) when compared to the Salpeter value (x = 1.35). LK 10 is a rather massive old OC, with a mass within 1360 < m (M ⊙ ) ≤ 4400, for stars in the observed magnitude range and the extrapolation to 0.08 M ⊙ , respectively. The mass estimated in the restricted magnitude range for the remaining, more distant OCs is within 260 ≤ m (M ⊙ ) < 380. However, similarity with the CMD morphology and red clump of LK 10 suggests that they may be as massive as LK 10. The RDPs are well represented by a King-like function, except LK 10, which has a central cusp suggesting post-core collapse. Structurally, LK 1, FSR 1521 and FSR 1555 are similar to a sample of nearby OCs of comparable age.

Old open clusters in the outer Galactic disk

Context. The outer parts of the Milky Way disk are believed to be one of the main arenas where the accretion of external material in the form of dwarf galaxies and subsequent formation of streams is taking place. The Monoceros stream and the Canis Major and Argo over-densities are notorious examples. Understanding whether what we detect is the signature of accretion or, more conservatively, simply the intrinsic nature of the disk, represents one of the major goals of modern Galactic astronomy. Aims. We try to shed more light on the properties of the outer disk by exploring the properties of distant anti-center old open clusters. We want to verify whether distant clusters follow the chemical and dynamical behavior of the solar vicinity disk, or whether their properties can be better explained in terms of an extra-galactic population. Methods. VLT high resolution spectra have been acquired for five distant open clusters: Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73 and Berkeley 75. We derive accurate radial velocities to distinguish field interlopers and cluster members. For the latter we perform a detailed abundance analysis and derive the iron abundance [Fe/H] and the abundance ratios of several α elements. Results. Our analysis confirms previous indications that the radial abundance gradient in the outer Galactic disk does not follow the expectations extrapolated from the solar vicinity, but exhibits a shallower slope. By combining the metallicity of the five program clusters with eight more clusters for which high resolution spectroscopy is available, we find that the mean metallicity in the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H] ≈− 0.35, with only marginal indications for a radial variation. In addition, all the program clusters exhibit solar scaled or slightly enhanced α elements, similar to open clusters in the solar vicinity and thin disk stars. Conclusions. We investigate whether this outer disk cluster sample might belong to an extra-galactic population, like the Monoceros ring. However, close scrutiny of their properties – location, kinematics and chemistry – does not convincingly favor this hypothesis. On the contrary, they appear more likely genuine Galactic disk clusters. We finally stress the importance to obtain proper motion measurements for these clusters to constrain their orbits.

Photometric study of distant open clusters in the second quadrant: NGC 7245, King 9, King 13 and IC 166

We present a UBV CCD photometric study of four open clusters, NGC 7245, King 9, IC 166 and King 13, located between $l = 90^o - 135^o$. All are embedded in a rich galactic field. NGC 7245 and King 9 are close together in the sky and have similar reddenings. The distances and ages are: NGC 7245, 3.8$\pm$0.35 kpc and 400 Myr; King 9 (the most distant cluster in this quadrant) 7.9$\pm$1.1 kpc and 3.0 Gyr. King 13 is 3.1$\pm$0.3 kpc distant and 300 Myr old. King 9 and IC 166 (4.8$\pm$0.5 kpc distant & 1 Gyr old) may be metal poor clusters (Z=0.008), as estimated from isochrone fitting. The average value of the distance of young clusters from the galactic plane in the above longitude range and beyond 2 kpc ($-47\pm$16 pc, for 64 clusters), indicates that the young disk bends towards the southern latitudes.

Abundance gradients in the Milky Way for α elements, iron peak elements, barium, lanthanum, and europium

We model the abundance gradients in the disk of the Milky Way for several chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba, La and Eu), and compare our results with the most recent and homogeneous observational data. We adopt a chemical evolution model able to well reproduce the main properties of the solar vicinity. The model assumes that the disk formed inside-out with a timescale for the formation of the thin disk of 7 Gyr in the solar vicinity, whereas the halo formed on a timescale of 0.8 Gyr. We adopt new empirical stellar yields derived to best fit the abundances and the abundance ratios in the solar vicinity. We compute, for the first time, the abundance gradients for all the mentioned elements in the galactocentric distance range 4-22 kpc. Comparison with the observed data on Cepheids in the galactocentric distance range 5-17 kpc gives a very good agreement for many of the studied elements. In addition, we well fit the data for the evolution of lanthanum in the solar vicinity, for which we present results here for the first time. We explore, also for the first time, the behavior of the abundance gradients at large galactocentric distances by comparing our results with data relative to distant open clusters and red giants and select the best chemical evolution model on that basis. We find a very good fit to the observed abundance gradients, as traced by Cepheids, for most of the elements, thus confirming the validity of the inside-out scenario for the formation of the Milky Way disk as well as the adopted nucleosynthesis prescriptions. The flat gradients at large galactocentric distances (>12 kpc), as traced by the Cepheids, open cluster and red giant data, lead us to conclude that a model where the density of the halo stellar component is constant in the«inner 20 kpc should be preferred. Other models with different distributions of the halo stellar mass do not produce a good fit of the data.

The San Pedro Mártir Open Cluster Survey: Progress, Techniques, Preliminary Results

AbstractA CCD UBVRI survey of northern open clusters is being undertaken at San Pedro Mártir, México, and performed using always the same instrumental setup (telescope, CCD, filters), reduction methods, and system of standards of Landolt (1992). To date more than 300 clusters (mostly unstudied previously) have been observed, and about half the data reduced using aperture-photometry and PSF techniques. Our analysis procedures are being refined by studying in detail a small subset of these clusters. For example, the heavily reddened clusters Be80 and Be95 are being examined in the color-color diagrams: (B-V, U-B) and (B-V, R-I) to better understand the problems of curvature and variable reddening. For clusters for which our U data reaches the F-type stars, such as NGC2192 and NGC7296, techniques are being examined for estimating both the reddening E(B-V) and metallicity [Fe/H] via the use of the (U-B) excess. If the clusters also have “red clump” stars, such as NGC1798 and Do02, these procedures can be iterated between the clump and main sequence stars to establish even better the values of E(B-V) and [Fe/H]. Finally, color-magnitude diagrams, such as (B-V, V) and (V-I, V), are being employed together with the SchmidtKaler (1982) colors and Padova isochrones of Girardi et al. (2000) to obtain distances and ages for these clusters. A java-based computer program is being developed to help in the visualization and analysis of these photometric data. This system is capable of displaying each cluster simultaneously in different color-color and color-magnitude diagrams and has an interactive way to identify a star, or group of stars, in one diagram and to see were it falls in the other diagrams, facilitating the elimination of field stars and the apperception of cluster features. This program is capable of displaying up to 16 different diagrams for one cluster and processing up to 20 clusters at the same time. Our aims are the following: (1) a common UBVRI photometric scale for open clusters, (2) an atlas of color-color and color-magnitude diagrams for open clusters, (3) a homogeneous set of cluster reddenings, distances, and ages, (4) an increased number of old and distant open clusters, and (5) a selection of interesting clusters for further study.

Photometry and Spectroscopy of Old, Outer Disk Star Clusters: vdB-Hagen 176, Berkeley 29, and Saurer 1

It has been previously proposed that some distant open clusters in the Milky Way may have been accreted during a dwarf galaxy merger, perhaps associated with the same event that led to the formation of the Galactic anticenter stellar structure (GASS), also known as the Monoceros Ring. We have obtained VI and Washington + DDO51 photometric and medium-resolution (R ~ 8000) multifiber spectroscopic data for the three distant old open clusters Berkeley 29, Saurer 1, and vdB-Hagen 176 (BH?176). These clusters are spatially coincident with GASS, but radial velocities and spectroscopic metallicities were not available during previous studies of the GASS candidate cluster system. Similar data for the clusters Be 20 and Be 39 have been obtained for calibration purposes. We provide the first reliable radial velocity for BH?176 (Vhelio = 11.2 ? 5.3 km s-1). We also find that Vhelio = +95.4 ? 3.6 and +28.4 ? 3.6 km s-1 for Sa 1(A) and Be 29, respectively. In contrast to previous findings, we show that ?-enhanced isochrones, while spectroscopically motivated, provide a poor fit to the Be 29 color-magnitude diagram. We find that the clusters Be 29 and Sa 1 have properties that are consistent with the previously reported characteristics for GASS candidate clusters and the GASS stellar stream as derived from M giant observations. However, the radial velocity and photometric metallicity ([Fe/H] ~ 0.0 dex) for BH?176 suggest that a connection of this cluster with the putative GASS cluster system is unlikely. We reassess the age-metallicity relation for the most likely members of the GASS clusters system for which spectroscopic metallicities are now available.

Abundance Ratios in Open Clusters and Field Giants of the Outer Galactic Disk

We present elemental abundance ratios for bright red giants in the old, distant open clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31 as well as for distant field red giants in the direction of the southern Galactic warp. At Galactocentric distances beyond 10 to 12 kpc, the metallicity gradient disappears. The open cluster and field red giants of the outer disk exhibit an almost constant iron abundance [Fe/H] = −0.5. We also measured the abundances for 15 other elements and found very similar compositions in the field and cluster stars. Specifically, they all show enhancements of the α elements O, Mg, Si, Ca, and Ti as well as the r-process element Eu. These abundance patterns reveal a rapid star formation history for the outer Galactic disk. A possible explanation for the measured compositions is that the outer disk formed as a result of star formation triggered by a series of merger or accretion events.

Berkeley 22, an old and distant open cluster towards the Galactic anticentre

We present deep CCD BVI photometry of the distant, old open cluster Berkeley 22, covering from the red giants branch (RGB) to about 6 magnitudes below the main sequence (MS) turn-off. Using the synthetic Colour - Magnitude Diagram method with three different types of stellar evolutionary tracks, we estimate values and theoretical uncertainty of distance modulus mod0, reddening E(B-V), age tau and approximate metallicity. The best fit to the data is obtained for 13.8 <= mod0 <= 14.1, 0.64 <= E(B-V) <= 0.65, 2 <= tau <= 2.5 Gyr (depending on the amount of overshooting from convective regions adopted in the stellar models) and solar metallicity.

The relatively young, metal-poor and distant open cluster NGC 2324

We have obtained CCD photometry in the Johnson V, Kron-Cousins I and CT1 Washington systems for NGC 2324, a rich open cluster located ∼35 ◦ from the Galactic anticentre direction. We measured V magnitudes and V − I colours for 2865 stars and T1 magnitudes and C − T1 colours for 1815 stars in an area of 13. 6 × 13. 6. The comparison of the cluster colour-magnitude diagrams with isochrones of the Geneva group yield E(V − I) = 0.33 ± 0.07 and V − MV = 13.70 ± 0.15 for log t = 8.65 (t = 440 Myr) and Z = 0.008 ((Fe/H) = −0.40), and E(C − T1) = 0.40 ± 0.10 and T1 − MT 1 = 13.65 ± 0.15 for the same age and metallicity level. The resulting E(V − I) reddening value implies E(B − V) = 0.25 ± 0.05 and a distance from the Sun of (3.8 ± 0.5) kpc. Star counts carried out within and outside the cluster region allowed us to estimate the cluster angular radius as 5. 3 ± 0. 3 (5.9 pc). When using the E(B − V) reddening value here derived and the original Washington photometric data of Geisler et al. (1991) for the stars confirmed as red cluster giants from Coravel radial velocities, we found (Fe/H) = −0.31 ± 0.04, which is in good agreement with the best fits of isochrones. Therefore, NGC 2324 is found to be a relatively young, metal-poor and distant open cluster located beyond the Perseus spiral arm. A comparison of NGC 2324 with 10 well-known open clusters of nearly the same age shows that the cluster metal abundance and its position in the Galaxy are consistent with the existence of a radial abundance gradient of −0.07 dex kpc −1 in the Galactic disc.