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|Tiny bat vs mighty Eagle.|
|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.
|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.
|NGC 6738: Not a real open cluster|
A photometric, astrometric and spectroscopic investigation of the poorlystudied open cluster NGC 6738 has been performed in order to ascertainits real nature. NGC 6738 is definitely not a physical stellar ensemble:photometry does not show a defined mean sequence, proper motions andradial velocities are randomly distributed, spectro-photometricparallaxes range between 10 and 1600 pc, and the apparent luminosityfunction is identical to that of the surrounding field. NGC 6738therefore appears to be an apparent concentration of a few bright starsprojected on patchy background absorption.
|An Infrared and Radio Study of the Galactic Worm GW 46.4+5.5|
In order to study the physical properties and origin of the Galacticworm GW 46.4+5.5, we have carried out high-resolution (~3') H I and CO(J=1-0) line observations and analyzed available infrared and radioemission survey data. GW 46.4+5.5 appears as a long (~8 deg),filamentary structure extending vertically from the Galactic plane inboth median-filtered far-infrared and radio continuum maps. TheI60/I100 ratio in GW 46.4+5.5 is estimated to be0.29+/-0.05, which is significantly higher than the value determined forthe solar neighborhood. The high ratio is consistent with a hypothesisthat the dust grains in the worms have been processed by interstellarshocks. The radio continuum emission from GW 46.4+5.5 has spectral indexα~=-0.47 and does not correlate with I60 except foremission at low (|b|<=0.5d) latitudes. Thus, most of the radiocontinuum emission is likely to be nonthermal. Our one-dimensional H Iobservations show that the H I gas associated with GW 46.4+5.5 is mainlyat vLSR~=15-40 km s-1. The H I gas is clumpy, andwe detected two molecular clouds associated with the H I peaks. Themolecular clouds have large internal velocity dispersions, 8.0 and 6.6km s-1, compared with their masses, 2.8x103 and1.7x103 Msolar, which implies that they are notgravitationally bound. Using the Leiden-Dwingeloo H I data, we identifyan expanding H I supershell associated with GW 46.4+5.5, which iscentered on (l,b)~=(42deg,5deg) with an angularsize of 14degx22deg (or 340x540 pc2 at1.4 kpc). The supershell appears between vLSR~=18 and 40 kms-1 and slowly decreases in size as the velocity increases.An averaged position-velocity diagram reveals that the supershell has acentral velocity of ~18 km s-1, giving a kinematic distanceof 1.4 kpc and an expansion velocity of ~15 km s-1. Assumingthat it has been created by multiple stellar winds and supernovaexplosions, we estimate its kinematic age and the energy required toproduce it to be about 5 Myr and 1.5x1052 ergs, respectively.The structure is also visible in median-filtered radio continuum maps,but not in the ROSAT maps. The observed molecular clouds might havecondensed out of shock-compressed gas in GW 46.4+5.5 because they areclosely associated with the H I gas in velocity as well as in position.Their altitudes are 80 and 100 pc, respectively, higher than the scaleheight of the thin molecular gas disk. The physical properties of theclouds are very similar to those of the high-altitude clouds observedrecently in sensitive wide-latitude CO surveys. Our results suggest thatat least some of the high-altitude clouds might have formed in Galacticworms (or swept-up H I shells and supershells).
|Catalogue of Eclipsing and Spectroscopic Binary Stars in the Regions of Open Clusters|
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