Number Of Open Clusters Research Articles

Study of 26 Galactic Open Clusters with Extended Main-sequence Turnoffs

Recent studies indicate that some Galactic open clusters (OCs) exhibit extended main-sequence turnoff (eMSTO) in their color–magnitude diagrams (CMDs). However, the number of Galactic OCs with eMSTO structures detected so far is limited, and the reasons for their formation are still unclear. This work identifies 26 Galactic OCs with undiscovered eMSTOs and investigates the causes of these features. Stellar population types and fundamental parameters of cluster samples are acquired using CMD fitting methods. Among them, the results of 11 OCs are reliable as the observed CMDs are well-reproduced. We propose the crucial role of stellar binarity and confirm the importance of stellar rotation in reproducing eMSTO morphologies. The results also show that the impact of age spread is important, as it can adequately explain the structure of young OCs and fit the observed CMDs of intermediate-age OCs better.

The new detection of blue straggler stars in 50 open clusters using Gaia DR3

Context. The particularly abundant presence of blue straggler stars (BSS) in Galactic open clusters offers favorable conditions for detailed studies on the statistical properties and the origin of the blue straggler population. With the help of Gaia DR3, the number of identified open clusters continuously increases, and the determination of star cluster members is more reliable. Aims. We performed a more thorough search for BSS in newly found open clusters based on Gaia data. Methods. We implemented a uniform membership determination for over one thousand newly identified open clusters with larger sky coverage based on the astrometric and photometric data from Gaia DR3. The membership probabilities of stars were assigned by the pyUPMASK algorithm. Then we estimated the physical parameters of these clusters by isochrone fitting on their CMDs and picked out BSS in the specific region of these CMDs. Results. We identified 138 BSS that had not been reported before in 50 open clusters. Conclusions. Compared with recent catalogs that present more than 1500 BSS in 339 open clusters, our new catalog increased the number of BSS in Galactic open clusters by about 10%, and the number of open clusters with BSS by nearly 17%. In the future, more accurate abundance measurements are anticipated to better probe the origin of BSS in open clusters.

The OCCASO survey: presentation and radial velocities of 12 Milky Way open clusters

Open clusters (OCs) are crucial for studying the formation and evolution of the Galactic disc. However, the lack of a large number of OCs analysed homogeneously hampers the investigations about chemical patterns and the existence of Galactocentric radial and vertical gradients, or an age–metallicity relation. To overcome this, we have designed the Open Cluster Chemical Abundances from Spanish Observatories (OCCASO) survey. We aim to provide homogeneous radial velocities, physical parameters and individual chemical abundances of six or more red clump stars for a sample of 25 old and intermediate-age OCs visible from the Northern hemisphere. To do so, we use high-resolution spectroscopic facilities (R ≥ 62 000) available at Spanish observatories. We present the motivation, design and current status of the survey, together with the first data release of radial velocities for 77 stars in 12 OCs, which represents about 50 per cent of the survey. We include clusters never studied with high-resolution spectroscopy before (NGC 1907, NGC 6991, NGC 7762), and clusters in common with other large spectroscopic surveys like the Gaia-ESO Survey (NGC 6705) and Apache Point Observatory Galactic Evolution Experiment (NGC 2682 and NGC 6819). We perform internal comparisons between instruments to evaluate and correct internal systematics of the results, and compare our radial velocities with previous determinations in the literature, when available. Finally, radial velocities for each cluster are used to perform a preliminary kinematic study in relation with the Galactic disc.

A Derivative Formula for the Free Energy Function

We consider bond percolation on the ${\bf Z}^d$ lattice. Let $M_n$ be the number of open clusters in $B(n)=[-n, n]^d$. It is well known that $E_pM_n / (2n+1)^d$ converges to the free energy function $\kappa(p)$ at the zero field. In this paper, we show that $\sigma^2_p(M_n)/(2n+1)^d$ converges to $-(p^2(1-p)+p(1-p)^2)\kappa'(p)$.

Probing disk properties with open clusters

We use the open clusters (OCs) with known parameters available in the WEBDA database and in recently published papers to derive properties related to the disk structure. The sample totals 654 OCs, consisting basically of Trumpler types I to III clusters. Because of the completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile. We simulate the effects of completeness assuming that the observed distribution of the number of OCs with a given number of stars above the background, measured in a restricted zone outside the Solar circle, is representative of the intrinsic distribution of OCs throughout the Galaxy. As a result we derive completeness-corrected radial distributions which agree with exponential disks throughout the observed Galactocentric distance range 5--14\,kpc, with scale lengths in the range $1.5 - 1.9\,kpc$. In particular we retrieve the expected exponential-disk radial profile for the highly depleted regions internal to the Solar circle. Extrapolation of the completeness-corrected radial distributions down to the Galactic center indicates a total number of OCs in the range $\rm(1.8 - 3.7)\times10^5$. These estimates are upper-limits because they do not take into account depletion in the number of OCs by dynamical effects in the inner parts of the Galaxy. The observed and completeness-corrected age-distributions of the OCs can be fitted by a combination of two exponential-decay profiles characterized by age scales of $\rm\sim100\,Myr$ and $\rm\sim1.9\,Gyr$, respectively. This rules out evolutionary scenarios based on constant star-formation and OC-disruption rates. We estimate that 3.4--8\% of the embedded clusters do actually emerge from the parent molecular clouds as OCs.

A search for luminous Be stars

AbstractAs Be stars are restricted to luminosity classes III‐V, but early B‐type stars are believed to evolve into supergiants, it is to be expected that the Be phenomenon disappears at some point in the evolution of a moderately massive star, before it reaches the supergiant phase. As a first stage in an attempt to determine the physical reasons of this cessation, a search of the literature has provided a number of candidates to be Be stars with luminosity classes Ib or II. Spectroscopy has been obtained for candidates in a number of open clusters and associations, as well as several other bright stars in those clusters. Among the objects observed, HD 207329 is the best candidate to be a high‐luminosity Be star, as it appears like a fast‐rotating supergiant with double‐peaked emission lines. The lines of HD 229059, in Berkeley 87, also appear morphologically similar to those of Be stars, but there are reasons to suspect that this object is an interacting binary. At slightly lower luminosities, LS I +56°92 (B4 II) and HD 333452 (O9 II), also appear as intrinsically luminous Be stars. Two Be stars in NGC 6913, HD 229221 and HD 229239, appear to have rather higher intrinsic magnitudes than their spectral type (B0.2 III in both cases) would indicate, being as luminous as luminosity class II objects in the same cluster. HD 344863, in NGC 6823, is also a rather early Be star of moderately high luminosity. The search shows that, though high‐luminosity Be stars do exist, they are scarce and, perhaps surprisingly, tend to have early spectral types. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

UBVIphotometry of the intermediate-age open cluster NGC 6939

ABSTRACT We present CCD UBVI photometry of the nearby, intermediate age open clusterNGC 6939. Using the synthetic Colour - Magnitude Diagrams technique we estimatethe following parameters: age between 1.3 and 1.0 Gyr (depending on whether or notovershooting is considered), reddening 0.34 ≤ E(B−V) ≤ 0.38 and distance modulus11.3 ≤ (m−M) 0 ≤ 11.4.Key words: Hertzsprung-Russell (HR) diagram – open clusters and associations:general – open clusters and associations: individual: NGC 6939 1 INTRODUCTIONA vast amount of information on the formation and evolu-tion of our Galaxy can be gathered from the study of openclusters (OCs). In particular, old OCs may be useful to de-rive information not only on the present day situation, butalso on the time evolution of the disc, since their ages coverthe whole existence of the disc, reaching to about 10 Gyr(Friel 1995). In order to fully exploit the information pro-vided by OCs we must derive for them in a homogeneousway: accurate absolute ages (and a consistent age ranking),distances, reddenings, and metal content. With this aim weare studying a number of old OCs (see Bragaglia & Tosi2003 and Sandrelli et al. 1999 for references), and we nowadd another cluster to our sample.Given its relative proximity, NGC 6939 (C2030+604)has been the target of several studies in the past: the firstbibliographic entry is 80 years ago (Kustner 1923) but, sur-prisingly, the first CCD data appeared only in 2002 (Rosvick& Balam 2002, hereafter RB02). As usual, the cluster pa-rameters found in literature do not agree with each other,and we present in this paper new and improved determi-nations for this intermediate age open cluster located atRA(2000) = 20:31:32, DEC(2000) = +60:39:00, or l = 95.88,b = 12.30.Photometry has been previously presented by severalauthors, but old photographic photometry only reachedabout one magnitude below the main sequence Turn-Off.Mermilliod, Huestamendia, & del Rio (1994) took UBV pho-toelectric photometry of 37 member stars all in the redclump phase, with the intent of discriminating between dif-ferent evolutionary models (with or without overshooting)by comparison with theoretical isochrones. It turned outthat most of the bright stars lying in the cluster directionare indeed cluster members (their Table 1 lists only 4 non-members among 45 objects). The recent work by Rosvick &Balam (2002) has presented the first deep BVI CCD data.They used the 1.85m Dominion Astrophysical Observatorytelescope, covering the central part of NGC 6939 (more orless our field A, see later). Their CMDs show considerablescatter, which they do not attribute to contamination fromfield stars (even if they could not prune their diagrams onthe main sequence, since no proper motion survey on thiscluster has reached so deep), but to differential reddening[E(B−V) = 0.29 to 0.41], which also influences the distancederivation [(m−M)

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar overdensity, close to the Galactic plane at (ℓ= 240°, b=−8°), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, 1904, 2298 and 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this overdensity is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disc. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits the extant position, distance and velocity information on the Galactic ‘Ring’, as well as that of the M-giant overdensities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic plane, with a pericentre at approximately the solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disc, as well as a vertical scaleheight similar to that of the thick disc. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disc, that the thick disc is continually growing, even up to the present time, and that thick disc globular clusters were accreted on to the Milky Way from dwarf galaxies in co-planar orbits.

Clusters of Open Clusters

We examine the grouping of open clusters (OCs) in layer clusters and the ages of open clusters that form clusters of OCs. For this goal we use Lynga [3] catalog which consists of 370 OCs with known distances and ages. 180 of 370 OCs are used to form 51 clusters of OCs. Here the number of young cluster of OCs (which contain at least 1 OC with age no more than 10^7 yrs) is found 29. 19 of them are coincident with the OB associations. Old OCs which are having the age 10^8 yrs are oviding to attend the young clusters of OCs. This shows that star formation has a duration up to 10^8 yrs in a molecular complex. Old OCs do not form rich clusters. They come together rarely with larger age spans. 22 old clusters of old OCs are found and number of OCs in them does not exceed 4. No coincidence is observed between OB associations and old clusters of OCs. Keywords: Open clusters, OB associations, star formation regions.

On critical probabilities in percolation theory

Several versions of the concept of critical percolation probability are discussed in the bond percolation problem on the square lattice. Critical probabilities are also employed as technical devices in the proofs of two new results. First, there is a critical probability pT below which all moments of the cluster size are finite. Secondly, an infinite connected cluster of open bonds exists with positive probability if and only if any angular sector contains an infinite connected cluster of open bonds with positive probability. An expression is derived for the expected number of open clusters per bond in the percolation model, relating to the problem of rigorously justifying a critical probability result of Sykes and Essam.

A study of apparently nearby stars with coinciding spectral type and apparent magnitude

At visual inspection of objective-prism plates one frequently detects a pair or higher multiple of apparently identical stellar spectra so close together that they form a conspicuous configuration for the eye. An attempt to analyse this type of objects has given no positive indication that it should be identified with any unique physical phenomenon. A series of explanations is presented. A minor fraction of the objects may constitute widely separated binary components, the identity of which is physically conditioned by the process of formation. For the majority, however, it seems more probable that they form parts of cluster remnants or very small clusters, and that the coincidence of the spectral types is in the first instance to consider as a fortunate random effect that facilitates the detection of the hidden cluster. Although there are several interpretations of the appearance of such ‘miniclusters’, I find it particularly important to take the possibility into consideration that the number of open clusters (and associations) may be considerably greater than one has hitherto assumed—from observational as well as from theoretical point of view.

A survey of observational problems of stellar evolution

I shall start by giving an account of the remarkable work on moving star groups on which Dr Eggen has been engaged for the last 2 or 3 years. Dr Eggen cannot present the results of his researches himself to this Discussion, as he is at this moment in South Africa carrying out photoelectric observations on the Magellanic Clouds and on other objects. As is well known, Dr Eggen is distinguished for his photoelectric observations of the nearby open clusters such as the Hyades, work of which character has been the main observational basis for the entire theory of stellar evolution in its contemporary form. It is in these open clusters that one in able to isolate sets of stars with a common age and a common chemical constitution; that is to say sets of stars whose colour luminosity array, or H. -R. diagram, represents the locus of points corresponding to starts in the same state of evolution but differing in mass. Since the open clusters are not necessarily all of the same age, and are in fact adjudged to be of very different ages, the H. -R. diagrams of a number of open clusters afford the means of reconstructing the path, in a plane representing absolute magnitude and colour, of a star as it passes through a succession of stages of development. The very new is represented by the association h and X Persei, the comparatively new by open clusters such as Pleiades and Hyades, and so on to clusters which are held to be very old such as M67 (figure 1). The particular feature of Dr Eggen's recent investigations is that he has passed on from considering the open clusters such as Hyades to considering group of stars widely separated in the sky but possessing parallel motion. The Hyades themselves are distributed over a comparatively large area of the sky, that is to say about 15° square. Even from the trigonometrical parallaxes, the distances of the stars forming the cluster are quite well known and we can say at once that the cluster occupies a volume, more or less spherical, with a radius of about 10 parsecs. Only a fraction of the stars seen in this direction are members of the moving group. They are distinguished by the fact that their observed transverse motions, or proper motions, converge to a point in the sky, the radiant of the cluster. From the average of the radial velocities of the members we can construct the actual motion in kilometres per second of the group relative to the sun, which is probably within 1 or 2 km/s of the value 44 km/s, and then from the observed proper motion of each star and the star's angular distance from the radiant we can infer the parallax. Since proper motions can be found in well-observed stars with an accuracy of a few thousandths of a second of arc per year, and since the proper motions of Hyades members are about a tenth of a second of are per year, we can derive parallaxes of Hyades members with an error of only a few parts per cent from the proper motions, whereas trigonometrical parallaxes are much more uncertain than this as a rule.