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Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects
Some H II regions surrounding young stellar clusters contain tiny dustyclouds, which on photos look like dark spots or teardrops against abackground of nebular emission. From our collection of Hα imagesof 10 H II regions gathered at the Nordic Optical Telescope, we found173 such clouds, which we call ``globulettes,'' since they are muchsmaller than normal globules and form a distinct class of objects. Manyglobulettes are quite isolated and located far from the molecular shellsand elephant trunks associated with the regions. Others are attached tothe trunks (or shells), suggesting that globulettes may form as aconsequence of erosion of these larger structures. None of our objectsappear to contain stellar objects. The globulettes were measured forposition, dimension, and orientation, and we find that most objects aresmaller than 10 kAU. The Rosette Nebula and IC 1805 are particularlyrich in globulettes, for which the size distributions peak at mean radiiof ~2.5 kAU, similar to what was found by Reipurth and coworkers and DeMarco and coworkers for similar objects in other regions. We estimatetotal mass and density distributions for each object from extinctionmeasures and conclude that a majority contain <13 MJ,corresponding to planetary-mass objects. We then estimate the internalthermal and potential energies and find, when also including the effectsfrom the outer pressure, that a large fraction of the globulettes couldbe unstable and would contract on short timescales, <10 6yr. In addition, the radiation pressure and ram pressure exerted on theside facing the clusters would stimulate contraction. Since theglobulettes are not screened from stellar light by dust clouds fartherin, one would expect photoevaporation to dissolve the objects. However,surprisingly few objects show bright rims or teardrop forms. Wecalculate the expected lifetimes against photoevaporation. Theselifetimes scatter around 4×106 yr, much longer thanestimated in previous studies and also much longer than the free-falltime. We conclude that a large number of our globulettes have time toform central low-mass objects long before the ionization front, drivenby the impinging Lyman photons, has penetrated far into the globulette.Hence, the globulettes may be one source in the formation of browndwarfs and free-floating planetary-mass objects in the galaxy.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
| Infrared Nebulae around Young Stellar Objects
We present a K-band atlas of 106 reflection nebulae, 41 of which are newdiscoveries. We observed these nebulae with the University of Hawaii 2.2m telescope in the course of an imaging survey of 197 objects that wereselected as nearby young Class I sources. K-band images andflux-calibrated surface brightness contour plots of each nebula arepresented. We found that the near-IR luminosities and physical sizes ofthe nebulae increase with the bolometric luminosity of the illuminatingsources. Only 22 nebulae, about 10% of these candidate Class I sources,have indications of shocked H2 emission. The great variety ofnebulae that we observed prevented us from classifying them based onmorphology. However, we note that as the spectral index decreases, thecentral star is more frequently visible at K band, and the flux from thecentral star tends to be dominant over the flux from the nebula. Forobjects that have a higher spectral index, most of the K-band flux isfrom the reflection nebula, and the central star is less frequentlyvisible. The nebula around IRAS 05450+0019 has a unique morphology, andwe speculate that it may be an example of a disk shadow being projectedinto the surrounding cloud. We present J-, H-, and K-band images of thisobject with surface brightness contours, as well as its spectral energydistribution from 1.2 to 100 μm.
| Rotational Velocities for B0-B3 Stars in Seven Young Clusters: Further Study of the Relationship between Rotation Speed and Density in Star-Forming Regions
We present the results of a study aimed at assessing the differences inthe distribution of rotation speeds N(vsini) among young (1-15 Myr) Bstars spanning a range of masses 6 Msolar>1 Msolar pc-3)ensembles that will survive as rich, bound stellar clusters for ageswell in excess of 108 yr. Our results demonstrate (1) thatindependent of environment, the rotation rates for stars in this massrange do not change by more than 0.1 dex over ages t~1 to ~15 Myr; and(2) that stars formed in high-density regions lack the cohort of slowrotators that dominate the low-density regions and young field stars. Wesuggest that the differences in N(vsini) between low- and high-densityregions may reflect a combination of initial conditions andenvironmental effects: (1) the higher turbulent speeds that characterizemolecular gas in high-density, cluster-forming regions; and (2) thestronger UV radiation fields and high stellar densities thatcharacterize such regions. Higher turbulent speeds may lead to highertime-averaged accretion rates during the stellar assembly phase. In thecontext of stellar angular momentum regulation via ``disk-locking,''higher accretion rates lead to both higher initial angular momenta andevolution-driven increases in surface rotation rates as stars contractfrom the birth line to the zero-age main sequence (ZAMS). Stronger UVradiation fields and higher densities may lead to shorter disk lifetimesin cluster-forming regions. If so, B stars formed in dense clusters aremore likely to be ``released'' from their disks early during theirpre-main-sequence lifetimes and evolve into rapid rotators as theyconserve angular momentum and spin up in response to contraction. Bycontrast, the majority of their brethren in low-density,association-forming regions can retain their disks for much or all oftheir pre-main-sequence lifetimes, are ``locked'' by their disks torotate at constant angular speed, and lose angular momentum as theycontract toward the ZAMS, and thus arrive on the ZAMS as relativelyslowly rotating stars.
| 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.
| 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.
| Spectral observations of stars associated with nebulae. I. PP52, PP57, PP63, and PP89
Spectral observations of stars associated with nebulae, PP52, PP57,PP63, and PP89, are reported. The star PP52 is of spectral type A and isassociated with a reflecting nebula. PP57 is a nonstationary star. Aweak object (1079-0122733) immediately adjacent to PP57 is highlyvariable. It is shown that the nebulae PP63 and PP89 are cometary.
| 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.
| 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.
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| A Study of Stellar Rotation in Seven Young Clusters
The distribution of rotational velocities among young stars may provideinsight into initial conditions in star-forming regions (e.g. Wolff,Strom, & Hillenbrand 2004). In this contribution, we reportprojected rotational velocities (vsini) for a large sample of starsdrawn from 7 clusters ranging in age from 1--20 Myr (IC 1805, Cyg OB2,NGC 2244, NGC 6611, NGC 6823, NGC 7235, NGC 7380). These data providethe basis for assessing (a) the cosmic dispersion in the distribution ofvsini values, N(vsini), for late-O and early- B stars among clusters ofsimilar age; and (b) evolution-driven changes in N(vsini) via comparisonof young and ``old'' clusters in our sample.Wolff, S.C., Strom, S.E. & Hillenbrand, L.A. 2004 ApJ 601, 979
| On the highly reddened members in six young galactic star clusters - a multiwavelength study
The spectral and reddening properties of 211 highly reddenedproper-motion members with V < 15 mag in six young galactic starclusters are investigated using low-resolution spectroscopic, broad-bandUBVRIJHK and mid-infrared (IR) data. We report emission features in CaIIHK and HI lines for a sample of 29 stars including 11 stars reported forthe first time and also provide either a new or more reliable spectralclass for a sample of 24 stars. CaII triplet width measurements are usedto indicate the presence of an accretion disc for a dozen stars and toindicate luminosity for a couple of stars. On the basis of spectralfeatures, near-IR excesses, dereddened colour-colour diagrams and mid-IRspectral indices we identify a group of 28 pre-main-sequence clustermembers including five highly probable Herbig Ae/Be and six classical TTauri stars. A total of 25 non-emission main-sequence (MS) stars,amounting to ~10 per cent early-type MS members, appears to showVega-like characteristics or are precursors to such a phenomenon. Thevarious membership indicators suggest that ~16 per cent of theproper-motion members are non-members. A significant fraction (>70per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC6611 show anomalous reddening with RV= 4.78 +/- 0.10, 3.54+/- 0.04, 3.87 +/- 0.05 and 3.56 +/- 0.02, respectively, indicating thepresence of grain size dust larger than that typical of the diffusemedium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 alsoshow normal behaviour while the cluster NGC 6823 appears to have normalreddening. Three highly luminous late-type giants, one in NGC 2244 andtwo in NGC 6530, appear to be members and are inpost-hydrogen-core-burning stages, suggesting a prolonged duration (~25Myr) of star formation.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. I. SNRS G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5
Most astronomers now accept that stars more massive than about 9Msolar explode as supernovae and leave stellar remnants,either neutron stars or black holes, with neutron stars being moreprevalent. Recent modeling of the explosions suggests a significantdiversity in the key natal properties-rotation rate, velocity, andmagnetic field strength-of the resulting neutron stars that account forthe association of active radio pulsars, pulsar wind nebulae, andmagnetars with supernova remnants (SNRs). The discovery of a centralX-ray source in Cas A, the youngest known Galactic SNR, dramatized theexpected diversity. However, less than half of the SNRs within 5 kpchave identified central sources, and only three are identified as theremnants of Type Ia SNe. Here we report a systematic effort to searchfor compact central sources in the remaining 23 SNRs of this distancelimited sample. Our search was inspired, on empirical considerations, bythe enigmatic faint X-ray source in Cas A motivated, on theoreticalgrounds, by the expectation that young neutron stars emit cooling X-rayemission; and made possible by the superb angular resolution offered bythe Chandra X-ray mission and the sensitivity of the XMM-Newton mission.In this first paper we report Chandra observations of four SNRs(G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5). We have undertakena systematic optical/IR identification program of the X-ray sourcesdetected in the field of each SNR. Foreground (flare stars, activestars) and background (active galactic nuclei) sources have identifiableIR/optical counterparts. In contrast, the counterparts of neutron stars(or black holes) are expected to be very faint. We are able to accountfor all the well-detected X-ray sources and thus able to state with someconfidence that there are no associated central sources down to a levelof one-tenth of that of the Cas A central source,LX<~1031ergss-1. We compare ourlimits with cooling curves for neutron stars and find that any putativeneutron stars in these SNRs must be cooling faster than expected fortraditional 1.35Msolar neutron stars and that any putativepulsar must have low spin-down luminositiesE<~1034ergss-1. However, our limits are unableto constrain the presence or absence of more unusual options, such asrelatively more massive neutron stars with M>~1.45Msolar,neutron stars with exotic interiors, or quiescent black holes. Insubsequent papers, we will report on the X-ray and optical/IRobservations of the remaining members of the 5 kpc sample.
| Close binary systems in regions of OB-associations. II. AT Vulpeculae in Vul OB1 and EV Vulpeculae in Vul OB4.
Not Available
| 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.
| GSC 2139 2190 is a slowly changing Be variable and not a hot low mass post asymptotic giant branch star
Formerly presented as a Population II post AGB star, GSC 2139 2190 isactually a variable Population I Be star of (GCVS) type BE.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. V. NGC 2169
We present results of a search for variable stars in the field of theyoung open cluster NGC 2169. The General Catalogue of Variable Stars(GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variablestars in the field we observed, viz., two beta Cep stars, V 916 and V917Ori, an alpha^2CVn variable, V 1356Ori, and an RRc variable, V1154Ori. We find V 916 and V 1154Ori to be constant in light. We confirmthe variability of V 917Ori, but not the period given in GCVS. For thechemically peculiar A0 V Si star V 1356Ori we definitely establish theperiod of 1.565 d, thus settling the uncertainty persisting in theliterature since the star was discovered to be variable. In addition, wefind two other stars to be variable in light. Both show irregularvariations.For V 917Ori, one of the two GCVS beta Cep variables, we determine aperiod of 0.267 d (frequency 3.7477d^-1). However, prewhitening withthis period leaves a significant amount of the star's light-variationunaccounted for. Since the star shows emission at Hα, wehypothesize that the unaccounted for variation is caused by an erratic,Be-type activity. As to the periodic term, we consider three hypotheses:(1) beta Cep-type pulsation, (2) rotational modulation of the lambda Eritype, and (3) ellipsoidal variation due to distorted primary componentin a close binary system. After deriving the star's effectivetemperature from Stromgren indices and the luminosity from the distancemodulus of the cluster, we show that while the third hypothesis isuntenable, the first two should be retained. However, neither isentirely satisfactory.For a number of stars we provide the V magnitudes. For 14 brighteststars in our field we also obtain the photometric alpha-index, a measureof the Hα equivalent width. From the alpha index, we detect mildemission at Hα in two stars, V 917Ori and NGC 2169-8.
| 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.
| tex2html_wrap_inlineuvbyb#beta; Photoelectric Photometry of the Open Cluster NGC823
Absolute uvby beta photoelectric photometry of the brightest stars inthe direction of the Open Cluster NGC823 has been secured and ispresented. From the data analysis, reddening, distance, temperature andgravity are determined for the B, A, F stars and, from these, reddeningand a mean distance modulus ( 12.475+/-0.414) to the cluster aredetermined. Also, their membership in the cluster has been establishedand a numerical value of the age is determined through direct comparisonwith theoretical models. A brief analysis of the short period variablestars in the direction of NGC823 is made. We have concluded that theymight not be pre-MS delta Scuti stars.
| 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.
| 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
| Short-period line profile and light variations in the Herbig Ae star V351 Orionis
We present 318 high-dispersion echelle spectra and 450UBVRCIC observations of the pre-main-sequence starV351 Orionis. These data were obtained simultaneously over a two-weekperiod. The star is unusual in that it no longer shows the frequentsharp fadings that occurred prior to 1986. We find featurescharacteristic of Herbig Ae stars: Hα emission, sharp and broadNaII absorption components and weak HeI absorption. The Hα profileis strongly variable with a time-scale of 1d and normally is an inverseP Cygni profile, but on occasions overt emission disappears altogether.The equivalent widths of the sharp NaII absorption lines have decreasedquite markedly since 1995, indicating a decrease in the column densityof gas in the line of sight. We also detect very weak forbidden [OI]emission. There is no evidence for veiling of the star, but a possible1.0-d or 0.5-d period in the equivalent widths of the lines couldindicate the presence of an accretion hotspot. The star is of particularinterest because it is pulsating. We detect at least two periods in thelight variation and a further period from the radial velocityvariations. Attempts at photometric mode identification were notsuccessful, but from the morphology of the line profiles we can identifyone mode as non-radial. In addition, we see on some nights severalresidual absorption subfeatures moving across the line profile from blueto red. These are almost certainly caused by one or more modes of highdegree (l~8).
| 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).
| Non-uniform extinction in young open star clusters
The extinction law and the variation of colour excess with position,luminosity and spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14,Tr 15, Tr 16, Coll 228, Tr 37 and Be 86 have been studied. Thedifference in the minimum and maximum values of E(B-V) of clustermembers has been considered as a measure of the presence of non-uniformgas and dust inside the clusters. Its value ranges from 0.22 to 1.03magin the clusters under study, which indicates that non-uniform extinctionis present in all the clusters. It has been noticed for the first timein NGC 1502 and Tr 37. It is also found that the differential colourexcess in open clusters, which may be caused by the presence of gas anddust, decreases systematically with the age of clusters, indicating thatmatter is either used in star formation or blown away by hot stars orboth. There is no uniformity in the variation of E(B-V) with position orspectral class or luminosity. Except in Tr 14, all clusters show arandom spatial distribution of E(B-V), indicating a random distributionof gas and dust inside the clusters. The E(B-V) value correlates withboth luminosity and spectral class only in the case of Coll 228, Tr 16and Be 86. The members of these clusters atλ>=λR show larger values of colour excessratios than the normal ones. The value of E(U-V)/E(B-V) for most of thecluster members is close to the normal interstellar value of 1.73.However, the colour excess ratios with E(B-V) atλ>=λJ are smaller than the normal value forNGC 663, NGC 869, NGC 884 and NGC 1502, while they are larger for NGC6611, Coll 228, Tr 16 and Tr 14. Thus there is no uniformity in therelationship of extinction properties amongst the clusters under study.
| 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].
| Pulsation in the pre-main-sequence Herbig Ae star HD 142666
Asteroseismology of pre-main-sequence δ Scuti stars has thepotential not only to provide unprecedented constraints on models ofthese stars, but also to allow for the possibility of detectingevolutionary period changes, thus providing a direct measure of thepre-main-sequence evolutionary time-scale. In the last two years, thepublished number of such stars known has doubled from four to eight.Searches are now being conducted amongst the Herbig Ae stars, which areconsidered to be excellent candidates. We announce the discovery ofδ Scuti pulsation in one Herbig Ae star, HD 142666, which lieswithin Marconi & Palla's theoretically predicted instability stripfor pre-main-sequence stars, making this the ninth knownpre-main-sequence δ Scuti star. We also demonstrate a lack ofδ Scuti pulsation in another such star, HD 142527.
| The H I supershell GS061+00+51 and its neighbours
We describe H I observations of a 4degr x 4degr field in the Milky Waycentered on l=61degr , b=0degr made by the Effelsberg radiotelescope.The field contains one previously identified H I supershell, GS061+00+51(Heiles \cite{Heiles79}); apart from it we find several new structures.We also study the H I distribution in the vicinity of four H I regions,S86, S87, S88 and S89. We confirm the existence of the shellGS061+00+51, and we find that it has two smaller neighbours, sphericalshells with Rsh ~ 30 pc. We identify at least one moreregular shell at vLSR = -18 km s-1; and oneblown-out shell at vLSR = -54 km s-1. In two caseswe are able to connect H I regions with features in the H I distribution(S86 and S87), in two other cases no connection is found. Apart fromquite regular H I shells we see a number of non-coherent objects, whichare probably a result of the turbulence in the interstellar medium.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IV. NGC663
We present results of the variability search in the field of the youngopen cluster NGC663. In addition to the one beta Cep-type variable knownin this cluster, we found another one. It is a mono-periodic pulsatorchanging brightness with a period of 0.27640 d. In total, 19 newvariables were discovered and the variability of 5 other ones wasconfirmed. Out of all 24 variables in the observed field, 21 areprobable cluster members. One SPB candidate and three eclipsing orellipsoidal variables could be classified. Moreover, ten out of fourteenBe stars we observed vary in brightness. Only one of them shows periodicvariations of the lambda Eri-type, while the remaining ones exhibitirregular changes with the range up to 0.4 mag in the I_C band. We alsoprovide new VI_C photometry of 477 stars in the field of the cluster andcheck the consistence of the present photometry with cluster parametersderived earlier. The average cluster E(V-I_C) color excess amounts toabout 0.92mag.
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