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|Unraveling the Origins of Nearby Young Stars|
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings|
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|On the current status of open-cluster parameters|
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
|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.
|Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1|
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
|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.
|Revisiting the population of Galactic open clusters|
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
|Elemental Abundance Studies of CP Stars. The Silicon Stars HD 87240 and HD 96729|
We compared elemental abundances of field and open cluster Ap Si stars.In particular, an analysis of the Ap Si stars HD 87240 and HD 96729 ispresented using an ATLAS9 model atmosphere and observational materialtaken with a REOSC echelle spectrograph attached to the Jorge Sahade2.15 m telescope at CASLEO. These chemically peculiar (CP) stars belongto the southern hemisphere open clusters NGC 3114 and NGC 3532,respectively. For HD 87240 and HD 96729, C is mostly solar, Mg and S areslightly underabundant, Si and Ca are overabundant by factors between1--10. Heavier elements are all overabundant, TiCrFe by factors of 10, SrYZr by factors between 100--1000 and rare earths by factors of 1000 or more.
|The Age and Progenitor Mass of Sirius B|
The Sirius AB binary system has masses that are well determined frommany decades of astrometric measurements. Because of the well-measuredradius and luminosity of Sirius A, we employed the TYCHO stellarevolution code to determine the age of the Sirius AB binary systemaccurately, at 225-250 Myr. Note that this fit requires the assumptionof solar abundance and the use of the new Asplund et al. primordialsolar metallicity. No fit to Sirius A's position is possible using theold Grevesse & Sauval scale. Because the Sirius B white dwarfparameters have also been determined accurately from space observations,the cooling age could be determined from recent calculations by Fontaineet al. or Wood to be 124+/-10 Myr. The difference in the two ages yieldsthe nuclear lifetime and mass of the original primary star,5.056+0.374-0.276 Msolar. This resultyields, in principle, the most accurate data point at relatively highmasses for the initial-to-final mass relation. However, the analysisrelies on the assumption that the primordial abundance of the Siriusstars was solar, based on membership in the Sirius supercluster. Arecent study suggests that its membership in the group is by no meanscertain.
|The open-cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution|
Recent studies of white dwarfs in open clusters have provided newconstraints on the initial-final mass relationship (IFMR) formain-sequence stars with masses in the range 2.5-6.5Msolar.We re-evaluate the ensemble of data that determines the IFMR and arguethat the IFMR can be characterized by a mean IFMR about which there isan intrinsic scatter. We investigate the consequences of the IFMR forthe observed mass distribution of field white dwarfs using populationsynthesis calculations. We show that while a linear IFMR predicts a massdistribution that is in reasonable agreement with the recent resultsfrom the Palomar-Green survey, the data are better fitted by an IFMRwith some curvature. Our calculations indicate that a significant (~28)percentage of white dwarfs originating from a single star evolution hasmasses in excess of ~0.8Msolar, obviating the necessity forpostulating the existence of a dominant population of high-mass whitedwarfs that arise from binary star mergers.
|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.
|The University of New South Wales Extrasolar Planet Search: methods and first results from a field centred on NGC 6633|
We report on the current status of the University of New South WalesExtrasolar Planet Search project, giving details of the methods we useto obtain millimagnitude precision photometry using the 0.5-m AutomatedPatrol Telescope. We use a novel observing technique to optimallybroaden the point spread function and thus largely eliminate photometricnoise due to intrapixel sensitivity variations on the CCD. We haveobserved eight crowded Galactic fields using this technique during 2003and 2004. Our analysis of the first of these fields (centred on the opencluster NGC 6633) has yielded 49 variable stars and four shallow transitcandidates. Follow-up observations of these candidates have identifiedthem as eclipsing binary systems. We use a detailed simulation of ourobservations to estimate our sensitivity to short-period planets, and toselect a new observing strategy to maximize the number of planetsdetected.
|The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters|
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
|Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas|
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
|X-ray astronomy of stellar coronae|
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
|Spectroscopic binaries in southern open clusters|
This is a report on an ongoing program about binaries in southern openclusters. The long-term purpose of this project is to contribute tounderstanding the formation and evolution of spectroscopic binaries,providing observational constraints that will permit tests of some ofthe current theories on binary formation in open clusters.
|Eclipsing binaries in open clusters - I. V615 Per and V618 Per in h Persei|
We derive absolute dimensions for two early-type main-sequence detachedeclipsing binaries in the young open cluster h Persei (NGC 869). V615Persei has a spectral type of B7 V and a period of 13.7 d. V618 Perseiis A2 V and has a period of 6.4 d. New ephemerides are calculated forboth systems. The masses of the component stars have been derived usinghigh-resolution spectroscopy and are 4.08 +/- 0.06 and 3.18 +/- 0.05Msolar for V615 Per and 2.33 +/- 0.03 and 1.56 +/- 0.02Msolar for V618 Per. The radii have been measured by fittingthe available light curves using EBOP and are 2.29 +/- 0.14 and 1.90 +/-0.09 Rsolar for V615 Per and 1.64 +/- 0.07 and 1.32 +/- 0.07Rsolar for V618 Per. By comparing the observed spectra ofV615 Per with synthetic spectra from model atmospheres we find that theeffective temperatures of the stars are 15000 +/- 500 K for the primaryand 11000 +/- 500 K for the secondary. The equatorial rotationalvelocities of the primary and secondary components of V615 Per are 28+/- 5 and 8 +/- 5 km s-1, respectively. Both components ofV618 Per rotate at 10 +/- 5 km s-1. The equatorial rotationalvelocities for synchronous rotation are about 10 km s-1 forall four stars. The time-scales for orbital circularization for bothsystems, and the time-scale for rotational synchronization of V615 Per,are much greater than the age of h Per. Their negligible eccentricitiesand equatorial rotational velocities therefore support the hypothesisthat they were formed by delayed break-up. We have compared the radii ofthese stars with models by the Granada and the Padova groups for starsof the same masses but different compositions. We conclude that themetallicity of the stars is Z~ 0.01. This appears to be the firstestimate of the bulk metallicity of h Per. Recent photometric studieshave assumed a solar metallicity so their results should be reviewed.
|WIYN Open Cluster Study. XIX. Main-Sequence-Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams|
We have combined existing optical magnitudes for stars in seven openclusters and 54 field stars with the corresponding JHKsphotometry from the Two Micron All Sky Survey (2MASS). Combining opticalwith near-IR photometry broadens the color baseline, minimizing theinfluence of photometric errors and allowing better discriminationbetween cluster stars and contaminating foreground and backgroundpopulations. The open clusters in this study include NGC 2516, M35, M34,NGC 3532, M37, M67, and NGC 188. The field stars we are using possesshigh-quality Hipparcos parallaxes and well-determined metal abundances,allowing us to empirically determine the dependence of V-K color onmetal abundance in the range -0.45<=[Fe/H]<=+0.35.Using this relation along with the parallaxes of the field stars, we areable to construct an unevolved main sequence in the [MV,(V-K)0] diagram for a specific abundance. These diagrams arethen used to fit to the cluster main sequences in the (V, V-K)color-magnitude diagram in order to estimate a distance for each opencluster. We find that the resultant distances are within the range ofdistances found in the literature via the main-sequence-fittingtechnique. It is hoped that this will spur an expansion of the current(limited) database of star clusters with high-quality V-K photometrydown to the unevolved main sequence.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|Stellar Coronal Astronomy|
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
|Angular Momentum Evolution of Young Stars: Toward a Synthesis of Observations, Theory, and Modeling|
The conference was held in Albuquerque, New Mexico, 2002 June 5, as aTopical Session at the 200th Meeting of the American AstronomicalSociety.
|On the Rotational Evolution of Solar- and Late-Type Stars, Its Magnetic Origins, and the Possibility of Stellar Gyrochronology|
We propose a simple interpretation of the rotation period data forsolar- and late-type stars. The open cluster and Mount Wilson starobservations suggest that rotating stars lie primarily on two sequences,initially called I and C. Some stars lie in the intervening gap. Thesesequences, and the fractional numbers of stars on each sequence, evolvesystematically with cluster age, enabling us to construct cruderotational isochrones allowing ``stellar gyrochronology,'' a procedure,on improvement, likely to yield ages for individual field stars. The ageand color dependences of the sequences allow the identification of theunderlying mechanism, which appears to be primarily magnetic. Themajority of solar- and late-type stars possess a dominant Sun-like, orinterface, magnetic field, which connects the convective envelope toboth the radiative interior of the star and the exterior, where windscan drain off angular momentum. These stars spin down Skumanich style.An age-decreasing fraction of young G, K, and M stars, which are rapidrotators, possess only a convective field, which is not only inefficientin depleting angular momentum but also incapable of coupling the surfaceconvection zone to the inner radiative zone, so that only the outer zoneis spun down, and on an exponential timescale. These stars do not yetpossess large-scale dynamos. The large-scale magnetic field associatedwith the dynamo, apparently created by the shear between the decoupledradiative and convective zones, (re)couples the convective and radiativezones and drives a star from the convective to the interface sequencethrough the gap on a timescale that increases as stellar mass decreases.Fully convective stars do not possess such an interface, cannot generatean interface dynamo, and hence can never make such a transition.Helioseismic results for the present-day Sun agree with this scheme,which also explains the rotational bimodality observed by Herbst andcollaborators among pre-main-sequence stars and the termination of thisbimodality when stars become fully convective. This paradigm alsoprovides a new basis for understanding stellar X-ray and chromosphericactivity, light-element abundances, and perhaps other stellar phenomenathat depend on rotation.This is Paper 13 of the WIYN Open Cluster Study (WOCS).
|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.
|Physical parameters of the Algol system BP Muscae from simultaneous analysis of GENEVA 7-colour light curves|
The semi-detached eclipsing binary system BP Muscae has been analysedusing the Wilson-Devinney program. Light curves have been obtained inthe GENEVA 7-colour photometric system, and radial velocity curves forboth components have been measured with the spectrograph CORALIE. Thephysical and orbital parameters have been determined through aself-consistent simultaneous solution of light curves in seven coloursand of the radial velocity curves of both components. The absoluteelements of the components are, for the primary (mass gainer),M1 = 2.40 +/- 0.01 Msun, R1 = 2.64 +/-0.01 Rsun, Mbol_1 = 0.66 +/- 0.04,Teff_1 = 9180 +/- 90 K, and for the secondary (mass loser),M2 = 0.68 +/- 0.01 Msun, R2 = 3.76Rsun, Mbol_2 = 2.40 +/- 0.08, Teff_2 =5160 +/- 90 K. The semi-major axis A of the relative orbit is 13.617 +/-0.019 Rsun. The spectral type of the components are A0.5/1.5V (primary) and about G5 III. The distance to BP Mus is evaluated as 562+/- 17 pc, and the colour excess E[B2-V1] as 0.220 +/- 0.014.Based on observations collected at the Swiss 70 cm and 120 cm telescopesat the European Southern Observatory (La Silla, Chile).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/1073
|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.
|Physics of the outer atmosphere of the Sun and of stars along their evolutionary track|
We discuss the activity of a Co-financed project dedicated to studystellar activity and, more in general, stellar properties, throughobservations in X-ray, UV, optical etc. bands and through modeling. Akey aspect of the project is the study of activity of stars in variousphases of their evolution so as to ascertain the role of evolution indetermining activity and the role of activity in influencing evolution,e.g. through the significant loss of angular momentum. An even moredetailed study, thanks to its proximity, of the Sun is a fundamentalpart of this project; the relevant information is indeed both importantfor solar physics `per se' and to help defining with a high level ofdetail, and therefore of insight, the physical conditions in the solaratmosphere which thus is an important reference for the studies ofstellar activity.
|Spectroscopic Binaries and Kinematic Membership in the Open Cluster NGC 3532|
We report spectroscopic observations for bright stars in the opencluster NGC 3532 up to 1 mag below the turnoff point. We compute radialvelocities by cross-correlations and determine spectral types androtational velocities. Using 21 stars identified as certain members, wederive a mean cluster velocity of +3.4+/-0.3 km s-1. Fromradial velocities and angular distances to the cluster center, wecompute membership probabilities for all but two stars that are radialvelocity variables. Only one out of 34 program stars is a clearkinematic nonmember. Three spectroscopic binaries and three additionalpossible radial velocity variables are detected among the 23 starsmeasured more than once. We report the star HD 96609 as a double-linedspectroscopic binary. Using the two-dimensional cross-correlationtechnique TODCOR developed by Zucker & Mazeh, we derive the radialvelocity curves for both components and obtain the orbital parameterswith errors of 0.3% and 0.7% for the projected orbital semiaxis andmasses, respectively. This system is composed of two main-sequence starsin a circular orbit, with a period of 8.19 days. The observationspresented here were obtained at the Complejo Astronómico ElLeoncito (CASLEO), which is operated under agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina (CONICET) and the National Universities of LaPlata, Córdoba, and San Juan.
|Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution|
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
|Spectroscopic Binaries and Kinematic Membership in the Open Cluster NGC 3532|
|Elemental Abundances in CP Stars of Galactic Open Clusters|
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