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|Observations of 14 young open star clusters with the HEGRA system of Cherenkov telescopes|
Context: .A sample of 14 young open star clusters has been observed inthe TeV energy regime with the stereoscopic system of the HEGRA (HighEnergy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002,resulting in more than 300 h of observation time. Aims: .Youngopen star clusters may contribute to the acceleration of cosmic rays.The detection of γ-rays (from decaying π^0s produced inhadronic interactions) from these objects could be evidence for such acontribution. The results of our observations are compared to availableγ-ray data and to a simple hadronic model in the framework ofshock front acceleration of cosmic rays in the stellar winds of thecluster members to test the potential of the presently available data onyoung open star clusters to constrain this type of model. Methods:.The stereoscopic system of HEGRA Cherenkov telescopes makes use of theatmospheric imaging technique. Air showers initiated by primaryGamma-Rays are recorded as elliptical images in the telescope cameras.The images from the different telescopes are then superimposed toreconstruct the parameters of the primary particle. This technique(stereoscopy) was pioneered by the HEGRA experiment. Results: .Nosignificant excess has been found in the analysed data set of young openstar clusters. The derived upper limit on the TeV gamma-ray flux fromBerkeley 87 and the available EGRET data from the same direction do notallow us to fully constrain the simple hadronic model used here. Thecomparison of the upper limits derived for all 14 objects with the fluxdetected from TeV J2032+4130 (under the assumption of an association ofthe TeV-signal with the compact stellar association Cyg OB2) suggeststhat γ-ray emission from young open star clusters as an objectclass cannot be ruled out.
|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.
|Revisiting the population of Galactic open clusters|
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
|Kinematics of the Gould belt based on open clusters.|
|Astrophysical parameters of Galactic open clusters|
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
|New Herbig-Haro objects and giant outflows in Orion|
We present the results of a photographic and CCD imaging survey forHerbig-Haro (HH) objects in the L1630 and L1641 giant molecular cloudsin Orion. The new HH flows were initially identified from a deepHα film from the recently commissioned AAO/UKST Hα Survey ofthe southern sky. Our scanned Hα and broad-band R images highlightboth the improved resolution of the Hα survey and the excellentcontrast of the Hα flux with respect to the broad-band R.Comparative IVN survey images allow us to distinguish between emissionand reflection nebulosity. Our CCD Hα, [Sii], continuum and I-bandimages confirm the presence of a parsec-scale HH flow associated withthe Ori I-2 cometary globule, and several parsec-scale strings of HHemission centred on the L1641-N infrared cluster. Several smalleroutflows display one-sided jets. Our results indicate that, fordeclinations south of -6 deg in L1641, parsec-scale flows appear to bethe major force in the large-scale movement of optical dust andmolecular gas.
|Probable binary open star clusters in the Galaxy.|
The existence of double/binary clusters in the Magellanic Clouds isfairly well established, whereas only one such pair, h + χ Persei,is known in the Galaxy. From the catalogues of open clusters of theGalaxy, we have identified 18 probable pairs of clusters (with knowndistances), with spatial separations less than 20pc. The tidaldisruption timescales for these pairs, due to Galactic differentialrotation are calculated, using cluster data where available or byassuming typical values. In some cases, these timescales are larger thanthe average open cluster lifetime, =~10^8^yr. About 8% of open clustersappear to be members of binary systems, and hence binary cluster systemsmay not be very uncommon in the Galaxy.
|M is for Messier.|
|A binocular star-hop in Orion.|
|Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)|
|Some characteristics of complexes of open star clusters|
Mean coordinates and velocities, phase sizes, mean elements of galacticorbits, mean ages, and metal abundances are given for 11 complexes ofopen clusters, and correlations between these characteristics arediscussed. The possible existence of a supercomplex encompassing 9 or 10complexes, and probably a number of individual clusters, is discussed.This rotates at an angular velocity of 10 to 13 km/s kpc.
|Catalog of AP and AM stars in open clusters|
The previous results of Raab (1922), Markarian (1951), and Collinder(1931) have been used to catalog Ap and Am stars that are in the fieldof open clusters. Tabular data are presented for the clusterdesignation, the HD or HDE number, the right ascension (1900), thedeclination (1900), and the magnitude. Also listed are the spectraltypes and, for certain stars, the probability of cluster membership.
|Kinematics of young open clusters and the rotation curve of our Galaxy|
Published observational data on a sample of 105 kinematically andspatially distinct open clusters of early spectral type (up to B3) arecompiled in tables, graphs, and diagrams and characterizedstatistically. Findings reported include (1) solar motion expanding atLSR velocity 3 km/s or less (with no noncircular motion in the directionof rotation), (2) Oort constant A = 17.0 + or - 1.5 km/s kpc andsecond-order rotation term alpha = -2.0 + or - 0.6 km/s sq kpc at R-R0between -3 and 5 kpc, (3) maximum rotation-curve deviation + or - 10km/s at R-R0 about - or + 2 kpc, and (4) nondecreasing rotationalvelocities beyond about R-R0 = 3 kpc. The rotational velocities of H IIregions and molecular clouds in the Perseus arm are found to besignificantly lower than those of the open clusters.
|The large system of molecular clouds in Orion and Monoceros|
Emission is noted over about one-eighth of an 850-sq deg region centeredon Orion and Monoceros that has been surveyed in the J = 1 to 0 line ofCO; most of the emission arises from giant molecular clouds associatedwith Orion A and B, and Mon R2. A much smaller area was surveyed forC-13O emission. A comparison of cloud masses obtained by threeindependent methods indicates that CO luminosity is as accurate ameasure of cloud mass as other indicators. The possible relationshipsamong clouds in the survey are discussed, including the conjecture thatthe overall Orion complex of clouds is a much larger system thanpreviously considered, incorporating most of the clouds in the presentsurvey.
|Catalogue of Eclipsing and Spectroscopic Binary Stars in the Regions of Open Clusters|
|Positions of stars in regions of 14 southern galactic clusters|
Positions have been obtained for a total of 3487 stars scattered over 14regions that are centered on each of the southern galactic clusters NGC1981, 2287, 2437, 2451, 2516, 2546, 2547, 2548, 3114, 3532, IC 2391,2395, 2602, and Truempler 10. A frame of reference has been establishedfor each region using ESO Schmidt plates centered on the clusters, witheach plate containing 20-35 measurable Perth 70 stars that are used fordetermining the positions of 200-400 fainter stars within a centralfield of 25 min of arc radius (covering the corresponding 1.5-m plates).
|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.
|Interstellar Calcium Lines in the Spectra of Stars in Open Clusters.|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1949ApJ...110..117S&db_key=AST
|The Distance of the Orion Nebula|
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