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SigSpec. I. Frequency- and phase-resolved significance in Fourier space
Context: Identifying frequencies with low signal-to-noise ratios in timeseries of stellar photometry and spectroscopy, and measuring theiramplitude ratios and peak widths accurately, are critical goals forasteroseismology. These are also challenges for time series with gaps orwhose data are not sampled at a constant rate, even with modern DiscreteFourier Transform (DFT) software. Also the False-Alarm Probabilityintroduced by Lomb and Scargle is an approximation which becomes lessreliable in time series with longer data gaps. Aims: A rigorousstatistical treatment of how to determine the significance of a peak ina DFT, called SigSpec, is presented here. SigSpec is based on ananalytical solution of the probability that a DFT peak of a givenamplitude does not arise from white noise in a non-equally spaced dataset. Methods: The underlying Probability Density Function (PDF) of theamplitude spectrum generated by white noise can be derived explicitly ifboth frequency and phase are incorporated into the solution. In thispaper, I define and evaluate an unbiased statistical estimator, the“spectral significance”, which depends on frequency,amplitude, and phase in the DFT, and which takes into account thetime-domain sampling. Results: I also compare this estimator to resultsfrom other well established techniques and assess the advantages ofSigSpec, through comparison of its analytical solutions to the resultsof extensive numerical calculations. According to those tests, SigSpecobtains as accurate frequency values as a least-squares fit of sinusoidsto data, and is less susceptible to aliasing than the Lomb-ScarglePeriodogram, other DFTs, and Phase Dispersion Minimization (PDM). Idemonstrate the effectiveness of SigSpec with a few examples of ground-and space-based photometric data, illustratring how SigSpec deals withthe effects of noise and time-domain sampling in determining significantfrequencies.
| CCD BV survey of 42 open clusters
Aims.We present results of a photometric survey whose aim was to derivestructural and astrophysical parameters for 42 open clusters. While oursample is definitively not representative of the total open clustersample in the Galaxy, it does cover a wide range of cluster parametersand is uniform enough to allow for simple statistical considerations. Methods: BV wide-field CCD photometry was obtained for open clusters forwhich photometric, structural, and dynamical evolution parameters weredetermined. The limiting and core radii were determined by analyzingradial density profiles. The ages, reddenings, and distances wereobtained from the solar metallicity isochrone fitting. The mass functionwas used to study the dynamical state of the systems, mass segregationeffect and to estimate the total mass and number of cluster members. Results: This study reports on the first determination of basicparameters for 11 out of 42 observed open clusters. The angular sizesfor the majority of the observed clusters appear to be several timeslarger than the catalogue data indicate. The core and limiting clusterradii are correlated and the latter parameter is 3.2 times larger onaverage. The limiting radius increases with the cluster's mass, and boththe limiting and core radii decrease in the course of dynamicalevolution. For dynamically not advanced clusters, the mass functionslope is similar to the universal IMF slope. For more evolved systems,the effect of evaporation of low-mass members is clearly visible. Theinitial mass segregation is present in all the observed young clusters,whereas the dynamical mass segregation appears in clusters older thanabout log({age}) = 8. Low-mass stars are deficient in the cores ofclusters older than log({age}) = 8.5 and not younger than one relaxationtime.Tables 1-5 are only available in electronic form at http://www.aanda.org
| Pulsating pre-main sequence stars in IC 4996 and NGC 6530
Context: .Asteroseismology of pulsating pre-main sequence (PMS) starshas the potential of testing the validity of current models of PMSstructure and evolution. As a first step, a sufficiently large sample ofpulsating PMS stars has to be established, which allows to selectcandidates optimally suited for a detailed asteroseismological analysisbased on photometry from space or ground based network data. Aims:.A search for pulsating PMS members in the young open clusters IC 4996and NGC 6530 has been performed to improve the sample of known PMSpulsators. As both clusters are younger than 10 million years, allmembers with spectral types later than A0 have not reached the zero-agemain sequence yet. Hence, IC 4996 and NGC 6530 are most suitable tosearch for PMS pulsation among their A- and F-type cluster stars.Methods: .CCD time series photometry in Johnson B and V filters has beenobtained for IC 4996 and NGC 6530. The resulting light curves for 113stars in IC 4996 and 194 stars in NGC 6530 have been subject to detailedfrequency analyses. Results: .2 δ Scuti-like PMS stars havebeen discovered in IC 4996 and 6 in NGC 6530. For another PMS star ineach cluster, pulsation can only be suspected. According to the computedpulsation constants, the newly detected PMS stars seem to prefer topulsate in a similar fashion to the classical δ Scuti stars, andwith higher overtone modes.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| The Optical Gravitational Lensing Experiment. Variable Stars in the Field of Open Cluster NGC6259
We present the results of a search for variable stars in the field ofNGC6259. Altogether 85 variable stars have been discovered. 36 of themare eclipsing systems. This group contains 13 EA, 2 EB and 21 EW-typestars. Light curves of two variable stars resemble those of Miras. Theremaining 47 variable stars detected in the field of this open clusterare most probably highly obscured red giants (OSARG, irregular). For alldetected variable stars we provide their light curves, preliminaryclassification, discussion on the possible cluster membership,equatorial coordinates, finding charts and periods when possible.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| NGC 146: a young open cluster with a Herbig Be star and intermediate mass pre-main sequence stars
We present UBV CCD photometry and low-resolution spectra of stars in thefield of the young open cluster NGC 146. UBV photometry of 434 starswere used to estimate the E(B-V) reddening of 0.55 ± 0.04 mag andBV photometry of 976 stars were used to estimate a distance modulus of(m-M)0 = 12.7 ± 0.2 mag, corresponding to a distanceof 3470+335-305 pc. We estimated 10-16 Myr as theturn-off age for the upper main sequence of the cluster using isochronesand synthetic colour magnitude diagrams. We identified two B type starswith Hα in emission and located on the MS using slit-less spectra.A higher resolution spectrum of the brighter Be star indicated thepresence of a number of emission lines, with some lines showing thesignature of gas infall. This star was found to be located in the regionof Herbig Ae/Be stars in the (J-H) vs. (H-K) colour-colour diagram.Thus, we identify this star as a Herbig Be star. On the other hand, 54stars were found to show near infrared excess, of which 17 were found tobe located in the region of Herbig Ae/Be stars and 18 stars were foundto be located in the region of Be stars in the NIR colour-colourdiagram. Thus NGC 146 is a young cluster with a large number ofintermediate mass pre-main sequence stars. The turn-on age of thecluster is found to be ~3 Myr. Though NGC 146 shows an older turn off,the bulk of stars in this cluster seems to belong to the youngerpopulation of 3 Myr.
| 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.
| Supernova 1954J (Variable 12) in NGC 2403 Unmasked
We have confirmed that the precursor star of the unusual supernova 1954J(also known as Variable 12) in NGC 2403 survived what appears to havebeen a superoutburst, similar to the 1843 Great Eruption of ηCarinae in the Galaxy. The apparent survivor has changed little inbrightness and color over the last 8 years, and a Keck spectrum revealscharacteristics broadly similar to those of η Car. This is furthersuggested by our identification of the actual outburst-surviving star inhigh-resolution images obtained with the Advanced Camera for Surveys onthe Hubble Space Telescope. We reveal this ``supernova impostor'' as ahighly luminous (M0V~-8.0 mag), very massive(Minitial>~25 Msolar) eruptive star, nowsurrounded by a dusty (AV~4 mag) nebula, similar to ηCar's famous Homunculus.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by AURA, Inc., under NASA contract NAS5-26555.
| Collaborative Research of Open Star Clusters
Preliminary results on observations of open clusters are pre-sented. Theproject has been initiated in the framework of the Uzbek-Taiwan andTaiwan-Baltic collaboration, mainly to upgrade and make use offacilities at Maidanak Observatory. We present detailed,multi-wavelength studies of the young cluster NGC 6823 and theassociated complex nebulosity, to diagnose the young stellar populationand star formation history in the region. In addition, 7 compact openclusters have been monitored for stellar variability. We show howobservations like these could feasibly be used to look for exoplanettransit events. We also expect to join the Whole-Earth Telescope effortin future campaigns for asteroseismology.
| Pulsating pre-Main sequence stars in young open clusters
New pulsating pre-main sequence (PMS) stars have been discovered in theyoung open clusters IC 4996 and NGC 6383 using CCD time seriesphotometry in Johnson B and V filters. As the cluster ages are bothsmaller than 10 million years, all members later than spectral type A0are still contracting towards the ZAMS, hence providing ideal candidatesfor searches of pulsation. A dozen stars in NGC 6383 and 35 stars in IC4996 lie within the boundaries of the classical instability region inthe Hertzsprung-Russell (HR) diagram, but pulsation was detected foronly two of them in each cluster.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| 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.
| 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.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Characteristics of new star cluster candidates in the Cygnus area
The morphology and stellar content of 22 clusters in the Cygnus area hasbeen determined using 2MASS infrared observations in the J and K bands.7 of the clusters are well-known objects and our results are in goodagreement with previous work. 12 objects are recently-discovered clustercandidates (Dutra & Bica \cite{dutra01}) and 3 are new clustercandidates. Adopting distance estimates from the literature or byconsidering cluster counterparts, we derive a consistent set ofparameters suitable for analysis and comparison: center coordinates,radius, stellar population, IMF slope and mass range. We find a mean IMFslope of Gamma =-1.30 for the known clusters and Gamma =-1.40 for theothers. Infrared data having better completeness, we derive largerstellar contents and cluster masses than previous works. An importantfraction of the cluster stellar population in Cygnus is hidden in veryobscured areas. A clear tendency towards mass segregation appears formost of the clusters. Some of the new clusters seem to be quite massive,probably hosting very massive stars at the center. They are privilegedtargets for more detailed investigations using infrared spectroscopy.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| 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.
| Variable Stars in the Field of Young Open Cluster NGC 581
We present results of the search for variable stars in the field ofyoung open cluster NGC 581. Based on 19 nights of observations, 6 newvariable stars were discovered. Two of them turned out to be eclipsingbinary systems. Another two detected variable stars are most probably ofgamma Dor type. During our observations one of the known Be starslocated in our field of view showed irregular variations of brightness,typical for this class of stars. The sixth variable star is a pulsatingred giant.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Characteristics of the Variable Star P Cygni Determined from Cluster Membership
Empirical information on the luminosity, reddening, age, and mass of thevariable B2 Oe supergiant P Cygni is derived from its assumed membershipin the sparse anonymous cluster on which it is projected, as well as itsassociation with the spatially adjacent cluster IC 4996, which forms adouble cluster with the P Cyg cluster. Evidence for the high luminosityof P Cyg is confirmed by its derived absolute magnitude of M(v) = -8.46+/-0.03, which translates to log (L/L(solar)) = 5.54 +/-0.02 for aneffective temperature consistent with the star's derived space reddening(E(B-V)=0.53 +/-0.02). More surprising is an age for the associatedclusters of 6 (+/-1.5) x 10^6 years, corresponding to a turnoff pointmass of 25.1 (+/-5.5 solar mass). By inference, P Cygni, as a postmain-sequence object, should have a mass of no more than ~23-25 solarmass.
| Fundamental parameters and new variables of the galactic open cluster NGC 7128
CCD photometry in Johnson UBV and Strömgren uvby systems andmedium-resolution spectroscopy of the galactic open cluster NGC 7128 arepresented. Spectral types of the brightest 12 stars in the cluster fieldwere determined based on equivalent widths of the Hα and the Hei6678-Å line. The spectroscopic observations also revealed twoobvious and one probable Be-type stars showing Hα emission. Theanalysis of the photometric diagrams gave a colour excess ofE(B-V)=1.03+/-0.06mag, a distance modulus DM=13.0+/-0.2mag and an ageabove 10Myr. Time-resolved photometric observations obtained on onenight resulted in the detection of short time-scale light variations ofseven new and three already known variable stars in the cluster field.
| Variable Stars in the Field of Young Open Cluster NGC 659
We present results of the search for variable stars in young opencluster NGC 659. Based on 35 nights of observations, 7 new variablestars were discovered. Three of them are Be stars. Another three aremost probably pulsating variables of gamma Dor type. One star is adetached binary system.
| 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.
| Search for Pre-Main-Sequence Stars in the Young Galactic Cluster NGC 6910
Photometric CCD UBV measurements have been obtained in the field of theyoung Cygnus cluster NGC 6910. The observations reach down to magnitudeV~=18 for 206 stars measured in all three UBV bands, uncovering theregion of the color-magnitude diagram where pre-main-sequence (PMS)stars are expected. The transformation of our instrumental magnitudes tothe standard system is discussed on the basis of two different sets ofstandard stars: 22 stars from six fields in the Landolt catalogs and 48stars with published photoelectric photometry in common with ours in thefour clusters observed in this campaign. The latter set is preferred,since it enables the effect of systematic errors in the U-B colors to beminimized. The adopted cluster parameters are E(B-V)=1.02+/-0.13,V0-MV=11.2+/-0.2, and age=(6.5+/-3)x106yr. The search for PMS cluster members on the basis of the calculatedcolor excess and distance modulus results in the proposed membership for11 PMS stars of spectral types from A to G.
| Spectroscopy of Pre-Main-Sequence Candidates of Spectral Type AF in the Young Galactic Cluster IC 4996
We present the results of a spectroscopic analysis of thepre-main-sequence (PMS) candidates in IC 4996, proposed by Delgado etal. Spectral types and heliocentric radial velocities are calculated for16 stars in the field observed by these authors, 13 of them located inthe region of the color-magnitude diagram where their proposed PMS starsare located. The estimated heliocentric radial velocity of the clusteris centered around -12+/-5 km s^-1. From the radial velocitydistribution, six stars are rejected as cluster members, one of themshowing spectral features characteristic of an Am star. The remaining 10stars are confirmed as cluster members: three B-type stars and seven PMSstars of spectral types A4-F0 (six stars) and early G (one star). One ofthe proposed PMS members clearly shows radial velocity and spectral typevariations, as well as relatively broad Hα absorption. The G-typecluster member is a weak-lined T Tauri star with strong Li Iλ6708 absorption [W_λ(Li I)~=0.26 Å]. These resultsstrongly support the presence in the cluster of a populated sequence ofPMS stars of AF spectral type.
| 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.
| Pre-Main-Sequence Stars in the Young Galactic Cluster IC 4996: A CCD Photometric Study
The results of a UBV CCD photometric study of the cluster IC 4996 arepresented. We obtain new values for the cluster parameters: E(B-V) =0.71 +/- 0.08, V_0 - M_V = 11.9 +/- 0.1, and age = (7.5 +/- 3) x 10^6yr. The combined evidence from the photometric diagrams suggests thepresence of a number of pre-main-sequence (PMS) cluster members withspectral types ~A0 to F5. The interpretation of these objects as PMScandidates is further supported by independent results from CCD uvbyHβ observations. If confirmed, these stars would bridge theexisting gap in the sample of PMS stars, between the coolest Herbig AeBestars (HAeBe), and the hottest T Tauri stars. These PMS candidates arelocated some 0.5 and 1 mag above the main sequence in the V-(B-V)diagram, around the location of spectral types AF. This feature,together with their locations in the (U-B)-(B-V) diagram and theinterpretation that they are PMS members, suggest an additionalreddening law with a slope alpha = E(U-B)/E(B-V) ~= 0.55, probablycaused by circumstellar material.
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