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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.
| New Variable Stars in the Open Cluster M103 (NGC581)
We report a detection of 15 new variable stars in the open cluster M103(NGC581)
| 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.
| The Massive Runaway Stars HD 14633 and HD 15137
We present results from a radial velocity study of two runaway O-typestars, HD 14633 (ON8.5 V) and HD 15137 [O9.5 III(n)]. We find that HD14633 is a single-lined spectroscopic binary with an orbital period of15.4083 days. The second target, HD 15137, is a radial velocity variableand a possible single-lined spectroscopic binary with a period close to1 month. Both binaries have large eccentricity, small semiamplitude, anda small mass function. We show the trajectories of the stars in the skybased on an integration of motion in the Galactic potential, and wesuggest that both stars were ejected from the vicinity of the opencluster NGC 654 in the Perseus spiral arm. The binary orbital parametersand runaway velocities are consistent with the idea that both thesestars were ejected by supernova explosions in binaries and that theyhost neutron star companions. We find that the time of flight sinceejection is longer than the predicted evolutionary timescales for thestars. This discrepancy may indicate that the stars have a lower massthan normally associated with their spectral classifications, that theywere rejuvenated by mass transfer prior to the supernova, or that theirlives have been extended through rapid rotation.Based in part on observations made at the Observatoire de Haute Provence(CNRS), France.
| 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.
| 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.
| 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.
| 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.
| Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars
We obtained high-resolution spectroscopic observations of 34 γDoradus candidates. From the red-wavelength spectra, we determinedspectral classes, radial velocities, and projected rotationalvelocities. The spectra of seven late A or early F stars show metalliclines that have composite profiles consisting of a narrow component nearthe center of a broad line, indicating that they may be shell stars orbinaries. Several stars, including HD 152896, HD 173977, HD 175337, andHD 195068/9, show large line profile asymmetries. Two stars, HD 11443(=α Trianguli) and HD 149420, are ellipsoidal variables and notγ Doradus stars. The percentage of binary systems in our samplemay be as high as 74%.
| 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.
| 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.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| 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' 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.
| 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.
| 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).
| 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].
| 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
| Photometric and kinematic studies of open star clusters. II. NGC 1960 (M 36) and NGC 2194
We present CCD photometry and proper motion studies of the two open starclusters NGC 1960 (M 36) and NGC 2194. Fitting isochrones to the colourmagnitude diagrams, for NGC 1960 we found an age of t=16 Myr and adistance of roughly d=1300 pc and for NGC 2194 t=550 Myr and d=2900 pc,respectively. We combined membership determination by proper motions andstatistical field star subtraction to derive the initial mass functionof the clusters and found slopes of Gamma =-1.23 +/- 0.17 for NGC 1960and Gamma =-1.33 +/- 0.29 for NGC 2194. Compared to other IMF studies ofthe intermediate mass range, these values indicate shallow massfunctions. Partly based on data observed at the German-SpanishAstronomical Centre, Calar Alto, operated by the Max-Planck-Institutefor Astronomy, Heidelberg, jointly with the Spanish National Commissionfor Astronomy
| 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.
| Astrometric CCD observations of Jupiter's inner satellites Thebe and Amalthea and Saturn's satellite Phoebe in the 1998 opposition.
Not Available
| 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.
| Photometric and kinematic studies of open star clusters. I. NGC 581 (M 103)
We present CCD photometry and a proper motion study of the young openstar cluster, NGC 581 (M 103). Fitting isochrones to the colourmagnitude diagram, we found an age of 16 +/- 4 Myr and a distance ofroughly 3 kpc for this cluster. The proper motion study identifies 77stars of V=14.5 mag or brighter to be cluster members. We combinemembership determination by proper motions and statistical field starsubtraction to derive the IMF of the cluster and find a quite steepslope of Gamma =-1.80.
| Differences in the fractions of Be stars in galaxies
The number ratios Be/(B+Be) of Be to B-type stars in young, well studiedclusters of the Galaxy, the LMC and SMC are examined. In order todisentangle age and metallicity effects we choose clusters in the sameage interval and for which reliable photometric and spectroscopic dataare available. Number counts are made for various magnitude intervals,and the results are found to be stable with respect to this choice. Inthe magnitude interval MV = -5 to -1.4 (i.e. O9 to B3) weobtained a ratio Be/(B+Be) = 0.11, 0.19, 0.23, 0.39 for 21 clusterslocated in the interior of the Galaxy, the exterior of the Galaxy, theLMC and the SMC, respectively. Various hypotheses for these differencesare examined. An interesting possibility is that the average rotation isfaster at low metallicities as a result of star formation processes. Themuch higher relative N-enrichment found by Venn et al. (\cite{vencar})in A-type supergiants of the SMC, compared to galactic supergiants, alsostrongly supports the presence of more rotational mixing at lowmetallicities. We discuss whether high rotational mixing may be thesource of primary nitrogen in the early chemical evolution of galaxies.
| A report on the studies of star clusters with the UPSO 104-cm Sampurnanand telescope during last 25 years
Not Available
| A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics
We use optical and near-infrared star counts to explore the structureand dynamics of the Orion Nebula Cluster (ONC). This very young (<1 Myr)cluster is not circularly symmetric in projection but is elongatednorth-south in a manner similar to the molecular gas distribution in theregion, suggesting that the stellar system may still reflect thegeometry of the protocluster cloud. Azimuthally averaged stellar sourcecounts compare well with simple spherically symmetric, single-mass Kingcluster models. The model fits suggest that the inner Trapezium regionshould be regarded as the core of the ONC, not as a distinct entity assometimes advocated. We estimate that the core radius of the cluster is0.16-0.2 1 pc and that the central stellar density approaches 2 x104 stars/pc3. Adopting the stellar velocitydispersion from published proper-motion studies, virial equilibriumwould require a total mass within about 2 pc of the Trapezium of 4500Mo, slightly more than twice the mass of the known stellarpopulation and comparable to the estimated mass in molecular gasprojected onto the same region of the sky. If >~20% of the remainingmolecular gas is converted into stars, thus adding to the binding mass,given that the present stellar population alone has a total energy closeto zero, the ONC is likely to produce a gravitationally bound cluster.The ONC also exhibits mass segregation, with the most massive(Trapezium) stars clearly concentrated toward the center of the clusterand some evidence for the degree of central concentration to decreasewith decreasing mass down to 1-2 Mo, as would be expected forgeneral mass segregation. Given the extreme youth of the stars comparedwith the estimated range of collisional relaxation times, the masssegregation is unlikely to be the result of cluster relaxation. Instead,we suggest that the mass segregation reflects a preference for highermass stars to form in dense, central cluster regions.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
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