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Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Aims.We derive accurate parameters related to the CMD, structure anddynamical state of M 52 and NGC 3960, whose fields are affected bydifferential reddening. Previous works estimated their ages in theranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods: .J, Hand Ks 2MASS photometry with errors <0.2 mag is used tobuild CMDs, radial density profiles, luminosity and mass functions, andcorrect for differential reddening. Field-star decontamination isapplied to uncover the cluster's intrinsic CMD morphology, andcolour-magnitude filters are used to isolate stars with high probabilityof being cluster members. Results: .The differential-reddeningcorrected radial density profile of M 52 follows King's law with coreand limiting radii of R_core =0.91 ± 0.14 pc and R_lim =8.0± 1.0 pc. NGC 3960 presents an excess of the stellar density overKing's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ±0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 areR_tidal=13.1 ± 2.2 pc and R_tidal=10.7 ± 3.7 pc. Clusterages of M 52 and NGC 3960 derived with Padova isochrones are constrainedto 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF(χ_core=0.89 ± 0.12) is flatter than the halo's(χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up inMS/evolved stars in M 52 is ~1200 M_ȯ, and the total mass(extrapolated to 0.08M_ȯ) is ~3800 M_ȯ. The total mass in NGC3960 is ~1300 M_ȯ. Conclusions: .Compared to open clusters indifferent dynamical states studied with similar methods, the core andoverall parameters of M 52 are consistent with an open cluster moremassive than 1000 M_ȯ and ~60 Myr old, with some mass segregationin the inner region. The core of NGC 3960 is in an advanced dynamicalstate with strong mass segregation in the core/halo region, while thesomewhat flat overall MF (χ≈ 1.07) suggests low-mass starevaporation. The excess stellar density in the core may suggestpost-core collapse. The dynamical evolution of NGC 3960 may have beenaccelerated by the tidal Galactic field, since it lies ≈0.5 kpcinside the Solar circle.
| Photometric survey of marginally investigated open clusters. I. Basel 11b, King 14, Czernik 43
Aims.To progress in galactic studies based on open clusters, e.g.cluster formation rate or kinematical properties, one needs to improvethe number of open clusters observed. However, only half of the 1700known galactic open clusters have been properly observed so far, makingany statistical investigation insignificant, especially at largerdistances from the Sun. Methods: .We study marginallyinvestigated or neglected open clusters with Bessell CCD BVR photometry,whose data were used to fit isochrones to the individual color-magnitudediagrams. Results: .We examined the galactic clusters Basel 11b,King 14 and Czernik 43, the last being observed for the first time tothis extent. As well as a careful comparison to available photometry,their parameters such as age, interstellar reddening, distance andapparent diameter were determined. The obtained cluster properties wereverified by near infrared 2MASS data. The three investigatedintermediate age clusters are all located in the galactic disk withdistances between 1.8 and 3.0 kpc from the Sun.
| Stellar contents of two young open clusters: NGC 663 and 654
UBVRI CCD photometry in a wide field around two young open clusters, NGC663 and 654, has been carried out. Hα and polarimetricobservations for the cluster NGC 654 have also been obtained. We use thephotometric data to construct colour-colour and colour-magnitudediagrams, from which we can investigate the reddening, age, mass andevolutionary states of the stellar contents of the these clusters. Thereddening across the cluster regions is found to be variable. There isevidence for anomalous reddening law in both clusters; however, moreinfrared and polarimetric data are needed to conclude about thereddening law. Both clusters are situated at about a distance of 2.4kpc. Star formation in both clusters is found to be a continuousprocess. In the case of NGC 663, star formation seems to have takenplace sequentially, in the sense that formation of low-mass starsprecedes the formation of most massive stars. Whereas, in the case ofNGC 654, formation of low-mass stars did not cease after the formationof most massive stars in the cluster.
| CCD-Δ a and BVR photometry of NGC 7296
The first CCD photometric investigation of the open cluster NGC 7296 upto now was performed within the narrow band Δ a photometricsystem, which enables us to detect peculiar objects. A deeperinvestigation of that cluster followed, using the standard BVR-Besselfilter set. The age and E(B-V) was determined independently to log t =8.0 ± 0.1 and 0.15 ± 0.02, respectively by using Δ aand broadband photometry. In total five Be/Ae objects and two metal-weakstars showing significant negative Δ a-values as well as oneclassical chemically peculiar star could be identified within thatintermediate age open cluster.Based on observations at the Hvar Observatory, University of Zagreb andthe Leopold-Figl Observatory for Astrophysics, University of Vienna.
| 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.
| On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions
We construct the observed luminosity functions of the remote young openclusters NGC 2383, 2384, 4103, 4755, 7510 and Hogg 15 from CCDobservations of them. The observed LFs are corrected for field starcontamination determined with the help of a Galactic star count model.In the case of Hogg 15 and NGC 2383 we also consider the additionalcontamination from neighbouring clusters NGC 4609 and 2384,respectively. These corrections provide a realistic pattern of clusterLF in the vicinity of the main-sequence (MS) turn-on point and atfainter magnitudes reveal the so-called H-feature arising as a result ofthe transition of the pre-MS phase to the MS, which is dependent on thecluster age. The theoretical LFs are constructed representing a clusterpopulation model with continuous star formation for a short time-scaleand a power-law initial mass function (IMF), and these are fitted to theobserved LF. As a result, we are able to determine for each cluster aset of parameters describing the cluster population (the age, durationof star formation, IMF slope and percentage of field starcontamination). It is found that in spite of the non-monotonic behaviourof observed LFs, cluster IMFs can be described as power-law functionswith slopes similar to Salpeter's value. The present main-sequenceturn-on cluster ages are several times lower than those derived from thefitting of theoretical isochrones to the turn-off region of the uppermain sequences.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. I. SNRS G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5
Most astronomers now accept that stars more massive than about 9Msolar explode as supernovae and leave stellar remnants,either neutron stars or black holes, with neutron stars being moreprevalent. Recent modeling of the explosions suggests a significantdiversity in the key natal properties-rotation rate, velocity, andmagnetic field strength-of the resulting neutron stars that account forthe association of active radio pulsars, pulsar wind nebulae, andmagnetars with supernova remnants (SNRs). The discovery of a centralX-ray source in Cas A, the youngest known Galactic SNR, dramatized theexpected diversity. However, less than half of the SNRs within 5 kpchave identified central sources, and only three are identified as theremnants of Type Ia SNe. Here we report a systematic effort to searchfor compact central sources in the remaining 23 SNRs of this distancelimited sample. Our search was inspired, on empirical considerations, bythe enigmatic faint X-ray source in Cas A motivated, on theoreticalgrounds, by the expectation that young neutron stars emit cooling X-rayemission; and made possible by the superb angular resolution offered bythe Chandra X-ray mission and the sensitivity of the XMM-Newton mission.In this first paper we report Chandra observations of four SNRs(G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5). We have undertakena systematic optical/IR identification program of the X-ray sourcesdetected in the field of each SNR. Foreground (flare stars, activestars) and background (active galactic nuclei) sources have identifiableIR/optical counterparts. In contrast, the counterparts of neutron stars(or black holes) are expected to be very faint. We are able to accountfor all the well-detected X-ray sources and thus able to state with someconfidence that there are no associated central sources down to a levelof one-tenth of that of the Cas A central source,LX<~1031ergss-1. We compare ourlimits with cooling curves for neutron stars and find that any putativeneutron stars in these SNRs must be cooling faster than expected fortraditional 1.35Msolar neutron stars and that any putativepulsar must have low spin-down luminositiesE<~1034ergss-1. However, our limits are unableto constrain the presence or absence of more unusual options, such asrelatively more massive neutron stars with M>~1.45Msolar,neutron stars with exotic interiors, or quiescent black holes. Insubsequent papers, we will report on the X-ray and optical/IRobservations of the remaining members of the 5 kpc sample.
| The Anomalous X-ray Pulsar 4U 0142+61: Variability in the infrared and a spectral break in the optical
We present new optical and infrared observations of the counterpart tothe Anomalous X-ray Pulsar (AXP) 4U 0142+61 taken with the Keck Itelescope. The counterpart is found to be variable in the infrared. Thiscontrasts with our optical observations, which do not show any evidencefor variability. Apart from the variability the AXP shows a remarkablespectral energy distribution. In particular, we find a sudden drop influx going from V to B, presumably due to a spectral feature. We compareour results to those obtained for the two other securely identified AXPcounterparts, to 1E 2259+586 and 1E 1048.1-5937. 4U 0142+61 is verysimilar to the former source in its X-ray timing and spectralproperties, and we find that this similarity extends to the quiescentinfrared to X-ray flux ratio. For 1E 1048.1-5937, which has differentX-ray properties, the situation is less clear: in one observation, theinfrared to X-ray flux ratio was much larger, but another observationgave an upper limit which is consistent with that observed for 4U0142+61. Assuming the quiescent ratios are all similar, we estimate theoptical and infrared brightnesses for the three AXPs that remain to beidentified as well as for the four Soft Gamma-ray Repeaters. We alsodiscuss briefly how the observed optical and infrared emission mightarise, in particular in the context of the magnetar model.Table 3 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/416/1037
| Open Star Clusters: Star Formation and Structure
Observational evidences indicate that the formation of open starclusters has been efficient throughout the life of our Galaxy. As aconsequence, we find open clusters with ages ranging from a few Myr tofew Gyr. This makes open clusters natural laboratories where we can testthe theory of star formation and stellar evolution. Study of young openclusters indicates that the star formation in a few clusters isnon-coeval and the star formation efficiency depends on the IMF. Ourwork supports the existence of corona around open clusters. The coronalregions in bound open clusters are dynamically stable in the tidalforces of the Galaxy.
| 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.
| Carbon Star Survey in the Local Group. V. The Outer Disk of M31
We employ the CFH12K mosaic to identify carbon stars, using the R, I,CN, and TiO photometric technique, in a 2240 arcmin2 area,ranging from 17 to 30 kpc of the southwest disk of M31, barely reachingthe edge of the observed H I disk. We found 945 C stars with=19.94 and σ=0.47. The surface density of Cstars along the major axis of M31 follows an exponential profile with ascale length of 4.85+/-0.35 kpc, in agreement with adopted values forthe scale length of the disk population. Our survey partially overlapswith the recently discovered G1 density enhancement by Ferguson et al.We confirm that no AGB star excess is detectable in the surveyed part ofthe clump. The C/M ratio, along the major axis, is derived over adistance range of 7 kpc. The strong C/M gradient seen contrasts withresults of previous studies of the C stars in M31.
| Analysing the database for stars in open clusters. I. General methods and description of the data
We present an overview and statistical analysis of the data included inWEBDA. This database includes valuable information such as coordinates,rectangular positions, proper motions, photometric as well asspectroscopic data, radial and rotational velocities for objects of openclusters in our Milky Way. It also contains miscellaneous types of datalike membership probabilities, orbital elements of spectroscopicbinaries and periods of variability for different kinds of variablestars. Our final goal is to derive astrophysical parameters (reddening,distance and age) of open clusters based on the major photometric systemwhich will be presented in a follow-up paper. For this purpose we havechosen the Johnson UBV, Cousins VRI and Strömgren uvbybetaphotometric systems for a statistical analysis of published data setsincluded in WEBDA. Our final list contains photographic, photoelectricand CCD data for 469 820 objects in 573 open clusters. We have checkedthe internal (data sets within one photometric system and the samedetector technique) and external (different detector technique) accuracyand conclude that more than 97% of all investigated data exhibit asufficient accuracy for our analysis. The way of weighting and averagingthe data is described. In addition, we have compiled a list of deviatingmeasurements which is available to the community through WEBDA.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| The Mass Function of Young Star Clusters in our Galaxy and Nearby Galaxies: Is It Universal?
Mass functions (MFs) derived from photometric observations of young starclusters of our Galaxy, the Magellanic Clouds (MCs), M31 and M33 havebeen used to investigate the question of universality of the initialmass function and presence of mass segregation in these systems.Observational determination of the MF slope of young star clusters havean inherent uncertainty of at least 1.0 dex in the Milky Way andof 0.4 dex in the MCs. There is no obvious dependence of the MFslope on either galactocentric distance or age of the young starclusters or on the spatial concentration of the stars formed or on thegalactic characteristics including metallicity. Effects of masssegregation have been observed in a good number of young stellar groupsof our Galaxy and MCs. As their ages are much smaller than theirdynamical evolution times, star formation processes seem to beresponsible for the observed mass segregation in them.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 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/cgi-bin/qcat?J/A+A/388/168
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| A comprehensive study of the rich open star cluster NGC 2099 based on deep BVI CCD observations
The CCD observations of the rich open star cluster NGC 2099 and itssurrounding field region have been carried out up to a limitingmagnitude of V ~ 22 mag in B, V and I passbands for the first time. Atotal of ~ 12 000 stars have been observed in the area of about 24arcminx 34arcmin in the cluster region, as well as ~ 2180 stars in the ~12arcmin x 12arcmin area of the field region located ~ 45arcmin awayfrom the cluster center. The cluster parameters determined by fittingthe convective core overshoot isochrones in the V, (B-V) and V, (V-I)diagrams are E(B-V) = 0.30+/-0.04 mag, distance = 1360+/- 100 pc, age =400 Myr and metallicity Z = 0.008. A well-defined cluster main sequencespread over about 8 mag in range is observed for the first time. Itsintrinsic spread amounting to ~ 0.06 mag in colour is almost the sameover the entire brightness and can be understood in terms of thepresence of physical/optical binaries. The core and cluster radiidetermined from the radial stellar density profiles are 185 arcsec and1000 arcsec respectively. Only about 22% of cluster members are presentin the core region. The effects of mass segregation, most probably dueto dynamical evolution, have been observed in the cluster. The massfunction slope of the entire cluster is ~ -0.67+/-0.12. It becomescloser to the Salpeter value of -1.35, if flattening in the cluster massfunction due to presence of both binaries and a much more extendedcorona is considered. Full Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/65
| Hα Emission Stars in the Cepheus OB3 Region
Hα emission objects were searched for, using the objective-prismSchmidt plates in a 36 deg^2 sky area covering the Cepheus OB3association. Hundred and eight emission stars have been found, 68 ofwhich are new findings to our knowledge. The properties of the IRASpoint sources identified with Hα emission stars indicate that mostof the Hα emission stars are T Tauri stars or related objects.Hα emission stars associated with IRAS sources and thoseunassociated have different distributions, suggesting an age sequenceamong them. Altogether, they show a ring- or shell-like surfacedistribution, which almost coincides with that of heated dusts, andthese distributions appear to be encompassed by an HI shell.
| Search for Variable Stars in Two Old Open Clusters: NGC2506 and NGC2420
We present results of photometric observations of two old open clusters:NGC 2506 and NGC 2420. V-band time-series and UBVI absolute CCDphotometric observations were carried out to search for variable starsand to investigate their physical properties. From the UBVI photometryof NGC 2506, we obtain interstellar reddening of E(B-V)=0.04+/-0.03 mag,distance modulus (V-M_V)_0=12.5+/-0.1 mag, and age log(t/yr)=9.25. Frommonitoring of 590 stars with 304 CCD frames taken for ten nights, threedelta Sct stars and one eclipsing binary star are discovered in NGC2506. For the other cluster, NGC 2420, we have examined light variationsof 505 stars using 347 time-series data obtained for five nights,finding no variable star. It should be noted that we have found no gammaDor star among main-sequence stars near F0 in these two old openclusters, which is consistent with the suggestion that gamma Dor-typephenomenon occurs in stars younger than log(t/yr)=8.4. On the otherhand, it has been known that color-magnitude diagrams of these twoclusters show well-established binary sequences, implying high incidenceof binary systems: >=20% for NGC 2506 and approx 50% for NGC 2420.However, only one eclipsing binary star was found in these two clusters.
| NGC 7654: An interesting cluster to study star formation history
CCD UBVIC photometry in a wide field around the open clusterNGC 7654 has been carried out for ~ 17,860 stars, down to V~20. Thereddening across the cluster region is found to be variable withE(B-V)min = 0.46 to E(B-V)max=0.80. The cluster issituated at a distance of 1380 ± 70 pc. The colour magnitudediagrams show a large age spread in the ages. Star formation was biasedtowards relatively higher masses during the early phase of starformation whereas most of the low mass stars of the cluster were formedduring the later phase. The star formation seems to have been a gradualprocess that proceeded sequentially in mass and terminated with theformation of most massive stars. The present data do not support auniform mass function (MF) for different regions in the cluster.Although for the whole cluster region, the MF in the observed mass range0.8 <= Msolar <= 4.5, can be represented by a singlepower law with a slope Γ= -1.40 ± 0.07, however itindicates various features when examined carefully. In three subregionsof the cluster the slope Γ, for the mass range ~ 1.5
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| 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
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| Statistical parallaxes and kinematical parameters of classical Cepheids and young star clusters
The statistical-parallax method is applied for the first time to spacevelocities of 270 classical Cepheids with proper motions adopted fromHIPPARCOS (1997) and TRC (Hog et al. 1998) catalogs and distances basedon the period-luminosity relation by Berdnikov et al. (1996). Thedistance scale of short-period Cepheids (with periods less than 9 days)is shown to require an average correction of 15-20%, whereas statisticalparallaxes of Cepheids with periods > 9 days are found to agree wellwith photometric distances. It is shown that the luminosities ofshort-period Cepheids must have been underestimated partly due to thecontamination of this subsample by a substantial (20 to 40%) fraction offirst-overtone pulsators. The statistical-parallax technique is alsoapplied for the first time to 117 open clusters younger than 100 millionyears and with proper motions reduced to the HIPPARCOS reference system.It is concluded that a 0.12-0.15 mag increase of the distance scales ofopen clusters and Cepheids would be sufficient to reconcile thestatistical-parallax results inferred for these two types of objects.Such approach leads to an LMC distance modulus of less than 18.40 mag,which agrees, within the errors, with the short distance scale for RRLyrae variables and is at variance with the conclusions by Feast andCatchpole (1998) and Feast et al. (1998), who argue that the LMCdistance modulus should be increased to 18.70 mag. The distance scalebased on the Cepheid period-luminosity relation by Berdnikov and Efremov(1985) seems to be a good compromise. Extragalactic distances, whichrely on long-period Cepheids, seem to require no substantial correction.In addition to statistical parallaxes, kinematical parameters have beeninferred for the combined sample consisting of Cepheids andopen-clusters: solar-motion components (U0 ,V0,W0) = (9, 12, 7) km/s (+/- 1 km/s); velocity-ellipsoid axes(σU; σV; σW) = (15.0,10.3, 8.5) km/s (+/- 1 km/s); the angular velocity of rotation of thesubsystem, ω0 = 28.7 +/- 1 km/s/kpc, the Oort constantA = 17.4 +/- 1.5 km/s, and the second derivative of angular velocity,⋰ω0= 1.15 +/- 0.2 km/s/kpc3.
| Red supergiants in the LMC - III: luminous F and G stars
New BVRI observations for 40 and spectrophotometric measurements for 23F to G LMC supergiant candidates (and 3 galactic F to G supergiants) arepresented. The errors of the BVRI data are 0.01 to 0.03 mag in mostcases. The wavelength range of the spectra is 3400 to 6400 Angstroms,their resolution 10 Angstroms. The mean error of the fluxes is 0.03 mag.Spectral indices measuring the strengths of the Hβ , Hγ ,Hdelta , NaD and CaII H+K lines, the CHα_ {0} and CNbeta_ {0}bands, of the Balmer jump and the slope of the continuum redwards arediscussed as measures of effective temperature and luminosity on thebasis of galactic stars with accurate MK types and parallaxes. TheHγ line and the continuum gradient are very good temperaturecriteria, the CHα_ {0} band and especially the Balmer jump forluminosity. The luminosity classification given for F to G supergiantcandidates in the LMC in the literature is often doubtful. 5 of the 23stars observed spectrophotometrically turn out to be probably galacticforeground dwarfs on the basis both of the Balmer jump and thecomparison of their flux distributions with synthetic ones based on theKurucz model atmospheres. Surface gravities derived purely on the basisof flux distributions and such ones given by models of stellar evolutionagree with each other for dwarfs and giants only. For supergiants theformer are about 1.0 dex higher than the latter. As a consequenceeffective temperatures and metallicities given by these two methodsdeviate from each other for such stars, too. The intrinsic colours andtemperatures of galactic and LMC supergiants do not differ. Withabsolute magnitudes up to -9.6 mag the upper luminosity limit in the LMCdoes not exceed that in the Galaxy, where Ia-0 supergiants haveMV of up to -9.5 mag. The metallicities of the supergiantsshow a rather large scatter. Nevertheless the mean metallicities of 0.02+/- 0.09 dex for the Galaxy and -0.26 +/- 0.10 dex for the LMC agreewell with other observations.
| Search for variable stars in the open cluster NGC 7654
Time-series CCD photometry of the open cluster NGC 7654 was performedover five nights from October 2nd to 18th, 1997 U.T., in order to searchfor new variable stars. We also carried out UBV photometric observationsto obtain physical parameters of variable stars in the cluster. We havederived the distance modulus of (V-M_V)deg = 10.9 +/- 0.2 andthe interstellar reddening of E(B-V) = 0.62 +/- 0.05 by fitting the ZAMSto the observed color-color and color-magnitude diagrams of the cluster.Using the theoretical isochrones with solar metal abundance by Bertelliet al. (1994), an age of the cluster, log t = 8.0, was estimated.Applying the ensemble normalization technique to the observed 1215time-series data, light variations of 284 stars, fainter than 11\fm0 inthe observed field, were carefully examined. As a result, three slowlypulsating B stars and one eclipsing binary were discovered in thecluster. We also confirmed the light variation of a low amplitude deltaSct star which has been recently discovered by Viskum et al. (1997).
| Search for star clustering: methodology and application to the Two Micron Galactic Survey
A new approach to the study of the large-scale stellar clusterdistribution in the Galaxy based on two-point correlation techniques ispresented. The basic formalism for this method is outlined, and itsapplications are then investigated by the use of a simple model ofcluster distribution in the Galaxy. This provides an estimate of thepotentials of the two-point correlation function for indicatingclustering in the measured star positions, which can be related to thepresence of star clusters in the observed field. This technique is thenapplied to several areas of the Two Micron Galactic Survey catalogue,from which information is obtained on the distribution of clustersaccording to position in the Galaxy, as well as about age, density ofstars, etc.
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