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Characterization of open cluster remnants
Context: Despite progress in the theoretical knowledge of open clusterremnants and the growing search for observational identifications inrecent years, open questions still remain. The methods used to analyzeopen cluster remnants and criteria to define them as physical systemsare not homogeneous. In this work we present a systematic method forstudying these objects that provides a view of their properties andallows their characterization. Aims: Eighteen remnant candidates areanalyzed by means of photometric and proper motion data. These dataprovide information on objects and their fields. We establish criteriafor characterizing open cluster remnants, taking observationaluncertainties into account. Methods: 2MASS J and H photometry isemployed (i) to study structural properties of the objects by means ofradial stellar density profiles, (ii) to test for any similarity betweenobjects and fields with a statistical comparison method applied to thedistributions of stars in the CMDs, and (iii) to obtain ages, reddeningvalues, and distances from the CMD, taking an index of isochrone fitinto account. The UCAC2 proper motions allowed an objective comparisonbetween objects and large solid angle offset fields. Results: Theobjective analysis based on the present methods indicates 13open-cluster remnants in the sample. Evidence of the presence of binarystars is found, as expected for dynamically evolved systems. Finally, weinfer possible evolutionary stages among remnants from the structure,proper motion, and CMD distributions. The low stellar statistics forindividual objects is overcome by means of the construction of compositeproper motion and CMD diagrams. The distributions of remnants in thecomposite diagrams resemble the single-star and unresolved binary stardistributions of open clusters.
| Observational templates of star cluster disruption. The stellar group NGC 1901 in front of the Large Magellanic Cloud
Context: Observations indicate that present-day star formation in theMilky Way disk takes place in stellar ensembles or clusters rather thanin isolation. Bound, long-lived stellar groups are known as openclusters. They gradually lose stars and are severely disrupted in theirfinal evolutionary stages, leaving an open cluster remnant made up of afew stars. Aims: In this paper, we study in detail the stellar contentand kinematics of the poorly populated star cluster NGC 1901. Thisobject appears projected against the Large Magellanic Cloud. The aim ofthe present work is to derive the current evolutionary status, binaryfraction, age, and mass of this stellar group. These are fundamentalquantities to compare with those from N-body models in order to studythe most general topic of star cluster evolution and dissolution. Methods: The analysis is performed using wide-field photometry in theUBVI pass-band, proper motions from the UCAC.2 catalog, and 3 epochs ofhigh-resolution spectroscopy, as well as results from extensive N-bodycalculations. Results: The star group NGC 1901 is found to be anensemble of solar metallicity stars, 400±100 Myr old, with a coreradius of 0.23 pc, a tidal radius of 1.0 pc, and a location at400±50 pc from the Sun. Out of 13 confirmed members, only 5single stars have been found. Its estimated present-day binary fractionis at least 62%. The calculated heliocentric space motion of the clusteris not compatible with possible membership in the Hyades stream. Conclusions: Our results show that NGC 1901 is a clear prototype of anopen cluster remnant characterized by a high value of the binaryfraction and a significant depletion of low-mass stars. In light ofnumerical simulations, this is compatible with NGC 1901 being whatremains of a larger system initially made of 500-750 stars.Data for Fig. 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/466/931 Table 1 is onlyavailable in electronic form at http://www.aanda.org
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Long-term monitoring of variable stars in open clusters .
Using the `C2' and `C3' CCD cameras mounted respectively on the 1.2mSwiss Telescope at La Silla, Chile and on its Flemish twin at La Palma,Canary Islands, Spain, we have been conducting long-term photometricobservations of chosen galactic open clusters, with the aim ofidentifying and analysing their variable stars. A mmag overall accuracyensures various kinds of variable stars are within our reach. Given thelarge time baseline, this project is particularly well suited fortackling long-term variability. Here, we present preliminary results onNGC 1901, which we began to analyse recently. Other clusters are stillbeing monitored, with results to be published soon.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Probing the nature of possible open cluster remnants with the Southern Proper Motion Program
We discuss the nature of eleven Possible Open Cluster Remnants (POCRs)by using absolute proper motions from the Southern Proper Motion (SPM)Program 3 (Girard et al. [CITE], AJ, 127, 3060) combined with nearinfrared photometry from 2MASS. The analysis is done by considering thedistribution of stars in the Color-Magnitude and the Vector Pointdiagrams. We successfully probed the capabilities of the SPM catalog todetect a physical group by looking at the open cluster Blanco 1.However, within the uncertainties of the SPM3 catalog and basing on2MASS photometry we conclude that only one - ESO 282SC26 - out of elevenobjects turns out to be a probable physical group. We suggest it is anopen cluster 1.3 Gyr old and located 1.4 kpc from the Sun.
| The Prospects for Finding Brown Dwarfs in Eclipsing Binary Systems and Measuring Brown Dwarf Properties
We present the results of a simulation to investigate the prospects ofmeasuring mass, age, radius, metallicity, and luminosity data for browndwarfs in fully eclipsing binary systems around dwarf spectral typesfrom late K to early M that could be identified by ultra-wide-fieldtransit surveys such as SuperWASP. These surveys will monitorapproximately a million K and M dwarfs with |b|>20deg(where blending of sources is not a significant problem) at a levelsufficient to detect transits of low-luminosity companions. We look atthe current observational evidence for such systems and suggest that ~1%of late K and early-to-mid M dwarfs could have a very close (~0.02 AU)brown dwarf companion. With this assumption, and using SuperWASP as anexample, our simulation predicts that ~400 brown dwarfs in fullyeclipsing binary systems could be discovered. All of these eclipsingbinaries could yield accurate brown dwarf mass and radius measurementsfrom radial velocity and follow-up light curve measurements. Byinferring the brown dwarf effective temperature distribution, assuming auniform age spread and an α=0.5 companion brown dwarf massfunction, the simulation estimates that brown dwarf brightness couldalso be measurable (at the 10% level) for ~60 of these binary systemsfrom near-infrared follow-up light curves of the secondary eclipse. Weconsider irradiation of these brown dwarfs by their primary stars andconclude that it would be below the 10% level for ~70% of them. Thismeans that in these cases, the measured brown dwarf brightnesses shouldessentially be the same as those of free-floating counterparts. Thepredicted age distribution of the primaries is dominated by youngsystems, and ~20 binaries could be younger than 1 Gyr. Irradiation willbe below the 10% level for ~80% of these. We suggest that many of theseyoung binary systems will be members of ``kinematic moving groups,''allowing their ages to be accurately constrained.
| NGC 5385, NGC 2664 and Collinder 21: Three candidate open cluster remnants
We present CCD UBVI photometric and medium/high resolution spectroscopicobservations obtained in the field of the previously unstudieddissolving open cluster candidates NGC 5385, NGC 2664 and Collinder 21.Our analysis is based on the discussion of star counts, photometry,radial velocity distribution, and proper motions available from theTycho 2 catalogue. All three aggregates clearly emerge from the meanGalactic field, but, regrettably, the close scrutiny of proper motionsand radial velocities reveals that we are not facing any physical group.Instead, what we are looking at are just chance alignments of a fewbright unrelated stars. Our analysis casts some doubt on the Bica et al.(\cite{Bic01}, A&A, 366, 827) criterion to look for Possible OpenCluster Remnants. It seems mandatory to define a better criterion toadopt for further investigations.Based on observations carried out at Mt Ekar Observatory, Asiago, Italy.Photometry is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/428/67
| Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins Iand Washington CMT1 systems is presented in the field of thepoorly known open cluster NGC 2627. Four independent Washingtonabundance indices yield a mean cluster metallicity of [Fe/H]=-0.12 +/-0.08, which is compatible with the existence of a radial gradient in theGalactic disc. The resultant colour-magnitude diagrams indicate that thecluster is an intermediate-age object of 1.4 Gyr. Based on the best fitsof the Geneva group's isochrones to the (V, V-I) and (T1,C-T1) diagrams, we estimate E(V-I) = 0.25 +/- 0.05 andV-MV= 11.80 +/- 0.25 for logt= 9.15, and E(C-T1) =0.23 +/- 0.07 and T1-MT1= 11.85 +/-0.25 for logt= 9.10, respectively, assuming solar metal content. Thederived reddening value E(C-T1) implies E(B-V) = 0.12 +/-0.07 and a distance from the Sun of 2.0 +/- 0.4 kpc. Using the WEBDAdata base and the available literature, we re-examined the overallproperties of all the open clusters with ages between 0.6 and 2.5 Gyr.We identified peaks of cluster formation at 0.7-0.8, 1.0-1.1, 1.6-1.7and 2.0-2.1 Gyr, separated by relative quiescent epochs of ~0.2-0.3 Gyr.We also estimated a radial abundance gradient of -0.08 +/- 0.02, whichis consistent with the most recent determinations for the Galactic disc,but no clear evidence for a gradient perpendicular to the Galactic planeis found.
| Open Clusters or Their Remnants: B and V Photometry of NGC 1901 and NGC 1252
Not Available
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
| Open clusters or their remnants: B and V photometry of NGC 1901 and NGC 1252
Photometry in the B and V bands is presented for the southern stellargroups NGC 1901 and NGC 1252. NGC 1901 is often described as an opencluster while NGC 1252 consists of a concentration of about 20 starscentered ~20' north of the original New General Cataloguecoordinates, and at the southwest edge of the large region previouslyassigned to this object in the literature. NGC 1901 has a clear mainsequence and shares similarities with the Hyades. We derive a reddeningvalue E(B-V) = 0.04, a distance from the Sun dsun = 0.45 kpc(Z = -0.23 kpc) and an age 0.6 +/- 0.1 Gyr. NGC 1901 is conclusively aphysical system, dynamically comparable to or more evolved than theHyades. The colour-magnitude diagram of NGC 1252 suggests a turnoff andmain sequence, and a total of 12 probable members. We simulated theGalactic field colour-magnitude diagram in the same direction and foundit to be a poor match to NGC 1252, suggesting that NGC 1252 is not afield fluctuation. Isochrone fitting to the probable members isconsistent with E(B-V) = 0.02, dsun = 0.64 kpc (Z = -0.46kpc) and an age 3 +/- 1 Gyr. NGC 1252 cannot be ruled out as a physicalgroup with the available data. If so, evidence is found that it is not aclassical open cluster, but rather an open cluster remnant.
| Dissolving star cluster candidates
We present a list of 34 neglected entries from star cluster catalogueslocated at relatively high galactic latitudes (|b| >15deg) which appear to be candidate late stages of starcluster dynamical evolution. Although underpopulated with respect tousual open clusters, they still present a high number density contrastas compared to the galactic field. This was verified by means of (i)predicted model counts from different galactic subsystems in the samedirection, and (ii) Guide Star Catalog equal solid angle counts for theobject and surrounding fields. This suggests that the objects arephysical systems, possibly star clusters in the process of disruption ortheir fossil remains. The sample will be useful for followup studies inview of verifying their physical nature.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| The Age Range of Hyades Stars
On the basis of canonical models, the age of Hyades supercluster stars,whether in the Hyades and Praesepe clusters or the noncluster field,ranges from (5-6) x 10^8 to 10^9 yr. The difference between the parallaxderived from the supercluster motion and that obtained from Hipparcosobservations has a dispersion only twice that of the mean dispersion ofthe individual Hipparcos values. The supercluster appears not to containred giants on the first ascent of the red giant branch, but onlyasymptotic giant branch (``clump'') stars. The masses obtained forindividual components of binary stars in the supercluster show adispersion of less than 10% when compared with model predictions.
| The Distribution of Nearby Stars in Velocity Space Inferred from HIPPARCOS Data
The velocity distribution f(v) of nearby stars is estimated, via amaximum likelihood algorithm, from the positions and tangentialvelocities of a kinematically unbiased sample of 14,369 stars observedby the Hipparcos satellite. The distribution f shows rich structure inthe radial and azimuthal motions, v_R and v_phi, but not in the verticalvelocity, v_z: there are four prominent and many smaller maxima, many ofwhich correspond to well-known moving groups. While samples ofearly-type stars are dominated by these maxima, also up to about aquarter of red main-sequence stars are associated with them. Thesemoving groups are responsible for the vertex deviation measured even forsamples of late-type stars; they appear more frequently for ever reddersamples, and as a whole they follow an asymmetric drift relation, in thesense that those only present in red samples predominantly have large|v_R| and lag in v_phi with respect to the local standard of rest (LSR).The question arises, how did these old moving groups get on theireccentric orbits? A plausible mechanism known from solar system dynamicsthat is able to manage a shift in orbit space is sketched. Thismechanism involves locking into an orbital resonance; in this respect isintriguing that Oort's constants, as derived from Hipparcos data, implya frequency ratio between azimuthal and radial motion of exactlyOmega:kappa = 3:4. Apart from these moving groups, there is a smoothbackground distribution, akin to Schwarzschild's ellipsoidal model, withaxis ratios sigma_R:sigma_phi:sigma_z ~ 1:0.6:0.35. The contours arealigned with the v_R-direction, but not with respect to the v_phi- andv_z-axes: the mean v_z increases for stars rotating faster than the LSR.This effect can be explained by the stellar warp of the Galactic disk.If this explanation is correct, the warp's inner edge must not be withinthe solar circle, while its pattern rotates with frequency >~13 kms^-1 kpc^-1 retrograde with respect to the stellar orbits.
| The Stellar Content of Star Stream I
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111.1615E&db_key=AST
| 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.
| Integrated photometric properties of open clusters
Galactic open clusters provide an abundant sample of stellar aggregatesof various sizes, ages and metal abundances, apt to constitute atemplate for comparison with star systems in other galaxies. In thispaper we present and discuss a standard methodology to synthesize U,B,Vfluxes and colours, and apply it to a set of 138 open clusters. Resultsare compared with previous ones available in the literature. We wereable to calibrate a mass-luminosity relation by which we evaluated themass of ~400 open clusters, leading to a well defined present-day massfunction. The number-complete sample of galactic open clusters presentedin Battinelli & Capuzzo-Dolcetta (1991) is enlarged of a 15%.
| Photoelectric Search for Peculiar Stars in Open Clusters - Part Fourteen - NGC1901 NGC2169 NGC2343 CR:132 NGC2423 and NGC2447
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102....1M&db_key=AST
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| A comparison of observations with main-sequence evolutionary models.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977PASP...89..554P&db_key=AST
| The Age Distribution and Total Lifetimes of Galactic Clusters
The age distribution of galactic clusters is obtained from catalogues ofwell observed clusters compiled by Becker and Fenkart (1971) and Lindoff(1968). The observed age distribution of clusters within 1000 pc doesnot seem to be seriously affected by selection effects. Assuming aconstant rate of formation of clusters, we deduce from the observed agedistribution of clusters within 1000 pc statistical information aboutthe total lifetimes of galactic clusters: 50% of new clustersdisintegrate within 2 10^8 years, 10% have a total lifetime longer than5 10^8 years, and only 2% live longer than 10^9 years. Hence, thetypical lifetime is short, but there exists a wide spread in theindividual lifetimes. The lifetimes obtained in this paper may serve asa powerful observational test of theories of the dynamical evolution ofstar clusters. We find that only a small fraction of field stars areformer members of now dissolved galactic clusters. Moving groups shouldgenerally not be identified with disintegrated clusters. The relativedynamical age of a galactic cluster is rather weakly correlated with itsabsolute age. Therefore, and because of natural selection effects in thesurviving old clusters, it is very unlikely that more directobservational information about the dynamical evolution of clusters canbe obtained by studying objects of various ages.
| An atlas of open cluster colour-magnitude diagrams
Not Available
| A stellar group in line of sight with the Large Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1968AJ.....73..566S&db_key=AST
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