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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.
| Diameters of Open Star Clusters
The present paper presents a tabulation of data on all 600 Galactic openclusters for which it is currently possible to calculate lineardiameters. As expected, the youngest ``clusters,'' with ages <15 Myr,contain a significant (>=20%) admixture of associations. Amongintermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, themedian cluster diameter is found to increase with age. Small, compactclusters are rare among objects with ages >1.5 Gyr. Open clusterswith ages >1 Gyr appear to form what might be termed a ``clusterthick disk,'' part of which consists of objects that were probablycaptured gravitationally by the main body of the Galaxy.
| 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
| NGC 2180: A disrupting open cluster?
The spatial dependence of luminosity and mass functions of evolved openclusters is discussed in this work using J and H 2MASS photometry, whichallows a wide spatial coverage and proper background determination. Thetarget objects are the intermediate-age open cluster NGC 3680(ℓ=286.76o and b=16.92o), which has beenreported as being in an advanced state of dissolution, and NGC 2180(ℓ=203.85o, b=-7.01o), a possible dynamicallyevolved open cluster. At the 1-σ level, NGC 2180 presents a radialdistribution of stars that can be represented by a King law. We concludethat, although in an advanced dynamical state (mass segregated), NGC3680 does not present strong signs of dissolution, having luminosity andmass functions very similar to those of the τage≈3.2Gyr open cluster M 67. On the other hand, NGC 2180 appears to have flat,eroded luminosity functions throughout its structure, suggesting that inaddition to mass segregation, Galactic tidal stripping has beeneffective in depleting this object. Accordingly, NGC 2180 may be themissing link between evolved open clusters and remnants. For NGC 2180 weinfer a distance to the Sun dȯ=0.91 ± 0.15 kpc,an age τage≈710 Myr and an observed stellar mass ofmobs˜47 M&odot. For NGC 3680 we derivean age τage≈1.6 Gyr, E(B-V)=0.0 anddȯ=1.00 ± 0.09 kpc, confirming previousestimates. The observed stellar mass mobs≈130Mȯ agrees with previous values. We study both clustersin the context of dynamical states estimated from diagnostic diagramsinvolving photometric and structural parameters. Both objects appear tobe dynamically evolved systems. In particular, NGC 2180 is closer toopen cluster remnants than NGC 3680.
| Ruprecht 3: An old star cluster remnant?
2MASS J and H photometry and integrated spectroscopy are employed tostudy the nature of the poorly populated compact concentration of starsRuprecht 3, which was previously catalogued as an open cluster. Theintegrated spectrum remarkably resembles that of a moderately metal-richglobular cluster. The distribution of the object stars in thecolour-magnitude diagram is compatible with that of a 1.5 +/- 0.5 Gyropen cluster or older, depending on whether the bluer stars areinterpreted as turnoff stars or blue stragglers, respectively. We derivefor the object a distance from the Sun dsun = 0.72+0.04-0.03 kpc and a colour excess E(B-V) = 0.04.Although a globular cluster remnant cannot be ruled out, the integratedspectrum resemblance to that of a globular cluster probably reflects astochastic effect owing to the few brighter stars. The structural andphotometric properties of Ruprecht 3 are compatible with what would beexpected for an intermediate-age open cluster remnant.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba and San Juan, Argentina.
| Photometry of dissolving star cluster candidates. The cases of NGC 7036 and NGC 7772
We present CCD UBVI observations obtained in the field of the twopreviously unstudied dissolving open cluster candidates NGC 7036 and NGC7772. Our analysis suggests that both the objects are Open ClusterRemnants (OCR). NGC 7036 is an open cluster remnant with a core radiusof about 3-4 arcmin. We derive for the first time estimates of itsfundamental parameters. We identify 17 likely members that define agroup of stars at 1 kpc from the Sun, with a low reddening E(B-V) ~ 0.1,and with an age of about 3-4 Gyr. As for NGC 7772, we identify 14 likelymembers, that define a group of stars with a very low reddening (E(B-V)~ 0.03), are 1.5 Gyr old and are located about 1.5 kpc from the Sun.Based on observations carried out at Mt Ekar, Asiago, Italy. Photometryis only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/385/471
| 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.
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