Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
The Benchmark Cluster Reddening Project. I. Reddening Values for the Hyades, Coma, and Praesepe
From histories of reddening studies for the Hyades, Praesepe, and nearbyfield stars, it is concluded that reddening scholarship requiresfundamental reform. The adopted protocol for the reformed version ofscholarship includes (1) rigorous published foundations for reddeningvalues, (2) explicit quality control of reddening techniques, (3) use oftechniques with limited metallicity sensitivity, (4) explicit zero-pointcontrol for both input data and derived reddening values, (5) anexplicit choice of a reddening zero point, and (6) adherence tostatistical norms. It is argued that only limited use should be made ofreddening values from (1) spectral types for F0 and later type stars,(2) UBV analysis of blanketed stars, and (3) certain published relationsbetween reddening and distance. However, reddening values may be basedsafely on (1) polarization measurements, (2) comparisons of β and(R-I)C for F stars, and (3) Strömgren β analysis ofA stars. For the Hyades and Coma, upper 95% confidence limits on E(B-V)are found to be <=1.0 and <=3.2 mmag, respectively. For Praesepe,the derived value of E(B-V) is 27+/-4 mmag. The latter result isobtained after an allowance is made for the effects of an anomalousrelation between b-y and (R-I)C in Praesepe A stars. A testof the possibility that differential stellar rotation causes the anomalyyields an inconclusive result. Adoption of the revised reddening valueinstead of the canonical value of zero yields an increase in the derivedPraesepe distance modulus by 0.14 mag and a decrease in the derivedcluster age by about 11%. In addition, by using publishedhigh-dispersion results, it is found that the value of [Fe/H] forPraesepe that is consistent with the derived reddening value is+0.01+/-0.04 dex. Further results for Praesepe stars will be requiredfor a definitive comparison of the Hyades and Praesepe metallicities.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| New stellar members of the Coma Berenices open star cluster
We present the results of a survey of the Coma Berenices open starcluster (Melotte 111), undertaken using proper motions from theUSNO-B1.0 (United States Naval Observatory) and photometry from theTwo-Micron All-Sky Survey (2MASS) Point Source catalogues. We haveidentified 60 new candidate members with masses in the range 1.007
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Improved Hipparcos Parallaxes of Coma Berenices and NGC 6231
A method to reprocess the Hipparcos Intermediate Astrometry Data thatreduces the propagation of the along-scan spacecraft attitude errors wasdeveloped and successfully used to obtain a more consistent parallax ofthe Pleiades (Makarov, published in 2002). The same technique is usednow to correct the Hipparcos parallaxes of the Coma Berenices and NGC6231 open clusters, which are also in error. The new mean parallax ofComa is 12.40+/-0.17 mas (against previously 11.43 mas) and of NGC 62311.7+/-0.4 mas (against previously -0.8 mas). The new data for Coma arein excellent agreement with the pre-Hipparcos main-sequence fittingestimates. All six members of NGC 6231 that have negative parallaxes inHipparcos obtain positive parallaxes. These results suggest that themain source of astrometric error has been correctly identified, and thata more accurate Hipparcos catalog can be computed.
| Beryllium Abundances in F and G Dwarfs in the Coma Cluster and the Ursa Major Moving Group from Keck HIRES Observations
The study of both light elements Li and Be in open clusters of knownproperties can reveal the internal structure and the mechanisms ofmixing in main-sequence stars as a function of age and composition. Inprevious work, we have investigated the older Hyades cluster and theyounger Pleiades and α Per clusters. The Coma Berenices clusterand the UMa moving group are intermediate in age between the Hyades andPleiades and provide a good linchpin for the influence of stellar age onlight-element abundances; there are dips in the mid-F stars in both Liand Be in the Hyades but no Be dip and only a minor Li dip in thePleiades. We have made observations of the resonance doublet of Be IInear 3130 Å in 13 Coma and six UMa stars with the Keck I telescopeand the High Resolution Echelle Spectrometer. The Be abundances weredetermined by spectrum synthesis. In the F dwarfs in Coma, there areboth Li and Be deficiencies, indicating that the depletions occur duringthe main-sequence phase of evolution but do not become evident until anage of 200-300 Myr. For both UMa and Coma stars, the Li depletion isgreater than the Be depletion at all temperatures, but there is little,if any, Be depletion in stars with Teff<6000 K. In thefour clusters studied for Be, the mean Be abundance for stars withtemperatures less than 6000 K is logN(Be/H)+12.00=1.27, independent ofage or metal content. For the hotter stars (5850-6680 K), the Li and Beabundances are correlated, indicating that the depletion probably occurssimultaneously; this matches the results for the field stars and theHyades and the predictions of Li and Be depletion by rotationallyinduced mixing.
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| Spectroscopic survey of field stars : A search for metal-poor stars
We have undertaken a spectroscopic survey of field stars to finemetal-poor objects among them. Though the main objective of the surveyis to find new metal-poor stars, stellar parameterization is carried outfor all the sample stars so that the other categories of interestingobjects like composite stars, weak or strong CN, CH stars etc. can alsobe identified. Observations are carried out using OMR spectrographattached to VBT, Kavalur. The sample of candidate stars are chosen fromprismatic survey of Beers and his collaborators covering a large part ofthe Galaxy. At the first phase of this project, the analysis ofcompleted for a set of 19 relatively hot stars (Teff in 6000 to 8000Krage). The metallicities of the program stars are derived bysynthesizing the spectrum in the wavelength range 4900 to 5400 Åfor different metallicities and matching them with the observed spectra.This spectral region contains strong feature of Fe I at 5269 Å andone moderately strong Fe I blend at 5228 Å. These features weregenerally relied upon for Fe/H determination. More than half of thecandidate stars were found to show (Fe/H) in -0.7 to -1.2 range. Twomost metal-poor stars have (Fe/H) values of -1.3 and -1.8. It appearsthat metal-poor candidates suggested by Beers et al. from theirprismatic survey has a very significant fraction of metal-poor stars.The significantly metal-poor stars found so far would be studied indetail using high resolution spectra to understand nucleosynthesisprocesses that might have occurred in early Galaxy.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Dusty Debris around Solar-Type Stars: Temporal Disk Evolution
Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to searchfor evidence of emission from dust orbiting young main-sequence stars,both in clusters and isolated systems. Over half of the detections arenew examples of dusty stellar systems and demonstrate that such dust canbe detected around numerous stars older than a few times 106yr. Fluxes at 60 and either 90 or 100 μm for the new excess sourcestogether with improved fluxes for a number of IRAS-identified sourcesare presented. Analysis of the excess luminosity relative to the stellarphotosphere shows a systematic decline of this excess with stellar ageconsistent with a power-law index of -2.
| Lithium in the Coma Berenices open cluster
Lithium abundances, radial velocities, and rotational velocities arereported for 20 candidate F, G and K stars of the sparse Coma Berenicesopen cluster. All the stars are proper-motion selected, and our radialvelocities support the membership credentials of at least 12 of thecandidates. Combining our data with that in the literature, we have aclose-to-complete census of Li abundances for late-type stars withMV<5.8. These data show that the Li-depletion pattern inComa Ber is similar, but not identical, to that in the Hyades cluster,which has a similar age but higher metallicity. Several Coma Ber F starshave suffered significantly more Li depletion than their counterparts inthe Hyades. The G and early-K stars of Coma Ber have undergone less Lidepletion than those of the Hyades, but much more than that predicted bystandard evolutionary models featuring only convective mixing. Thisprovides strong evidence for additional mixing and Li depletionoperating in these stars during their first 400 Myr on the mainsequence, amounting to 0.3 dex at 6000 K and rising to 0.8-1.2 dex at5400 K. We find that 4 of the radial-velocity non-members are among asmall number of low-mass stars which were previously reported as part ofan extra-tidal moving group associated with the Coma Ber cluster. As aresult, we now find that the luminosity function of this moving group isindistinguishable from that of the central cluster. It is uncertainwhether there may be a significant number of stars with even lowermasses among the moving group.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Lithium in the low-mass stars of the Coma Berenices open cluster
I report lithium abundances and radial velocities for 15 late F, G and Kstars in the Coma Berenices open cluster. All the stars are propermotion members, and the radial velocities enable us to confirmmembership for 11 single stars and identify three SB1 systems and oneshort-period SB2 system that are very probable cluster members. Thepattern of Li abundances is similar to that in the Hyades, with someevidence for higher Li abundances in Coma Ber for T_eff<5700 K. Iargue that these data do not support standard models for Li depletioninvolving only convective mixing, because the Coma Ber stars haveconsistently determined lower metallicities and shallower convectionzones than the Hyades and should have suffered a great deal less Lidepletion than observed. Instead, it seems that the Li abundances inthese and other young clusters are primarily ranked according to theirage rather than their metallicity. I propose a scenario where theextreme metallicity dependence of pre-main-sequence Li depletion istempered by a mechanism that severely inhibits convective mixing, wherecontinued main-sequence depletion occurs via non-standard mixingmechanisms, and where any dependence of Li depletion on metallicitytakes at least several hundred Myr to emerge.
| An astrometric catalogue for the area of Coma Berenices
A catalogue of stellar positions and proper motions down to the 14thphotographic magnitude in the area of the open cluster in Coma Berenicesis compiled from data of 12 different sources. The accuracy of theproper motion data is comparable to that of the Hipparcos Catalogue. Thecatalogue Table 5 is 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/Abstract.html
| Red supergiants in the LMC - III: luminous F and G stars
New BVRI observations for 40 and spectrophotometric measurements for 23F to G LMC supergiant candidates (and 3 galactic F to G supergiants) arepresented. The errors of the BVRI data are 0.01 to 0.03 mag in mostcases. The wavelength range of the spectra is 3400 to 6400 Angstroms,their resolution 10 Angstroms. The mean error of the fluxes is 0.03 mag.Spectral indices measuring the strengths of the Hβ , Hγ ,Hdelta , NaD and CaII H+K lines, the CHα_ {0} and CNbeta_ {0}bands, of the Balmer jump and the slope of the continuum redwards arediscussed as measures of effective temperature and luminosity on thebasis of galactic stars with accurate MK types and parallaxes. TheHγ line and the continuum gradient are very good temperaturecriteria, the CHα_ {0} band and especially the Balmer jump forluminosity. The luminosity classification given for F to G supergiantcandidates in the LMC in the literature is often doubtful. 5 of the 23stars observed spectrophotometrically turn out to be probably galacticforeground dwarfs on the basis both of the Balmer jump and thecomparison of their flux distributions with synthetic ones based on theKurucz model atmospheres. Surface gravities derived purely on the basisof flux distributions and such ones given by models of stellar evolutionagree with each other for dwarfs and giants only. For supergiants theformer are about 1.0 dex higher than the latter. As a consequenceeffective temperatures and metallicities given by these two methodsdeviate from each other for such stars, too. The intrinsic colours andtemperatures of galactic and LMC supergiants do not differ. Withabsolute magnitudes up to -9.6 mag the upper luminosity limit in the LMCdoes not exceed that in the Galaxy, where Ia-0 supergiants haveMV of up to -9.5 mag. The metallicities of the supergiantsshow a rather large scatter. Nevertheless the mean metallicities of 0.02+/- 0.09 dex for the Galaxy and -0.26 +/- 0.10 dex for the LMC agreewell with other observations.
| The Coma Berenices star cluster and its moving group
We report on the analysis of astrometric and photometric data fromHipparcos, Tycho and the ACT catalogue in a 1200 sq. degree field aroundthe nearby open star cluster in Coma Berenices and infer thecharacteristic features of this cluster. From a sample of 51 kinematicmembers we derive the cluster's distance, size and spatial structure aswell as its stellar content, mass and age. We find that the clusterconsists of an elliptical core-halo system with major axis parallel tothe direction of galactic motion, and of a moving group of extratidalstars. The latter have tangential distances >= 10 pc from the clustercenter, but have the same distance from the Sun, the same motion and thesame age as the stars in the core and halo. The luminosity function ofthe core-halo system steeply declines beyond absolute magnitude M_V=4.5,but that of the moving group rises towards fainter magnitudes andsuggests the existence of further low-mass members below the currentmagnitude limit. The cloud of extratidal stars witnesses the process ofdissolution of the cluster.
| Spectrophotometry of 237 Stars in 7 Open Clusters
Spectrophotometry is presented for 237 stars in 7 nearby open clusters:Hyades, Pleiades, Alpha Persei, Praesepe, Coma Berenices, IC 4665, andM39. The observations were taken by Lee McDonald and David Bursteinusing the Wampler single-channel scanner on the Crossley 0.9m telescopeat Lick Observatory from July 1973 through December 1974. Sixteenbandpasses spanning the spectral range 3500 Angstroms to 7780 Angstromswere observed for each star, with bandwidths 32Angstroms, 48 Angstromsor 64 Angstroms. Data are standardized to the Hayes-Latham system tomutual accuracy of 0.016 mag per passband. The accuracy of thespectrophotometry is assessed in three ways on a star-by-star basis.First, comparisons are made with previously published spectrophotometryfor 19 stars observed in common. Second, (B-V) colors and uvby colorsare compared for 236 stars and 221 stars, respectively. Finally,comparsions are made for 200 main sequence stars to the spectralsynthesis models of Kurucz, fixing log g = 4.0 and [Fe/H] = 0.0, andonly varying effective temperature. The accuracy of tests using uvbycolors and the Kurucz models are shown to track each other closely,yielding an accuracy estimate (1 sigma ) of 0.01 mag for the 13 colorsformed from bandpasses longward of the Balmer jump, and 0.02 mag for the3 colors formed from the three bandpasses below the Balmer jump. Incontrast, larger scatter is found relative to the previously publishedspectrophotometry of Bohm-Vitense & Johnson (16 stars in common) andGunn & Stryker (3 stars). We also show that the scatter in the fitsof the spectrophotometric colors and the uvby filter colors is areasonable way to identify the observations of which specific stars areaccurate to 1 sigma , 2 sigma , .... As such, the residuals from boththe filter color fits and the Kurucz model fits are tabulated for eachstar where it was possible to make a comparison, so users of these datacan choose stars according to the accuracy of the data that isappropriate to their needs. The very good agreement between the modelsand these data verifies the accuracy of these data, and also verifiesthe usefulness of the Kurucz models to define spectrophotometry forstars in this temperature range (>5000 K). These data define accuratespectrophotometry of bright, open cluster stars that can be used as asecondary flux calibration for CCD-based spectrophotometric surveys.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Coronal activity in the Coma Berenices open cluster.
We present ROSAT PSPC observations of the ~500Myr old Coma Berenicescluster. The X-ray survey in Coma consists of a) a raster scan of shortduration PSPC pointings, b) ROSAT All-Sky Survey data, and c) a 16ksecdeep PSPC pointing. The raster scan and the survey data were mergedtogether covering an area of about 36 square degrees, while the deeppointing was analyzed separately. No major differences were foundbetween the two datasets. Our ROSAT observations indicate that Coma ismuch more similar in its X-ray properties to the coeval Hyades than tothe also coeval Praesepe cluster. As in the Hyades, almost all late-Fand G stars were detected, also showing a comparable range in X-rayluminosity. X-ray luminosity distribution functions (XLDFs) forsolar-type members are in close agreement with those of the Hyades,confirming that the majority of Praesepe's members are less luminousthan what one would expect for stars of their age. The Coma Berenicescluster is known for its apparent deficit of low mass stars of spectraltype K and later. We present 12 new possible low-mass Coma candidates,identified through this X-ray survey.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| X-Ray and Optical Observations of the COMPTEL Error Box for GRB 910601
We present X-ray and optical observations of the COMPTEL and IPN errorbox of the gamma-ray burst GRB 910601. Our data consists of a 20 ks PSPCROSAT image and optical CCD images and spectra. Thirty X-ray sourceswere identified within the COMPTEL error box. B and V photometry ofobjects within the error boxes of the 12 strongest X-ray sources ispresented as well as optical spectroscopy of several of the brightestoptical sources. Two X-ray sources (#29 and #30) are located within theCOMPTEL 1.5- error box and are also within a few arcmin of the IPN errorannulus for this burst. Source #29 was the strongest X-ray source in thetwo-degree diameter ROSAT field. We have associated this source with thestar SAO 70303, an early G-type star. A fainter star with unusual colorsis located about 16" to the NE of SAO 70303. No obvious opticalcounterpart was found within the ROSAT error circle for #30 to V <=22.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| Analyses of archival data for cool dwarfs. 2: A catalog of temperatures
A calibration presented in a previous paper is used in this paper toderive temperatures for FGK stars near the main sequence. Thecalibration is checked against published counterparts, and it is foundthat previous calibrations have not established K-dwarf temperatures inparticular beyond reasonable doubt. The database assembled to derive thetemperatures is described, and the problems posed by close binaries areevaluated. The newly derived temperatures are used to check a line-depthratio proposed as a thermometer by Gray and Johanson (1991, PASP, 103,439), and it is found that the ratio is metallicity-sensitive.Temperatures are given for a total of 417 stars.
| Member stars of the open cluster Mel 111 in Coma Berenices
Proper motions of AGK3 stars in a 6 x 6 deg field centered on the opencluster Mel 111 in Coma Berenices have been used for selecting starswhich are probable members of the cluster. Available photometric dataand radial velocities have been used to refine the selection. Seventeennew candidates are proposed.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| A new library of stellar optical spectra
Attention is given to a new digital optical stellar library consistingof spectra covering 3510-8930 R at 11-A resolution for 72 differentstellar types. These types extend over the spectral classes O-M andluminosity classes I-V. Most spectra are of solar metallicity stars butsome metal-rich and metal-poor spectra are included. This new library isquantitatively compared to two previously published libraries. It offersseveral advantages over them: it is photometrically well-calibratedindividually and consistently from star to star. Good temperature andluminosity coverage has been achieved. The incorporation of stars withwell-determined temperature, metallicity, and surface gravity parametersincreases the accuracy of the spectral type assigned to each compositelibrary star.
| The initial mass function of the Coma Berenices open cluster (MEL 111)
The HR (log T(eff), M(bol)) diagram of the Coma Berenices open cluster(MEL 111), as derived from a reliable list of cluster members, isemployed to show that its IMF can be approximated by a power law ofstellar mass with a slope of x = 1.90 in the mass range 1-2.5 solarmasses. The HR diagram of the Coma Berenices open cluster, and starcounts of Coma Berenices are presented.
| Chemical composition of open clusters. III - Iron and carbon in F dwarfs in Coma, Praesepe, and M67
Iron and carbon abundances are determined for F dwarfs in several openclusters and the field in order to investigate the homogeneity ofchemical mixing in the Galactic disk. Abundances are based onmeasurements of eight Fe I lines and six high-excitation C I lines inthe spectral region 7065-7155 A analyzed with model atmospheres. Clustermean values of Fe/H and C/H show cluster-to-cluster variations well inexcess of observational errors, indicating both that the overall metalcontent of the gas from which these clusters formed preserved small butsignificant differences over periods of several billion years, and thatthe production of carbon closely followed that of iron so that theseregions of star formation had the same ratio of C/Fe.
| Early type high-velocity stars in the solar neighborhood. IV - Four-color and H-beta photometry
Results are presented from photometric obaservations in the Stromgrenuvby four-color and H-beta systems of early-type high-velocity stars inthe solar neighborhood. Several types of photometrically peculiar starsare selected on the basis of their Stromgren indices and areprovisionally identified as peculiar A stars, field horizontal-branchstars, metal-poor stars near the Population II and old-disk turnoffs,metal-poor blue stragglers, or metallic-line A stars. Numerousphotometrically normal stars were also found.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Береникина коса |
| Ректацензија: | 12h19m01.47s |
| Deклинација: | +24°50'46.2" |
| Apparent магнитуда: | 8.802 |
| Даљина: | 94.162 parsecs |
| Proper motion RA: | -10.1 |
| Proper motion Dec: | -8.7 |
| B-T magnitude: | 9.376 |
| V-T magnitude: | 8.85 |
Каталог и designations:
|
