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NGC 6996



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CCD BV survey of 42 open clusters
Aims.We present results of a photometric survey whose aim was to derivestructural and astrophysical parameters for 42 open clusters. While oursample is definitively not representative of the total open clustersample in the Galaxy, it does cover a wide range of cluster parametersand is uniform enough to allow for simple statistical considerations. Methods: BV wide-field CCD photometry was obtained for open clusters forwhich photometric, structural, and dynamical evolution parameters weredetermined. The limiting and core radii were determined by analyzingradial density profiles. The ages, reddenings, and distances wereobtained from the solar metallicity isochrone fitting. The mass functionwas used to study the dynamical state of the systems, mass segregationeffect and to estimate the total mass and number of cluster members. Results: This study reports on the first determination of basicparameters for 11 out of 42 observed open clusters. The angular sizesfor the majority of the observed clusters appear to be several timeslarger than the catalogue data indicate. The core and limiting clusterradii are correlated and the latter parameter is 3.2 times larger onaverage. The limiting radius increases with the cluster's mass, and boththe limiting and core radii decrease in the course of dynamicalevolution. For dynamically not advanced clusters, the mass functionslope is similar to the universal IMF slope. For more evolved systems,the effect of evaporation of low-mass members is clearly visible. Theinitial mass segregation is present in all the observed young clusters,whereas the dynamical mass segregation appears in clusters older thanabout log({age}) = 8. Low-mass stars are deficient in the cores ofclusters older than log({age}) = 8.5 and not younger than one relaxationtime.Tables 1-5 are only available in electronic form at http://www.aanda.org

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.

CCD Photometry and Classification of Stars in the North America and Pelican Nebulae Region. II. The Region of NGC 6996
Magnitudes and color indices of 620 stars down to V = 17.5 mag in theeight-color Vilnius + I photometric system are obtained in the area ofthe open cluster NGC 6996 in the North America Nebula. By combining theresults of optical photometry and the infrared 2MASS data an increasedvalue of the ratio of total-to-selective extinction is found. Forearly-type stars it corresponds to RBV=3.5. Spectral types,interstellar color excesses, extinctions and distances of stars aredetermined from the photometric data. The plot of extinction vs.distance shows that the extinction exhibits a steep rise at 400 pcreaching 3--4 mag. Forty seven main-sequence stars and three red giantsare identified as probable cluster members. The cluster distance isfound to be 794 pc by ZAMS fitting and 781 pc by averaging individualdistances of the 50 probable cluster members. The cluster stars show arange of interstellar extinction, with an average value of AVis 1.92 mag. Fitting the main sequence and red giants of the cluster tothe isochrones in the MV vs. (B--V)0 plane givesan age of 3.5× 108 yr. Probably NGC 6996 has no geneticrelation to the star-forming region in the North America and Pelicannebulae. About 35 stars in the magnitude range 16--17 in the field ofNGC 6996 are suspected to be O--B5 stars belonging to the Perseus spiralarm.

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.

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Observation and Astrometry data

Right ascension:20h56m25.00s
Apparent magnitude:99.9

Catalogs and designations:
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NGC 2000.0NGC 6996

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