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An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| Towards absolute scales for the radii and masses of open clusters
Aims. In this paper we derive tidal radii and masses of open clusters inthe nearest kiloparsecs around the Sun. Methods: For each cluster, themass is estimated from tidal radii determined from a fitting ofthree-parameter King profiles to the observed integrated densitydistribution. Different samples of members are investigated. Results:For 236 open clusters, all contained in the catalogue ASCC-2.5, weobtain core and tidal radii, as well as tidal masses. The distributionsof the core and tidal radii peak at about 1.5 pc and 7-10 pc,respectively. A typical relative error of the core radius lies between15% and 50%, whereas, for the majority of clusters, the tidal radius wasdetermined with a relative accuracy better than 20%. Most of theclusters have tidal masses between 50 and 1000 m_ȯ, and for abouthalf of the clusters, the masses were obtained with a relative errorbetter than 50%.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/468/151
| CCD photometric search for peculiar stars in open clusters. VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, Ruprecht 44, Ruprecht 115, and Ruprecht 120
Context: We continue our survey of magnetic chemically peculiar (CP2)stars in galactic open clusters to shed more light on their origin andevolution. Aims: To study the group of CP2 stars, it is essential tofind these objects in different galactic environments and at a widerange of evolutionary stages. The knowledge of open cluster ages andmetallicities can help for finding a correlation between theseparameters and the (non-)presence of peculiarities, which has to betaken into account in stellar evolution models. Methods: Theintermediate band Δ a photometric system samples the depth of the5200 Å flux depression by comparing the flux at the centre withthe adjacent regions with bandwidths of 110 Å to 230 Å. Itis capable of detecting magnetic CP2 and CP4 stars with high efficiency,but also the groups of (metal-weak) λ Bootis and classicalBe/shell stars can be successfully investigated. In addition, it allowsthe age, reddening, and distance modulus to be determined withappropriate accuracy by fitting isochrones. Results: From the 1677observed members of the eight open clusters, one Ae and twenty-five CP2stars were identified. Furthermore nineteen deviating stars aredesignated as questionable for several reasons. The estimated age,reddening, and distance for the programme clusters were compared withpublished values of the literature and discussed in this context. Conclusions: .The current paper shows that CP2 stars are continuouslypresent in very young (7 Myr) to intermediate age (500 Myr) openclusters at distances greater than 2 kpc from the Sun.Based on observations at CASLEO, CTIO (Proposal 2003A-0057), and OSN.The Observatorio de Sierra Nevada is operated by the Consejo Superior deInvestigaciones Científicas through the Instituto deAstrofísica de Andalucía (Granada, Spain). Photometricdata are only avaialable in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/591 Full Fig. [seefull textsee full textsee full textsee full textsee full text], Tables[see full textsee full textsee full textsee full textsee full text] and[see full textsee full textsee full textsee full textsee full text] areonly available in electronic form at http://www.aanda.org
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the Hγ profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method toestimate the polar gravity for these stars, which we argue is a usefulindicator of their evolutionary status. We combine these results withprojected rotational velocity measurements obtained in a previous paperon these same open clusters. We find that the more massive B starsexperience a spin-down as predicted by the theories for the evolution ofrotating stars. Furthermore, we find that the members of binary starsalso experience a marked spin-down with advanced evolutionary state dueto tidal interactions. We also derive non-LTE-corrected heliumabundances for most of the sample by fitting the He Iλλ4026, 4387, 4471 lines. A large number of heliumpeculiar stars are found among cooler stars withTeff<23,000 K. The analysis of the high-mass stars (8.5Msolar
| Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in sampleswith well-defined ages and initial chemical composition. Here we presenta survey of projected rotational velocities for a large sample of mainlyB-type stars in young clusters to study the time evolution of therotational properties of massive stars. The survey is based onmoderate-resolution spectra made with the WIYN 3.5 m and CTIO 4 mtelescopes and Hydra multi-object spectrographs, and the target starsare members of 19 young open clusters with an age range of approximately6-73 Myr. We made fits of the observed lines He I λλ4026,4387, 4471, and Mg II λ4481, using model theoretical profiles tofind projected rotational velocities for a total of 496 OB stars. Wefind that there are fewer slow rotators among the cluster B-type starsrelative to nearby B stars in the field. We present evidence consistentwith the idea that the more massive B stars (M>9 Msolar)spin down during their main-sequence phase. However, we also find thatthe rotational velocity distribution appears to show an increase in thenumbers of rapid rotators among clusters with ages of 10 Myr and higher.These rapid rotators appear to be distributed between the zero age andterminal age main-sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of aspin-up at the terminal age main sequence due to core contraction. Wesuggest instead that some of these rapid rotators may have been spun upthrough mass transfer in close binary systems.
| 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 Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| 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 massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| A pair of reddened young star clusters in Centaurus
Photometry in the BVI and JHK bands is presented for the first time forthe southern open clusters Danks 1 and 2, located in the Centaurusregion. They are relatively rich and compact, and projected atapproximately 4 arcmin from each other. The clusters are very reddened,with AV= 8.1 +/- 0.45 and 9.3 +/- 0.6, respectively. They areyoung clusters, with ages of approximately 10 Myr, and may be related tothe 305° HII region complex in the area. Distances from the Sun ofdsolar~ 3.6 +/- 0.5 and 3.4 +/- 0.2 kpc are derived for Danks1 and 2. They might form a physical cluster pair.
| 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.
| Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
| The Lack of Blue Supergiants in NGC 7419, a Red Supergiant-rich Galactic Open Cluster with Rapidly Rotating Stars
According to previous studies based on photometry alone, NGC 7419reveals a surprisingly low ratio of blue to red supergiants: only oneblue supergiant (BSG) along with a record number of five red supergiants(RSGs). However, for a cluster like NGC 7419 with solar metallicity, oneexpects twice as many BSGs as RSGs. To verify the small ratio of BSGs toRSGs, we have obtained spectroscopic observations of the seven mostluminous blue member stars using the 1.6 m telescope of the MountMégantic Observatory. (The RSGs have already been studiedspectroscopically.) To classify the stars, we have developed a systemespecially adapted for these heavily reddened stars in a spectral regionfrom 8400 to 8920 Å, near the hydrogen Paschen series limit. Thisclassification system is based on standard stars of known MK spectraltype extending over O9-B5 and all luminosity classes and is linkedthrough a grid of synthetic spectra to the atmospheric physicalparameters Teff and logg. We also include Be stars. Among theseven blue stars observed in NGC 7419, four have red spectra that aredominated by absorption lines and three by emission lines. The spectraltypes for the former are B2.5 II-III, B2.5 III, B0 III, and B4 III (e),while those for the latter are Be, B1 III-Ve, and Be, respectively. Theaverage heliocentric radial velocity of these stars is -66+/-6 kms-1, compatible with the value of -74+/-9 km s-1measured for the five RSG members. A distance of 1.7+/-0.4 kpc for thiscluster was estimated using the blue-star spectral types, in agreementwith the value of 2.3+/-0.3 kpc found by Beauchamp and coworkers, basedon isochrone fitting in the color-magnitude diagram. With no BSG starsdetected spectroscopically, we confirm the low number, in this caseabsence, of BSGs in this cluster. The high fraction of Be stars detectedby us and others among the bright blue member stars could be explainedby an average rotational velocity for the stars in NGC 7419, which issignificantly higher than in other clusters of similar age andmetallicity. Since higher stellar rotation rates shorten the BSG phase,we suggest that this explains why the evolved stars in NGC 7419 havebecome RSGs. Thus, NGC 7419 is an exceptional case, since high stellarrotation normally tends to occur at lower metallicity.
| 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.
| Interstellar extinction in the open clusters towards galactic longitude around 130deg
In this paper we present a detailed study of the intra-cluster reddeningmaterial in the young open clusters located around l ~ 130degusing colour-excess diagrams and two-colour diagrams. The study supportsthe universality of the extinction curves for lambda >= lambdaJ, whereas for shorter wavelengths the curve depends upon thevalue of the Rcluster (total-to-selective absorption in thecluster region). The value of Rcluster in the case of NGC654, NGC 869 and NGC 884 is found to be normal, whereas the value ofRcluster in the cluster regions NGC 1502 and IC 1805indicates an anomalous reddening law in these regions. In the case ofNGC 1502 the value of Rcluster is found to be lower(2.57+/-0.27) whereas in the case of IC 1805 it is higher (3.56+/-0.29)than the normal value of 3.1. Although the intra-cluster materialindicates a higher value of Rcluster in the NGC 663 region,the error in the estimation of Rcluster is too large toconclude anything. It is also found that the extinction process in the Uband in the case of NGC 663 seems to be less efficient, whereas in thecase of NGC 869 the process is more efficient.
| 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.
| 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.
| The blue to red supergiant ratio in young clusters at various metallicities
We present new determinations of the blue to red supergiant ratio (B/R)in young open clusters at various metallicities. For this purpose, weexamine the HR diagrams of 45 clusters in the Galaxy and of 4 clustersin the Magellanic Clouds. The identification of supergiants is based onspectroscopic measurements (with photometric counts to check theresults). The new counts confirm the increase of the B/R ratio when themetallicity increases with the following normalized relation:(B/R)/((B/R)sun) =~ 0.05* e3(Z)/(Zsun)}, where Zsun=0.02 and(B/R)sun is the value of B/R at Zsun which dependson the definition of B and R and on the age interval considered (e.g.for spectroscopic counts including clusters with log age between 6.8 and7.5, (B/R)sun =~ 3 when B includes O, B and A supergiants).
| Membership in the Region of the Double Cluster h and χ Persei Working from Proper Motions and Positions: Distance Moduli and Extinction in That Galactic Direction
A segregation between cluster members and field stars in the region ofthe double cluster h and χ Persei is accomplished working fromproper motions and positions as kinematic random variables. Theextinction RV and the distance moduli are determined once thephysical cluster members are found.
| 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].
| Periodic Pattern in the Residual-Velocity Field of OB Associations
Not Available
| Some Theoretical consequences of the Thick, Magnetized Galactic Disk
A number of recent observational studies reveal a new paradigm for diskgalaxies, one in which layers of warm ionized gas, dust structures andmagnetic fields commonly extend to considerable heights (of kpc) abovetheir midplanes. This thick disk has an intricate and largely unknownstructure. Its inclusion while modeling large scale dynamical processesin the interstellar medium, such as the interacion of HVC with galacticdisks, the Parker instability, and the gas response to the spiraldensity wave, alters considerably the ``classic'' results obtained inthe thin disk picture of galactic disks which was popular in previousstudies.
| 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.
| Trigonometric Parallaxes and a Kinematically Adjusted Distance Scale for OB Associations
By directly comparing the photometric distances of Blaha and Humphreys(1989) (BH) to OB associations and field stars with the correspondingHipparcos trigonometric parallaxes, we show that the BH distance scaleis overestimated, on average, by 10-20%. This result is independentlycorroborated by applying the rigorous statistical-parallax method andits simplified analog (finding a kinematically adjusted rotation-curvesolution from radial velocities and proper motions) to a sample of OBassociations. These two methods lead us to conclude that the BH distancescale for OB associations should be shrunk, on average, by 11 +/- 6 and24 +/- 10 %, respectively. Kinematical parameters have been determinedfor the system of OB associations: u_0 = 8.2 +/- 1.3 km/s, v_0= 11.9 +/-1.1 km/s, w_0 = 9.5 +/- 0.9 km/s, sigma_u = 8.2 +/- 1.1 km/s, sigma_v =5.8 +/- 0.8 km/s, sigma_w = 5.0 +/- 0.8 km/s, Omega_0 = 29.1 +/- 1.0km/s/kpc, (Omega_0)' = -4.57 +/- 0.20 km/s/kpc^2, and (Omega_0)'' = 1.32+/- 0.14 km/s/kpc^3. The distance scale for OB associations reduced by20% matches the short Cepheid distance scale (Berdnikov and Efremov1985; Sitnik and Mel'nik 1996). Our results are a further argument forthe short distance scale in the Universe.
| The Galactic metallicity gradient
We have previously published intermediate to high resolutionspectroscopic observations of approximately 80 early B-typemain-sequence stars situated in 19 Galactic open clusters/associationswith Galactocentric distances distributed over 6<= Rg<=18 kpc. This current study collates and re-analyses theseequivalent-width datasets using LTE and non-LTE model atmospheretechniques, in order to determine the stellar atmospheric parameters andabundance estimates for C, N, O, Mg, Al and Si. The latter should berepresentative of the present-day Galactic interstellar medium. Ourextensive observational dataset permits the identification ofsub-samples of stars with similar atmospheric parameters and ofhomogeneous subsets of lines. As such, this investigation represents themost extensive and systematic study of its kind to date. We concludethat the distribution of light elements (C, O, Mg & Si) in theGalactic disk can be represented by a linear, radial gradient of-0.07+/-0.01 dex kpc-1. Our results for nitrogen and oxygenviz. (-0.09+/-0.01 dex kpc-1 and -0.067+/-0.008 dexkpc-1) are in excellent agreement with that found from thestudy of H II regions. We have also examined our datasets for evidenceof an abrupt discontinuity in the metallicity of the Galactic disk neara Galactocentric distance of 10 kpc (see Twarog et al. \cite{twa97}).However, there is no evidence to suggest that our data would be betterfitted with a two-zone model. Moreover, we observe a N/O gradient of-0.04+/-0.02 dex kpc-1 which is consistent with that foundfor other spiral galaxies (Vila-Costas & Edmunds \cite{vil93}).
| 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.
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