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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.
| 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.
| 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
| 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.
| A study of the two northern open clusters NGC 1582 and NGC 1663
We present CCD UBV(I)C observations obtained in the field ofthe previously unstudied northern open clusters NGC 1582 and NGC 1663.For the former, we also provide high-resolution spectra of the brighteststars and complement our data with Two-Micron All-Sky-Survey (2MASS)near-infrared photometry and with astrometric data from the Tycho-2catalog. From the analysis of all these data, we argue that NGC 1582 isa very poor, quite large and heavily contaminated open cluster. It turnsout to have a reddening EB-V = 0.35 +/- 0.03, to be situated1100 +/- 100 pc from the Sun and to have an age of 300 +/- 100 Myr. Onthe other hand, we were not able to unambiguously clarify the nature ofNGC 1663. By assuming it is a real cluster and from the analysis of itsphotometric diagrams, we found a color excess value EB-V =0.20, an intermediate age value ( ~ 2000 Myr) and a distance of about700 pc. The distribution of the stars in the region however suggests weare probably facing an open cluster remnant. As an additional result, weobtained aperture photometry of three previously unclassified galaxiesplaced in the field of NGC 1663 and performed a preliminarymorphological classification of them.Based on observations carried out at Mt Ekar, Asiago, Italy.Data are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/527
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