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Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| 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
| Photometric properties of the δ Scuti star BR Cancri
We present the results of a three-year Johnson V and StrömgrenuvbyHβ photometric study of the δ Scuti star BR Cancri (BRCnc). Our data sets consist of 1293 discrete differential magnitudes inJohnson V and yellow y filters, 883 in Strömgren v and 239 in ubfilters. The Fourier analysis of the data suggests four pulsationfrequencies for the variable: f1=24.978,f2=11.358, f3=11.808 andf4=27.914cycled-1. During the three observingyears, the main frequency f1 kept its V(y) amplitude constantat about 6mmag but its v amplitude seems to be changing. Amplitudevariations for all the three other frequencies are also claimed. Thepulsation modes of the frequencies are discussed based on the colourdata. Using uvbyβ data and calibrations in the literature, wederive the physical parameters for BR Cnc.
| 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
| Binaries in the Praesepe and Coma Star Clusters and Their Implications for Binary Evolution
This completes a study of the evolution of binary systems in five openclusters of various ages. Among 21 stars observed in Praesepe, eight arefound or confirmed to be spectroscopic binaries and orbital elements arederived, while one more shows long-term binary motion. Among 18 starsobserved in the Coma Berenices cluster, five are found or confirmed tobe spectroscopic binaries and orbital elements are derived, while asixth has tentative elements. Among five clusters studied we searchedfor three expected evolutionary effects, namely an increase with age inthe mass ratios, a decrease with age of the binary periods, and anincrease in binary frequencies. We find that there is a progression (atthe 3 sigma level) from no binaries out of 10 with mass ratios greaterthan 0.5 in the youngest cluster (combined with the published resultsfor NGC 6193) to 25% such stars in the intermediate-age clusters to 43%such stars in these two oldest clusters. There is no evidence for anincrease in short-period binaries with age. And there is slight evidence(at the 1 sigma level) for an increase with age from 15% to 28% in thefraction of large-amplitude binaries. These results are mostlyconsistent with the idea that most binaries are formed or modified inthree-body interactions, and successive generations of formation anddisruptions tend to form binaries with larger mass ratios. However, partof the initial generation of binaries is probably primordial.
| High resolution spectroscopy over lambda lambda 8500-8750 Å for GAIA. I. Mapping the MKK classification system
We present an Echelle+CCD high resolution spectroscopic atlas (0.25Ä/pix dispersion, 0.43 Ä FWHM resolution and 20 000 resolvingpower) mapping the MKK classification system over the interval lambdalambda 8500-8750 Ä. The wavelength interval is remarkably free fromtelluric lines and it is centered on the near-IR triplet of Ca II, thehead of hydrogen Paschen series and several strong metallic lines. Thespectra of 131 stars of types between O4 and M8 and luminosity classes Ithrough V are included in the atlas. Special care was put in maintainingthe highest instrumental homogeneity over the whole set of data. Thecapability to derive accurate MKK spectral types from high resolutionobservations over the interval lambda lambda 8500-8750 Ä isdiscussed. The observations have been performed as part of an evaluationstudy of possible spectroscopic performances for the astrometric missionGAIA planned by ESA. Tables~3 and 4 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/ Abstract.html}\fnmsep\thanks{ Thespectra of the stars listed in Table~2 are also available in electronicform at the CDS or via the personal HomePagehttp://ulisse.pd.astro.it/Astro/Atlases/}\fnmsep\thanks{ Figures 3--28are only available in electronic form at http://www.edpsciences.com
| 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
| Chromospheric Activity in the Late A- and Early F-Type Stars of Open Clusters. I. Hyades, Praesepe, and Coma
We report observations of the He i lambda5876 (D3) line in the late A-and early F-type stars in the Hyades, Praesepe, and Coma star clustersused to determine chromospheric activity levels. The average activitylevel in the early F-type stars is statistically identical in the threeclusters and in field dwarfs, and the dispersion in activity levels ineach sample is similar. The earliest star detected in each clusterranges from B-V = 0.26 in the Hyades to B-V = 0.31 in Praesepe. However,four stars in the Hyades in the range 0.28-0.32 are not detected. Thedetection of Hyades star VB 84 at B-V = 0.26 represents the bluest starobserved to date with a statistically nonzero chromospheric D3equivalent width. In nine early F-type stars we compare D3 strengtheither during two different observing runs or in several individualintegrations. In two stars we find evidence for short-term variabilityof a factor of 2 on timescales of 1 day or less. The timescale of thevariability is consistent with rotational modulation. Finally, thesimilarity in chromospheric activity in the Hyades and Praesepe starscontrasts with the significant weakness of coronal X-ray emission ofearly F-type stars in Praesepe compared to Hyades. We have noexplanation for this difference; however, this result suggests that theobserved disappearance of D3 in the late A-type stars is not directlycaused by the weak coronal emission of these stars.
| The Age Range of Hyades Stars
On the basis of canonical models, the age of Hyades supercluster stars,whether in the Hyades and Praesepe clusters or the noncluster field,ranges from (5-6) x 10^8 to 10^9 yr. The difference between the parallaxderived from the supercluster motion and that obtained from Hipparcosobservations has a dispersion only twice that of the mean dispersion ofthe individual Hipparcos values. The supercluster appears not to containred giants on the first ascent of the red giant branch, but onlyasymptotic giant branch (``clump'') stars. The masses obtained forindividual components of binary stars in the supercluster show adispersion of less than 10% when compared with model predictions.
| Blue stragglers in open clusters. I. NGC 2632
We present the results of a spectroscopic study of four blue stragglersfrom old galactic open cluster NGC 2632 (Praesepe). The LTE analysisbased on Kurucz's atmosphere models and synthetic spectra technique hasshown that three stars, including the hottest star of the clusterHD73666, possess an uniform chemical composition: they show a solar-likeabundance (or slight overabundance) of iron and an apparent deficiencyof oxygen and silicon. Two stars exhibit a remarkable bariumoverabundance. The chemical composition of their atmospheres is typicalfor Am stars. One star of our sample does not share such uniformelemental distribution, being generally deficient in metals. For allstars Teff and log g values were estimated. A shortdiscussion of the obtained results is given.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
| Accurate Two-dimensional Classification of Stellar Spectra with Artificial Neural Networks
We present a solution to the long-standing problem of automaticallyclassifying stellar spectra of all temperature and luminosity classeswith the accuracy shown by expert human classifiers. We use the 15Angstroms resolution near-infrared spectral classification systemdescribed by Torres-Dodgen & Weaver in 1993. Using the spectrum withno manual intervention except wavelength registration, artificial neuralnetworks (ANNs) can classify these spectra with Morgan-Keenan types withan accuracy comparable to that obtained by human experts using 2Angstroms resolution blue spectra, which is about 0.5 types (subclasses)in temperature and about 0.25 classes in luminosity. Accuratetemperature classification requires a hierarchy of ANNs, whileluminosity classification is most successful with a single ANN. Wepropose an architecture for a fully automatic classification system.
| 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.
| Blue Stragglers in the Solar Vicinity: Newborn or Reborn
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110..823E&db_key=AST
| 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.
| High-precision study of proper motions and membership of 924 stars in the central region of Praesepe.
High-precision proper motions for 257 and 296 stars in a 90'x90' regioncentered on BD+20 2170 were obtained from seven plates taken with the40cm refractor (f=6895mm) at Zo-Se station of Shanghai Observatory withsix AC plates and the stellar positions in Russell's (1976) catalogue,respectively. Combining with the data given by Klein-Wassink (1927),Jones (1971), Artyukhina (1971), and Jones & Cudworth (1983), theseproper motions give a sample of 924 stars in the region mentioned above.With membership probabilities estimated by an improved maximumlikelihood method, a very good sample of 198 members is obtained withproper motion accuracies ranging from +/-0.2 to +/-5.0mas/yr, of which60 per cent are better than +/-1.0mas/yr, and the completeness nearlydown to B=15.5.
| 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.
| ICCD speckle observations of binary stars. IX - A duplicity survey of the Pleiades, Praesepe, and IC 4665 clusters
Multiplicity of stars within clusters is a well-studied phenomenon.However, recent survey work done on the Hyades by Mason et al. (1993)would seem to indicate that even in the most often studied clusters,there may be binaries yet undiscovered. In order to expand the sample ofcluster binaries with potentially short-period visual orbits, a specklesurvey of 45 Pleiades, 54 Praesepe, and 22 IC 4665 bright stars (V isless than 10) for possible multiplicity was conducted at the KPNO 4 mMayall telescope between 1987 October and 1991 November. Of these, threenew binaries have been discovered: one in the Pleiades where the newcomponent may be spectroscopic, another in Praesepe which has beenconfirmed from examinations of archival observations and also has beenresolved by occultation, and the third in IC 4665. Continued study ofthese new binary stars could further refine the cluster distance modulias well as the cluster mass-luminosity relations.
| Washington photometry of open cluster giants - Nine old disk clusters in the third Galactic quadrant
Washington photoelectric photometry for 136 stars in nine old openclusters is presented and used to determine the luminosity class, T(e),and metallicity. The virtually unstudied cluster NGC 2324 is found tohave a metallicity one tenth that of the sun, as is the cluster NGC 2660for which previous estimates have ranged from about +0.1 to -0.5. Athird cluster, NGC 3960, with a Galactocentric distance of only 8 kpc,is found to have a metallicity of -0.7, also substantially lower thanpublished values. Such clusters indicate that substantial scatteractually exists in the tight relation found by Friel and Janes (1992)between the metallicity of an open cluster and its currentGalactocentric distance. Outer disk clusters have a metallicity at agiven age that is much more like that of the LMC counterparts than thatof solar neighborhood disk field stars or clusters.
| The Hyades supercluster in the FK5
The members of the Hyades supercluster brighter than about M(V) = + 4mag and contained in the FK5, or having nearly FK5 quality propermotions, show a convergent point of (A,D) = (6h, + 6.5 deg). The Hyadescluster stars in the FK5 have a mean distance of 46.7 pc. Thesupercluster, as well as the Hyades and Praesepe cluster, populationsrepresent at least three age groups. Standard models indicate ages of 3to 4, 6, and 8 x 10 exp 8 yr, whereas model ages with convectiveovershoot are nearly twice this. Most of the Am and USPC stars in thesupercluster are of the same age. The Ap stars mark the onset of shellhydrogen burning. The photometry of the red giants confirms the agespread and indicates a weakening of CN strength with age. Attention iscalled to the need for further study of NGC 2423 as an effectiveprolusion to understanding the evolution of the supercluster.
| J 2000.0 positions and proper motions of 257 stars in the central part of the Praesepe astrometric standard region
Data are presented on the J2000.0 positions and proper motions of 257stars in the central 1.5 deg x 1.5 deg area of the Praesepe astrometricstandard region. These data obtained with accuracies of 0.005-0.10arcsec for the positions in each direction and 0.0002-0.006 arcsec/yrfor the proper motions in each direction. The list includes stars forwhich proper motions were not given in the Russell (1976) catalog.
| The late A-type stars - Refined MK classification, confrontation with Stromgren photometry, and the effects of rotation
The MK spectral classification for late A-type stars is refined and theeffects of rotation of spectral classification and uvby(beta) photometryfor these stars are examined. It is found that, for A3 stars, the4417/4481 A wavelength ratio produces results that are inconsistent withthe Stark broadening of the H lines. It is suggested that this ratio isnot useful as a luminosity criterion at any spectral type.Self-consistent sequences of narrow- and broadline standards areestablished. The results of the refined classification system arecompared with Stromgren photometry, showing a set of low-v sin i A-typestars with anomalously large delta(c1) indices for theirluminosity types. It is proposed that these stars are rapid rotatorsseen at fairly low inclination angles.
| 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).
| Photoelectric search for CP2-stars in open clusters. XII - Alpha Persei, Praesepe and NGC 7243
Observations of 95 stars in the open clusters Alpha Persei, Praesepe,and NGC 7243 have been performed by Delta-a photometry in order tosearch for the presence of the 5200-A feature of CP2 (or CP4) stars.Although peculiarity has been claimed for half a dozen stars in AlphaPersei, the photometry detects only the CP4 star (= He-weak) HL 985among the cluster members. The behavior of 3 Be/shell stars (HL 861,1164, 904) concerning the 5200-A feature is discussed. Praesepe, by farthe oldest cluster in the survey program, presents a very new andspecial case: 3 stars, previously classified as Am have Delta-a valuesaround 0.020 mag, i.e., they exhibit photometric peculiarity likeCP2-stars. Such values are incompatible with preceding results for fieldand cluster Am-stars (= CP3). These objects deserve further attentionboth photometrically and spectroscopically. NGC 7243 exhibits anoutstanding high frequency of CP2-stars; two certain Delta a-peculiarstars have been found: L 370, already known as peculiar, and the newlyidentified L 114. Two stars are near the detection level: L 58 and 121.According to Geneva photometry, L 487 is markedly peculiar.
| MK classification of the brighter praesepe stars
MK classifications have been obtained for the 42 brightest stars in therich open cluster Praesepe. In the H-R diagram, 10 of the 12 starsbrighter than V = 7.0 mag have luminosity classes brighter than V or areAm stars. Only two of the stars fainter than V = 7.0 mag are ofluminosity class IV. The nine Am stars discovered by Bidelman (1956) areconfirmed. The frequency of these among the late A stars is consistentwith results for field stars, but there is only one Ap star. The reasonsfor that deficiency are discussed.
| Metallicism among A and F giant stars
132 stars considered as A and F giants have been studied for theirproperties in the Geneva photometric system. It is shown that thissystem to derive the temperature, absolute magnitude and Fe/H value forstars in this part of the HR diagram. 36 percent of the stars of oursample exhibit an enhanced value Delta m2 that can be interpreted interms of Fe/H. The red limit of stars having an enhanced Fe/H value is0.225 in B2-V1 or 6500 K in Teff. This corresponds to the limit definedby Vauclair and Vauclair (1982) where the diffusion timescale is equalto the stellar lifetime and permits the assumption that the diffusion isthe process responsible for the metallicism observed in the A and Fgiants.
| A new photometric WBVR system
A description of the new broad-band photometric system, WBVR, is given.It is shown that WBVR is close to the UBVR system but is defined bystrictly fixed response curves and the primary standard. Theconstruction of a network of secondary standards distributed uniformlyin the northern sky, as well as the coordination of the magnitudes ofthe standards to the self-consistent system are discussed. All standardsare examined for a possible brightness variability and it is found thatno light variability exceeding 0.015 mag in amplitude is present andthat their average magnitude shows stability on a scale of severalyears. The individual WBVR observations of the standards are presentedin tables.
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| Takýmyýldýz: | Yengec |
| Sað Açýklýk: | 08h37m46.76s |
| Yükselim: | +19°16'02.0" |
| Görünürdeki Parlaklýk: | 6.736 |
| Uzaklýk: | 196.078 parsek |
| özdevim Sað Açýklýk: | -35.3 |
| özdevim Yükselim: | -13.5 |
| B-T magnitude: | 6.963 |
| V-T magnitude: | 6.755 |
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