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Ýlgili Makaleler
Unconstrained Astrometric Orbits for Hipparcos Stars with Stochastic Solutions
A considerable number of astrometric binaries whose positions on the skydo not obey the standard model of mean position, parallax, and linearproper motion were observed by the Hipparcos satellite. Some of themremain undiscovered, and their observational data have not been properlyprocessed with the more adequate astrometric model that includesnonlinear orbital motion. We develop an automated algorithm, based on``genetic optimization,'' to solve the orbital fitting problem in themost difficult setup, when no prior information about the orbitalelements is available (from, e.g., spectroscopic data or radial velocitymonitoring). We also offer a technique to accurately compute theprobability that an orbital fit is bogus, that is, that an orbitalsolution is obtained for a single star, and to estimate the probabilitydistributions for the fitting orbital parameters. We test this method onHipparcos stars with known orbital solutions in the catalog and furtherapply it to 1561 stars with stochastic solutions, which may beunresolved binaries. At a confidence level of 99%, orbital fits areobtained for 65 stars, most of which have not been known as binary. Itis found that reliable astrometric fits can be obtained even if theperiod is somewhat longer than the time span of the Hipparcos mission,that is, if the orbit is not closed. A few of the new probable binarieswith A-type primaries with periods 444-2015 days are chemically peculiarstars, including Ap and λ Bootis types. The anomalous spectra ofthese stars are explained by admixtures of light from the unresolved,sufficiently bright and massive companions. We estimate the apparentorbits of four stars that have been identified as members of the ~300Myr old Ursa Major kinematic group. Another four new nearby binaries mayinclude low-mass M-type or brown dwarf companions. Follow-upspectroscopic observations in conjunction with more accurate inclinationestimates will lead to better estimates of the secondary mass. Similarastrometric models and algorithms can be used for binary stars andplanet hosts observed by SIM and Gaia.
| High-Resolution Spectroscopy of Ursa Major Moving Group Stars
We use new and extant literature spectroscopy to address abundances andmembership for UMa moving group stars. We first compare the UMa, Coma,and Hyades H-R diagrams via a homogeneous set of isochrones and findthat these three aggregates are essentially coeval; this (near)coevality can explain the indistinguishable distributions of UMa andHyades dwarfs in the chromospheric emission versus color plane. Ourspectroscopy of cool UMa dwarfs reveals striking abundanceanomalies-trends with Teff, ionization state, and excitationpotential-like those recently seen in young, cool M34, Pleaides, andHyades dwarfs. In particular, the trend of rising λ7774-based O Iabundance with declining Teff is markedly subdued in UMacompared to the Pleiades, suggesting a dependence on age or metallicity.Recent photometric metallicity estimates for several UMa dwarfs aremarkedly low compared to the group's canonical metallicity, and similardeviants are seen among cool Hyads as well. Our spectroscopy does notconfirm these curious photometric estimates, which seem to be calledinto question for cool dwarfs. Despite disparate sources of Li data, ourhomogeneous analysis indicates that UMa members evince remarkably smallscatter in the Li-Teff plane for Teff>=5200 K.Significant star-to-star scatter suggested by previous studies is seenfor cooler stars. Comparison with the consistently determined HyadesLi-Teff trend reveals differences that are qualitativelyconsistent with this cluster's larger [Fe/H] (and perhaps slightlygreater age). However, quantitative comparison with standard stellarmodels indicates the differences are smaller than expected, suggestingthe action of a fourth parameter beyond age, mass, and [Fe/H]controlling Li depletion. The UMa-Coma cool star Li abundances may showa slight 0.2 dex difference; however, this may be mass-independent andthus more consistent with a modest initial Li abundance difference.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.Based on observations obtained at Kitt Peak National Observatory, adivision of the National Optical Astronomy Observatories, which isoperated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation.
| Spectral Types for Four OGLE-III Transit Candidates: Could These Be Planets?
We present spectral types for OGLE (Optical Gravitational LensingExperiment) transiting planet candidates OGLE-TR-134 through 137 basedon low-resolution spectra taken at Kitt Peak. Our main objective is toaid those planning radial velocity monitoring of transit candidates. Weobtain spectral types with an accuracy of 2 spectral subtypes, alongwith tentative luminosity classifications. Combining the spectral typeswith light-curve fits to the OGLE transit photometry, and with TwoMicron All Sky Survey counterparts in two cases, we conclude thatOGLE-TR-135 and 137 are not planetary transits, while OGLE-TR-134 and136 are good candidates and should be observed with precision radialvelocity monitoring to determine whether the companions are of planetarymass. OGLE-TR-135 is ruled out chiefly because a discrepancy between thestellar parameters obtained from the transit fit and those inferred fromthe spectra indicates that the system is a blend. OGLE-TR-137 is ruledout because the depth of the transit combined with the spectral type ofthe star indicates that the transiting object is stellar. OGLE-TR-134and 136, if unblended main-sequence stars, are each orbited by atransiting object with radius below 1.4 RJ. The caveats arethat our luminosity classification suggests that OGLE-TR-134 could be agiant (and therefore a blend), while OGLE-TR-136 shows a (much smaller)discrepancy of the same form as OGLE-TR-135, which may indicate that thesystem is a blend. However, since our luminosity classifications areuncertain at best, and the OGLE-TR-136 discrepancy can be explained ifthe primary is a slightly anomalous main-sequence star, the stars remaingood candidates.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| The Brown Dwarf Desert at 75-1200 AU
We present results of a comprehensive infrared coronagraphic search forsubstellar companions to nearby stars. The research consisted of (1) a178-star survey at Steward and Lick observatories, with opticalfollow-up from Keck Observatory, capable of detecting companions withmasses greater than 30 MJ, and semimajor axes between about140 to 1200 AU; (2) a 102-star survey using the Keck Telescope, capableof detecting extrasolar brown dwarfs and planets typically more massivethan 10 MJ, with semimajor axes between about 75 and 300 AU.Only one brown dwarf companion was detected, and no planets. Thefrequency of brown dwarf companions to G, K, and M stars orbitingbetween 75 and 300 AU is measured to be 1%+/-1%, the most precisemeasurement of this quantity to date. The frequency of massive (greaterthan 30 MJ) brown dwarf companions at 120-1200 AU is found tobe f=0.7%+/-0.7%. The frequency of giant planet companions with massesbetween 5 and 10 MJ orbiting between 75 and 300 AU ismeasured here for the first time to be no more than ~3%. Together withother surveys that encompass a wide range of orbital separations, theseresults imply that substellar objects with masses between 12 and 75MJ form only rarely as companions to stars. Theories of starformation that could explain these data are only now beginning toemerge.
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| Stellar atmospheres of nearby young solar analogs
High-resolution (/R~90,000) spectra of 34 nearby, young Sun-like starswere analyzed using stellar atmosphere models to estimate effectivephotosphere temperatures, surface gravities, and the abundance ofcertain heavy elements (C, Na, Mg, Si, S, Ca, Ti, Fe, and Ni). Theeffective temperatures derived from spectroscopy were compared withtemperatures estimated using optical and near-infrared photometry. Inmany cases the spectroscopic temperatures are significantly higher thanthe photometric estimates, possibly as a result of spottedness orchromospheric activity on these active stars. Values of effectivetemperature, surface gravity, and luminosity were compared totheoretical stellar evolution tracks and the evolutionary status ofthese objects was evaluated. The correlation between heavy elementabundance patterns and kinematics (space motion) was also examined. Twonearby stars that were tentatively assigned to the Hyades cluster basedon kinematics have Fe abundances that are also consistent withmembership in that cluster. Members of the Ursa Major kinematic groupexhibit a range of [Fe/H] values but have monotonic [Si/Fe]. These twoobservations suggest that heterogeneous incorporation of the heavyelements into protostars is creating the variation in metallicity. LocalAssociation members have a distinctly different Si/Fe that probablyreflects their distinct origin and chemical inheritance.
| 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
| The geometry of stellar motions in the nucleus region of the Ursa Major kinematic group
The structure of the central part of the UMa stellar kinematic group wasconsidered according to the data from Hipparcos, Tycho and ACTcatalogues. We have shown that in the space occupied by the nucleusthere are several populations of stars. The first includes ten starswhich are very compactly located in the proper motion diagram and have anormal proper motion distribution. The second group includes six starswith large kinematic difference and there are six field stars placedhere too. The question about their coexistence in a common region ofspace remains open. The well-known stars Mizar and Alcor appear tobelong to the different subsystems and do not belong to a unique stellarsystem. The published radial velocities, as a rule, are determined withinsufficient accuracy compared to the errors of the currently availableproper motions. A high precision in the radial velocities measurementsis necessary for a thorough study of the kinematics of stars inside UMastar system.
| Spectroscopy and Photometry of Nearby Young Solar Analogs
We present new photometry and spectroscopy of 34 stars from a catalog of38 nearby (d<25 pc) G and K dwarfs selected as analogs to the earlySun. We report that the least active star in our sample is also slowlyrotating and probably of solar age. Two other stars appear to be evolvedobjects that have recently acquired angular momentum. A fourth star maybe a spectroscopic binary. Many of the other stars belong to previouslyidentified common proper-motion groups. Space motions, lithiumabundances, and Ca II emission of these stars suggest ages between 70and 800 Myr.
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Nearby Young Solar Analogs. I. Catalog and Stellar Characteristics
We present a catalog of 38 young solar analogs within 25 pc, stars thatare uniquely well suited for observations of their circumstellarenvironments to improve our understanding of conditions within the solarsystem during the Hadean/early Archean eons (prior to 3.8 Gyr ago).These G and early K stars were selected from the Hipparcos astrometriccatalog based on lack of known stellar companions within 800 AU,bolometric luminosities close to that of the zero-age Sun and consistentwith the zero-age main sequence, and ROSAT X-ray luminositiescommensurate with the higher rotation rate and level of dynamo-drivenactivity in solar-mass stars less than 0.8 Gyr old. While many of theseobjects have been previously identified, this sample is novel in tworespects: The selection criteria specifically consider the planetaryenvironment, and the selection is uniform and all-sky. The X-rayemission from these young analogs is spectrally soft and consistent witha coronal origin. Calcium H and K emission, rotation periods, lithiumabundances, and kinematics support an age range of 0.2-0.8 Gyr for mostof these stars. Three stars have exceptionally high space motions withrespect to the local standard of rest and may be old disk or halo starsthat are anomalously X-ray luminous.
| Beryllium abundances in parent stars of extrasolar planets: 16 Cygni A & B and rho (1) CANCRI
The (9) Be ii lambda 3131 Angstroms doublet has been observed in thesolar-type stars 16 Cyg A & B and in the late G-type star rho (1)Cnc, to derive their beryllium abundances. 16 Cyg A & B show similar(solar) beryllium abundances while 16 Cyg B, which has been proposed tohave a planetary companion of ~ 2 M_Jup, is known to be depleted inlithium by a factor larger than 6 with respect to 16 Cyg A. Differencesin their rotational histories which could induce different rates ofinternal mixing of material, and the ingestion of a similar planet by 16Cyg A are discussed as potential explanations. The existence of twoother solar-type stars which are candidates to harbour planetary-masscompanions and which show lithium and beryllium abundances close tothose of 16 Cyg A, requires a more detailed inspection of thepeculiarities of the 16 Cyg system. For rho (1) Cnc, which is thecoolest known object candidate to harbour a planetary-mass companion (M> 0.85 M_Jup), we establish a precise upper limit for its berylliumabundance, showing a strong Be depletion which constrains the availablemixing mechanisms. Observations of similar stars without companions arerequired to assess the potential effects of the planetary companion onthe observed depletion. It has been recently claimed that rho (1) Cncappears to be a subgiant. If this were the case, the observed strong Liand Be depletions could be explained by a dilution process taking placeduring its post-main sequence evolution. Based on observations made withthe Nordic Optical and William Herschel Telescopes, which are operatedon the island of La Palma by the NOT Scientific Association and theIsaac Newton Group, respectively, in the Spanish Observatorio del Roquede los Muchachos of the Instituto de Astrof\'\i sica de Canarias.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| Are Both Stars in a Classic T Tauri Binary Classic T Tauri Stars?
Large near-IR color excesses and emission-line spectra indicate thepresence of optically thick circumstellar accretion disks and activeboundary layers in young stars. We investigate whether this classic TTauri (TT) behavior is found in one or both members of a binary systempreviously identified as a TT on the basis of the unresolved light. Weconsider the angularly resolved Br gamma and, in one case, Na I, spectraof four close (1."3--2."6) TT systems. We also take into account theangularly resolved near-IR (K--L) colors of eight additional youngbinaries with separations between 0."3 and 2."5. We find that for all 12systems, both components show, or have shown in the history of theirobservation, evidence for TT behavior. We demonstrate that this cannotbe the result of random pairing of a population of single TT andweak-lined T Tauri (WT) stars. We speculate that the result that inner,AU-sized disks tend to survive for a similar length of time in bothcomponents of a close (0."3--2."6) binary suggests that a circumbinaryenvelope effectively regulates the common evolution of the inner disks.
| An Atlas of the infrared spectral region. II. The late-type stars (G - M)
This Atlas illustrates the behavior of late type stars (F, G, K and M)in the near infrared 8400-8800 Angstrom region with a resolution ofabout 2 degrees. Seventeen figures illustrate the spectral sequence andluminosity classes V, III, Ib and Ia. Four figures illustrate peculiarspectra, namely those of Am stars, composites, weak metal stars and Sand C type objects. The complete Atlas is also available as FITS filesfrom the CDS de Strasbourg and other data centers.
| The catalogue of nearby stars metallicities.
Not Available
| 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.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Beryllium abundances in late-type Hyades dwarf stars.
We have observed, with high spectral resolution, the BeII resonancedoublet at λ 3131A in five G and K-type main-sequence starsbelonging to the Hyades open cluster, as well as in four K-type probablemembers of the Ursa Major Group. We carried out a detailed abundanceanalysis via spectral synthesis fitting and studied the utility of theBeII lines for deriving beryllium abundances in late-type dwarf stars.Reliable abundances were derived for three Hyades stars with effectivetemperatures in the range 5700-5200K. For stars cooler than ~5200K theobserved feature in the position of the weaker BeII line is stronglyaffected by a line of other element which we tentatively identify as Mniλ 3131.037A. Due to this fact, we could establish only upperlimits to the abundances in these very cool stars. Lithium and berylliumabundances in Hyades stars with effective temperatures in the range6000-5200K show a complete different behaviour. While lithium is rapidlydepleted towards decreasing temperatures, reaching a relative differenceof ~3 orders of magnitude, beryllium remains fairly constant for thesame stars, with an abundance similar to that exhibited by hotter starsof the cluster and close to the solar value. Standard evolutionarymodels cannot account for the present Li and Be observations in Hyadeslate-type stars. Rotational mixing associated with angular momentum losscan explain the observed abundances if the initial angular momenta ofthe stars were progressively larger for decreasing stellar mass. On theother hand, mixing induced by internal gravity waves satisfies theobservational constraints.
| Far-infrared observations of chromospherically active stars in stellar clusters.
Using the IRAS database we have searched for far-infrared emission froma large number of chromospherically active dwarf stars (including flarestars) in stellar clusters of different ages and distances. Only starsin the nearest clusters (<50pc) were detected by IRAS. Their 12μmluminosity (L_12_) is in the range 0.2-5x10^31^erg/s. We find that thedetected emission is photospheric in origin and that there is nocorrelation of flux either with age or with stellar-activity levels. Thecluster stars' far-infrared radiation characteristics are very similarto those of solar-vicinity field stars of similar spectral type andchromospheric activity.
| Spectral classifications in the near infrared of stars with composite spectra. I. The study of MK standards.
Up to now the spectral classifications of the cool components ofcomposite spectra obtained in the 3800-4800A wavelength region have beenvery disparate. These disparities are due to the fact that the spectraof the evolved cool component are strongly veiled by that of the hotterdwarf component, which makes a classification very difficult. We proposeto study these systems in the near infrared (8380-8780A). In thisspectral domain the magnitude difference between the spectra of thecomponents is in general sufficiently large so that one observespractically only the spectrum of the cool component. In this first paperwe provide, for a sample of MK standards, the relations between theequivalent width (Wlambda_ ) of certain lines and thespectral classifications. For the cool G, K and M type stars, the linesconsidered are those of the calcium triplet (Ca II 8498, 8542 and 8662),of iron (Fe I 8621 and 8688), of titanium (Ti I 8426 and 8435) and ofthe blend λ8468. The use of certain line intensity ratiospermits, after eliminating partially the luminosity effects, a firstapproach to the spectral type. For the hotter stars of types O, B, A andF we study the behavior of the hydrogen lines (P12 and P14), the calciumlines (Ca II 8498 and 8542) as well as those of the oxygen (O I 8446).The latter line presents a very characteristic profile for stars of lowrotation and therefore in Am stars, which are frequently found among thecomposite spectrum binaries. Among the cooler stars of our sample, only6% present real anomalies with respect to the MK classifications. Thisresult is very encouraging for undertaking the classification of asample of composite spectra. The spectra were taken at the Observatoirede Haute-Provence (OHP) with the CARELEC spectrograph at the 193 cmtelescope, with a dispersion of 33 A/mm.
| Submillimeter studies of main-sequence stars
JCMT maps of the 800-micron emission from Vega, Fomalhaut, and BetaPictoris are interpreted to indicate that they are not ringed by largereservoirs of distant orbiting dust particles that are too cold to havebeen detected by IRAS. A search for 800-micron emission from stars inthe Pleiades and Ursa Majoris open clusters is reported. In comparisonwith the mass of dust particles near T Tauri and Herbig Ae stars, theJCMT data indicate a decline in dust mass during the initial 3 x 10 exp8 yr that a star spends on the main sequence that is at least as rapidas (time) exp -2. It is estimated that in the Kuiper belt the ratio oftotal mass carried by small particles to that carried by comets isorders of magnitude smaller than this ratio is 1 AU from the sun. If800-micron opacities calculated by Pollack et al. (1993) are correct,then the particles with radii less than 100 microns that dominate theFIR fluxes measured by IRAS cannot entirely account for the measured800-micron fluxes at Vega, Beta Pic, and Fomalhaut; larger particlesmust be present as well.
| Rotation and activity among solar-type stars of the Ursa Major Group
We examine rotation and chromospheric activity among G and K dwarfsrecently shown to be members of the Ursa Major Group (UMaG). Rotationperiods for UMaG stars are smaller than for stars of the same colors inthe Hyades, and by an amount corresponding to the Skumanich relation.Most UMaG stars have about the same level of Ca II and K emission,implying that they also have nearly uniform intrinsic rotation rates.That means that the diversity of rotation rates and levels of activityseen among solar-type stars in the Alpha Persei and Pleiades clustershas largely converged by the age of UMaG (0.3 Gyr).
| The evolution of the lithium abundances of solar-type stars. II - The Ursa Major Group
We draw upon a recent study of the membership of the Ursa Major Group(UMaG) to examine lithium among 0.3 Gyr old solar-type stars. For most Gand K dwarfs, Li confirms the conclusions about membership in UMaGreached on the basis of kinematics and chromospheric activity. G and Kdwarfs in UMaG have less Li than comparable stars in the Pleiades. Thisindicates that G and K dwarfs undergo Li depletion while they are on themain sequence, in addition to any pre-main-sequence depletion they mayhave experienced. Moreover, the Li abundances of the Pleiades K dwarfscannot be attributed to main-sequence depletion alone, demonstratingthat pre-main-sequence depletion of Li also takes place. The sun's Liabundance implies that the main-sequence mechanism becomes lesseffective with age. The hottest stars in UMaG have Li abundances likethose of hot stars in the Pleiades and Hyades and in T Tauris, and thetwo genuine UMaG members with temperatures near Boesgaard's Li chasmhave Li abundances consistent with that chasm developing fully by 0.3Gyr for stars with UMaG's metallicity. We see differences in theabundance of Li between UMaG members of the same spectral types,indicating that a real spread in the lithium abundance exists withinthis group.
| Stellar kinematic groups. I - The Ursa Major group
The Ursa Major Group (UMaG) is studied as a test case for theauthenticity of Stellar Kinematic Groups, using Coravel radialvelocities, recent compilations of astrometric data, and newspectroscopic observations. Spectroscopic age indicators, particularlyindices of the strength of chromospheric emission, are applied tosolar-type candidate members of UMaG, and it is shown that stars thatmeet the spectroscopic criteria also have kinematics that agree betterwith the space motions of the nucleus of UMaG than does the startingsample as a whole. The primary limitation on the precision of kinematicsis now parallaxes instead of radial velocities. These more restrictivekinematic criteria are then applied to other UMaG candidates and a listsummarizing membership is presented. UMaG is also examined as a cluster,confirming its traditional age of 0.3 Gyr, and a mean Fe/H of -0.08 +/-0.09 for those stars most likely to be bona fide members.
| The importance of surface inhomogeneities for K and M dwarf chromospheric fluxes
We present published and archived spectroscopic and spectrophotometricdata of H-alpha, Ca II, Mg II, and X-rays for a large sample of K and Mdwarfs. The data set points to the importance that surfaceinhomogeneities have in the flux luminosity diagrams in these late-typedwarfs, irrespective of whether the Balmer lines are in emission orabsorption. Although supporting the fact that cooler stars exhibitincreasing levels of surface activity, evident through an increasingincidence of Balmer emission, surface inhomogeneities, or variations inthe local temperature and density structure, at the chromospheric level,dominate the total Ca II and Mg II fluxes. We show that the flux-fluxand luminosity-luminosity relations indicate differing extents ofinhomogeneity from the chromosphere through to the corona. A goodcorrelation between Ca II and Mg II fluxes indicates that they areformed in overlapping regions of the chromosphere, so that thecontribution of surface inhomogeneities is not evident from thisparticular flux-flux diagram. In the region of the upper chromospherethrough to the transition and corona, the correlation between Ly-alphaand X-ray fluxes indicates regions with similar levels of arealinhomogeneity. This appears to be uncorrelated with that at thechromospheric level.
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| Takýmyýldýz: | Buyuk Ayi |
| Sað Açýklýk: | 12h31m18.92s |
| Yükselim: | +55°07'07.8" |
| Görünürdeki Parlaklýk: | 8.113 |
| Uzaklýk: | 23.736 parsek |
| özdevim Sað Açýklýk: | 107.8 |
| özdevim Yükselim: | -0.5 |
| B-T magnitude: | 9.293 |
| V-T magnitude: | 8.211 |
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