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|NGC2236: a moderately metal-poor open cluster of Hyades-like age located beyond the Perseus spiral arm|
New CCD photometry in the Washington system C and T1passbands down to T1 ~ 18.5 mag in the field of the northernopen cluster NGC2236 is presented. T1 magnitudes and C -T1 colours for a total of 1162 stars within an area of 13.6× 13.6 arcmin2 were measured. These CCD data weresupplemented with photoelectric CMT1T2 photometryof 13 red giant candidates. The comparison of the cluster(T1, C - T1) colour-magnitude diagram withtheoretical isochrones computed for the Washington system yields E(C -T1) = 1.10 +/- 0.10 and for log t = 8.80 (t =600+100-40 Myr) and Z = 0.008. The derived E(C -T1) value implies E(B - V) = 0.55 +/- 0.05. NGC2236 is thenlocated at 2.5 +/- 0.5 kpc from the Sun beyond the Perseus spiral armand at ~10.8 kpc from the Galactic centre. A cluster angular diameter of9.4 arcmin, equivalent to 6.8 pc, was estimated from star counts bothwithin and outside the cluster field. We also derived from the stellardensity profile a cluster core radius of rc = 1.7 arcmin (1.2pc) and an annular corona of Δrc = 1.8rc(2.2 pc). Five independent Washington abundance indices yield a meancluster metallicity of [Fe/H] = -0.3 +/- 0.2, which is not only inreasonably good agreement with the one obtained from the isochrone fit,but also compatible with the existence of a radial abundance gradient inthe Galactic disc. We examined the properties of a sample of 20 knownopen clusters aligned along the line-of-sight to NGC2236. Berkeley27appears as the farthest and oldest cluster of the studied sample.
|Proper motion determination of open clusters based on the UCAC2 catalogue|
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
|Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data|
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
|On the recent star formation history of the Milky Way disk|
We have derived the star formation history of the Milky Way disk overthe last 2 Gyr from the age distribution diagram of a large sample ofopen clusters comprising more than 580 objects. By interpreting the agedistribution diagram using numerical results from an extensive libraryof N-body calculations carried out during the last ten years, wereconstruct the recent star formation history of the Milky Way disk.Under the assumption that the disk has never been polluted by anyextragalactic stellar populations, our analysis suggests thatsuperimposed on a relatively small level of constant star formationactivity mainly in small-N star clusters, the star formation rate hasexperienced at least five episodes of enhanced star formation lastingabout 0.2 Gyr with production of larger clusters. This cyclic behaviourshows a period of 0.4+/-0.1 Gyr and could be the result of density wavesand/or interactions with satellite galaxies. On the other hand, the starformation rate history from a volume-limited sample of open clusters inthe solar neighbourhood appears to be consistent with the overall starformation history obtained from the entire sample. Pure continuous starformation both in the solar neighbourhood and the entire Galactic diskis strongly ruled out. Our results also indicate that, in the Milky Waydisk, about 90% of open clusters are born with N<=150 and the slopein the power-law frequency distribution of their masses is about -2.7when quiescent star formation takes place. If the above results arere-interpreted taking into consideration accretion events onto the MilkyWay, it is found that a fraction of the unusually high number of openclusters with ages older than 0.6 Gyr may have been formed in disruptedsatellites. Problems arising from the selection effects and the ageerrors in the open cluster sample used are discussed in detail.
|The age of the oldest Open Clusters|
We determine ages of 71 old Open Clusters by a two-step method: we usemain-squence fitting to 10 selected clusters, in order to obtain theirdistances, and derive their ages from comparison with our own isochronesused before for Globular Clusters. We then calibrate the morphologicalage indicator δ(V), which can be obtained for all remainingclusters, in terms of age and metallicity. Particular care is taken toensure consistency in the whole procedure. The resulting Open Clusterages connect well to our previous Globular Cluster results. From theOpen Cluster sample, as well as from the combined sample, questionsregarding the formation process of Galactic components are addressed.The age of the oldest open clusters (NGC 6791 and Be 17) is of the orderof 10 Gyr. We determine a delay by 2.0±1.5 Gyr between the startof the halo and thin disk formation, whereas thin and thick disk startedto form approximately at the same time. We do not find any significantage-metallicity relationship for the open cluster sample. The cumulativeage distribution of the whole open cluster sample shows a moderatelysignificant (˜ 2σ level) departure from the predictions foran exponentially declining dissolution rate with timescale of 2.5 Gyr.The cumulative age distribution does not show any trend withgalactocentric distance, but the clusters with larger height to theGalactic plane have an excess of objects between 2-4 and 6 Gyr withrespect to their counterpart closer to the plane of the Galaxy.
|The relatively young and metal-poor Galactic open cluster NGC 2194|
We present CCD VIKC photometry down to V~ 21 mag in the fieldof the rich open cluster NGC 2194, which is projected towards theGalactic anticentre direction. We measured V magnitudes and V-I coloursfor a total of 2515 stars in a field of 13.6 × 13.6arcmin2. These data are supplemented with CCD photometry inthe C, M and T1 filters of the Washington system andphotoelectric CMT1T2 photometry of 20 red giantcandidates. Based on the best fits of isochrones computed by the Genevaand Padova groups to the (V, V-I) colour-magnitude diagram, we derive acolour excess E(V-I) = 0.75, a cluster distance of 3.2 kpc and an age of400 Myr. Five independent Washington abundance indices yield a meancluster metallicity of [Fe/H]=-0.27 +/- 0.06, which is compatible withthe existence of both a radial and Z gradient in the Galactic disc. NGC2194 is a relatively young open cluster, whose deficiency in metalcontent can be accounted for by the fact that it was born not only farfrom the Galactic centre in the outer disc, but also at a relativelyhigh Z value.
|Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy|
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
|Foreground and background dust in star cluster directions|
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
|Catalogue of blue stragglers in open clusters.|
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
|The Old Open Clusters Of The Milky Way|
The Galactic open clusters, in particular the oldest members, serve asexcellent probes of the structure and evolution of the Galactic disk.Individual clusters provide excellent tests of stellar and dynamicalevolution. Cluster spatial and age distributions provide insight intothe processes of cluster formation and destruction that have allowedsubstantial numbers of old open clusters to survive.Spectroscopic andphotometric data for the old clusters yield kinematic, abundance, andage information that clarifies the relationship between the old opencluster population and other Galactic populations. New samples of oldopen clusters, which span a large range in distance and age, are used todefine disk abundance gradients and the cluster age-metallicityrelationship, and they point to a complex history of chemical enrichmentand mixing in the disk.
|The galactic system of old star clusters: The development of the galactic disk|
The vast majority of open clusters persist as clusters for no more thana few hundred million years, but the few which survive for much longerperiods constitute a unique sample for probing the evolution of thegalactic disk. In a charge coupled device (CCD) photometric survey forpossible old open clusters combined with previously publishedphotometry, we have developed a list of 72 clusters of the age of theHyades or older (Phelps (1994). Using our version of parameters based onthe luminosity difference between the main sequence turnoff and thehorizontal branch and on the color difference between the turnoff andthe giant branch, we have calculated a 'Morphological Age Index' (MAI)for the clusters in our list and for a sample of globular clusters. Wefind that the MAI is well correlated with the logarithm of cluster ages,as determined by fitting to theoretical isochrones. We conclude that theindex is a good measure of the relative ages of both globular and openclusters, although uncertainties in the models and residual metallicityeffects prevent the use of the MAI as a definitive calibration of actualcluster ages. The age distribution of the open clusters overlaps that ofthe globular clusters, indicating that the galactic disk began todevelop toward the end of the period of star formation in the galactichalo. The open cluster age distribution can be fit approximately with atwo-component exponential decay function; one component can beidentified as the tail of the dominant population of thin disk openclusters with an age scale factor of 200 Myr, and the other consists oflonger-lived clusters with an age scale of 4 Gyr. The young openclusters are distributed on the galactic plane almost symmetricallyabout the Sun with a scale height perpendicular to the galactic plane of55 pc. The old population consists of rich clusters found only in theouter disk, nore than RGC = 7.5 kpc from the galactic center;this population has a scale height of 375 pc. After accounting for thetwo exponential distributions of cluster ages, there are indications ofan excess of clusters in the age range of 5-7 Gyr; there may have beenlarge bursts of star formation in that period, or perhaps a largerproportion of the clusters forming at that time had advantageous orbitsfor survival. Either circumstance is consistent with the idea that thegalactic disk has been repeatedly disturbed, possibly in collisions orother interactions with external systems, resulting in the occasionalformation of clusters with relatively large velocities perpendicular tothe plane; these are the clusters that have survived until the present.Finally, the repeated accretion of low angular momentum material ontothe disk from the halo or beyond would also explain the observed radialcomposition gradient and the lack of a correlation between open clustermetallicity and age found by Friel & Janes (1993).
|Development of the Galactic disk: A search for the oldest open clusters|
In an extensive charge coupled devices (CCD) photometric survey ofpotential old open clusters, we have identified a number of systems thatare indeed old; some of them are among the oldest of the open clusters.Using our versions of two well-known morphological age indices, onebased on the luminosity difference between the main sequence turnoff andthe horizontal branch and the other on the color difference between theturnoff and the giant branch, we have ranked the open clusters inapproximate order of age. Our data together with previously publishedphotometry of other old open clusters, yields a catalogue of 72 clustersof the age of Hyades or older with 19 of the clusters as old or olderthan M67 (about 5 Gyr). Among the oldest open clusters are Be 17, Cr261, NGC 6791, Be 54, and AM 2. Be 17 and another old cluster, Lynga 7,are possibly as old as the youngest globulars. The data also suggestthat the formation rate of open clusters may have been higher early inthe history of the disk than at intermediate times since numerousclusters have survived from that time.
|Study of faint young open clusters as tracers of spiral features in our Galaxy|
Continuing the study of faint young open clusters as tracers of spiralfeatures in the Galaxy, photoelectric and photographic photometry of 39stars was done in the field of the faint open cluster NGC 2236 = OCl 501in the direction of Monoceros constellation. Out of these stars, a totalof 22 down to m(v) equal to about 15.4 mag have been found to beprobable members. There is apparently a variable extinction across thefield of the cluster with E(B-V) ranging between 0.84 mag and 0.68 mag.The median age of this cluster is estimated to be 7.6 x 10 exp 7 years,and the cluster is thereby considered as belonging to the marginally oldcategory. Thus, it cannot be specifically used as a spiral arm tracer inthe study of the Galaxy. This cluster is located at a distance of 3.72+/- 0.13 kpc, which places it at the inner edge of the outer Perseusspiral feature of the Milky Way.
|The young open clusters NGC 2244 and NGC 2264|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991RMxAA..22...99P&db_key=AST
|On the universality of initial mass function of open star clusters|
The luminosity functions of 23 open star clusters of age less than 1 Gyrare determined by the method of star counts on astronegatives obtainedwith the Schmidt telescope at the Radioastrophysical Observatory of theAcademy of Sciences of the Latvian SSR. The differential and integralluminosity functions are then used to calculate the initial luminosity(mass) functions, and the results are presented in tables. It isconcluded that, for time intervals up to 1 Gyr and distances up to 2kpc, the process of star formation leads to identical mass distributionsin different protoclouds, confirming the hypothesis that theopen-cluster zero-age luminosity function is universally valid.
|Component Analysis of Open Clusters|
|Investigation of the open star cluster NGC 2236|
Photographic UBV photometry carried out by Johnson and Morgan for 1500stars brighter than V = 16.5m in the region of NGC 2236 is examined.Based on this photometry, the following cluster characteristics aredetermined: the H-R diagram, the cluster size, the extinction of lightin the direction of the cluster, and the distance. The luminosityfunction of the cluster is obtained, and its age and mass are estimated.
|Star Count Integral Characteristics for 22 Open Star Clusters|
|Red giants in old open clusters - A test for stellar evolution|
A group of 38 among the oldest open clusters has been analyzed usingtheoretical isochrones derived by Ciardullo and Demarque (1977) andevolutionary tracks for central He-burning of Sweigart and Gross (1976).This analysis has been performed in two steps: - comparison of HRdiagrams with isochrones statistical check of the fit between observedluminosity distributions of red giant stars and the theoretical ones.The following conclusions have been reached: - clusters older than 2 or3 billion years (B-V) equal to or greater than 0.35 at MS turnoff pointgenerally agree with theoretical description and allow the determinationof reliable ages from the distribution of red giants, - for clustersyounger than 2 or 3 billion years, the theory fails in theinterpretation of their red giant stars and the statistical analysisrejects all the tested distributions. In fact, their behavior is similarto that of even younger clusters, in which the evolved stars, duringshell H-burning, have non-degenerate cores and therefore do not undergothe helium flash. It is suggested that such behavior could be attributedto overshooting from the convective core during central H-burning in themass range 1.3-2.2 solar masses.
|Radio observations of galaxies of the Byurakan classification at 102 MHz|
Interplanetary-scintillation observations at 102 MHz have been made of131 galaxies of the Biurakan classification (1975). It is found thatgalaxies with more pronounced nuclei have higher activity in the radiorange and that scintillating components (with angular dimensions about0.1 arcsec) are more frequently found in them. The scintillatingcomponents have steep spectra.
|Open clusters and galactic structure|
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
|King 8 - A metal-poor disk cluster|
The faint cluster, King 8 (IAU designation C00546+336), was observed aspart of a study of galactic anticenter stars, which are used as probesof the environmental conditions in the outer galactic disk. Thedistribution of stellar ages and metallicities as a function of positioncan be compared to the properties of solar neighborhood stars, andthereby serve as constraints for theoretical models. BV observations areconsidered, taking into account photoelectric data, photographic data,and video camera data. The final photographic values for all starsmeasured are listed in a table. Video camera magnitudes were averagedwith photographic magnitudes when available. Attention is given to acolor-magnitude diagram, the luminosity function, a two-color diagram,slitless spectra, and IIDS spectrophotometry. The color-magnitudediagram morphology of King 8 suggests this open cluster is a moderateage, 800 million years, metal-poor (Y = 0.30, Z = 0.01) object locatedin the anticenter 3-4 kpc from the sun.
|A catalogue of galactic star clusters observed in three colours|
Abstract image available at:http://adsabs.harvard.edu/abs/1971A&AS....4..241B
|The open cluster NGC 2236.|
Abstract image available at:http://adsabs.harvard.edu/abs/1970A&A.....9..221R
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