Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Caroline Herschel's catalogue of nebulae
Not Available
| New UBV, DDO and Washington Photometric Data for the Red Giants in then Open Cluster NGC 2447: Membership and Chemical Composition
UBV, DDO and Washington photometry for 14 red giant candidates of theopen cluster NGC 2447 is presented. Membership results emerging from theapplication of two photometric criteria are in excellent agreement withthose derived from published Coravel radial velocities. A mean clusterreddening E(B-V) = 0.05 +/- 0.04 is derived. Both the ultravioletexcesses and cyanogen anomalies of the cluster giants imply [Fe/H]≃ -0.1. Five Washington abundance indicators yield a mean clustermetallicity of [Fe/H]_W = -0.09 +/- 0.06.
| 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.
| Caroline Herschel as observer
Not Available
| Properties of five low-contrast open clusters in the third quadrant
We derive photometric, structural and dynamical evolution-relatedparameters of five as yet unstudied low-contrast open clusters locatedin the third quadrant using 2MASS data. The target clusters are Czernik31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply astatistical field-star decontamination procedure to infer on theintrinsic colour-magnitude diagram (CMD) morphology which is criticalfor such low-contrast objects. Consequently, it became possible toderive accurate reddening, age, distance from the Sun and Galactocentricdistance for the five clusters. In the structural andluminosity/mass-function analyses we apply a colour-magnitude filterwhich encompasses the cluster evolutionary CMD sequences and excludesstars with discrepant colours. Using this procedure we derive core andlimiting radii, mass function (MF) slope, total mass, mass density andrelaxation time. We derive ages in the range 140-1100 Myr,Galactocentric distances within 7.7-11.4 kpc, and total masses within360-2900 M_ȯ. Reflecting large-scale mass segregation, the MF slopein the core is significantly flatter than that in the halo of the fiveclusters. Although some of the present clusters are relatively youngerthan the Gyr-old clusters, they present evidence of advanced dynamicalevolution. This kind of study has become possible because of thephotometric uniformity and spatial coverage of 2MASS which allows aproper subtraction of the field-star contamination on the target CMDs.The present study indicates that low-contrast clusters can be studiedwith 2MASS, particularly after field-star subtraction, which isimportant since most of the unstudied open clusters belong to thisclass.
| Structure and stellar content analysis of the open cluster M 11 with 2MASS photometry
An overall analysis of the structure and stellar content of M 11 ispresented, thanks to the wide-angle 2MASS spatial coverage. We derivephotometric and structural parameters and discuss the spatial dependanceof the luminosity and mass functions. Photometric parameters basicallyagree with previous ones mostly based on the optical. We obtained a coreradius of 1.23 pc and a tidal radius of 29 pc. In particular, thecluster is populous enough so that the tidal radius could be obtained byfitting the three-parameter King profile to the radial distribution ofstars. We analyzed the spatial distribution of mass functions, findingthat the slope changes from -0.73 in the core to +2.88 in the outerhalo. The spatial distribution of mass function slopes derived from2MASS agrees with that derived from optical CCD data, which furtherconfirms the reliability of 2MASS data for future analyses of this kindat comparable observational limits. We detect mass segregation up todistances from the center of ≈20 arcmin. We emphasize that the massfunction slope in the core is flatter than anywhere else as aconsequence of mass segregation. The derived total cluster mass is≈11 000 M_ȯ.
| 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.
| Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane
We derive photometric, structural and dynamical evolution-relatedparameters of 11 nearby open clusters with ages in the range 70 Myr to 7Gyr and masses in the range ≈400 M_ȯ to ≈5300 M_ȯ. Theclusters are homogeneously analysed by means of J, H and KS2MASS photometry, which provides spatial coverage wide enough toproperly take into account the contamination of the cluster field byGalaxy stars. Structural parameters such as core and limiting radii arederived from the background-subtracted radial density profiles.Luminosity and mass functions (MFs) are built for stars later than theturnoff and brighter than the 2MASS PSC 99.9% completeness limit. Thetotal mass locked up in stars in the core and the whole cluster, as wellas the corresponding mass densities, are calculated by taking intoaccount the observed stars (evolved and main sequence) and extrapolatingthe MFs down to the H-burning mass limit, 0.08 M_ȯ. We illustratethe methods by analysing for the first time in the near-infrared thepopulous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive anage of 1.1 ± 0.1 Gyr, distance from the Sun d_ȯ=1.2 ±0.1 kpc, core radius Rcore=1.4 ± 0.1 pc, limitingradius Rlim=11.6 ± 0.7 pc and total massmtot≈(5.3±1.6) × 103 M_ȯ.Large-scale mass segregation in NGC 2477 is reflected in the significantvariation of the MF slopes in different spatial regions of the cluster,and in the large number-density of giant stars in the core with respectto the cluster as a whole. For NGC 2516 we derive an age of 160 ±10 Myr, d_ȯ=0.44 ± 0.02 kpc, Rcore=0.6 ±0.1 pc, Rlim=6.2 ± 0.2 pc andmtot≈(1.3±0.2) × 103 M_ȯ.Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusterspresent a slope break in the MF occurring at essentially the same massas that found for the field stars in Kroupa's universal IMF. The MFbreak is not associated to cluster mass, at least in the clusters inthis paper. In two clusters the low-mass end of the MF occurs above theMF break. However, in three clusters the MF break does not occur, atleast for the mass range m≥0.7 M_ȯ. One possibility isdynamical evolution affecting the MF slope distribution. We also searchfor relations of structural and evolutionary parameters with age andGalactocentric distance. The main results for the present sample are:(i) cluster size correlates both with age and Galactocentric distance;(ii) because of size and mass scaling, core and limiting radii, and coreand overall mass correlate; (iii) massive (m≥1000 M_ȯ) andless-massive clusters follow separate correlation paths on the planecore radius and overall mass; (iv) MF slopes of massive clusters arerestricted to a narrow range, while those of the less-massive onesdistribute over a wider range. Core and overall MF flattening is relatedto the ratio (τ) of age to relaxation time. For large values ofτ the effects of large-scale mass segregation and low-mass starsevaporation can be observed in the MFs. In this sense, τ appears tocharacterize the evolutionary state of the clusters. We conclude thatappreciable slope flattenings in the overall MFs of the less-massiveclusters take ~6 times longer to occur than in the core, while in themassive clusters they take a time ~13 times longer. We investigatecluster parameters equivalent to those determining the fundamental planeof ellipticals. These parameters are: overall mass, projected massdensity and core radius. We conclude that in the present sample there isevidence of a fundamental plane. Larger samples are necessary to pindown this issue.
| Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyrusing UCAC2 proper motion data and 2MASS photometry, which allows us toreach stellar masses down to ≈0.7 M_ȯ. We employ in this work anapproach in which the background proper motion contribution isstatistically subtracted in order to obtain the cluster's intrinsicproper motion distribution. For each cluster we consider the projectedvelocity distributions in the core and off-core regions separately. Inthe projected velocity distribution of all sample clusters we find awell-defined low-velocity peak, as well as an excess in the number ofstars at larger velocities. The low-velocity peak is accounted for bythe random motion of the single stars, while the high-velocity excesscan be attributed to the large velocity changes produced by asignificant fraction of unresolved binaries in a cluster. We derivekinematic parameters of the single-star distribution, in particular theprojected velocity dispersion. The relatively large velocity dispersionsderived in this work may reflect the non-virialized state of theclusters. Based on the relative number of high-velocity (binary) andsingle stars, we inferred for the sample clusters unresolved binaryfractions in the range 15%≤f_bin≤54%, for both core and off-coreregions. Stars with a projected velocity exceeding the maximum reachedby the single-star distribution are identified in 2MASS J×(J-H)colour magnitude diagrams. The asymmetry observed in the distribution ofthese stars around the main sequence is consistent with models ofmain-sequence widening resulting from unresolved binaries combined with2MASS photometric uncertainties. The present results suggest that caremust be taken when applying proper-motion filters to sort out members,especially binaries in a star cluster. This paper shows that propermotions turn out to be a useful tool for identifying high-velocity starsas unresolved binary cluster members, and as a consequence, map andquantify the binary component in colour magnitude diagrams.
| Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
| Multicolor Photometry of Red Giant Candidates in the Southern Open Cluster NGC 2447
Photoelectric photometry in the UBV, DDO and Washington systems ispresented for 14 late-type giant candidates of the southern open clusterNGC 2447. By applying two independent photometric criteria, nine starsare found to have a high probability of being cluster giants. Thephotometric membership results are in good agreement with those derivedfrom published Coravel radial velocities. The mean interstellarreddening EB-V = 0.05±0.04 has been derived from theconfirmed cluster giants. NGC 2447 has a mean ultraviolet excess<δ (U-B)> = 0.01±0.02 (σ n) withrespect to solar composition K giants, and a mean cyanogen anomaly<Δ CN> = 0.01±0.02 (σ n), bothimplying a small metal deficiency ([Fe/H] ≃ -0.1). Fiveindependent Washington abundance indices yield a mean clustermetallicity of [Fe/H] = -0.09±0.06, in agreement with the twoprevious estimates. Therefore, NGC 2447 is found to be slightlymetal-poor.
| 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.
| The relatively young, metal-poor and distant open cluster NGC 2324
We have obtained CCD photometry in the Johnson V, Kron-Cousins I andCT1 Washington systems for NGC 2324, a rich open clusterlocated 35° from the Galactic anticentre direction. We measuredV magnitudes and V-I colours for 2865 stars and T1 magnitudesand C-T1 colours for 1815 stars in an area of 13.6 arcmin× 13.6 arcmin. The comparison of the cluster colour-magnitudediagrams with isochrones of the Geneva group yield E(V-I) = 0.33± 0.07 and V-MV = 13.70 ± 0.15 for log t = 8.65(t = 440 Myr) and Z = 0.008 ([Fe/H] = -0.40), and E(C-T1) =0.40 ± 0.10 and T1-MT1 = 13.65 ±0.15 for the same age and metallicity level. The resulting E(V-I)reddening value implies E(B-V) = 0.25 ± 0.05 and a distance fromthe Sun of (3.8 ± 0.5) kpc. Star counts carried out within andoutside the cluster region allowed us to estimate the cluster angularradius as 5.3 arcmin ± 0.3 arcmin (5.9 pc). When using the E(B-V)reddening value here derived and the original Washington photometricdata of \citet{gcm91} for the stars confirmed as red cluster giants fromCoravel radial velocities, we found [Fe/H] = -0.31 ± 0.04, whichis in good agreement with the best fits of isochrones. Therefore, NGC2324 is found to be a relatively young, metal-poor and distant opencluster located beyond the Perseus spiral arm. A comparison of NGC 2324with 10 well-known open clusters of nearly the same age shows that thecluster metal abundance and its position in the Galaxy are consistentwith the existence of a radial abundance gradient of -0.07 dexkpc-1 in the Galactic disc.
| 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.
| 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
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| 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
| Chemical abundances in seven red giants of NGC 2360 and NGC 2447
Chemical abundances of about fifteen elements from oxygen to europiumare measured in seven red giants of the two open clusters NGC 2360 andNGC 2447. The effective temperatures of the giants are determinedspectroscopically by taking advantage of their known masses (~ 2Msolar in NGC 2360 and ~ 3 Msolar in NGC 2447) andbolometric magnitudes. The average iron abundances we obtain for the twoclusters are [Fe/H]=0.07 for NGC 2360 and [Fe/H]=0.03 for NGC 2447.Evolutionary stellar model calculations are performed in the mass range1 - 4 Msolar in order to analyze the surface Na and Oabundances predicted after the first dredge-up. The sodium abundanceshows a well defined correlation with stellar mass in the 2 - 3Msolar range. The agreement between our Na abundancedeterminations in NGC 2360 and our model predictions at 2Msolar is very good. In contrast, the overabundance in one ofthe three stars in NGC 2447 exceeds that predicted at 3Msolar by ~ 0.08 dex, which is significant compared to theobservational error bars. The effects of core overshooting, convectionprescription, metallicity and nuclear reaction rates on the Na surfacepredictions of our models are investigated. An oxygen deficiencyrelative to iron by 0.2 dex is measured in our stars, in disagreementwith our model predictions. Assuming that the Sun is 0.1-0.3 dexenriched in oxygen relative to neighbor stars could explain thediscrepancy. Based on observations collected at the European SouthernObservatory, La Silla, Chile (period 56) Table 3 is available only inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Core velocity dispersions for 25 Galactic and 10 old Magellanic globular clusters.
We present, for 25 Galactic and 10 old Magellanic globular clusters,projected velocity dispersion (σ_p_) measurements obtained byapplying a cross-correlation technique to integrated-light spectra. Inorder to understand and estimate the statistical errors of thesemeasurements due to small numbers of bright stars dominating theintegrated light, we provide an extensive discussion based on detailednumerical simulations. These errors are smaller if the integration areais larger and/or the cluster concentration higher. The simulations showthat measurements are reliable when the integrated light within theintegration area is brighter than a given magnitude. The statisticalerrors on the σ_p_ measurements of Magellanic globular clustersare small because of a physically large integration area, whereas theycan be important for measurements carried out over small central areasin Galactic clusters. The present observational results are used tooutline a few characteristics of the globular cluster fundamental plane.In this respect, the old Magellanic globular clusters appear similar tothe Galactic clusters.
| Parameters of open star clusters from uvby-beta photometry.
Not Available
| Tidal circularization and the eccentricity of binaries containing giant stars.
We show that the predicted amount of tidal circularization in a detachedbinary containing a giant is, according to Zahn's theory, determinedonly by observable properties of its orbit, and a single integral alongthe giant's stellar evolutionary sequence. We perform these integrals,display them graphically, and provide fitting formulae. Spectroscopicbinaries in open clusters are ideal for testing the theory of tidalcircularization, since the stars' mass, luminosity and temperature canall be determined with precision. We apply our calculations to 28binaries in 12 open clusters. We show that the orbital parameters of allcan be understood within the circularization theory, and that the mixinglength parameter in Zahn's theory is well constrained:1<~α<~3. We show that the orbital period alone does notseparate circular and eccentric orbits, and point out that theeccentricity of the orbit can sometimes distinguish between giant starsand core helium burning `clump' stars even in circumstances where thisis difficult spectroscopically. In some special types of binary systems,the circularization is sensitive to aspects of stellar evolution theorythat are not well understood: the maximum depth of penetration of theouter convective zone, the radius at first dredge-up, the maximum radiiof stars at the tips of the giant and asymptotic giant branches. It thusappears that the orbits of binaries containing giants may prove to be auseful tool for studying stellar evolution.
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| The evolution of galactic carbon stars.
Based on a comparison of observations with new synthetic AGB evolutioncalculations we propose a revised evolutionary scenario for carbon starsin the solar neighbourhood. From observations we derive that the lowestinitial mass from which carbon stars form is about 1.5Msun_.This constraint combined with four other constraints (the observedinitial-final mass relation, the birth rate of carbon stars, theobserved abundance ratios in planetary nebulae (PNe) and the numberratios C/M and S/C of AGB stars) are used to derive the followingparameters for the synthetic AGB evolution model. Third dredge-up occursfor core masses above 0.58Msun_ and the dredge-up efficiencyis λ=0.75. We consider a Reimers mass loss law (with a scalingfactor η_AGB_) and the mass loss rate law recently proposed byBloecker & Schoenberner (1993; with a scaling factor η_BS_). Wefind η_AGB_=4 and η_BS_=0.08. Both models fit the observationsequally well. The model predicts that stars in the range1.5Msun_<~M<~1.6Msun_ become carbon stars attheir last thermal pulse (TP) on the AGB and live only a few 10^4^yr ascarbon stars. More massive stars experience additional TPs as carbonstars (up to about 25 for a 3Msun_ star) and live up to10^6^yr. For M>4Msun_ hot-bottom burning prevents theformation of carbon stars. For M<~2Msun_, M-stars skip theS-star phase when they become carbon stars. The average lifetime of thecarbon star phase is ~3x10^5^yr. The carbon stars for which C/O ratioshave been derived in the literature (with values <~1.5) arepredominantly optical carbon stars with a 60μm excess. Yet, disk PNeare known with C/O ratios up to about 4. We predict that carbon starswith C/O ratios >1.5 are to be found among the infrared carbon stars.The model predicts that the probability that a carbon star hasC/O>1.5 is about 30%, in reasonable agreement with the observed ratioof the surface density in the galactic plane of infrared carbon stars toall carbon stars. The infrared carbon stars are predicted to be (onaverage) more massive than the optical carbon stars. The fact thatcarbon stars with C/O>1.5 apparently never reach the optical carbonstar phase (with a detached shell) is probably due to differences inevolution. If indeed infrared carbon stars are on average more massive(i.e. have larger core masses) than optical carbon stars, the interpulseperiod is shorter, and the increase in luminosity during the TP issmaller (due to the larger envelope mass). Both effects will decreasethe likelihood of a detached shell to occur. We predict that two-thirdsof all detached shells around optical carbon stars are oxygen-rich.
| The apparent distribution of red giant spectroscopic binaries in open clusters
The projected radial distribution of 88 red giant spectroscopic binarieshas been compared to that of 260 single giants in 14 intermediate-ageand old clusters. The radial extension of the available data is ofcrucial importance, and the clusters have been partitioned in twogroups. In the first group, with most complete data, only one cluster(M67) shows a statistically significant concentration of spectroscopicbinaries towards the inner regions relative to single giants. Centralconcentration of binaries is observed, without statistical significance,for three younger clusters. The segregation does not depend on theperiod of the binaries, but qualitively seems to depend on the totalmass of the binary. For the 7 clusters of the second group, no effect isvisible. These clusters are either not old enough (8.45 less than logtless than 9.25) or, more probably, the radial extension of the existingphotometric studies is too small. Simulation with real data tends tofavor the latter explanation.
| New Photometric Variable Stars in the Field of Southern Open Clusters
Not Available
| Photoelectric Search for Peculiar Stars in Open Clusters - Part Fourteen - NGC1901 NGC2169 NGC2343 CR:132 NGC2423 and NGC2447
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102....1M&db_key=AST
| Asymptotic giant branch stars near the sun
Available red and near-infrared photometry and apparent motions of M, S,and C asymptotic giant branch (AGB) stars in the Bright Star Catalogueare tabulated and discussed. It is shown that the red and near infraredindices normally used for late-type stars are interchangeable except forcarbon stars. The M-type giants are variable with visual amplitudegreater than 0.05 mag. The reddening-free parameter m2 from Genevaphotometry is essentially a temperature parameter for M giants, whilethe reddening-free parameter d is a sensitive detector of blue stellarcompanions. The space density of AGB stars near the sun decreases by afactor of 35 in a temperature range 3800 to 3400 K. Two of the S starsnear the sun were found to have nearly equal space motions and may becomembers of the Arcturus group.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
NEWS@SKY (Science&Space News)
