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Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Modeling Results
We have carried out an L'- and M-band adaptive optics (AO) extrasolarplanet imaging survey of 54 nearby, Sun-like stars using the Clio cameraat the MMT. Our survey concentrates more strongly than all others todate on very nearby F, G, and K stars, in that we have prioritizedproximity higher than youth. Our survey is also the first to includeextensive observations in the M band, which supplemented the primary L'observations. These longer-wavelength bands are most useful for verynearby systems in which low-temperature planets with red IR colors(i.e., H - L', H - M) could be detected. The survey detectedno planets, but set interesting limits on planets and brown dwarfs inthe star systems we investigated. We have interpreted our null result bymeans of extensive Monte Carlo simulations and constrained thedistributions of extrasolar planets in mass M and semimajor axis a. Ifplanets are distributed according to a power law with dN vprop M? a ? dMda, normalized to be consistentwith radial velocity (RV) statistics, we find that a distribution with? = -1.1 and ? = -0.46, truncated at 110 AU, isruled out at the 90% confidence level. These particular values of? and ? are significant because they represent the mostplanet-rich case consistent with current statistics from RVobservations. With 90% confidence no more than 8.1% of stars like thosein our survey have systems with three widely spaced, massive planetslike the A star HR 8799. Our observations show that giant planets inlong-period orbits around Sun-like stars are rare, confirming theresults of shorter-wavelength surveys and increasing the robustness ofthe conclusion.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Constraints on Long-period Planets from an L'- and M-band Survey of Nearby Sun-like Stars: Observations
We present the observational results of an L'- and M-band adaptiveoptics imaging survey of 54 nearby, Sun-like stars for extrasolarplanets, carried out using the Clio camera on the MMT. We haveconcentrated more strongly than all other planet-imaging surveys to dateon very nearby F, G, and K stars, prioritizing stellar proximity higherthan youth. Ours is also the first survey to include extensiveobservations in the M band, which supplement the primary L'observations. Models predict much better planet/star flux ratios at theL' and M bands than at more commonly used shorter wavelengths (i.e., theH band). We have carried out extensive blind simulations with fakeplanets inserted into the raw data to verify our sensitivity, and toestablish a definitive relationship between source significance in? and survey completeness. We find 97% confident-detectioncompleteness for 10? sources, but only 46% for 7?sources—raising concerns about the standard procedure of assuminghigh completeness at 5?, and demonstrating that blind sensitivitytests to establish the significance-completeness relation are animportant analysis step for all planet-imaging surveys. We discovered apreviously unknown ~0.15 M sun stellar companion to the F9star GJ 3876, at a projected separation of about 80 AU. Twelveadditional candidate faint companions are detected around other stars.Of these, 11 are confirmed to be background stars and one is apreviously known brown dwarf. We obtained sensitivity to planetary-massobjects around almost all of our target stars, with sensitivity toobjects below 3 M Jup in the best cases. Constraints onplanet populations based on this null result are presented in ourModeling Results paper.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.
| Spectroscopic properties of cool Ursa Major group members
Context: Until now, most members of the Ursa Major (UMa) group of starshave been identified by means of kinematic criteria. However, in manycases kinematic criteria alone are insufficient to ascertain, whether anindividual star is really a member of this group. Since photometriccriteria are ineffective in the case of cool dwarf members, one must usespectroscopic criteria. Nevertheless, resulting membership criteria areinconclusive. Aims: We reanalyse spectroscopic properties of coolUMa group dwarfs. In particular, we study the distribution of ironabundance, the strength of the Li I absorption at 6708 Å and theLi abundance, and the infilling of the core of the Hα line.Methods: Twenty-five cool and northern bona-fide members are carefullyselected from the literature. Homogeneously measured stellar parametersand iron abundances are given for all Sun-like stars selected, based onspectra of high resolution and high signal-to-noise ratio. In addition,we measure the Li equivalent width and abundance as well as the relativeintensity of the Hα core and the corresponding chromosphericflux. Results: The studied stars infer an average Ursa Major groupiron abundance of -0.03±0.05 dex, which is higher by about 0.06dex than determined elsewhere. The Li abundance derived of Ursa Majorgroup dwarf stars is higher than in the Hyades at effective temperaturescooler than the Sun, but lower than in the younger Pleiades, a resultwhich is independent of the exact value of the effective temperatureadopted. The Sun-like and cooler dwarfs also display chromosphericinfilling of the Hα core. We present spectroscopic criteria thatmay be used to exclude non-members.Based on observations with the Coudé-Échellespectrograph of the Alfred-Jensch-Teleskop at the ThüringerLandessternwarte Tautenburg.Based on observations collectedat the Centro Astronómico Hispano Alemán (CAHA) at CalarAlto,operated jointly by the Max-Planck-Institut für Astronomie and theInstituto de Astrofísica de Andalucía (CSIC).
| The main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filtersystem well defined stellar samples are needed. The nearby stars providea complete stellar sample representative for the thin disc population.We compare the filter transformations of different authors applied tothe main sequence stars from F to K dwarfs to SDSS filter system anddiscuss the properties of the main sequence. The location of the meanmain sequence in colour-magnitude diagrams is very sensitive tosystematic differences in the filter transformation. A comparison withfiducial sequences of star clusters observed in g', r', and i' show goodagreement. Theoretical isochrones from Padua and from Dartmouth havestill some problems, especially in the (r-i) colours.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Multicolour CCD measurements of nearby visual double stars. II
We present accurate CCD astrometric and photometric data for 31 nearbyvisual double stars in the standard filters BVRI. The observations werecollected with a 1.3-m telescope in 2001-2002 The results consist ofrelative astrometric positions (epoch, angular separation and positionangle) and differential BVRI photometry of the components. Mean errorsare: 0.01 arcsec for the separation; 0.06 ° for the position angle;and 0.015m for the photometric data. Comparing the relative positions atdifferent epochs, we evaluate the physical association of the systems.We additionally derive fractional masses and true separations for themost probable binary systems and, whenever orbits are available, alsototal and component masses.Table \ref{t2} is only available in the electronic form athttp:www.edpsciences.org. Tables \ref{t4} and \ref{t5} are onlyavailable 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/422/1023Based on data obtained at the Skinakas Observatory and by the Hipparcosastrometry satellite. The Skinakas Observatory is a collaborativeproject of the University of Crete, the Foundation for Research andTechnology - Hellas, and the Max-Planck-Institut fürExtraterrestrische Physik.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| 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.
| Lithium Abundances in Wide Binaries with Solar-Type Twin Components
We present high-resolution spectroscopic observations of the Li Iresonance line in a sample of 62 stars that belong to 31 common propermotion pairs with twin F- or G-type components. Photospheric abundancesof lithium were derived by spectral synthesis analysis. For seven of thepairs, we have measured large lithium abundance differences. Elevenother pairs have components with similar lithium abundances. We cannotdetermine if the remaining 13 pairs have lithium differences because wedid not detect the Li I lines, and hence we can only provide upperlimits to the abundances of both stars. Our results demonstrate thattwin stars do not always share the same lithium abundances. Lithiumdepletion in solar-type stars does not only depend on age, mass, andmetallicity. This result is consistent with the spread in lithiumabundances among solar-type stars in the solar-age open cluster M67. Ourstars are brighter than the M67 members of similar spectral type, makingthem good targets for detailed follow-up studies that could shed lighton the elusive mechanism responsible for the depletion of lithium duringthe main-sequence evolution of the Sun and solar-type stars.
| 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.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| 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 evolutionary status of activity-selected solar-type stars and of T Tauri stars as derived from HIPPARCOS parallaxes: evidence for long-lived T Tauri disks?
We have used the Hipparcos parallaxes to study the evolutionary statusof a sample of stars with spectral types from late F to M0 (hereafter``solar-type stars''), selected on the basis of their activity, mainlyfrom Einstein-based surveys. The parallaxes have been used to place theobjects in the H-R diagram, determining their age by comparison withtheoretical evolutionary tracks and observational main sequences. Thisage is compared with age estimates derived from the lithium abundance,the activity level and the presence of circumstellar disks. Tocomplement our sample at the young end we have also studied theHipparcos-determined distances of a sample of optically-selectedpre-main sequence stars, mostly classical T Tauri stars (CTTS). SomeCTTS appear to be much nearer to us than previously determined, and faraway from their putative parent cloud. This implies a significantlylarger age providing observational evidence for the existence oflong-lived T Tauri disks which could produce slow rotators on theZero-Age Main Sequence (ZAMS). None of the above-mentioned age proxiesappears to reliably and unambiguously select very young stars in therange of spectral types considered here, with some apparently very youngobjects effectively lying onto or very close to the main sequence. Theattribution of ages to young solar-type stars on the basis of any of thestandard proxies may thus significantly under- or over-estimate theevolutionary age of the object. Caution must therefore be exercised whenattributing ages to individual stars, and claims about the large numberof PMS stars found in X-ray based surveys may need to be at least inpart reconsidered in this light. Based on data from the ESA Hipparcossatellite.
| Initial Results of a Comprehensive Ultrasoft Survey of the Einstein IPC Database: Source List and Confirmation of the Selection Procedure
We have carried out a survey of the entire Einstein IPC database (atotal of 5934 sources) to select objects with strong components ofultrasoft X-ray emission (i.e., emission below 0.5 keV). The selectioncriteria are based on ratios of counts within three broad energy bands(i.e., X-ray ``colors''). A total of 516 objects have been selected,with 202 (39%) having been identified through correlations with otherdatabases. Of the currently identified objects, 45% are active galacticnuclei, 28% are stellar objects, 6% are ``normal'' galaxies and galaxyclusters, and the remaining 21% are ``nonstellar'' Galactic objects suchas cataclysmic variables, pulsars, and white dwarfs. We present adiscussion of the survey selection procedure, the list of selectedsources, and discussions of interesting objects from the aforementionedgroups. We discuss the remarkable success of our rather ``lowresolution'' method of estimating spectral shape and present plans forfollow-up studies of the as yet unidentified sources.
| HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population.
We present the parallaxes, measured by Hipparcos, for a sample of X-rayselected stars. The stars belong to the stellar sample of the EinsteinExtended Medium Sensitivity Survey. They are all at galactic latitude|b|>20deg, and are generally far away from known star formingregions. Several of these stars show lithium abundance and activitylevel typical of very young stars with ages comparable to that of thePleiades. We show that the majority of our sample stars are on the mainsequence, with only =~20% being giants. We do not find a significantpresence of pre-main sequence stars in our sample, notwithstanding thefact that some of our stars have a considerable lithium abundance,showing that the stars observed are most likely young and activemain-sequence objects.
| The evolutionary status of high lithium, high activity cool dwarfs
Not Available
| Dynamical studies of nine wide visual binaries in the solar neighborhood
Not Available
| Star Streams and Galactic Structure
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1595E&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.
| The Einstein Extended Medium-Sensitivity Survey Second Epoch: Results for the Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..701F&db_key=AST
| Statistical studies of visual double and multiple stars. II. A catalogue of nearby wide binary and multiple systems.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994RMxAA..28...43P&db_key=AST
| Radial velocities of the components of wide visual double stars.
Not Available
| Photometry of dwarf K and M stars
An observational program using UBVRI photometry is presented for 688stars from among the dwarf K and M stars already found spectroscopicallyby Vyssotsky (1958). Of these, 211 have not been observedphotometrically. These observations were obtained over a period ofseveral years at the Kitt Peak National Observatory using a GaAsphotomultiplier with an 0.9 m reflector. Based on night-to-nightvariations in the measures of individual stars, the internal errors maybe estimated to be roughly 0.01 mag for the colors and 0.015 for the Vmagnitudes. The photometric parallaxes reported for each star werecomputed in the manner discussed by Weis (1986).
| 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.
| Photographic measurements of visual binaries
The results of 164 photographic measurements of 50 large and, generally,neglected visual double stars are presented. The measurements have beenmade at Brera-Merate Observatory during the years 1984-88 with Zenastrograph.
| The frequency of low-mass companions to K and M stars in the solar neighbourhood
The measurements of radial velocities of 200 stars from the Gliesecatalog during 5 years with an accuracy of 0.5 km/s indicate the absenceof substellar mass companions with periods less than 3000 d. Theprobability of companion detection is determined by numerical modeling.New data on spectroscopic orbits of late-type dwarfs are used toestimate the distribution of companion masses by the maximum likelihoodmethod. The statistical properties of low-mass binaries are differentfrom those of more massive main-sequence and giant systems: thefrequency of spectroscopic binaries is less (10 +/- 2 percent) while atleast half of them have a mass ratio exceeding 0.5. Evidence is foundfor a nonmonotonic distribution of the masses of secondary componentswith a deficit in the 0.2-0.3 solar mass range.
| The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications
The optical identifications are presented of the Einstein ExtendedMedium-Sensitivity Survey (EMSS), including the methodology used tooptically identify the EMSS sources and the uncertainties involved withthat process. The optical properties of the classes of X-ray, optical,and radio data for each of the identified and, as yet, unidentifiedsources of the survey are described. A new class of X-ray emitters,cooling flow galaxies, is proposed. The criteria used to determinewhether the proposed optical counterpart to the X-ray source is aplausible identification are described. Plausibility is based on theoptical classification of the counterpart, e.g., AGN, cluster, G star,and the X-ray-to-optical flux ratios previously observed for theseclasses of X-ray emitters. Two independent schemes of opticalclassification of the counterparts are used to check the plausibility ofthese identifications; one is based on moderate-resolution opticalspectroscopy, and the other, on inferred X-ray luminosity and theoverall energy distribution.
| The chromospheric emission-age relation for stars of the lower main sequence and its implications for the star formation rate
An attempt is made to formulate the relationship between age andchromospheric emission (CE) in late-type dwarf stars. Evidence isreviewed that a deterministic relationship of this type actually exists,and that for stars of known age, either a power-law relation or a curvecorresponding to a constant star formation rate fits equally well.Further observations should be able to demonstrate either that there isa real excess of young stars near the sun or that the evolution of CEfor a low-mass star goes through a slow initial decline, a rapid declineat intermediate ages, and finally a slow decline for old stars like thesun.
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