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A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Spectroscopic survey of post-AGB star candidates
Aims.Our goal is to establish the true nature of post-AGB starcandidates and to identify new post-AGB stars. Methods: We used lowresolution optical spectroscopy and we compared the spectra of thecandidate post-AGB stars with those of stars in the library spectaavailable in the literature and with spectra of "standard" post-AGBstars, and direct imaging in narrow-band filters. Results: Spectra wereobtained for 16 objects: 14 objects have not been observed previouslyand 2 objects are already known post-AGB stars used as "standards" foridentification. From the spectra we identify: six new post-AGB starswith spectral types between G5 and F5, two H ii regions the morphologyof which is revealed in the direct images for the first time, a G giantwith infrared emission, a young stellar object, a probable post-AGB starwith emission lines and three objects for which the classification isstill unclear. As a whole, our results provide new, reliableidentifications for 10 objects among listed post-AGB star candidates.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC), and at the Observatorio de Sierra Nevada, whichis operated by the Consejo Superior de InvestigacionesCientíficas through the Instituto de Astrofísica deAndalucía (Granada, Spain). Appendices A-D are only available inelectronic form at http://www.aanda.org
| Star formation in the region of young open cluster - NGC 225
NGC 225 is believed to be a 120 Myr old open cluster located at tilda650 pc. Eight stars with H alphaemission are found to belocated around the cluster, of which two are probable Herbig Be stars,indicating a very young age for the cluster. To explore whether theHerbig Be stars, which are pre- main sequence (PMS) stars are part ofthis cluster, we re-estimated the cluster parameters using optical(UBV)pg and 2MASS JHK photometry. We combined the above data to detectthe presence of any possible PMS stars in the cluster region. Among theidentified 28 proper motion members, 15 stars were found to havenear-infrared (NIR) excess indicating that they are PMS stars. Also,most of the upper MS stars were found to show NIR excess suggesting thatthe brighter proper motion member stars have not yet reached the MS. PMSisochrones were used to estimate the age of stars with NIR excess and isfound to be between 0.5-10 Myr. Thus, the cluster NGC 225 is a veryyoung cluster, younger than 10 Myr and its age is not 120 Myr aspreviously believed. We propose that a recent star formation hasresulted in the formation of NGC 225, two Herbig Be stars, stars with Hemission, dust lanes and nebulosity in the vicinity of the cluster.
| Caroline Herschel's catalogue of nebulae
Not Available
| 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.
| Deepsky delights.
Not Available
| NSV 16, the Enigmatic Variable in Cassiopeia
A study of NSV 16 using 411 plates of Moscow collection shows behaviordiffering from one season to another. The object is quite enigmatic anddeserves a detailed study, including spectroscopy.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| 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
| The CFHT Open Star Cluster Survey. I. Cluster Selection and Data Reduction
We present this paper in conjunction with a companion paper as the firstresults in the Canada-France-Hawaii Telescope Open Star Cluster Survey.This survey is a large BVR imaging data set of 19 open star clusters inour Galaxy. This data set was taken with the CFH12K mosaic CCD(42'×28'), and the majority of the clusters were imaged underexcellent photometric, subarcsecond seeing, conditions. The combinationof multiple exposures extending to deep (V~25) magnitudes with short(<=10 s) frames allows for studies ranging from faint white dwarfstars to bright turnoff, variable, and red giant stars. The primary aimof this survey is to catalog the white dwarf stars in these clusters andestablish observational constraints on the initial-final massrelationship for these stars and the upper mass limit to white dwarfproduction. Additionally, we hope to better determine the properties ofthe clusters, such as age and distance, and also test evolution anddynamical theories by analyzing luminosity and mass functions. In orderto more easily incorporate these data in further studies, we haveproduced a catalog of positions, magnitudes, colors, and stellarityconfidence for all stars in each cluster of the survey. This reduction,along with the computed calibration parameters for all three nights ofthe observing run will encourage others to use these data in differentastrophysical studies outside of our goals. Additionally, the data setis reduced using the new TERAPIX photometric reduction package, PSFex,which is found to compare well with other packages. This paper isintended both as a source for the astronomical community to obtaininformation on the clusters in the survey and as a detailed reference ofreduction procedures for further publications of individual clusters. Wediscuss the methods employed to reduce the data and compute thephotometric catalog. We reserve both the scientific results for eachindividual cluster and global results from the study of the entiresurvey for future publications. The first of these further publicationsis devoted to the old rich open star cluster, NGC 6819, and appears as acompanion paper in the same issue of the Journal.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Kinematics and binaries in young stellar aggregates. I. The trapezium system BD+00(deg) 1617 in Bochum 2
Internal kinematics, spectroscopic binaries and galactic motion areinvestigated for the trapezium system BD+00(deg) 1617 (which lies at theheart of the young open cluster Bochum 2) by means of 73 high resolutionEchelle+CCD spectra secured over the period 1994-98. Two of the threeO-type member stars are found to be binaries on close and highlyeccentric orbits of 6.8 and 11.0 day period. The spectra of the twobinaries show large variations in the half-intensity and equivalentwidths of the HeI absorption lines, which are intrinsic to the primariesand are not correlated to the orbital phase. Astrometric and radialvelocities exclude that one of the component is leaving as a runawaystar, but upper limits are still compatible with the trapeziumevaporating at very low relative velocities. The projected rotationalvelocities of the three constituent O-type stars are low. This conformsto expectations from the high frequency of binaries. The observed radialvelocity of Bochum 2 agrees with Hron (1987) expression for the Galaxyrotation inside the latter's quoted errors. Tables~2 and 3 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc:u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.
| GAIA Spectroscopy: Proposing the 8500--8750 Angstroms Region and Evaluating the Performances
We propose the Gaia spectroscopic observations to be performed over thewavelength interval 8500--8750 Angstroms, with an optimal dispersion of0.25 Angstroms/pix (or 1000 pixels budget per spectrum) and a 2 pixelPSF. In this paper, on the base of extensive observations as well assynthetic spectra and simulations, we review the spectroscopicperformances expected for Gaia: radial and rotational velocities,spectral classification, detection of mass-loss and spectralpeculiarities, chemical abundance analysis and reddening estimates fromthe 8620 Angstroms diffuse interstellar band. Lower dispersion spectra(corresponding to smaller pixel budgets) are considered too.
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| Binary stars in the vicinity of the open cluster NGC 6913.
Not Available
| Spectral Classification of "Photometric Subgiants" in Open Clusters
Color-magnitude diagrams of several intermediate-age open clusterscontain apparent post-main-sequence stars which are fainter thanpredicted by standard evolutionary tracks. We have obtainedclassification spectra for such stars in eight clusters. Some of theseare reddened background early-type stars, and several of the late-typestars turn out to have luminosities indicative of being foreground tothe cluster. A few stars appear to be cluster members but do not fit thetheoretical tracks. We find that the Bahcall-Soneira Galaxy model,modified for low galactic latitudes, can give statistically usefulpredictions of the numbers of field stars.
| UBVRI CCD photometry of the old open cluster NGC 6253
We present UBVRI photometry for the old open cluster NGC 6253. Fromcomparison of the observed colour-magnitude diagrams with simulationsbased on stellar evolutionary models we derive in a self-consistent waythe reddening, the distance and the age of the cluster:E(B-V)=0.23-0.32, (m-M)_0=10.9+/-0.1, with metallicity approximatelydouble the solar value and age ~=3 Gyr. The cluster has a binarysequence, discernible even through the field contamination, suggestingthat about 1/3 of the cluster members belong to binary, or multiple,systems.
| Cluster membership determinations from proper motion surveys
Not Available
| DDO Metal Abundances of High-Luminosity Late-Type Stars in Galactic Open Clusters
Results from UBV and DDO photometry are presented for 54 high-luminositylate-type stars in the fields of 23 open clusters. The probability ofcluster membership for each observed star is evaluated using twoindependent photometric criteria. It is found that 32 stars are verylikely cluster members, the remaining ones being almost certainly fieldobjects. The recently improved calibrations of the DDO system have beenused to derive MK spectral types, effective temperatures, andmetallicities, while E(B-V) color excesses have been determined throughknown photometric and spectroscopic procedures. The DDO metallicitiesrange between values typical of moderately metal-poor ([Fe/H]=~ -0.3) tomoderately metal-rich ([Fe/H] =~ 0.2) clusters. The masses of thecluster giants range between 1 and 4 solar masses, with the scatterwithin a cluster being less than 1 solar mass. (SECTION: StellarClusters and Associations)
| Estimates of geometric and dynamic parameters of star-gas complexes in the Galaxy
Parameters of geometric models of 11 gas-star complexes (GSCs) wereobtained. We used information about GSC projections onto the celestialsphere and the Galactic plane and about GSC extension along the line ofsight. GSCs were represented as triaxial ellipsoids. To estimate thesemiminor axis of the GSC ellipsoidal model and GSC slope angle to theGalactic plane, we used data on spatial location of open stellarclusters (OSCs) entering GSCs. GSC slopes to the Galactic plane varybetween 2.5 and 20.5 deg. Their semiminor axes are between 11 and 164pc. GSC total masses are estimated from GSC tidal effect on OSCs thatare members of the corresponding GSCs. The effect manifests itself insmaller sizes of young OSCs as compared to their tidal sizes in theforce field of the Galaxy. We used studies of stability of an OSC movingin the joint force field of the Galaxy and spheroidal stationary GSC, aswell as studies of evolution of a virialized cluster located at thecenter of a nonstationary ellipsoidal GSC. Estimated total masses fordifferent GSCs lie between 0.65 x 10 exp 5 solar masses and 11.5 x 10exp 7 solar masses.
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| Memberships and CM diagrams of young open clusters. I - NGC 225
Results are presented on astrometric, photometric, and spectroscopicstudies of the loose young open cluster NGC 225, conducted on the basisof observations and archived plate material. The main cluster parametersderived in this study are presented in the form of a table. Astrometricmember candidates were checked against photometric and spectroscopiccriteria, yielding an enhanced list of cluster members which is suitablefor further and more extensive investigations.
| The luminosity of reflection nebulae
Consideration is given to the connection between the dispersion inHubble's relation for reflection nebulae and their morphologicalcharacteristics. Statistical estimates show that the reflection nebulaeconnected with more opaque dust-gas clouds, compared to those near lessopaque clouds, have large values of Delta-B (the measure of deviationsin Hubble's relation) and a high surface brightness. These propertiesare more pronounced in the Herbig Ae/Be stars. Stars in the nebulaeconnected with more opaque dust-gas clouds were found to exhibit excessreddening, with R above 3.1. It is concluded that the dispersion in theHubble relation is mainly due to the optical depth of the nebulae.
| Theoretical color-magnitude diagrams of open clusters
Theoretical isochrones were constructed for clusters with ages between10 to the 7th and 10 to the 9th yrs. The isochrones are transformed toM(v)-(B-V) coordinates. The ages of 40 open clusters are obtained on thebasis of these isochrones.
| Component Analysis of Open Clusters
Not Available
| Young stellar-gas complexes in the Galaxy
It is found that about 90 percent of OB-associations and o-b2 clusterssituated within 3 kpc of the sun can be united into complexes withdiameters of 150-700 pc. Almost all of these clusters contain giantmolecular clouds with a mass greater than about 100,000 solar masses. Anumber of complexes are associated with giant H I clouds; a few of thesmall complexes are situated in the HI caverns. The concentration ofOB-associations and young clusters in star complexes attests to theircommon origin in the supergiant gaseous clouds.
| The Guide Star Photometric Catalog.
This paper presents data and finding charts for the Guide StarPhotometric Catalog (GSPC), which is an all-sky set of 1477pohotoelectrically determined BV sequences covering the magnitude rangefrom 9 to 15. The GSPC was created to provide photometric calibratorsfor the Guide Star Catalog, which is a catalog of approximately 2 x 10to the 7th objects needed to support the pointing of the Hubble SpaceTelescope. For declinations greater than +3 deg, the sequences generallylie near the centers of the original (Palomar Observatory-NationalGeographic Society) Sky Atlas, while for smaller declinations they lienear the centers for the ESO/SERC Southern Sky Atlas. The sequencesnominally contain (at least) six stars, each with a photometricprecision of 0.05 mag. In practice, a small number of sequences containsfewer stars, and the precisions achieved for the faintest stars are morenearly 0.1 mag.
| The classification of open clusters by the centroid method of cluster analysis
The distribution of open clusters in the Galaxy are considered, withspace coordinates including mass, absolute magnitude, integrated colorindex, diameter, metallicity, and age. It is shown that the majority ofclusters belong to several classes which have parameter values in asufficiently narrow range. The classes form a linear sequence by age andmonotonic sequence on a color-magnitude diagram. They are not isolated,but move into each other continuously. This suggests that the process ofcluster formation contains no significant gaps. The bifurcation of theage sequence of classes depending on the mass and diameter values isfound. This bifucation makes an evolutionary interpretation possible.
| Yellow evolved stars in open clusters
This paper describes a program in which Galactic cluster post-AGBcandidates were first identified and then analyzed for clustermembership via radial velocities, monitored for possible photometricvariations, examined for evidence of mass loss, and classified ascompletely as possible in terms of their basic stellar parameters. Theintrinsically brightest supergiants are found in the youngest clusters.With increasing cluster age, the absolute luminosities attained by thesupergiants decline. It appears that the evolutionary tracks ofluminosity class II stars are more similar to those of class I than ofclass III. Only two superluminous giant star candidates are found inopen clusters.
| Mass-losing red giants in open clusters
Mass-losing stars in open clusters with main-sequence turn-offs atintermediate mass have been searched for by using the IRAS data base.The absence of many strong 60 micron sources in open clusters impliesthat intermediate-mass stars lose much of their mass during an intensewind phase of rather short duration. For stars of about seven solarmasses, this phase, if it exists at all, lasts for not much more than100,000 yr. For stars of about four solar masses, the intense wind phaseappears to last considerably less than 10 million yr; it may well lastfor less than a million yr.
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