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|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters|
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
|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 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.
|On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient|
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
|BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters|
We have initiated a long-term project, the BOAO photometric survey ofopen clusters, to enlarge our understanding of Galactic structure usingUBVI CCD photometry of open clusters, which have been little studiedbefore. This is the second paper of the project, in which we present thephotometry of 12 open clusters. We have determined the clusterparameters by fitting the Padua isochrones to the color-magnitudediagrams of the clusters. All the clusters except for Be 60 and NGC 1348are found to be intermediate-age to old (0.2-4.0 Gyrs) open clusterswith a mean metallicity of [Fe/H]~0.0.
|Dust emission from star-forming regions. VI. The submillimetre YSO cluster in NGC 2264|
We present new submillimetre and millimetre continuum observations ofNGC 2264, in the region of Allen's infra-red source IRS1, at 350, 450,800 and 1300 μm taken with the 15-m JCMT and the IRAM 30-m MRT. Weobserve a ridge of bright submillimetre and millimetre emission which wedesignate NGC 2264 MM. Within this ridge we have discovered a cluster offive submillimetre sources which we label NGC 2264 MMS1-5. The masses ofthe sources lie in the range ~ 10-50M_sun and each source has a FWHM of<=0.1pc. The total mass of the ridge is ~ 730M_sun. We identify MMS3as harbouring the driving source of a previously discovered bipolaroutflow. We note that MMS5 is positionally coincident with a previouslydiscovered HST near-ir source, but we conclude that this is a chancealignment, since typical extinction values within the ridge lie in therange A_V ~ 200-1600. The cluster which we have discovered is of highermass and extends over a larger area than the low-mass sub-clusterdiscovered by the HST surrounding IRS1. Our continuum maps show agreater dynamic range than molecular spectral line maps, due to gasfreezing out onto dust grains at the densities of these sources. Wesuggest that each of the sources MMS1-5 is forming one or moreintermediate or high mass stars.
|A new CO survey of the Monoceros OB1 region.|
A new, fully sampled, sensitive CO survey of 52.5 square degrees towardsthe Monoceros OB1 (Mon OB1) region has been completed using the CfA 1.2m millimeter-wave radio telescope. This survey extends from b=-1.5°to b=+3.5° and from l=196.0° to l=206.5° on a uniformlyspaced grid of 3.75' in l and b, and has substantially bettersensitivity than previous surveys of the region. CO is detected in 80%of the area, 60% of which is weak with integrated intensities less than5K.km/s. The large-scale kinematics are dominated by the Perseus andLocal spiral arms. Within the Local arm the molecular gas appears tohave been strongly influenced by supernova remnants and expanding HIIregions with the strongest emission at velocities ranging from -17 to+10km/s, and a total mass of 1.3x10^5^Msun_. Weak CO emissionfrom Perseus arm clouds shows that they have a mean kinematic distanceof 3.5kpc and are comparable in size and mass to local molecular clouds.The coincident positions of colour selected IRAS point sources indicatesthat these distant clouds are undergoing massive star formation.
|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
|The Maxwellian alternative to the dark matter problem in galaxies|
The observed superrotation of the material at the outer edge of galaxiesis considered theoretically. Instead of explaining this motion in termsof a dark-matter halo, a Maxwellian-type interaction between masses isproposed. In this scheme, the luminous matter in the central galaxy andits motion provide the sources and vortices of a generalizedgravitational field, and a gravoinductional force generated by masscurrents leads to a new type of gravitational coupling between masses.It is shown analytically that this mechanism should have a greatereffect than Newtonian gravity at large distances from the rotatingcentral mass. Several specific problems are explored, and possibleobservational tests of the theory are discussed.
|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.
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