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The 78th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1706 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| 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.
| 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.
| K dwarfs and the chemical evolution of the solar cylinder
K dwarfs have lifetimes older than the present age of the Galactic disc,and are thus ideal stars for investigating the chemical evolution of thedisc. We have developed several photometric metallicity indicators for Kdwarfs, based on a sample of accurate spectroscopic metallicities for 34disc and halo G and K dwarfs. The photometric metallicities lead us todevelop a metallicity index for K dwarfs based only on their position inthe colour-absolute-magnitude diagram. Metallicities have beendetermined for 431 single K dwarfs drawn from the Hipparcos catalogue,selecting the stars by absolute magnitude and removing multiple systems.The sample is essentially a complete reckoning of the metal content innearby K dwarfs. We use stellar isochrones to mark the stars by mass,and select a subset of 220 of the stars, which is complete within anarrow mass interval. We fit the data with a model of the chemicalevolution of the solar cylinder. We find that only a modest cosmicscatter is required to fit our age-metallicity relation. The modelassumes two main infall episodes for the formation of the halo-thickdisc and thin disc, respectively. The new data confirm that the solarneighbourhood formed on a long time-scale of the order of 7 Gyr.
| Determination of accurate stellar radial-velocity measures
Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence
| 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
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solarneighbourhood. This distribution is centred on solar metallicity. Weshow that previous metallicity distributions, selected on the basis ofspectral type, are biased against stars with solar metallicity orhigher. A selection of G-dwarf stars is inherently biased againstmetal-rich stars and is not representative of the solar neighbourhoodmetallicity distribution. Using a sample selected on colour, we obtain adistribution where approximately half the stars in the solarneighbourhood have metallicities higher than [Fe/H]=0. The percentage ofmid-metal-poor stars ([Fe/H]<-0.5) is approximately 4 per cent, inagreement with present estimates of the thick disc. In order to have ametallicity distribution comparable to chemical evolution modelpredictions, we convert the star fraction to mass fraction, and showthat another bias against metal-rich stars affects dwarf metallicitydistributions, due to the colour (or spectral type) limits of thesamples. Reconsidering the corrections resulting from the increasingthickness of the stellar disc with age, we show that the simpleclosed-box model with no instantaneous recycling approximation gives areasonable fit to the observed distribution. Comparisons with theage-metallicity relation and abundance ratios suggest that the simpleclosed-box model may be a viable model of the chemical evolution of theGalaxy at solar radius.
| Multi-colour light variation of AGB stars observed with ISO
New visual light curves and infrared multi-epoch photometry arepresented for a sample of AGB-stars spectroscopically observed with ISO.While the ISO work is or will be presented elsewhere, the aim of thispaper is to give an overview of the properties of the light change ofthe objects. This information is crucial for the interpretation of thevaluable ISO material. Using the University of Vienna Twin AutomaticPhotoelectric Telescope (APT) we monitored the stars of our sample inthe photometric bands V and I_C. We present the light curve of eachobject and derive the parameters of the current light change, such asperiod(s) and amplitude. Furthermore we give V-I_C colours and colourvariations for these objects. Our results allow us to derive somegeneral results on semiregular and irregular variables. Only in lessthan 50% of the cases could we confirm the GCVS period. Moreover, we didnot find any pronounced difference between SRb and Lb variables in theregularity of the light curve. The existence of rapid oscillationsindicated by Hipparcos data could not be confirmed. In addition to thevisual light changes we present new near infrared photometry data.Although typically only few data points are available, they can beviewed relative to the better-monitored visual light curves providinginformation on possible phase shifts and differences in amplitude indifferent parts of the spectrum. Furthermore, multi-epoch photometryallows us to derive mean colours for these objects. Based onobservations made with the Carlos Sánchez Telescope operated onthe island of Tenerife by the Instituto de Astrofísica deCanarias (IAC) in the Observatorio del Teide, Izaña. Tables B2 toB5 are only available 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/qcat?J/A+A/375/527
| 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 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.
| 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
| Detection of moving clusters by a method of cinematic pairs.
Not Available
| 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
| The Sirius Supercluster and Missing Mass near the Sun
The Hipparcos results confirm some 50 members of the Sirius superclusterin the Bright Star Catalogue. The resulting, well-definedcolor-luminosity array indicates an age of 4 x 10^8 yr from conventional(no overshoot) models. A comparison of the luminosity function obtainedfrom members in the Bright Star Catalogue, unbiased as to proper motion,and in the Catalogue of Nearby Stars (~27 pc), which are strongly biasedtoward large proper motions, reveals that either most of the smallproper-motion stars near the Sun remain to be identified or theluminosity function of the supercluster is drastically different fromthat of the field stars. A search for low-mass members in a 6 deg x 6deg field in the center of the Ursa Major cluster yielded 10 possiblemembers that lie on the white dwarf sequence, 5 mag below the mainsequence. Luyten (field LP 131) found 368 stars in this field withproper motion exceeding 0.08", and only 19 of these have a positionangle of their proper motion between 0 deg and 98 deg (i.e., thequadrant containing UMa cluster members). The assumption of clustermembership yields a parallax very close to that obtained for brightmembers. Aside from white dwarfs, the possibilities are discussed thatthese stars are (1) brown dwarfs, (2) members of a cluster in the lineof sight to UMa but 100 pc more distant, or (3) reflecting a chain ofvery unlikely coincidences.
| 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 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.
| 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.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| 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.
| 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.
| 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).
| 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.
| Magnetic structure in cool stars. XVII - Minimum radiative losses from the outer atmosphere
The emissions in several chromospheric and transition region lines andin coronal soft X-rays are analyzed for a sample of cool stars. Thenature of the lower-limit flux densities is explored, and evidence isgiven for the possibility of a basal, nonmagnetic heating mechanismbeing responsible for these emission fluxes up to, and perhapsincluding, the upper transition region. It is argued that the excessflux density, derived by subtraction of the basal flux density from theobserved stellar flux, is the proper measure of magnetic activity. Thelevel of the basal flux density as a function of color is determined tobe 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sqcm/s for K-type stars.
| A search for remnant molecular disks around young nearby stars
This study presents sensitive millimeter wavelength observations ofeight Ursa Major stream stars, designed to detect the presence ofmolecular material resident in persistent outer remnant disks. Noemission arising from gaseous remnants of circumstellar disks was found.The candidates range in spectral type from K3 to A7, covering a massrange from 0.7 to 2 solar masses. Disks larger than about 350 AU couldnot have escaped detection if they were optically thick at the 115 GHzline of (C-12)O, implying a limiting CO mass of less than 6 x 10 exp 24g for a distributed component of this size. If the CO/H2 ratio iscomparable to that observed in the interstellar medium, then the totalouter disk masses must be less than 3 x 10 exp -5.
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| Сазвежђа: | Велики медвед |
| Ректацензија: | 12h41m44.52s |
| Deклинација: | +55°43'28.8" |
| Apparent магнитуда: | 8.271 |
| Даљина: | 23.223 parsecs |
| Proper motion RA: | 122.5 |
| Proper motion Dec: | -5.6 |
| B-T magnitude: | 9.509 |
| V-T magnitude: | 8.374 |
Каталог и designations:
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