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The Hα stellar and interstellar emission in the open cluster NGC 6910
Aims.We verify the nature of emission-line stars in the field of theopen cluster NGC 6910. Methods: Spectroscopy in the Hα region wasobtained. Raw CCD frames of spectra of all stars fainter than V = 9 magobserved by us are significantly affected by nebular emissionoriginating in the surrounding H II region IC 1318. After careful datareduction and subtraction of the nebular radiation we succeeded inobtaining reliable stellar spectra. Results: We confirm that the starNGC 6910 37 is a Be star, and we have corrected the classification ofV1973 Cyg from an Ae star to a normal A type star. Since the diffuseinterstellar bands do not appear in the spectrum of this star whilebeing present in the other stars we observed, we confirm that V1973 Cygis a foreground object with respect to IC 1318 and NGC 6910. We alsofind that the Hα line in HD 194279 has a P Cygni profile and theHα line profile is variable in HD 229196.
| A new view of the Cygnus X region. KOSMA 13CO 2 to 1, 3 to 2, and 12CO 3 to 2 imaging
Context: .The Cygnus X region is one of the richest star formation sitesin the Galaxy. There is a long-standing discussion about whether theregion is a chance superposition of several complexes along the line ofsight or a single coherent complex at a distance of 1.5 to 2 kpc. Aims: . Combining a 13CO 2 to 1 survey taken with the KOSMA 3m telescope with mid-IR images from MSX provides a way to improve ourunderstanding of the spatial structure of the complex. The physicalproperties of the molecular gas can be derived in more detail as it wasdone in former studies. Methods: . Cygnus X has been mapped in13CO J =2to1 (10.8 deg^2) at an angular resolution of 130'',as well as for smaller areas in 12CO and 13CO J =3 to 2 (90''), using the KOSMA 3 m submm-telescope. Results: .Weidentified 91 clumps in 13CO 2 to 1 that have a typicalexcitation temperature of 10-30 K, an average density of1.3×103 cm-3, radii of 1-8 pc, and masses ofa few hundred to several ten thousand M_ȯ. The main cloudcomplexes, the northern part (M≃2.8×10^5 M_ȯ) includingDR21 and W75N and the southern region (M≃4.5×10^5 M_ȯ)with IC 1318 b/c and AFGL2591, show differences in their physicalproperties. The 13CO emission is closely associated withmid-IR emission seen with MSX. We find evidence that Cygnus OB2 andCygnus OB9 are affecting the molecular material in Cygnus X. Conclusions: . Since essentially all molecular cloud complexes in CygnusX form groups that are connected by molecular emission (visible inchannel and position-velocity maps) and partly show evidence ofinteraction with UV radiation, we conclude that most of the objects seenin this region are located at the same distance, i.e., that of the OB2cluster at 1.7 kpc, which is also consistent with the distances ofother OB associations (OB9, OB1) in Cygnus X.
| Calibration of a UBVRI sequence around Nova Cyg 2006
A highly accurate UBV(RI)c photometric sequence around Nova Cyg 2006 ispresented. The photometric stability of the stars in the sequence hasbeen checked on 12 different nights. The sequence is linked to theLandolt's equatorial standards for (RI)c bands and to UBV photoelectricphotometry of selected open clusters by Hoag et al. (1961).
| 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.
| 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.
| Pulsating Stars in the ASAS-3 Database. I. beta Cephei Stars
We present results of an analysis of the ASAS-3 data for short-periodvariables from the recently published catalog of over 38000 stars. Usingthe data available in the literature we verify the results of theautomatic classification related to \beta Cep pulsators. In particular,we find that 14 stars in the catalog can be classified unambiguously asnew beta Cep stars. By means of periodogram analysis we derive thefrequencies and amplitudes of the excited modes. The main modes in thenew beta Cep stars have large semi-amplitudes, between 35 and 80 mmag.Up to four modes were found in some stars. Two (maybe three) new betaCep stars are members of southern young open clusters: ASAS164409-4719.1 belongs to NGC 6200, ASAS 164630-4701.2 is a member ofHogg 22, and ASAS 164939-4431.7 could be a member of NGC 6216.We also analyze the photometry of four known beta Cep stars in theASAS-3 catalog, namely IL Vel, NSV 24078, V1449 Aql and SY Equ. Finally,we discuss the distribution of beta Cep stars in the Galaxy.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VI. NGC 6910
We present results of variability search in the field of the young opencluster NGC 6910. We found four beta Cep-type stars in the cluster,which makes it rather exceptional among the northern clusters observedin our project. Two of these beta Cep stars show three modes withamplitudes above the detection level, in the remaining two we detectsingle modes. We discuss the possibility that the large number of betaCep stars in NGC 6910 is due to higher metallicity of the cluster. Inaddition, we found four other variables, one ellipsoidal or eclipsingbinary and three that show irregular variations. The latter three starshave H alpha in emission.We also provide VI_C photometry for 139 stars down to V=16.6 mag and Halpha photometry for 73 stars. Using this photometry we estimate thecluster age to be equal to 6+/-2 Myr, and the distance modulus,11.0+/-0.3 mag. The reddening is high and variable across the cluster.In terms of the E(V-I_C) color-excess it amounts to 1.25-1.7 mag,corresponding to E(B-V) between 1.0 mag and 1.4 mag. The averagereddening in the central field is E(V-I_ C=1.47+/-0.04 mag.
| The Cygnus X region. XXII. A probable HAeBe star with a giant bipolar outflow in DR 16
From medium-resolution radio images, DR 16 was suspected to be a largecometary nebula. To test this suggestion we obtained a higher resolution(15 arcsec) VLA continuum map. We also analyzed data from the CanadianGalactic Plane Survey in continuum, H I line, and IR. These data weresupplemented by published near-infrared (J, H, K) stellar photometricresults and MSX 8.28 μ m data. We suggest that DR 16 is the diffuse HII region of an ongoing star formation site at a distance of about 3kpc. The complicated radio picture arises from the superposition ofdiffuse H II with the remains of a giant bipolar outflow. The outflowwas generated by a probable Herbig AeBe star, and the lobes are theremnants of its working surfaces. Additional ring-like features arediscussed. DR 16 is part of a larger volume of space in the local spiralarm where star formation is an ongoing process.
| Deep optical observations of the supernova remnant G 78.2+2.1
The wide-field covered by the supernova remnant G 78.2+2.1 was observedin the optical emission lines of Hα+[N II] S II and O III. Theflux calibrated images reveal several H II regions in the field whichdominate the optical emission but we were able to identify possibleareas of shock-heated emission through the Hα+[N II] and S IIimages. These are mainly found to the north-east of gamma Cygni as wellas in the south and the morphology of the detected emission is patchyand diffuse. A few patchy structures are also detected in the mediumionization line image of [O III]. Long-slit spectra taken at one of thecandidate positions verify that we have detected radiation fromshock-heated gas (S II/Hα ˜= 0.6). The estimated shockvelocity lies below 100 km s-1, while the measured electrondensity of ˜700 cm-3 implies preshock cloud densitiesof ˜ 20 cm-3. High resolution maps in the infrared showthat the optical emission, which may be associated with G 78.2+2.1, liesin areas relatively free of infrared emission. The interstellarextinction measured through the optical spectra is compatible withcurrent estimates of the distance to the remnant. The optical data arein agreement with the explosion energy and interstellar medium densityestimated from the X-ray data suggesting that the remnant is still inthe adiabatic phase of its evolution. A second set of spectra taken inthe north-west suggests that we are probably dealing with a foreground HII region.
| Does Cyg OB2 harbour any open cluster?
We report the detection of three new open cluster candidates located inCygnus. Two of them are projected near the center of the rich compactassociation Cyg OB2. These two objects have been found during an opticalinspection of that region with Digitized Sky Survey images. Using 2MASSphotometry we determined their fundamental parameters. We assumed an ageof 1-4 Myr and obtained a distance from the Sun d_sun = 1.8 +/- 0.2 kpcfor both objects. Reddening values for Objects 1 and 2 are E(B-V) = 2.12+/- 0.06 and E(B-V)=1.82+/-0.06, respectively. A normal luminosityfunction is observed down to A0 stars, similar to that of NGC 6910. Thetwo open cluster candidates may form a physical pair in Cyg OB2, similarto Tr 14/Tr 16 in Car OB1. Subsequently, we carried out a systematicsearch with 2MASS in a region 3degx3degencompassing Cyg OB2. A third uncatalogued object resulted from thissearch which is projected just outside Cyg OB2.
| 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.
| A new Wolf-Rayet star in Cygnus
We report the discovery of a new Wolf-Rayet star in the direction ofCygnus. The star is strongly reddened but quite bright in the infrared,with J = 9.22, H = 8.08 and KS = 7.09 (2MASS). On the basisof its H + K spectrum, we have classified WR 142a a WC8 star. We haveestimated its properties using as a reference those of other WC8 starsin the solar neighbourhood as well as those of WR 135, whosenear-infrared spectrum is remarkably similar. We thus obtain aforeground reddening of AV =~ 8.1 mag, MJ =~ -4.3,log (L/Lsun) ~ 5.0-5.2, R = 0.8 Rsun, T =~ 125 000K, M = 7.9-9.7 Msun, {dot M} = (1.4-2.3) x 10-5Msun yr-1. The derived distance modulus, DM = 11.2+/- 0.7 mag, places it in a region occupied by several OB associationsin the Cygnus arm, and particularly in the outskirts of both Cygnus OB2and Cygnus OB9. The position in the sky alone does not allow us tounambiguously assign the star to either association, but based on themuch richer massive star content of Cygnus OB2 membership in this latterassociation appears to be more likely. Based on observations collectedat the German-Spanish Astronomical Centre, Calar Alto, operated by theMax-Planck-Institut für Astronomie, Heidelberg, jointly with theSpanish National Commission for Astronomy.
| CCD uvbybeta photometry of young open clusters. I. The double cluster h and chi Persei
We present CCD uvbybeta photometry for stars in the nuclei of the youngdouble cluster h and chi Persei. We find that the reddening is highlyvariable through the h Per nucleus, increasing from west to east, withvalues ranging from E(b-y) = 0.328+/-0.022 in the western part to E(b-y)= 0.465+/-0.024 in the south-east. Towards chi Per the reddening isfairly constant, with E(b-y) = 0.398+/-0.025. Both clusters share acommon distance modulus of 11.7+/-0.1 mag, and an age of log t =7.10+/-0.05 years. Tables 6 and 7 are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/479
| Characteristics of new star cluster candidates in the Cygnus area
The morphology and stellar content of 22 clusters in the Cygnus area hasbeen determined using 2MASS infrared observations in the J and K bands.7 of the clusters are well-known objects and our results are in goodagreement with previous work. 12 objects are recently-discovered clustercandidates (Dutra & Bica \cite{dutra01}) and 3 are new clustercandidates. Adopting distance estimates from the literature or byconsidering cluster counterparts, we derive a consistent set ofparameters suitable for analysis and comparison: center coordinates,radius, stellar population, IMF slope and mass range. We find a mean IMFslope of Gamma =-1.30 for the known clusters and Gamma =-1.40 for theothers. Infrared data having better completeness, we derive largerstellar contents and cluster masses than previous works. An importantfraction of the cluster stellar population in Cygnus is hidden in veryobscured areas. A clear tendency towards mass segregation appears formost of the clusters. Some of the new clusters seem to be quite massive,probably hosting very massive stars at the center. They are privilegedtargets for more detailed investigations using infrared spectroscopy.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/168
| 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).
| New infrared star clusters and candidates in the Galaxy detected with 2MASS
A sample of 42 new infrared star clusters, stellar groups and candidateswas found throughout the Galaxy in the 2MASS J, H and especiallyKS atlases. In the Cygnus X region 19 new clusters, stellargroups and candidates were found as compared to 6 such objects in theprevious literature. Colour-Magnitude Diagrams using the 2MASS PointSource Catalogue provided preliminary distance estimates in the range1.0 < dsun < 1.8 kpc for 7 Cygnus X clusters. Towardsthe central parts of the Galaxy 7 new IR clusters and candidates werefound as compared to 61 previous objects. A search for prominent darknebulae in KS was also carried out in the central bulge area.We report 5 dark nebulae, 2 of them are candidates for molecular cloudsable to generate massive star clusters near the Nucleus, such as theArches and Quintuplet clusters.
| Photometric study of the double cluster h & chi Persei
We present uvbybeta CCD photometry of the central region of the doublecluster h & chi Persei. We identify ~350 stars, of which 214 werenot included in Oosterhof's catalogue. Our magnitude limit V=16.5 allowsus to reach early F spectral type and obtain very accurate fits to theZAMS. We derive reddening values of E(b-y) = 0.44+/-0.02 for h Perseiand E(b-y) = 0.39+/-0.05 for chi Persei. From the ZAMS fitting, wederive distance moduli V0-MV = 11.66+/-0.20 andV0-MV = 11.56+/-0.20 for h and chi Perseirespectively. These values are perfectly compatible with both clustersbeing placed at the same distance and having identical reddenings. Theshift in the main-sequence turnoff and isochrone fitting, however, showthat there is a significant age difference between both clusters, withthe bulk of stars in h Persei being older than chi Persei. There is,however, a significant population of stars in h Persei which are youngerthan chi Persei. All this argues for at least three different epochs ofstar formation, corresponding approximately to log t = 7.0, 7.15 and 7.3Tables 2, 8 and 9 are 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/372/477. Based onobservations obtained with the Jacobus Kapteyn Telescope operated on theisland of La Palma by the Isaac Newton Group, in the SpanishObservatorio Roque de los Muchachos of the Instituto deAstrofísica de Canarias.
| Fundamental parameters and new variables of the galactic open cluster NGC 7128
CCD photometry in Johnson UBV and Strömgren uvby systems andmedium-resolution spectroscopy of the galactic open cluster NGC 7128 arepresented. Spectral types of the brightest 12 stars in the cluster fieldwere determined based on equivalent widths of the Hα and the Hei6678-Å line. The spectroscopic observations also revealed twoobvious and one probable Be-type stars showing Hα emission. Theanalysis of the photometric diagrams gave a colour excess ofE(B-V)=1.03+/-0.06mag, a distance modulus DM=13.0+/-0.2mag and an ageabove 10Myr. Time-resolved photometric observations obtained on onenight resulted in the detection of short time-scale light variations ofseven new and three already known variable stars in the cluster field.
| Photometric and Spectroscopic Study of the Young Open Cluster NGC 1893
We present uvbyβ CCD photometry of the field of the open clusterNGC 1893. Our photometry is deep enough to cover the completemain-sequence B spectral type. We identify ~50 very likely members ofthe cluster down to spectral type B9-A0, some of which have much higherreddenings than the average. We derive a color excess E(b-y)=0.33+/-0.03and a dereddened distance modulusV0-MV=13.9+/-0.2. From the β index, weidentify several candidates as emission-line stars, for which we haveobtained spectroscopy. Three of them display spectra corresponding tospectral type F but showing Hα in emission. Photometricmeasurements in this and previous studies indicate strong variability.These characteristics show them to be pre-main-sequence stars ofspectral type F. We also identify two likely Herbig Be stars. Theseresults hint at the existence of a sizable pre-main-sequence populationin NGC 1893.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| 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
| Evolutionary synthesis models for the emission from radioactive isotopes in H ii regions
We have extended our evolutionary synthesis models for Star FormingRegions to the /γ-ray domain. Applying different yields to theresulting synthetic populations, we obtain lightcurves of γ-rayline emission due to the radioactive decay of stellar nucleosynthesisproducts, such as 26Al and 60Fe. We present herethis time evolution and the application of the models to regions of starformation in our galaxy.
| On the evolutionary status of Be stars
We present a study of the incidence of Be stars in open clusters as afunction of the cluster age, using whenever possible ages determinedthrough Strömgren uvby photometry. For the first time in studies ofthis kind we have considered separately classical and Herbig Be stars.The main results can be summarized as follows: Clusters associated toemitting nebulosities and undergoing stellar formation are rich inemission line objects, which most likely are all pre main-sequencestars. No bona fide classical Be star has yet been identified amongthem. Clusters younger than 10 Myr and without associated nebulosity arealmost completely lacking Be stars, although they have a completeunevolved B main sequence. Classical Be stars appear at an age of 10Myr, and reach the maximum abundance in the age interval 13-25 Myr. Weinterpret our results in the sense that the Be phenomenon is anevolutionary effect which appears in the second half of the mainsequence lifetime of a B star. We propose that it can be related to mainstructural changes happening at this evolutionary phase, which also leadto the recently discovered non-monotonic helium abundance enhancement.The semiconvection or turbulent diffusion responsible of the surfacehelium enrichment, coupled with the high rotational velocity, cangenerate magnetic fields via the dynamo effect and thereby originate theBe phenomenon. Observational tests to this hypothesis are proposed.
| Search for Pre-Main-Sequence Stars in the Young Galactic Cluster NGC 6910
Photometric CCD UBV measurements have been obtained in the field of theyoung Cygnus cluster NGC 6910. The observations reach down to magnitudeV~=18 for 206 stars measured in all three UBV bands, uncovering theregion of the color-magnitude diagram where pre-main-sequence (PMS)stars are expected. The transformation of our instrumental magnitudes tothe standard system is discussed on the basis of two different sets ofstandard stars: 22 stars from six fields in the Landolt catalogs and 48stars with published photoelectric photometry in common with ours in thefour clusters observed in this campaign. The latter set is preferred,since it enables the effect of systematic errors in the U-B colors to beminimized. The adopted cluster parameters are E(B-V)=1.02+/-0.13,V0-MV=11.2+/-0.2, and age=(6.5+/-3)x106yr. The search for PMS cluster members on the basis of the calculatedcolor excess and distance modulus results in the proposed membership for11 PMS stars of spectral types from A to G.
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