Cluster Fundamental Parameters Research Articles

Binary Fraction in Galactic Star Clusters: FSR 866, NGC 1960, and STOCK 2

The study of binary stars in different astronomical environments offers insights into the dynamical state of the hosting stellar systems. The binary fraction in fact plays a crucial role in the dynamical evolution of stellar systems, regulating processes like mass segregation and dynamical heating, and in some cases leading to the formation of exotic objects, like, for instance, blue straggler stars. We used two methodologies to estimate the binary fraction in three different-age open star clusters: FSR 866, NGC 1960 (M36), and Stock 2. The first is a photometric approach based on color–magnitude diagram analysis, and the second is a spectroscopic technique that employs radial velocity measurements. We used Gaia DR3 data in tandem with new spectroscopic observations, and employed the Density-Based Spatial Clustering of Applications with Noise algorithm to identify probable cluster members based on proper motion and parallax in 3D space. The new sample of cluster members allows us to provide new estimates of the cluster fundamental parameters. As a by-product, we found two previously undetected, small physical groups of stars in the background of NGC 1960. The resulting binary fractions lie in the range 0.3–0.5 and are in good agreement with those expected theoretically for open clusters.

Small Magellanic Cloud Field Stars Meddling in Star Cluster Age Estimates

I revisited the age of the Small Magellanic Cloud (SMC) cluster HW 42, whose previous estimates differ in more than 6 Gyr, thus challenging the most updated knowledge of the SMC star formation history. I performed an analysis of number stellar density profiles at different brightness levels; carried out a field star decontamination of the cluster color–magnitude diagram; and estimated the cluster fundamental parameters from the minimization of likelihood functions and their uncertainties from standard bootstrap methods. I conclude that HW 42 is a Gyr old ([Fe/H] = −0.89 dex) SMC cluster projected on to a SMC composite star field population which shows variations in magnitude, color, and stellar density of Main Sequence stars. The present outcome solves the conundrum of the previous age discrepancies and moves HW 42 to a region in the SMC age–metallicity relationship populated by star clusters.

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.

Radial Velocity and Chemical Composition of Evolved Stars in the Open Clusters NGC 6940 and Tombaugh 5

Abstract We present and discuss medium resolution (R ∼ 13000), high signal-to-noise ( ), spectroscopic observations in the field of the open clusters NGC 6940 and Tombaugh 5. Spectra were recorded for seven candidate red giant stars in both clusters. For the latter, we present the very first chemical abundance analysis. We derive radial velocities for all the stars in NGC 6940, confirming membership to the cluster for all of them, while on the same ground, we exclude two stars in To 5. We perform a chemical abundance analysis of different atomic species, in particular Fe i, Si i, Ca i, Ti i, and Ni i. The mean metallicity of NGC 6940 is [Fe/H] = +0.09 ± 0.06 dex, in good agreement with previous works, while for To 5 is [Fe/H] = +0.06 ± 0.11 dex. Therefore, both clusters exhibit a chemical composition close to the solar value and do not deviate from the [Fe/H] Galactic radial abundance gradient. With these new values, we estimate the fundamental cluster parameters, after having derived clusters’ distances from the Gaia DR2 database. By adopting these distances, we derive updated estimated for the clusters ages: 1.0 ± 0.1 Gyr of NGC 6940 and 0.25 ± 0.05 Gyr for Tombaugh 5.

Astrophysical properties of star clusters in the Magellanic Clouds homogeneously estimated by ASteCA

Aims. To produce an homogeneous catalog of astrophysical parameters of 239 resolved star clusters located in the Small and Large Magellanic Clouds, observed in the Washington photometric system. Methods. The cluster sample was processed with the recently introduced Automated Stellar Cluster Analysis ($\texttt{ASteCA}$) package, which ensures both an automatized and a fully reproducible treatment, together with a statistically based analysis of their fundamental parameters and associated uncertainties. The fundamental parameters determined with this tool for each cluster, via a color-magnitude diagram (CMD) analysis, are: metallicity, age, reddening, distance modulus, and total mass. Results. We generated an homogeneous catalog of structural and fundamental parameters for the studied cluster sample, and performed a detailed internal error analysis along with a thorough comparison with values taken from twenty-six published articles. We studied the distribution of cluster fundamental parameters in both Clouds, and obtained their age-metallicity relationships. Conclusions. The $\texttt{ASteCA}$ package can be applied to an unsupervised determination of fundamental cluster parameters; a task of increasing relevance as more data becomes available through upcoming surveys.

Reanalysis of 24 Nearby Open Clusters using Gaia data

AbstractWe have developed a fully automated cluster characterization pipeline, which simultaneously determines cluster membership and fits the fundamental cluster parameters: distance, reddening, and age. We present results for 24 established clusters and compare them to literature values. Given the large amount of stellar data for clusters available from Gaia DR2 in 2018, this pipeline will be beneficial to analyzing the parameters of open clusters in our Galaxy.

The OPD photometric survey of open clusters II. robust determination of the fundamental parameters of 24 open clusters

In the second paper of the series we continue the investigation of open cluster fundamental parameters using a robust global optimization method to fit model isochrones to photometric data. We present optical UBVRI CCD photometry (Johnsons-Cousins system) observations for 24 neglected open clusters, of which 14 have high quality data in the visible obtained for the first time, as a part of our ongoing survey being carried out in the 0.6 m telescope of the Pico dos Dias Observatory in Brazil. All objects were then analyzed with a global optimization tool developed by our group which estimates the membership likelihood of the observed stars and fits an isochrone from which a distance, age, reddening, total to selective extinction ratio RV (included in this work as a new free parameter) and metallicity are estimated. Based on those estimates and their associated errors we analyzed the status of each object as real clusters or not, finding that two are likely to be asterisms. We also identify important discrepancies between our results and previous ones obtained in the literature which were determined using 2MASS photometry.

The OPD photometric survey of open clusters I. Techniques, program details and first results of robust determination of the fundamental parameters

Open clusters are considered valuable objects for the investigation of galactic structure and dynamics since their distances, ages and velocities can be determined with good precision. According to the New Catalog of Optically Visible Open Clusters and Candidates (Dias et al., 2002) about 10% of the optically revealed open clusters remain unstudied. However, previous analysis (Moitinho, 2010) has indicated that not considering this unstudied population introduces significant biases in the study of the structure and evolution of the Milky Way. In addition, a systematic revision of the data contained in the catalog, collected from the literature, is needed, due to its inhomogeneity.In this first paper of a series, we present the observational strategy, data reduction and analysis procedures of a UBRVI photometric survey of southern open star clusters carried out at Pico dos Dias Observatory (Brazil). The aim of the program is to contribute to an unbiased, homogenous collection of cluster fundamental parameters. We show that the implementation of a sequence of systematic procedures considerably improves the quality of the results.To illustrate the methods we present the first results based on one night of observations. The parameters, reddening, distance, age and metallicity, were obtained by fitting theoretical isochrones to cluster color–color and multidimensional color–magnitude diagrams, applying a cross-entropy optimization algorithm developed by our group, which takes into account UBVRI photometric data weighted using a membership-likelihood estimation.

Updated properties of the old open cluster Melotte 66: Searching for multiple stellar populations

Multiple generations of stars are routinely encountered in globular clusters but no convincing evidence has been found in Galactic open clusters to date. In this paper we use new photometric and spectroscopic data to search for multiple stellar population signatures in the old, massive open cluster, Melotte~66. The cluster is known to have a red giant branch wide in color, which could be an indication of metallicity spread. Also the main sequence is wider than what is expected from photometric errors only. This evidence might be associated with either differential reddening or binaries. Both hypothesis have, however, to be evaluated in detail before recurring to the presence of multiple stellar populations. New, high-quality, CCD UBVI photometry have been acquired to this aim with high-resolution spectroscopy of seven clump stars, that are complemented with literature data. Our photometric study confirms that the width of the main sequence close to the turn off point is entirely accounted for by binary stars and differential reddening, with no need to advocate more sofisticated scenarios, such as metallicity spread or multiple main sequences. By constructing synthetic color-magnitude diagrams, we infer that the binary fraction has to be as large as 30$%$ and their mass ratio in the range 0.6-1.0. As a by-product of our simulations, we provide new estimates of the cluster fundamental parameters. We measure a reddening E(B-V)=0.15$\pm$0.02, and confirm the presence of a marginal differential reddening. The distance to the cluster is $4.7^{+0.2}_{-0.1} $kpc and the age is 3.4$\pm$0.3 Gyr, which is somewhat younger and better constrained than previous estimates. Our detailed abundance analysis reveals that, overall, Melotte~66 looks like a typical object of the old thin disk population.

Chemical abundance analysis of the old, rich open cluster Trumpler 20

Trumpler 20 is an open cluster located at low Galactic longitude, just beyond the great Carina spiral arm, and whose metallicity and fundamental parameters were very poorly known until now. As it is most likely a rare example of an old, rich open cluster -- possibly a twin of NGC 7789 -- it is useful to characterize it. To this end, we determine here the abundance of several elements and their ratios in a sample of stars in the clump of Trumpler 20. The primary goal is to measure Trumpler 20 metallicity, so far very poorly constrained, and revise the cluster's fundamental parameters. We present high-resolution spectroscopy of eight clump stars. Based on their radial velocities, we identify six bona fide cluster members, and for five of them (the sixth being a fast rotator) we perform a detailed abundance analysis. We find that Trumpler 20 is slightly more metal-rich than the Sun, having [Fe/H]=+0.09$\pm$0.10. The abundance ratios of alpha-elements are generally solar. In line with recent studies of clusters as old as Trumpler 20, Ba is overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we re-estimate the cluster age to be 1.5$^{+0.2}_{-0.1}$ Gyr. Its distance to the Galactic centre turns out to be 7.3 kpc. With this distance and metallicity, Trumpler 20 fits fairly well in the metallicity gradient for the galactic inner disc. With this new study, the characterization of Trumpler~20 is now on much more solid ground. Further studies should focus on the estimate of the binary fraction and on its main sequence membership.

Photometric study of five open star clusters

UBVRI photometry of the five open clusters Czernik 4, Berkeley 7, NGC 2236, NGC 7226 and King 12 has been carried out using ARIES 104cm telescope, Nainital. Fundamental cluster parameters such as foreground reddening E(B-V), distance, and age have been derived by means of the observed two colour and colour-magnitude diagrams, coupled to comparisons with theoretical models. E(B-V) values range from 0.55 to 0.74mag, while ages derived for these clusters range from ∼10 to ∼500Myr. We have also studied the spatial structure, mass function and mass segregation effects. The present study shows that evaporation of low mass stars from the halo of the clusters increases as they evolve.

Global survey of star clusters in the Milky Way: The open cluster distance scale

AbstractWe present the Milky Way Star Clusters (MWSC) survey based on 470 million stars extracted from the all-sky catalogues PPMXL and 2mass. The target list comprises 3784 entries and includes all open and globular clusters, as well as all cluster candidates, known at present. By means of a combined photometric/astrometric pipeline process, we determine individual membership probabilities for cluster stars and provide fundamental cluster parameters such as cluster centre coordinates, mean proper motions, radial velocity, distance, reddening, age and tidal radius. This sample includes ~ 700 clusters with fundamental parameters based on optical data and offers a good opportunity for a comparison of the near-infrared (NIR) and optical cluster distance scales. We find that distances and reddening values obtained from the colour–magnitude diagrams (CMDs) in the NIR agree well with earlier distance estimates based on optical CMDs. These newly obtained homogeneous distances, together with the other cluster parameters, allow us to study the properties of the open cluster population in the Galactic disk.

New fundamental parameters of the Galactic open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37

We have obtained CCD UBVI_{KC} photometry down to V ~ 21.0 for the open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37. The latter has never been studied before. Cluster stellar density profiles were obtained from star counts in appropriate-sized boxes distributed throughout the entire observed fields. Based on different measured indices, we estimate the ages of Berkeley 26, Melotte 72 and NGC 2479. On the other hand, we indicate possible solutions for the cluster fundamental parameters by matching theoretical isochrones which reasonably reproduce the main cluster features in their CMDs. In the case of NGC 2479, the cluster E(B-V) and E(V-I) colour excesses and apparent distance modulus were estimated from the fit of the Zero-Age Main Sequence (ZAMS) to the colour-colour and colour-magnitude diagrams, respectively.

BVRIJHK photometry and proper motion analysis of NGC 6253 and the surrounding field

<i>Context. <i/>We present a photometric and astrometric catalog of 187 963 stars located in the field around the old super-metal-rich Galactic open cluster NGC 6253. The total field-of-view covered by the catalog is 34”33”. In this field, we provide CCD <i>BVRI<i/> photometry. For a smaller region close to the cluster's center, we also provide near-infrared <i>JHK<i/> photometry.<i>Aims. <i/>We analyze the properties of NGC 6253 by using our new photometric data and astrometric membership.<i>Methods. <i/>In June 2004, we targeted the cluster during a 10 day multi-site campaign, which involved the MPG/ESO 2.2 m telescope with its wide-field imager and the Anglo-Australian 3.9 m telescope, equipped with the IRIS2 near-infrared imager. Archival CCD images of NGC 6253 were used to derive relative proper motions and to calculate the cluster membership probabilities.<i>Results. <i/>We have refined the cluster's fundamental parameters, deriving , , , , and . The color excess ratios obtained using both the optical and near infrared colors indicate a normal reddening law in the direction of NGC 6253. The age of NGC 6253 at 3.5 Gyr, determined from our best-fitting isochrone appears to be slightly older than the previous estimates. Finally, we estimated the binary fraction among the cluster members to be ~20%-30% and identified 11 blue straggler candidates.

Homogeneous photometry and star counts in the field of 9 Galactic star clusters

We present homogeneous V,I CCD photometry of nine stellar fields in the two inner quadrants of the Galactic plane. The lines-of-view to most of these fields aim in the direction of the very inner Galaxy, where the Galactic field is very dense, and extinction is high and patchy. Our nine fields are, according to several catalogs, centred on Galactic star clusters, namely Trumpler 13, Trumpler 20, Lynga 4, Hogg 19, Lynga 12, Trumpler 25, Trumpler 26, Ruprecht 128, and Trumpler 34. Apart from their coordinates, and in some cases additional basic data (mainly from the 2MASS archive), their properties are poorly known. By means of star count techniques and field star decontaminated Color Magnitude diagrams, the nature and size of these visual over-densities has been established; and, when possible, new cluster fundamental parameters have been derived. To strengthen our findings, we complement our data-set with JHKs photometry from the 2MASS archive, that we analyze using a suitably defined Q-parameter. Most clusters are projected towards the Carina-Sagittarium spiral arm. Because of that, we detect in the Color Magnitude diagrams of most of the other fields several distinctive sequences produced by young population within the arm. All the clusters are of intermediate or old age. The most interesting cases detected by our study are, perhaps, that of Trumpler 20, which seems to be much older than previously believed, as indicated by its prominent – and double – red clump; and that of Hogg 19, a previously overlooked old open cluster, whose existence in such regions of the Milky Way is puzzling.

Deep MMT Transit Survey of the Open Cluster M37. I. Observations and Cluster Parameters

We have conducted a deep ( -->15 r 23), 20 night survey for transiting planets in the intermediate-age open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. In this paper we describe the observations and data reduction procedures for the survey and analyze the stellar content and dynamical state of the cluster. By combining high-resolution spectroscopy with existing -->BVICKs and new gri color-magnitude diagrams, we determine the fundamental cluster parameters: -->t = 485 ± 28 Myr without overshooting ( -->t = 550 ± 30 Myr with overshooting), -->E(B − V) = 0.227 ± 0.038, -->(m − M)V = 11.57 ± 0.13, and -->[ M/H ] = + 0.045 ± 0.044, which are in good agreement with, although more precise than, previous measurements. We determine the mass function down to 0.3 M☉ and use this to estimate the total cluster mass of -->3640 ± 170 M☉.

Extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud

The colour-magnitude diagram (CMD) of the intermediate-age Large Magellanic Cloud star cluster NGC 2154 and its adjacent field has been analysed using Padova stellar models to determine the cluster's fundamental parameters and its star formation history. Deep BR CCD photometry, together with synthetic CMDs and integrated luminosity functions, has allowed us to infer that the cluster experienced an extended star formation period of about 1.2 Gyr, which began approximately 2.3 Gyr ago and ended 1.1 Gyr ago. However, the physical reality of such a prolonged period of star formation is questionable, and could be the result of inadequacies in the stellar evolutionary tracks themselves. A substantial fraction of binaries (70 per cent) seems to exist in NGC 2154.

NGC 6791: An Exotic Open Cluster or the Nucleus of a Tidally Disrupted Galaxy?

We report on high-resolution echelle spectroscopy of 20 giant stars in the Galactic old open cluster NGC 6791, obtained with Hydra at the WIYN telescope. High-precision radial velocity allows us to isolate 15 bona fide cluster members. From 10 of them we derive a global [M/H] = +0.39 ± 0.05. We therefore confirm that NGC 6791 is extremely metal-rich, exhibits a few marginally subsolar abundance ratios, and within the resolution of our spectra does not show evidence of spread in metal abundance. With these new data we rederive the fundamental cluster parameters, suggesting that it is about 8 Gyr old and 4.3 kpc from the Sun. The combination of its chemical properties, age, position, and Galactic orbit hardly makes NGC 6791 a genuine Population I open cluster. We discuss possible interpretations of the cluster peculiarities, suggesting that the cluster might be what remains of a much larger system whose initial potential well could have been sufficient to produce high-metallicity stars and which has been depopulated by the tidal field of the Galaxy. Alternatively, its current properties may be explained by the perturbation of the Galactic bar on an object that originated well inside the solar ring, where the metal enrichment was very fast.

The young open cluster NGC 2129

The first charge-coupled device UBV( RI )C photometric study in the area of the doubtful open cluster NGC 2129 is presented. Photometry of a field offset 15 arcmin northwards is also provided, to probe the Galactic disc population towards the cluster. Using star counts, proper motions from the UCAC2 catalogue, colour‐magnitude and colour‐colour diagrams, we demonstrate that NGC 2129 is a young open cluster. The cluster radius is 2.5 arcmin, and across this region we find evidence of significant differential reddening, although the reddening law seems to be normal towards its direction. Updated estimates of the cluster fundamental parameters are provided. The mean reddening is found to be E(B − V) = 0.80 ± 0.08 and the distance modulus is (m − M)0 = 11.70 ± 0.30. Hence, NGC 2129 is located at 2.2 ± 0.2 kpc from the Sun inside the Local spiral arm. The age derived from 37 photometrically selected members is estimated to be approximately 10 Myr. These stars are used to provide new estimates of the cluster absolute proper-motion components. Ke yw ords: Hertzsprung‐Russell (HR) diagram ‐ open clusters and associations: general ‐ open clusters and associations: individual: NGC 2129.

M75, A Globular Cluster with a Trimodal Horizontal Branch. I. Color-Magnitude Diagram

Deep UBVI photometry for a large field covering the distant globular cluster M75 (NGC 6864) is presented. We confirm a previous suggestion that M75 possesses a bimodal horizontal branch (HB) bearing striking resemblance to the well-known case of NGC 1851. In addition, we detect a third, smaller grouping of stars on the M75 blue tail, separated from the bulk of the blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may correspond to the upper part of a very extended, though thinly populated, blue tail. Thus M75 appears to have a trimodal HB. The presence of the Grundahl jump is verified using the broadband U filter. We explore the color-magnitude diagram of M75 with the purpose of deriving the cluster's fundamental parameters and find a metallicity of [Fe/H] = -1.03 ± 0.17 dex and -1.24 ± 0.21 in the Carretta & Gratton and Zinn & West scales, respectively. We discuss earlier suggestions that the cluster has an anomalously low ratio of bright red giants to HB stars. A differential age analysis with respect to NGC 1851 suggests that the two clusters are essentially coeval.