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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.
| 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
| Recent developments on studies of open clusters
Studies on open clusters are of great significance for astronomy andastrophysics, for which distinct developments have been made recently inboth observational and theoretical respects. In this paper, currentresearch of open cluster is reviewed, and membership determination,basic parameters determination, evolution of cluster and clustermembers, structure and dynamics of open cluster are also described indetail.
| Searching for unknown open clusters in the Tycho-2 catalog
We present 11 new open cluster candidates found in a systematic searchfor unknown star clusters using the astrometric and photometric dataincluded in the Tycho 2 catalog. The possible existence of these stellaraggregates is supported by the analysis of proper motions,color-magnitude diagrams, stellar density distributions, and by thevisual inspection of the Digitized Sky Survey (DSS) plates. With thesetools we were able to determine mean absolute proper motions as well aspreliminary reddenings, distances and ages for the majority of thecandidates. We found that most of them are possibly nearby (closer than~ 600 pc) open clusters never studied before.Based on observations of the ESA Hipparcos satellite.Figures 1 and 2 are only available in electronic form athttp://www.edpsciences.org
| 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.
| Urban Astronomy: Observing the Messier Objects from the City
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| A revision of the fundamental parameters of the open cluster Hogg 15 and the projected star WR 47
We revise the fundamental parameters of the faint open cluster Hogg 15,for which two recent colour-magnitude diagram (CMD) studies haveobtained significantly different ages. In the present study, we combinea series of methods trying to constrain age, together with otherfundamental parameters. We employ spatial extractions to construct theCMDs, and the cluster integrated spectrum to compare it with those oftemplates of known age. We derive fundamental parameters, in particular,distance, of the closely projected Wolf-Rayet star HDE 311884 (WR 47) -often proposed to be physically related to Hogg 15. Based on the WR 47spectrum and available photometry, we conclude that the short distanceimplied by the Hipparcos parallax (216 pc) is affected by binary motion.From the WR 47 spectrum we estimate a reddening E(B-V) and a distance of1.10 +/- 0.05 and 5.2 +/- 0.9 kpc, respectively. For Hogg 15 we derivean age of 20 +/- 10 Myr, a reddening of 1.10 +/- 0.05, and a distance of3.1 +/- 0.5 kpc. We conclude that Hogg 15 is not related to WR 47 fromthe point of view of origin, since the cluster and the star do notbelong to the same formation event.
| 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
| NGC 6994 - clearly not a physical stellar ensemble
The sparse stellar ensemble NGC 6994 = M 73 has recently been discussedas a possible remnant of an old open cluster. In order to solve thecontroversy regarding the nature of this object we have takenhigh-resolution spectra of the six brightest stars within 6' angulardistance of its nominal position. These stars are the only obviouscandidate members of an eventual cluster or cluster remnant sincefainter stars do not show any significant concentration in the plane ofthe sky. The radial velocities, atmospheric parameters, and absolutemagnitudes derived from the spectra reveal that the six candidates donot share the same kinematics and lie at different distances from theSun. The proper motions provided by the Tycho-2 catalogue show thatthere is also a large spread in the tangential motions of these stars,in agreement with the spectroscopic results. This leads to theconclusion that the few bright stars that constitute NGC 6994 are not aphysical system (cluster, cluster remnant, or group). They must insteadbe understood as a projective chance alignment of physically unrelatedfield stars. Based on observations made with the spectrograph Elodie onthe 1.93 m telescope at Observatoire de Haute Provence.
| On the nature of five open cluster candidates projected close to the Galactic centre
We present CCD BVI observations obtained for 1759 stars in 4arcminx4arcmin regions around the unstudied southern stellar aggregatesRuprecht 103, 124, 129, 146 and 166, which are described as openclusters in several catalogues. Our analysis shows that none of thesestellar groups are genuine open clusters since no clear main sequencesor other meaningful features can be seen in their colour-magnitudediagrams. Star counts carried out within and outside the clustercandidate fields not only support these results but also show that thestudied objects do not appear to be candidate late stages of starcluster dynamical evolution. Based on observations made at theUniversity of Toronto (David Dunlap Observatory) 24-inch telescope, LasCampanas, Chile. Tables 2 to 6 are only available in electronic form atthe 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/379/453
| 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.
| NGC 6994: An open cluster which is not an open cluster
We report on CCD photometry in the Johnson B, V and I passbands for 146stars in a 9' x 9' region around the southernaggregate NGC 6994 (C 2056-128), which appears in the Lyng{\aa} (1987)catalogue of open star clusters. We argue that this object is not reallyan open cluster, but simply a random enhancement of four bright starsabove the background level. This stars sample includes HD 358033 and GSC05778-0082, together with M 73, which is referred to as a binary ormultiple star, but actually represents the whole asterism. Since NGC6994 is not the first case (see for instance Carraro & Patat 1995),this raises the possibility that other open clusters may have beenmisclassified. We also suggest that NGC 6994 is unlikely to be an opencluster remnant (OCR). Based on observations carried out at ESO, LaSilla.
| Optical imaging and spectroscopy of BL Lac objects
We present optical images and spectroscopy for a dozen of BL Lacobjects. Most of these objects were not previously studied and we givefor the first time the properties of their host galaxies. The propertiesof the new host galaxies are generally consistent with those derived inprevious optical studies. We found a case (1101-23) where the externalisophotes of the galaxy are clearly boxy. In addition we gatheredspectroscopy for several BL Lac objects with unknown redshift and forcompanion galaxies. This allowed us to derive a tentative redshift fortwo new BL Lacs and to investigate the environment around PKS 0829+04.These data complement existing data available in the literature on hostgalaxies of BL Lacs and their (close) environments. Based onobservations collected at the European Southern Observatory, La Silla,Chile.
| CCD photometry in the region of NGC 6994: The remains of an old open cluster
We present the results of BV(RI)KC CCD photometry down toV=21 mag in the region of NGC 6994. To our knowledge,no photometry has previously been reported for this object and we findevidences that it is a poor and sparse old open cluster, with a minimumangular diameter of 9 arcmin, i.e. larger than the 3 arcmin originallyassigned to it. We obtain a color excess EB-V = 0.07 +/- 0.02mag by means of the BVIC technique. Based on the theoreticalisochrones from \cite{van85} that are in better agreement with our data,we estimate for this cluster a distance from the Sun of 620 pc(V0-MV = 9 +/- 0.25 mag) and an age lying withinthe range of 2 - 3 Gyr, adopting solar metallicity. Thus, thecorresponding cluster's Galactocentric distance is 8.1 kpc and is placedat about 350 pc below the Galactic plane. According to these results,NGC 6994 belongs to the old open cluster population located in the outerdisk and at large distances from the Galactic plane, and must havesuffered significant individual dynamical evolution, resulting in masssegregation and evaporation of low mass stars. Table~1 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Shoot-out at Star Hill Inn
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| Tracking down the Helix.
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| The Cygnus Loop at 408 MHz - Spectral variations, and a better overall view
Observations of the Cygnus Loop made at 408 MHz are presented. Radiospectral variations within the remnant are revealed by comparing the newobservations with existing data taken at 2.695 GHz. The brightnortheastern arc of the Cygnus Loop has a radio spectral index alpha ofroughly 0.35, whereas the southern portion of the remnant generallyshows steeper spectra, with alpha up to roughly 0.55. There is probablya portion of the faint radio emission from the northeastern arc that hasa flatter spectrum than the brighter radio emission nearby. Thesedifferences are discussed in relation to mechanism for producing radioemission in SNRs and to the overall structure of the remnant. Theobservation confirm that there is faint radio emission outside thebright limb-brightened 'shell' of the remnant, but this is limited tothe west only. The faint halo of X-rays around the remnant is discussedand interpreted as originating from dust scattering.
| Catalogue des nebuleuses et des amas d'etoiles.
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