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A Photometric and Spectroscopic Search for White Dwarfs in the Open Clusters NGC 6633 and NGC 7063
We present photometric and spectroscopic studies of the white dwarf (WD)populations in the intermediate-age open clusters NGC 6633 and NGC 7063as part of the ongoing Lick-Arizona White Dwarf Survey. Using wide-fieldCCD imaging, we locate 41 candidate WDs in the two cluster fields: 32 inNGC 6633, and 9 in NGC 7063. Spectroscopic observations confirm 13 ofthese candidates to be bona fide WDs. We describe in detail ourBalmer-line fitting technique for deriving effective temperatures andsurface gravities from optical DA WD spectra and apply the technique tothe 11 DA WDs in the sample. Of these, only two DA WDs are at thecluster distance moduli, one in each cluster. Two more DAs lie 0.75 magforeground to NGC 6633, raising the possibility that these are doubledegenerate systems in the cluster. If they are nearly equal-massbinaries, both of these systems likely have combined masses above theChandrasekhar limit. One DB WD is found to be consistent with membershipin NGC 6633, which would make this the third confirmed He-atmosphere WDin an open cluster, although further data are needed to confirm clustermembership. The WD consistent with membership in the cluster NGC 7063has a low mass (~0.4 Msolar), suggesting it may be a He-coreWD resulting from close binary evolution. Three of the 11hydrogen-atmosphere WDs in this study are observed to have Ca IIabsorption; the number of DAZs in this study is consistent with previousobservations that ~25% of field WDs are DAZs.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Calibration of a UBVRI sequence around Nova Cyg 2006
A highly accurate UBV(RI)c photometric sequence around Nova Cyg 2006 ispresented. The photometric stability of the stars in the sequence hasbeen checked on 12 different nights. The sequence is linked to theLandolt's equatorial standards for (RI)c bands and to UBV photoelectricphotometry of selected open clusters by Hoag et al. (1961).
| 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.
| The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| 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.
| Observations of white dwarfs in open clusters: the initial-final mass relation and the "white dwarf deficit"
We present the first results from a photometric and spectroscopicinvestigation of white dwarfs (WDs) in intermediate-age open clusters (8Msun ≳ MMSTO ≳ 3Msun).Deep UBV photometry of several open clusters results in catalogues ofcandidate WDs in each cluster. High signal-to-noise spectra have beentaken of the WD candidates in three open clusters: IC 4665, NGC 6633,and NGC 7063. The rate of success in finding WDs is ~ 67%, with non-WDsidentified as hot subdwarf stars and QSOs. By fitting spectra to modelatmospheres, the Teff and log g for each WD was determined,from which the WD's mass and cooling age are inferred. Comparison withthe photometry permits separation of cluster WDs from background andforeground field WDs. From the cluster WDs, an empirical initial-finalmass relation is derived. The WD catalogues contain many fewer WDs thanwould be expected in each cluster based on the observed number ofmain-sequence cluster members. This "white dwarf deficit" persists evenafter careful consideration of sample completeness and allowances forWDs in multiple-star systems and dynamical evolution of the cluster havebeen made. We explore the possibility that the maximum mass of whitedwarf progenitors is lower than the commonly-stated 8Msun.The implications of such a result could include an increase in theexpected rate of core-collapse supernovae and challenges to existingmodels of the final stages in stellar evolution for stars withprogenitor masses between 6Msun and 8 Msun. Wealso report on the discovery of a potential helium-core WD in the opencluster NGC 7063. The age of the WD as derived from recent models is ~200Myr, in disagreement with the cluster age of 100 Myr.
| A Spectroscopic Search for λ Bootis and Other Peculiar A-Type Stars in Intermediate-Age Open Clusters
As part of our continuing search for peculiar A-type stars, especiallyλ Bootis stars, in open clusters of all ages, we have obtainedclassification spectra of 130 late B, A, and early F-type stars in 12intermediate-age open clusters, including NGC 1039, 2281, 2548, 6633,7039, 7063, 7092, and 7209, IC 4665, IC 4756, Stock 2, and Praesepe. Thespectra were obtained with resolutions of 1.8 and 3.6 Å on the 0.8m telescope of Appalachian State University and were classified on theMK system. Numerous classical Ap and Am stars were found among the 130,including two new Ap stars in NGC 7092. In addition, three emission-linestars and two candidate λ Bootis stars were found. Neither ofthese λ Bootis candidates turned out to be members of theirrespective clusters. Combined with 184 stars previously classified in 10other intermediate-age open clusters, also devoid of λ Bootisstars, a statistically significant null result is obtained. We discussthe implications of this null result for our understanding of theλ Bootis mechanism.
| 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
| A spectroscopic survey for lambda Bootis stars. III. Final results
In the third paper of a series dedicated to the spectroscopic survey fornew lambda Bootis stars, we present all new and confirmed members of thegroup as well as a detailed analysis of the observed sample. The natureof this small group of chemically peculiar stars of the upper mainsequence still challenges our understanding of processes like diffusion,mass-loss and accretion. The typical abundances pattern (nearly solarvalues for C, N, O and S whereas the Fe-peak elements are moderate tostrong underabundant) can still not be explained by any proposed theory.Hence, the significant increase of new members gives the opportunity toinvestigate the group properties in more detail. We report the discoveryof 26 new members of the group and the confirmation of 18 candidatesfrom the literature. This almost triples the number of known lambdaBootis stars. The existence of one member in the young open cluster NGC2264 and four members in the Orion OB1 association proves that thelambda Bootis phenomenon already works at very early stages of stellarevolution. Recent results from the Hipparcos mission have shown that thewell established lambda Bootis stars of the Galactic field comprise thewhole area from the Zero Age Main Sequence to the Terminal Age MainSequence (~ 109 yr for an A-type star). There is a continuoustransition between very young and rather evolved evolutionary stages. Wefind that the overall percentage of lambda Bootis type among all normaltype stars in the spectral range from B8 to F4 is 2% in the Galacticfield as well as in open clusters. Furthermore, 44 metal-weak objectsare listed which might be connected with the lambda Bootis phenomenon.Our biased sample (chosen by photometric boxes) is not distinguishedfrom all A-type stars in the corresponding spectral region by therotational velocity distribution. Only for the luminosity classes IV andIII (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| 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
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Baldone Schmidt Telescope Plate Archive and Catalogue
The article presents information on the archive and catalogue of theastrophotos taken with the Schmidt telescope of the Institute ofAstronomy of the University of Latvia (until July 1, 1997 --Radioastrophysical Observatory of the Latvian Academy of Sciences) inthe period 1967--1998. The archive and catalogue contain more than 22000direct and 2300 spectral photos of various sky regions. Information onthe types of photo materials and color filters used as well as on mostfrequently photographed sky fields or objects is given. The catalogue isavailable in a computer readable form at the Institute of Astronomy ofthe University of Latvia and at the Astrophysical Observatory in Baldone(Riekstukalns, Baldone, LV-2125, Latvia), e-mail: astra@latnet.lv.
| Proper motions of stars in the field of the open cluster NGC 7063.
Not Available
| Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
| 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.
| 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.
| 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.
| Ultraviolet and optical spectrophotometry of the Seyfert 1.8 galaxy Markarian 609
Ultraviolet and optical observations of the Seyfert 1.8 galaxy Mrk 609were collected simultaneously. The observations reveal strong line andcontinuum emission in the UV, an increase in the flux of H-beta and He I5876, and a decrease in the H-alpha/H-beta value since the measurementsby Osterbrock (1978, 1981), as well as an extended population ofearly-type stars, which is considered to be the source powering thelarger part of the far-IR emission. Special attention is given to theorigin of steep broad-line Balmer decrement measured by Osterbrock,since the strong UV continuum and the emission lines of Mrk 609 observedrule out reddening as the cause of the Balmer decrement. It is suggestedthat smaller-than-normal optical depths are likely to be the cause ofthe decrement.
| A photometric study of short period variable stars in open clusters
The photoelectric photometry of known Delta Scuti variable stars inselected open clusters (Coma, Praesepe, Pleiades, Alpha Per, and NGC2264) was carried out in order to determine the periods of pulsation.Multicolor uvby-beta photometry and a search for new variables in otheropen clusters (NGC 2539, NGC 6494, NGC 6882-5, NGC 7062, NGC 7063 andNGC 7686) were also performed.
| 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.
| Stromgren and H-beta photometry of early type stars in northern open clusters. I - NGC 7039, NGC 7063
Early-type stars up to 12.0 mag and spectral type F2 in two youngnorthern clusters were investigated by means of Stromgren and H-betaphotometry. The distance and reddening of the clusters were estimated,and the membership of the stars discussed. In the case of NGC 7039 adistance of 675 pc and a color excess of E(b-y) = 0.056 were found; therespective values for NGC 7063 were 635 pc and E(b-y) = 0.062. Thereality of NGC 7039 is somewhat puzzling: it seems that there exists aloose star aggregate called NGC 7039, containing about six to ninestars, and in the background another cluster at a distance of about 1500pc. Besides this, variable reddening across the cluster area isprobable.
| New variable stars in the region of the association T4 Cygni.
Not Available
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| Results of photographic observations of irregular variable stars in the T4 CYG association
Not Available
| Investigation of the initial mass spectrum of open star clusters
The mass spectra of 228 open star clusters were derived by comparison ofcolor-magnitude diagrams with evolutionary tracks. The application tobinary stars showed the reliability of the mass determination. Thederived mass spectra were fitted by power laws as well as exponentiallaws. It could be shown that both approximate the mass spectra of openstar clusters on the same average significance level. The presentinvestigation revealed a correlation of the slope of the mass spectrawith the cluster age, whereas a detected correlation of the slope withgalactocentric distance is slight. The results suggest that the slope ofthe mass spectrum increases with increasing cluster and galactocentricdistance. These findings are discussed with respect to their reasons andprevious results concerning open clusters and field stars.
| Yellow giants in young clusters. I - Photometric observations
To enable a study of the properties of luminous yellow stars, a list hasbeen compiled of 84 stars which appear to be in the appropriate regionin the color-magnitude diagrams of 41 young clusters. Proper motions,radial velocities, UBV photometry, and spectral types from theliterature have been used in those cases where such data are availableto assign membership probabilities. DDO and uvby photometry have alsobeen obtained of most of the stars in the list, and these data areanalyzed to remove stars from the sample which have the wrong luminosityor the wrong color excess for cluster membership. In addition, VRIphotometry is given for many of the stars in the list. Based on thesedata, 22 of the stars (26 percent) from the original sample are likelycluster members, while 48 (57 percent) are not likely to be members. Theremainder must be studied further to determine their status, but it isprobable that some of them are also in clusters.
| A search for new variable stars in the Milky Way field at L = 78 deg, B = - 6 deg
Positions, finding charts and elements of ninety-nine new variable starsin a Milky Way field (about 30 square degrees) around the Nova Cygni1970 and the symbiotic star HBV 475 are reported. In particular, periodsand light curves of thirty-eight new Mira variables are given. For theseMira variables the maximum of period distribution is attained around 345days. Statistical considerations are discussed for this sample.
| The absolute masses of 72 galactic clusters and 12 OB associations
The Reddish (1978) relative masses for 72 open clusters and 12 OBassociations are presently converted to absolute masses, within an errormargin of about 25 percent, using three calibration clusters of knownmass whose average mass is 300 solar masses. The Reddish techniqueassumes the initial stellar mass distribution function to be valid forall aggregates, together with a universal relationship between stellarmass and stellar luminosity.
| Open clusters and galactic structure
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
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