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uvby - β Photoelectric Photometry of the Open Cluster α Per
Absolute photoelectric photometry of stars in the direction of the OpenCluster α Per has been secured and is presented along with ananalysis reinforced with all the available data in Strömgrenuvby-β photometry compiled from the literature. Cluster membershipis analyzed and the physical characteristics of the stars have beendeduced. The membership determined in this paper is compared with thatof proper motion studies and Hipparcos.
| 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.
| Lithium Abundances in α Persei Stars
We revisit the scatter in lithium abundances among stars in the αPersei cluster. Among the factors that we examine, inhomogeneousreddening, stellar spots, and stellar surface activity are the maincontributors to this scatter. Thus there is no solid evidence requiringthe observed dispersion in lithium abundances to be intrinsic to thecluster member stars. Instead, the scatter is probably induced byobservational effects.
| 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
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| 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
| The Pleiades and alpha Persei Clusters
The upper-main-sequence members of the Pleiades and alpha Perseiclusters, considered as members of the Local Association, yield meanparallaxes that are only 4% larger than the mean values from Hipparcosobservations. The (log T_eff, M_V) diagram reveals that in thetemperature range from 6000 to 8000 K, the Hyades and alpha Perseimain-sequence members are nearly identical and several tenths of amagnitude brighter than similar stars on the Pleiades main sequence. Thedeparture of the Pleiades main sequence cannot be traced to either ageor heavy-element abundance differences in the range thought to apply tothese clusters. A 50% increase in the helium abundance of Pleiades overHyades stars could account for the luminosity difference. Alternativeexplanations are that the Pleiades cluster is rejected from superclustermembership and/or that the Hipparcos parallax results for the Pleiadesare in error by some 10%.
| 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
| Membership of low-mass stars in the open cluster Alpha Persei
The results of a combined astrometric, photometric, and spectroscopicsearch for low-mass members in the intermediate-age open cluster AlphaPersei are presented. Over 130 low-mass new members have been identifiedto M(v) about 12.5, almost doubling the previous number of knownmembers. The new membership information suggests a slight upwardrevision of Alpha Per's age to about 8 x 10 exp 7 yr. Alpha Per isnoticeably spatially elongated in a direction parallel to the Galacticplane, most likely due to tidal deformation. Analysis of thedistribution of relative H-alpha emission strengths among Alpha Permembers confirms the view that the mean H-alpha strength decreases inincreasingly older systems. Allowing for survey incompleteness, onecannot reject the idea that the luminosity function is consistent withthe field luminosity function to M(v) about 9 or 10.
| 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.
| Chemical abundances and ages of open clusters
The metal content of nine open clusters is discussed with the help ofresults from detailed spectral analyses of main-sequence and slightlyevolved stars, belonging to those clusters. This small sample ofclusters, out of about 1200 known, has, however, the advantage ofrepresenting the whole span of ages of open clusters with two young(Alpha Per, the Pleiades, with five intermediate age, including U Ma,Hyades, Coma, Praesepe, and NGC 752) and with two old (M 67 and NGC 188)clusters. It has also the advantage that the observations of almost allthe stars in these different clusters have been carried out on similar,when not identical telescopes, on similar high-resolution spectrographs,and similar detectors. The high signal/noise spectra of these clusterstars have been reduced with similar procedures, and interpreted withsimilar model atmosphere analyses. In taking into account the error barsattributed to the mean metal content parameter, Fe/H, of each cluster,one may conclude that only the Hyades cluster has a metal content whichdiffers significantly from that of the sun, being between 30 and 40percent higher than that of the sun.
| Lithium in lower-main-sequence stars of the Alpha Persei cluster
Lithium abundances are presented for main-sequence stars of spectraltypes F, G, and K in the young open cluster Alpha Per. For 46 clustermembers, a correlation between Li abundance and projected rotationalvelocity v sin i is found: all of the Li-poor stars are slow rotators.Two explanations are proposed to account for the correlation: (1) thatthe Li depletion is introduced following a rapid spin-down phaseexperienced by young low-mass stars, and that this episode of Lidepletion may be the dominant one determining the spread of Liabundances among young low-mass main-sequence stars, and (2) that starformation has occurred over a finite period such that the older starshave undergone a spin-down and depletion of Li by a means that may ormay not depend on rotation. The Li abundance in the warm and rapidlyrotating stars appears to be undepleted, as is predicted by recentmodels of pre-main-sequence stars. The depletion observed in the coolstars exceeds the level predicted by these models.
| Starbursts, binary stars, and blue stragglers in local superclusters and groups. I - The very young disk and young disk populations
The distributions in the HR diagram with theoretical time-constant locifor stars in several young clusters and superclusters are compared todemonstrate that 'blue stragglers' in these aggregates are mostfrequently simply single massive (mode B) stars formed in bursts of starformation that occur at discrete intervals in time following theformation of the bulk of the low-mass (mode A) stars in the aggregate.The characteristics of the close binary systems in these aggregates areexamined to show that, in several cases, mass transfer by Roche lobeoverflow has or will occur and that, in some instances, the system wouldhave appeared as a blue straggler prior to the mass-transfer event, and,in other instances, mass transfer will lead to the identification of thesystem as a blue straggler. Thus, it is concluded that the bluestraggler phenomenon has at least two distinct physical origins: it mayoriginate from delayed formation (starbursts) or from 'delayedevolution' in some close binaries (mass transfer from an evolvedprimary).
| Four-color and H beta photometry for open clusters. 10. The alf Per cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974AJ.....79..687C&db_key=AST
| The H-R diagram of the alf Per cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..242M&db_key=AST
| Photometry of the α Persei Cluster.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1960ApJ...132...68M&db_key=AST
| Eigenbewegungen in der Umgebung von α Persei
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