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Abundance analysis of Am binaries and search for tidally driven abundance anomalies - II. HD861, HD18778, HD20320, HD29479, HD96528 and HD108651
The main goal of this paper is to continue a systematic abundanceanalysis of a sample of Am binaries in order to search for possibleabundance anomalies driven by tidal interaction in these binary systems.New CCD observations in two spectral regions (6400-6500 and 6660-6760Å) of HD861, HD18778, HD20320, HD29479, HD96528 and HD108651 wereobtained. Synthetic spectrum analysis was carried out, and basic stellarproperties, effective temperatures, gravities, projected rotationalvelocities, masses, ages and abundances of several elements weredetermined. We conclude that HD861, 29479 and 108651 are typical Amstars, while HD20320 and 96528 are mild Am stars. HD18778 turned out notto be an Am star although its projected rotational velocity is very low(27 kms-1). On the contrary, HD96528 has one of the highestprojected rotational velocities (85 kms-1) among Am binarieswith orbital periods in the range 20-200 d, and yet it exhibits Amanomalies. Pseudo-synchronization and abundance anomalies are discussedin the context of possible tidal effects.
| 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
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| The 5200-Åflux depression of chemically peculiar stars - II. The cool chemically peculiar and λ Bootis stars
After establishing the synthetic Δa photometric system in thefirst paper of this series, we now present model atmospheres computedwith individual abundances for a representative sample of chemicallypeculiar (CP) stars and either confirm or redetermine their inputparameters through comparisons with photometric, spectrophotometric andhigh-resolution spectroscopic data. The final models obtained from thisprocedure were used to compute synthetic Δa indices which werecompared with observations. The observed behaviour of Δa isreproduced for several types of CP stars: models for Am stars shownegligible (or marginally positive) values of a few mmag, while forλ Bootis stars - and for metal deficient A stars in general - weobtain negative values (as low as -12 mmag in one case). For the coolestCP2 stars with effective temperatures below about 8500 K, we obtain mild(~+10 mmag) to moderately large (~+30 mmag) flux depressions inagreement with observations. However, Δa values for slightlyhotter members of the CP2 group (for which still Teff <10000 K) are underestimated from these new models. The effect of themicroturbulence parameter on the Δa index is revisited and itsdifferent role in various types of CP stars for reproducing the fluxdepression at 5200 Åis explained. We also provide reasons whymodels based on enhanced microturbulence and scaled solar abundancecould not explain the observed flux depression for all types of CPstars. We discuss potential improvements of the current models includingthe possibility of still missing line opacities (unidentified andautoionization lines), modifications due to an explicit account of aglobal stellar magnetic field, and the effect of vertical abundancestratification.
| Abundances of A/F and Am/Fm stars in open clusters as constraints to self-consistent models including transport processes
We describe the current status of a programme we started a few years agoto observe a large number of A/F and Am/Fm stars in open clusters ofvarious ages. Spectra were obtained with the AURELIE and ELODIEspectrographs at a resolving power of about 40000 and S/N ratios from100 up to 500. Abundances of 11 chemical elements have been derived byusing Takeda's (1995) procedure. A short review on previous abundancedeterminations of A and F dwarfs in open clusters and a progress reporton the current status of this project are presented. New abundancedeterminations for 24 A and F dwarfs in the Coma Berenices cluster arepresented. These abundance determinations serve to set constraints toself-consistent evolutionary models of A and F stars including transportprocesses.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| The CHARA Catalog of Orbital Elements of Spectroscopic Binary Stars
Optical interferometry is entering a new age, with several ground-basedlong-baseline observatories now making observations of unprecedentedresolution. Interferometers bring a new level of resolution to bear onspectroscopic binaries, enabling the full extraction of the physicalparameters for the component stars with high accuracy. In the case ofdouble-lined systems, a geometrically determined orbital parallaxbecomes available as well. The first step in preparing to observespectroscopic binaries is to list them, which has not been done sincethe 1989 publication of the Eighth Catalogue of the Orbital Elements ofSpectroscopic Binaries by Batten et al. We present a new catalog withroughly half again as many listings as the Eighth Catalogue. Angularseparation predictions are made for each catalog entry. The numbers ofspectroscopic binaries available for study as a function of severalimportant observational parameters are explored, and in particular, thenumber of spectroscopic binaries as a function of expected separation isdiscussed.
| Age Dependence of the Vega Phenomenon: Observations
We study the time dependency of Vega-like excesses using infraredstudies obtained with the imaging photopolarimeter ISOPHOT on board theInfrared Space Observatory. We review the different studies published onthis issue and critically check and revise ages and fractionalluminosities in the different samples. The conclusions of our studydiffer significantly from those obtained by other authors (e.g., Hollandand coworkers; Spangler and coworkers), who suggested that there is aglobal power law governing the amount of dust seen in debris disks as afunction of time. Our investigations lead us to conclude that (1) forstars at most ages, a large spread in fractional luminosity occurs, but(2) there are few very young stars with intermediate or small excesses;(3) the maximum excess seen in stars of a given age is aboutfd~10-3, independent of time; and (4) Vega-likeexcess is more common in young stars than in old stars.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| A highly sensitive search for magnetic fields in B, A and F stars
Circular spectropolarimetric observations of 74 stars were obtained inan attempt to detect magnetic fields via the longitudinal Zeeman effectin their spectral lines. The sample observed includes 22 normal B, A andF stars, four emission-line B and A stars, 25 Am stars, 10 HgMn stars,two lambda Boo stars and 11 magnetic Ap stars. Using the Least-SquaresDeconvolution multi-line analysis approach (Donati et al.\cite{donati97etal}), high precision Stokes I and V mean signatures wereextracted from each spectrum. We find absolutely no evidence formagnetic fields in the normal, Am and HgMn stars, with upper limits onlongitudinal field measurements usually considerably smaller than anypreviously obtained for these objects. We conclude that if any magneticfields exist in the photospheres of these stars, these fields are notordered as in the magnetic Ap stars, nor do they resemble the fields ofactive late-type stars. We also detect for the first time a field in theA2pSr star HD 108945 and make new precise measurements of longitudinalfields in five previously known magnetic Ap stars, but do not detectfields in five other stars classified as Ap SrCrEu. We also report newresults for several binary systems, including a new vsin i for therapidly rotating secondary of the Am-delta Del SB2 HD 110951. Based onobservations obtained using the MuSiCoS spectropolarimeter on theBernard Lyot telescope, l'Observatoire du Pic du Midi, France.
| 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.
| The Pleiades, Coma, Hyades, and Praesepe open clusters: Li, Al, Si, S, Fe, Ni, and Eu abundances compared in A stars
In the third of a series of papers on the A stars in open clusters, theComa and Hyades clusters are revisited; in the first and second papers,the Pleiades and Praesepe were respectively investigated. All thespectra were secured with the Canada-France-Hawaii telescope at highspectral resolution and high signal-to-noise ratios. Photosphericabundances have been determined for Li, Al, Si, S, Fe, Ni, and Eu frommodel atmosphere abundance analysis. All the A stars with enough-sharplines to be studied for Li were observed in the four clusters. Abundanceresults are summarized for 31 cluster members, including 21 Am, 7 normalA, and 3 early-A stars. The Am stars have very uniform Li, Al, Si, S,and Fe abundances in a large temperature range of nearly 1000 K.Compared to normal A stars, Li is significantly deficient in Am stars(by a factor of 3), Al marginally overabundant, Si, S, and Fe are thesame, Ni and Eu (with only a few results) overabundant. Those uniformabundances of Li, Al, Si, S, or Fe in Am stars involve that abundancesare little affected by the magnitude of the rotational velocity. Forboth Am and normal A samples, no abundance trend as a function of ageand/or evolution is detected in the case of Li, Al, Si, S, or Fe. Theages considered are in the range 0.8 - 7 108 years; theevolution is limited from the ZAMS to the cluster turn-off. The build-upof the chemical abundances studied, in particular the Li differentiationbetween Am and normal A stars, could have taken place very early whenthe stars arrive on the Main Sequence. The spread in lithium found forthe A stars is reminiscent of that reported in the field and one opencluster for stars of nearly the same mass and slightly evolved out ofthe Main Sequence. The Li abundance does not change as soon as the starevolves through the subgiant phase and the convection zone becomesdeeper. There are two exceptional Am stars: one, in the Hyades, isLi-deficient and the other, in Praesepe, Li-overabundant. They are noobvious circumstances that can distinguish both stars from others in thevery same region of their respective cluster sequence. In each of thefour clusters, the maximum Li abundance is found in A stars, generallyin normal A stars. The Fe abundance of both Am and normal A groups isfound to be twice the original Fe value (on the ZAMS) in each of thefour clusters, independently of their age or metallicity. It is wellestablished for the Am group and in only a narrow range of Teff for thenormal A group. This behavior is unexpected for normal A stars which arethought to have their original abundances and Fe abundance differentfrom that of Am stars. A larger normal A sample is needed to concludeanything. Our abundance results for cluster Am stars quantitativelyagree with predictions of new models coupling atomic diffusion withturbulent transport (Richer et al. \cite{richer99}). Based onobservations collected at the Canada-France-Hawaii telescope (Hawaii)
| 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.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| 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
| 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.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only 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
| Behaviour of calcium abundance in Am-Fm stars with evolution
Calcium abundance in the atmosphere of Am stars is examined as afunction of their evolutionary state within the main sequence. Newspectroscopic abundances as well as abundances obtained photometricallyby Guthrie (1987) are used, since they are mutually quite consistent.The main result of this work is that, contrary to earlier suggestions,calcium abundance does not tend to be larger in evolved Am stars than inunevolved ones, for objects distributed along a given evolutionary trackin the HR diagram. The trend appears to be the reverse, if it is presentat all. For our whole sample of Am stars, there is a significantcorrelation between calcium abundance and effective temperature, in thesense that the cooler objects are the most Ca-deficient, hence have themost pronounced Am peculiarity. This implies an apparent correlationbetween calcium deficiency and age, although the lack of Am starsyounger than log t = 8.6 seems real. Our results are fully consistentwith the low rate of Am stars observed in young clusters and withtheoretical predictions of time-dependent radiative diffusion (Alecian1996). Based on observations collected at Observatoire de Haute Provence(CNRS), France, and on data from the ESA HIPPARCOS astrometry satellite.
| 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
| On the nature of the AM phenomenon or on a stabilization and the tidal mixing in binaries. II. Metallicity and pseudo-synchronization.
We reveal sufficient evidences that for Am binaries the metallicitymight depend on their orbital periods, P_orb_, rather than on vsini. Inparticular, δm_1_ index seems to decrease with increasing orbitalperiod up to at least P_orb_=~50d, probably even up to P_orb_=~200d.This gives further support to our "tidal mixing + stabilization"hypothesis formulated in Part I. Moreover, while the most metallic Amstars seem to have rather large periods the slowest rotators are foundto exhibit substantially shorter P_orb_. A questioning eye is thus caston the generally adopted view that Am peculiarity is caused by asuppressed rotationally induced mixing in slowly rotating `single'stars. The observed anticorrelation between rotation and metallicity mayhave also other than the `textbook' explanation, namely being the resultof the correlation between metallicity and orbital period, as themajority of Am binaries are possibly synchronized. We further argue thatthere is a tendency in Am binaries towards pseudo-synchronization up toP_orb_=~35d. This has, however, no serious impact on our conclusionsfrom Part I; on the contrary, they still hold even if this effect istaken into account.
| Metal abundances of A-type stars in three galactic clusters.
Investigations have been carried out for 11 A stars in young openclusters (α Per, Coma, and Praesepe) and three field stars bymeans of high resolution spectroscopy. Detailed abundance analyses havebeen made for Mg, Ca, Sc, Cr, Fe, and Ni. The results are discussed inthe framework of element stratification processes as invoked bytime-dependent diffusion mechanisms in Am stars. The youngest cluster ofour sample, α Per, seems to show some untypical abundance patternswhich might be identified as early Am phases.
| Atmospheric chemical composition of Am stars in the Coma cluster
Not Available
| 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.
| On the nature of the AM phenomenon or on a stabilization and the tidal mixing in binaries. I. Orbital periods and rotation.
The paper casts a questioning eye on the unique role of the diffusiveparticle transport mechanism in explaining the Am phenomenon and arguesthat the so-called tidal effects might be of great importance incontrolling diffusion processes. A short period cutoff at =~1.2d as wellas a 180-800d gap were found in the orbital period distribution (OPD) ofAm binaries. The existence of the former can be ascribed to the state ofthe primaries with the almost-filled Roche lobes. The latter couldresult from the combined effects of the diffusion, tidal mixing andstabilization processes. Because the tidal mixing might surpassdiffusion in the binaries with the orbital periods P_orb_ less thanseveral hundred days and might thus sustain the He convection zone,which would otherwise disappear, no Am stars should lie below thisboundary. The fact that they are nevertheless seen there implies theexistence of some stabilization mechanism (as, e.g., that recentlyproposed by Tassoul & Tassoul 1992) for the binaries with orbitalperiods less than 180d. Further evidence is given to the fact that theOPD for the Am and the normal binaries with an A4-F1 primary arecomplementary to each other, from which it stems that Am stars are closeto the main sequence. There are, however, indications that they haveslightly larger radii (2.1-3 Rsun_) than expected for theirspectral type. The generally accepted rotational velocity cutoff at=~100km/s is shown to be of little value when applied on Am binaries ashere it is not a single quantity but, in fact, a function of P_orb_whose shape is strikingly similar to that of the curves of constantmetallicity as ascertained from observations. This also leads to thewell known overlap in rotational velocities of the normal and Am starsfor 402.5d.We have exploited this empirical cutoff function to calibrate thecorresponding turbulent diffusion coefficient associated with tidalmixing, having found out that the computed form of the lines of constantturbulence fits qualitatively the empirical shape of the curves ofconstant metallicity. As for larger orbital periods(20d55km/s found by Burkhart(1979) would then be nothing but a manifestation of insufficientlypopulated corresponding area of larger P_orb_.
| The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&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.
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| 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.
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Osservazione e dati astrometrici
| Costellazione: | Chioma di Berenice |
| Ascensione retta: | 12h28m44.60s |
| Declinazione: | +25°53'57.0" |
| Magnitudine apparente: | 6.65 |
| Distanza: | 78.989 parsec |
| Moto proprio RA: | -21.7 |
| Moto proprio Dec: | -17 |
| B-T magnitude: | 6.874 |
| V-T magnitude: | 6.638 |
Cataloghi e designazioni:
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