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Precision Kinematics and Related Parameters of the α Persei Open Cluster
A kinematical study of the nearby open cluster α Persei ispresented based on the astrometric proper motions and positions in theTycho-2 catalog and Second USNO CCD Astrographic Catalog (UCAC2). Usingthe astrometric data and photometry from the Tycho-2 and ground-basedcatalogs, 139 probable members of the cluster are selected, 18 of themnew. By the classical convergent point method, systematic motions ofstars inside the cluster and velocity dispersions are estimated. Asdirectly observed, the upper limit on the internal velocity dispersionper coordinate is 1.1 km s-1. The actual velocity dispersionis much smaller than that value, since all of it appears to come fromthe expected errors of the astrometric proper motions. The relativeposition of the convergent point with respect to the cluster starsyields the ``astrometric'' radial velocity, which turns out larger by afew km s-1 than the mean observed spectroscopic radialvelocity. This implies an overall contraction of the cluster. Kinematicparallaxes are computed for each member, and an improved H-R diagram isconstructed. An age of 52 Myr is determined by isochrone fitting. Thestar α Per itself fits an isochrone of this age computed withovershooting from the boundary of the convective zone. The theoreticalmass of the star α Per is 6.65 Msolar. With respect tothe common center of mass, half of the higher mass members (earlier thanG) are located within a radius of 10.3 pc. The cluster appears to beroughly twice as large, or as sparse, as the Pleiades, retainingnonetheless a similar dynamical coherence. The low rate of binaries isanother feature of this cluster, where we find only about 20% of membersto be known or suspected spectroscopic, astrometric, or visual binariesor multiple systems. X-ray emitters in the cluster appear to have thesame dispersion of internal velocities as the rest of the membership.The cluster is surrounded by an extended, sparse halo of comovingdwarfs, which are found by combining the proper-motion data from UCAC2with Two Micron All-Sky Survey infrared photometry. Since many of theseexternal stars are outside the tidal radius, the cluster being in anactive stage of disintegration or evaporation could be considered. Thishypothesis is not supported by the weak compression and thenonmeasurable velocity dispersion found in the kinematic analysis. Asearch for stars ejected from the α Persei cluster is carried outby tracking a large number of nearby stars 70 Myr back in time andmatching their positions with the past location of the cluster. Only oneplausible ejection is found prior to 10 Myr ago. The nearby star GJ 82,an active M dwarf with a strong Hα emission, is likely a formermember ejected 47 Myr ago at 5 km s-1.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Physical parameters of the Algol system VW Hydrae from simultaneous analysis of GENEVA seven-colour light curves
The semi-detached eclipsing binary system VW Hydrae (P = 2.70 days) hasbeen analysed using the Wilson Devinney program, on the basis of lightcurves obtained in the GENEVA seven-colour photometric system, andradial velocity curves for both components measured with thecross-correlation technique. The physical and orbital parameters havebeen determined through a self-consistent simultaneous solution of lightcurves in seven colours and of the radial velocity curves of bothcomponents. The effective temperature of the primary component isdetermined from the photometric analysis,T=10,650±200K. It is shown that this value can be verywell determined, despite the fact the primary eclipse is partial, by theuse of colour indexes, as e.g. [B2 ‑V1]0. The absolute elements of the components arefor the primary (mass gainer), with the value of T fixed,M1 = 3.10 ± 0.07Mȯ, R1 =2.60 ± 0.01Rȯ, M=0.05±0.01, andfor the secondary (mass loser), M2 = 0.76 ±0.02Mȯ, R2 = 3.38 ±0.01Rȯ,M=2.90±0.01,T=4848±6K. Thesemi-major axis A of the relative orbit is 12.771 ±0.090Rȯ. The estimated spectral types of the componentsare about B8/9 V (primary) and F9 III. The equatorial rotationalvelocity of the primary (54 km s‑1) shows that thesystem is synchronised. The distance to VW Hya is evaluated to 1122± 42 pc, and the colour excess E[B2 ‑V1] to 0.046 ± 0.022.
| 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.
| The Open Cluster NGC 6520 and the Nearby Dark Molecular Cloud Barnard 86
Wide-field BVI photometry and 12CO(1-->0) observations arepresented in the region of the open cluster NGC 6520 and the darkmolecular cloud Barnard 86. From the analysis of the optical data wefind that the cluster is rather compact, with a radius of1.0 arcmin +/-0.5 arcmin, smaller than previous estimates. The cluster ageis 150+/-50 Myr, and the reddening EB-V=0.42+/-0.10. Thedistance from the Sun is estimated to be 1900+/-100 pc, which is largerthan previous estimates. We finally derive basic properties of the darknebula Barnard 86 on the assumption that it lies at the same distance asthe cluster.
| The uBVI Photometric System. II. Standard Stars
Paper I of this series described the design of a CCD-based photometricsystem that is optimized for ground-based measurements of the size ofthe Balmer discontinuity in stellar spectra. This ``uBVI'' systemcombines the Thuan-Gunn u filter with the standard Johnson-Kron-CousinsBVI filters and can be used to discover luminous yellow supergiants inextragalactic systems and post-asymptotic giant branch stars in globularclusters and galactic halos. In the present paper we use uBVIobservations obtained on 54 nights with 0.9 m telescopes at Kitt Peakand Cerro Tololo to construct a catalog of standardized u magnitudes forstandard stars taken from the 1992 catalog of Landolt. We describe theselection of our 14 Landolt fields and give details of the photometricreductions, including red leak and extinction corrections,transformation of all the observations onto a common magnitude system,and the establishment of the photometric zero point. We present acatalog of u magnitudes of 103 stars suitable for use as standards. Weshow that data obtained with other telescopes can be transformed to ourstandard system with better than 1% accuracy.
| Properties of five low-contrast open clusters in the third quadrant
We derive photometric, structural and dynamical evolution-relatedparameters of five as yet unstudied low-contrast open clusters locatedin the third quadrant using 2MASS data. The target clusters are Czernik31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply astatistical field-star decontamination procedure to infer on theintrinsic colour-magnitude diagram (CMD) morphology which is criticalfor such low-contrast objects. Consequently, it became possible toderive accurate reddening, age, distance from the Sun and Galactocentricdistance for the five clusters. In the structural andluminosity/mass-function analyses we apply a colour-magnitude filterwhich encompasses the cluster evolutionary CMD sequences and excludesstars with discrepant colours. Using this procedure we derive core andlimiting radii, mass function (MF) slope, total mass, mass density andrelaxation time. We derive ages in the range 140-1100 Myr,Galactocentric distances within 7.7-11.4 kpc, and total masses within360-2900 M_ȯ. Reflecting large-scale mass segregation, the MF slopein the core is significantly flatter than that in the halo of the fiveclusters. Although some of the present clusters are relatively youngerthan the Gyr-old clusters, they present evidence of advanced dynamicalevolution. This kind of study has become possible because of thephotometric uniformity and spatial coverage of 2MASS which allows aproper subtraction of the field-star contamination on the target CMDs.The present study indicates that low-contrast clusters can be studiedwith 2MASS, particularly after field-star subtraction, which isimportant since most of the unstudied open clusters belong to thisclass.
| 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.
| Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane
We derive photometric, structural and dynamical evolution-relatedparameters of 11 nearby open clusters with ages in the range 70 Myr to 7Gyr and masses in the range ≈400 M_ȯ to ≈5300 M_ȯ. Theclusters are homogeneously analysed by means of J, H and KS2MASS photometry, which provides spatial coverage wide enough toproperly take into account the contamination of the cluster field byGalaxy stars. Structural parameters such as core and limiting radii arederived from the background-subtracted radial density profiles.Luminosity and mass functions (MFs) are built for stars later than theturnoff and brighter than the 2MASS PSC 99.9% completeness limit. Thetotal mass locked up in stars in the core and the whole cluster, as wellas the corresponding mass densities, are calculated by taking intoaccount the observed stars (evolved and main sequence) and extrapolatingthe MFs down to the H-burning mass limit, 0.08 M_ȯ. We illustratethe methods by analysing for the first time in the near-infrared thepopulous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive anage of 1.1 ± 0.1 Gyr, distance from the Sun d_ȯ=1.2 ±0.1 kpc, core radius Rcore=1.4 ± 0.1 pc, limitingradius Rlim=11.6 ± 0.7 pc and total massmtot≈(5.3±1.6) × 103 M_ȯ.Large-scale mass segregation in NGC 2477 is reflected in the significantvariation of the MF slopes in different spatial regions of the cluster,and in the large number-density of giant stars in the core with respectto the cluster as a whole. For NGC 2516 we derive an age of 160 ±10 Myr, d_ȯ=0.44 ± 0.02 kpc, Rcore=0.6 ±0.1 pc, Rlim=6.2 ± 0.2 pc andmtot≈(1.3±0.2) × 103 M_ȯ.Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusterspresent a slope break in the MF occurring at essentially the same massas that found for the field stars in Kroupa's universal IMF. The MFbreak is not associated to cluster mass, at least in the clusters inthis paper. In two clusters the low-mass end of the MF occurs above theMF break. However, in three clusters the MF break does not occur, atleast for the mass range m≥0.7 M_ȯ. One possibility isdynamical evolution affecting the MF slope distribution. We also searchfor relations of structural and evolutionary parameters with age andGalactocentric distance. The main results for the present sample are:(i) cluster size correlates both with age and Galactocentric distance;(ii) because of size and mass scaling, core and limiting radii, and coreand overall mass correlate; (iii) massive (m≥1000 M_ȯ) andless-massive clusters follow separate correlation paths on the planecore radius and overall mass; (iv) MF slopes of massive clusters arerestricted to a narrow range, while those of the less-massive onesdistribute over a wider range. Core and overall MF flattening is relatedto the ratio (τ) of age to relaxation time. For large values ofτ the effects of large-scale mass segregation and low-mass starsevaporation can be observed in the MFs. In this sense, τ appears tocharacterize the evolutionary state of the clusters. We conclude thatappreciable slope flattenings in the overall MFs of the less-massiveclusters take ~6 times longer to occur than in the core, while in themassive clusters they take a time ~13 times longer. We investigatecluster parameters equivalent to those determining the fundamental planeof ellipticals. These parameters are: overall mass, projected massdensity and core radius. We conclude that in the present sample there isevidence of a fundamental plane. Larger samples are necessary to pindown this issue.
| Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188
Luminosity and mass functions in the old open cluster NGC 188 areanalysed by means of J and H 2MASS photometry, which provides uniformityand spatial coverage for a proper background subtraction. With an age ofabout 6-8 Gyr, NGC 188 is expected to be suffering the effects ofadvanced dynamical evolution. Indeed, previous works in optical bandshave suggested the presence of mass segregation. Within theuncertainties, the observed projected radial density profile of NGC 188departs from the two-parameter King model in two inner regions, whichreflects the non-virialized dynamical state and possibly, some degree ofnon-sphericity in the spatial shape of this old open cluster. Fits withtwo and three-parameter King models to the radial distribution of starsresulted in a core radius Rcore=1.3±0.1 pc and a tidalradius Rtidal=21±4 pc, about twice as large as thevisual limiting radius. The concentration parameter c=1.2±0.1 ofNGC 188 makes this open cluster structurally comparable to the looseglobular clusters. The present 2MASS analysis resulted in significantslope variations with distance in the mass function φ(m)∝m-(1+χ), being flat in the central parts(χ=0.6±0.7) and steep in the cluster outskirts(χ=7.2±0.6). The overall mass function has a slopeχ=1.9±0.7, slightly steeper than a standard Salpeter massfunction. In this context, NGC 188 is similar to the 3.2 Gyr,dynamically evolved open cluster M 67. Solar metallicity Padovaisochrone fits to the near-infrared colour-magnitude diagram of NGC 188resulted in an age of 7.0±1.0 Gyr. The best fit, obtained withthe 7.1 Gyr isochrone, produced a distance modulus(m-M)0=11.1±0.1, E(B-V)=0.0, and a distance to the Sundȯ=1.66±0.08 kpc. The observed stellar mass (inthe range 0.98 Mȯ- 1.08 Mȯ) in NGC 188is mobs=380±12 Mȯ. A simpleextrapolation of the observed overall mass function to stars with 0.08Mȯ resulted in a total present mass ofmtot(1.8±0.7)×104Mȯ. On the other hand, for a more realistic initial massfunction which flattens in the low-mass range, the total mass in NGC 188drops to mtot(3.8±1.6)×103Mȯ. Since mass-loss processes such as evaporation andtidal stripping have been occurring in this old open cluster for about 7Gyr, the primordial mass in NGC 188 must have been significantly largerthan 4×103 Mȯ. We also examinedthe consequences of the presence of unresolved binaries and concludedthat, even if dominant in the CMD, binaries alone are not responsiblefor the flat central mass function, which supports the mass-segregationscenario.
| Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyrusing UCAC2 proper motion data and 2MASS photometry, which allows us toreach stellar masses down to ≈0.7 M_ȯ. We employ in this work anapproach in which the background proper motion contribution isstatistically subtracted in order to obtain the cluster's intrinsicproper motion distribution. For each cluster we consider the projectedvelocity distributions in the core and off-core regions separately. Inthe projected velocity distribution of all sample clusters we find awell-defined low-velocity peak, as well as an excess in the number ofstars at larger velocities. The low-velocity peak is accounted for bythe random motion of the single stars, while the high-velocity excesscan be attributed to the large velocity changes produced by asignificant fraction of unresolved binaries in a cluster. We derivekinematic parameters of the single-star distribution, in particular theprojected velocity dispersion. The relatively large velocity dispersionsderived in this work may reflect the non-virialized state of theclusters. Based on the relative number of high-velocity (binary) andsingle stars, we inferred for the sample clusters unresolved binaryfractions in the range 15%≤f_bin≤54%, for both core and off-coreregions. Stars with a projected velocity exceeding the maximum reachedby the single-star distribution are identified in 2MASS J×(J-H)colour magnitude diagrams. The asymmetry observed in the distribution ofthese stars around the main sequence is consistent with models ofmain-sequence widening resulting from unresolved binaries combined with2MASS photometric uncertainties. The present results suggest that caremust be taken when applying proper-motion filters to sort out members,especially binaries in a star cluster. This paper shows that propermotions turn out to be a useful tool for identifying high-velocity starsas unresolved binary cluster members, and as a consequence, map andquantify the binary component in colour magnitude diagrams.
| Spectroscopic binaries in southern open clusters
This is a report on an ongoing program about binaries in southern openclusters. The long-term purpose of this project is to contribute tounderstanding the formation and evolution of spectroscopic binaries,providing observational constraints that will permit tests of some ofthe current theories on binary formation in open clusters.
| Star cluster ecology - V. Dissection of an open star cluster: spectroscopy
We have modelled in detail the evolution of rich open star clusterssuch as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and3680, using simulations that include stellar dynamics as well as theeffects of stellar evolution. The dynamics is modelled via direct N-bodyintegration, while the evolution of single stars and binaries isfollowed through the use of fitting formulae and recipes. The feedbackof stellar and binary evolution on the dynamical evolution of thestellar system is taken into account self-consistently.Our model clusters dissolve in the tidal field of the Galaxy in atime-span of the order of a billion years. The rate of mass loss israther constant, ~1 Msolar per million years. The binaryfraction at first is nearly constant in time, then increases slowly nearthe end of a cluster's lifetime. For clusters which are more than about108 yr old the fractions of stars in the form of binaries,giants and merger products in the inner few core radii are considerablyhigher than in the outer regions, beyond the cluster's half-mass radius.When stars with masses >~2 Msolar escape from the cluster,they tend to do so with velocities higher than average.The stellar merger rate in our models is roughly one per 30 millionyears. Most mergers are the result of unstable mass transfer in closebinaries (~70 per cent), but a significant minority are caused by directencounters between single and binary stars. While most mergers occurwithin the cluster core, even beyond the half-mass radius stellarmergers occasionally take place. We notice a significant birth rate ofX-ray binaries, most containing a white dwarf as the mass acceptor. Wealso find one high-mass X-ray binary with a neutron-star accretor. Ifformed and retained, black holes participate in many (higher-order)encounters in the cluster centre, resulting in a large variety of exoticbinaries. The persistent triple and higher-order systems formed in ourmodels by dynamical encounters between binaries and single stars are notrepresentative for the multiple systems observed in the Galactic disc.We conclude that the majority of multiples in the disc probably formedwhen the stars were born, rather than through later dynamicalinteractions.
| Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
| uvbyCaHβ CCD Photometry of Clusters. IV. Solving the Riddle of NGC 3680
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the open cluster, NGC 3680. Restricting the data toprobable cluster members using the color-magnitude diagram (CMD) and thephotometric indices alone defines a sample of 34 stars at the clusterturnoff that imply E(b-y)=0.042+/-0.002 (s.e.m.) or E(B-V)=0.058+/-0.003(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk using bothb-y and Hβ as the temperature indices. The agreement among the fourapproaches is excellent, leading to final value of [Fe/H]=-0.14+/-0.03for the cluster and removing the apparent discrepancy between the pastuvby analyses and extensive results from the red giants. The primarysource of the photometric anomaly appears to be a zero-point offset inthe original m1 indices. Using the homogenized and combinedV, b-y data from a variety of studies transformed to B-V, the clusterCMD is compared with NGC 752, IC 4651, and the core-convective-overshootisochrones of Girardi et al. By interpolation to the proper metallicity,it is found that the E(B-V), m-M, and age for NGC 752, IC 4651, and NGC3680 are (0.03, 8.30, 1.55 Gyr), (0.10, 10.20, 1.7 Gyr), and (0.06,10.20, 1.85 Gyr), respectively. The revised age and metallicity sequenceand the color distribution of the giants provide evidence for thesuggestion that the giants defining the apparent clump in NGC 3680 arepredominantly first-ascent giants, as indicated by their Li abundance,while the clump stars in NGC 752, 0.1 mag bluer in B-V, areHe-core-burning stars. When combined with the color distribution in IC4651, it is suggested that over this modest age range where He-coreflash becomes important, the distribution of so-called clump starsswitches from being dominated by He-core-burning stars to first-ascentgiants in the bump phase.
| Theoretical isochrones compared to 2MASS observations: Open clusters at nearly solar metallicity
We study classical disk clusters at nearly solar metallicity with agesin the range 200 Myr - 6 Gyr using 2MASS photometry and theoreticalisochrones. As a first step we employ J, H and K Johnson's Padovaisochrones which, on a large scale, result in a good fit of the observeddata. However, we find some significant deviations, especially in thered giant branch for intermediate age clusters and the low-massmain-sequence end in general. Subsequently, isochrones involving the2MASS J, H and K_S transmission curves were generated which account fordifferences of ≈0.02 of the above deviations, in the sense requiredby the 2MASS observations. However, these corrections are not enough tofully account for the observed differences. We suggest that theremaining deviations should be accounted for by model atmospherelimitations, particularly the lack of molecular opacity data inlow-temperature giants. Future models will incorporate these updateddata in order to produce isochrones suitable for the whole range ofstellar masses.
| Recent developments on studies of open clusters
Studies on open clusters are of great significance for astronomy andastrophysics, for which distinct developments have been made recently inboth observational and theoretical respects. In this paper, currentresearch of open cluster is reviewed, and membership determination,basic parameters determination, evolution of cluster and clustermembers, structure and dynamics of open cluster are also described indetail.
| 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.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Physical parameters of the Algol system BP Muscae from simultaneous analysis of GENEVA 7-colour light curves
The semi-detached eclipsing binary system BP Muscae has been analysedusing the Wilson-Devinney program. Light curves have been obtained inthe GENEVA 7-colour photometric system, and radial velocity curves forboth components have been measured with the spectrograph CORALIE. Thephysical and orbital parameters have been determined through aself-consistent simultaneous solution of light curves in seven coloursand of the radial velocity curves of both components. The absoluteelements of the components are, for the primary (mass gainer),M1 = 2.40 +/- 0.01 Msun, R1 = 2.64 +/-0.01 Rsun, Mbol_1 = 0.66 +/- 0.04,Teff_1 = 9180 +/- 90 K, and for the secondary (mass loser),M2 = 0.68 +/- 0.01 Msun, R2 = 3.76Rsun, Mbol_2 = 2.40 +/- 0.08, Teff_2 =5160 +/- 90 K. The semi-major axis A of the relative orbit is 13.617 +/-0.019 Rsun. The spectral type of the components are A0.5/1.5V (primary) and about G5 III. The distance to BP Mus is evaluated as 562+/- 17 pc, and the colour excess E[B2-V1] as 0.220 +/- 0.014.Based on observations collected at the Swiss 70 cm and 120 cm telescopesat the European Southern Observatory (La Silla, Chile).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/1073
| Urban Astronomy: Observing the Messier Objects from the City
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| Brown dwarf populations in open clusters
We present the results of multiple simulations of open clusters,modelling the dynamics of a population of brown dwarf members. Weconsider the effects of a large range of primordial binary populations,including the possibilities of having brown dwarf members containedwithin a binary system. We also examine the effects of various clusterdiameters and masses. Our examination of a population of wide binarysystems containing brown dwarfs, reveals evidence for exchange reactionswhereby the brown dwarf is ejected from the system and replaced by aheavier main-sequence star. We find that there exists the possibility ofhiding a large fraction of the brown dwarfs contained within theprimordial binary population. We conclude that it is probable that themajority of brown dwarfs are contained within primordial binary systemswhich then hides a large proportion of them from detection.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| Star cluster ecology - IV. Dissection of an open star cluster: photometry
The evolution of star clusters is studied using N-body simulations inwhich the evolution of single stars and binaries is takenself-consistently into account. Initial conditions are chosen torepresent relatively young Galactic open clusters, such as the Pleiades,Praesepe and the Hyades. The calculations include a realistic massfunction, primordial binaries and the external potential of the parentGalaxy. Our model clusters are generally significantly flattened by theGalactic tidal field, and dissolve before deep core collapse occurs. Thebinary fraction decreases initially because of the destruction of softbinaries, but increases later because lower mass single stars escapemore easily than the more massive binaries. At late times, the clustercore is quite rich in giants and white dwarfs. There is no evidence forpreferential evaporation of old white dwarfs. On the contrary, the whitedwarfs formed are likely to remain in the cluster. Stars tend to escapefrom the cluster through the first and second Lagrange points, in thedirection of and away from the Galactic Centre. Mass segregationmanifests itself in our models well within an initial relaxation time.As expected, giants and white dwarfs are much more strongly affected bymass segregation than main-sequence stars. Open clusters are dynamicallyrather inactive. However, the combined effects of stellar mass-loss andevaporation of stars from the cluster potential drive the dissolution ofa cluster on a much shorter time-scale than if these effects areneglected. The often-used argument that a star cluster is barely olderthan its relaxation time and therefore cannot be dynamically evolved isclearly in error for the majority of star clusters. An observation of ablue straggler in an eccentric orbit around an unevolved star or a bluestraggler of more than twice the turn-off mass might indicate pastdynamical activity. We find two distinct populations of blue stragglers:those formed above the main-sequence turn-off, and those which appear asblue stragglers as the cluster's turn-off drops below the mass of therejuvenated star.
| The rise and rise of the deep sky image
Presidential Address to the British Astronomical Association, 2000October 25
| The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)
We present deep CCD photometry of the very rich, intermediate-age(similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-Vcolor-magnitude diagram (CMD) for the cluster shows an extremelywell-populated and very tightly constrained main sequence extending over12 mag from the turnoff. Both a well-defined main-sequence turnoff and ared giant population are also clearly evident. The CFH12K photometry forthis cluster is faint enough (V~24.5) to detect the remnants of the mostmassive progenitor cluster stars under the Type I SNe limit. Therefore,the CMD of the cluster also exhibits a well defined white dwarf``clump'' caused by the decreased rate of cooling of these stars as theyage, and a subsequent gap with very few objects. By using sourceclassification to eliminate faint blue galaxies and a statisticalsubtraction technique to eliminate foreground/background objects, wehave determined the age of the star cluster from the termination point(MV=11.95+/-0.30) in the white dwarf luminosity function. Thewhite dwarf cooling age is found to be566+/-154176 Myr from comparisons with white dwarfcooling models and is in excellent agreement with the main-sequenceturnoff isochrone age (520 Myr). After carefully accounting for theuncertainties in the white dwarf limiting magnitude, we show that thecooling age confirms that models including convective core overshootingare preferred for young-intermediate-aged clusters. This is particularlyimportant in the case of NGC 2099 as the age is similar to that of theHyades cluster, for which current models have difficulties inreproducing the details of the main-sequence turnoff. We also derive thereddening [E(B-V)=0.21+/-0.03] and distance[(m-M)V=11.55+/-0.13] to NGC 2099 by matching main-sequencefeatures in the cluster to a new fiducial main sequence for the Hyades,after correcting for small metallicity differences. As a continuing partof the goals of the CFHT Open Star Cluster Survey to better understanddynamical processes of open clusters, we also fit a King model to thecluster density distribution and investigate the cluster main-sequenceluminosity and mass functions in increasing concentric annuli. We findsome evidence for mass segregation within the boundary of NGC 2099 asexpected given the cluster's age relative to the dynamical age. Thepresent global mass function for the cluster is found to be shallowerthan a Salpeter IMF.
| Gravitational tidal effects on galactic open clusters
We have investigated the 2-D stellar distribution in the outer parts ofthree nearby open clusters: NGC 2287 (equiv M 41),NGC 2516, and NGC 2548 (equiv M48). Wide-field star counts have been performed in two colours on pairsof digitized ESO and SRC Schmidt plates, allowing us to select likelycluster members in the colour-magnitude diagrams. Cluster tidalextensions were emphasized using a wavelet transform. Taking intoaccount observational biases, namely the galaxy clustering anddifferential extinction in the Galaxy, we have associated these stellaroverdensities with real open cluster structures stretched by thegalactic gravitational field. As predicted by theory and simulations,and despite observational limitations, we detected a general elongated(prolate) shape in a direction parallel to the galactic Plane, combinedwith tidal tails extended perpendicularly to it. This geometry is dueboth to the static galactic tidal field and the heating up of thestellar system when crossing the Disk. The time varying tidal field willdeeply affect the cluster dynamical evolution, and we emphasize theimportance of adiabatic heating during the Disk-shocking. In the case ofNGC 2548, our dating of the last shocking with the Plane (based on atidal clump) is consistent with its velocity. During the 10-20Z-oscillations experienced by a cluster before its dissolution in theGalaxy, crossings through the galactic Disk contribute to at least 15%of the total mass loss. Using recent age estimations published for openclusters, we find a destruction time-scale of about 600 Myr for clustersin the solar neighbourhood. Plate scanning was done at the Centred'Analyse des Images (CAI) with M.A.M.A. (Machine AutomatiqueàMesurer pour l'Astronomie), a facility located at theObservatoire de Paris, developed and operated by INSU (Institut Nationaldes Sciences de l'Univers, CNRS). Web site http://dsmama.obspm.fr
| 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
| CCD UVBY Hβ Photometry in Clusters. I. The Open Cluster Standard, IC 4651
CCD photometry of the intermediate-age open cluster IC 4651 on theuvbyHβ system is presented and analyzed. By using a combination ofthe information from the color-magnitude diagram (CMD) and thecolor-color diagrams, a sample of 98 highly probable main-sequencecluster members with high photometric accuracy is isolated. From thissample, adopting the intrinsic color relation of Olsen,E(b-y)=0.062+/-0.003 and [Fe/H]=+0.077+/-0.012, where the errors quotedare the standard errors of the mean and refer to the internal errorsalone. Use of the Nissen intrinsic color relation produces E(b-y)=0.071and [Fe/H]=+0.115. Adopting the lower reddening, a direct main-sequencefit to the Hyades with (m-M)=3.33 leads to (m-M)=10.15, while isochroneswith convective overshoot and zeroed to the Hyades produce an age of1.7+/-0.1 Gyr, with an excellent match to the morphology of the turnoff.The higher reddening produces (m-M)=10.3 and an age lower by 0.1 Gyr.Comparison with the CMD of NGC 3680 shows that the two clusters havevirtually identical morphology, which in combination with their similarcompositions produces identical ages. Coincidentally, the shifts in theCMD necessary to superpose the two clusters require that the apparentmoduli of IC 4651 and NGC 3680 be the same, whileE(b-y)4651=E(b-y)3680+0.04.
| 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
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