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Bayesian Analysis to Identify New Star Candidates in Nearby Young Stellar Kinematic Groups
We present a new method based on a Bayesian analysis to identify newmembers of nearby young kinematic groups. The analysis minimally takesinto account the position, proper motion, magnitude, and color of astar, but other observables can be readily added (e.g., radial velocity,distance). We use this method to find new young low-mass stars in theβ Pictoris and AB Doradus moving groups and in the TW Hydrae,Tucana-Horologium, Columba, Carina, and Argus associations. Startingfrom a sample of 758 mid-K to mid-M (K5V-M5V) stars showing youthindicators such as Hα and X-ray emission, our analysis yields 214new highly probable low-mass members of the kinematic groups analyzed.One is in TW Hydrae, 37 in β Pictoris, 17 in Tucana-Horologium, 20in Columba, 6 in Carina, 50 in Argus, 32 in AB Doradus, and theremaining 51 candidates are likely young but have an ambiguousmembership to more than one association. The false alarm rate for newcandidates is estimated to be 5% for β Pictoris and TW Hydrae, 10%for Tucana-Horologium, Columba, Carina, and Argus, and 14% for ABDoradus. Our analysis confirms the membership of 58 stars proposed inthe literature. Firm membership confirmation of our new candidates willrequire measurement of their radial velocity (predicted by ouranalysis), parallax, and lithium 6708 Å equivalent width. We haveinitiated these follow-up observations for a number of candidates, andwe have identified two stars (2MASSJ01112542+1526214,2MASSJ05241914-1601153) as very strong candidate members of the βPictoris moving group and one strong candidate member(2MASSJ05332558-5117131) of the Tucana-Horologium association; thesethree stars have radial velocity measurements confirming theirmembership and lithium detections consistent with young age.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientique of France, and the University of Hawaii.
| Identifying the Young Low-mass Stars within 25 pc. II. Distances, Kinematics, and Group Membership
We have conducted a kinematic study of 165 young M dwarfs with ages oflsim300 Myr. Our sample is composed of stars and brown dwarfs withspectral types ranging from K7 to L0, detected by ROSAT and withphotometric distances of lsim25 pc assuming that the stars are singleand on the main sequence. In order to find stars kinematically linked toknown young moving groups (YMGs), we measured radial velocities for thecomplete sample with Keck and CFHT optical spectroscopy andtrigonometric parallaxes for 75 of the M dwarfs with the CAPSCaminstrument on the du Pont 2.5 m Telescope. Due to their youthfuloverluminosity and unresolved binarity, the original photometricdistances for our sample underestimated the distances by 70% on average,excluding two extremely young (lsim3 Myr) objects found to havedistances beyond a few hundred parsecs. We searched for kinematicmatches to 14 reported YMGs and identified 10 new members of the AB DorYMG and 2 of the Ursa Majoris group. Additional possible candidatesinclude six Castor, four Ursa Majoris, two AB Dor members, and onemember each of the Her-Lyr and ? Pic groups. Our sample alsocontains 27 young low-mass stars and 4 brown dwarfs with ages lsim150Myr that are not associated with any known YMG. We identified anadditional 15 stars that are kinematic matches to one of the YMGs, butthe ages from spectroscopic diagnostics and/or the positions on the skydo not match. These warn against grouping stars together based only onkinematics and that a confluence of evidence is required to claim that agroup of stars originated from the same star-forming event.Based on observations collected at the W. M. Keck Observatory, theCanada-France-Hawaii Telescope, the du Pont Telescope at Las CampanasObservatory, and the Subaru Telescope. The Keck Observatory is operatedas a scientific partnership between the California Institute ofTechnology, the University of California, and NASA, and was madepossible by the generous financial support of the W. M. Keck Foundation.The CFHT is operated by the National Research Council of Canada, theCentre National de la Recherche Scientifique of France, and theUniversity of Hawaii.
| Cool Young Stars in the Northern Hemisphere: ? Pictoris and AB Doradus Moving Group Candidates
As part of our continuing effort to identify new, low-mass members ofnearby, young moving groups (NYMGs), we present a list of young,low-mass candidates in the northern hemisphere. We used our provenproper-motion selection procedure and ROSAT X-ray and GALEX-UV activityindicators to identify 204 young stars as candidate members of the? Pictoris and AB Doradus NYMGs. Definitive membership assignmentof a given candidate will require a measurement of its radial velocityand distance. We present a simple system of indices to characterize theyoung candidates and help prioritize follow-up observations. New groupmembers identified in this candidate list will be high priority targetsfor (1) exoplanet direct imaging searches, (2) the study of post-T-Tauriastrophysics, (3) understanding recent local star formation, and (4) thestudy of local galactic kinematics. Information available now allows usto identify eight likely new members in the list. Two of these, a late-Kand an early-M dwarf, we find to be likely members of the ? Picgroup. The other six stars are likely members of the AB Dor movinggroup. These include an M dwarf triple system, and three very coolobjects that may be young brown dwarfs, making them the lowest-mass,isolated objects proposed in the AB Dor moving group to date.
| Potential Members of Stellar Kinematic Groups within 30 pc of the Sun
We analyze the kinematic histories of stars within 30 pc of the Sun, forwhich three-dimensional spatial coordinates and three-dimensionalvelocity vectors are available. From this sample, we extract members ofstellar kinematic groups (SKGs) in the following manner. First, weconsider in the three-dimensional velocity space centered on the localstandard of rest, a sphere with a radius of 8 km s-1centered on the mean velocity vector of a particular SKG. Around eachSKG velocity center, we have found a significant excess of starscompared to background field stars. For each candidate, in thethree-dimensional spatial coordinate space, its trajectory is tracedback in time by the age of the relevant SKG to obtain the estimateddistance from the SKG center at the time of the SKG's birth by theepicyclic approximation and harmonic vertical motion. It often happensthat a star is a candidate member of multiple SKGs. Then we rank thecandidacy to multiple SKGs based on the smallness of distanceseparations. In this manner, we have kinematically selected 238candidates. We further impose at least one of the following qualitativecriteria for being a member: spectral type A or B, variability, or EUVand X-ray emission. We have finally selected 137 candidate members ofSKGs out of a sample of 966 stars.
| Optimizing exoplanet transit searches around low-mass stars with inclination constraints
Aims: We investigate a method to increase the efficiency of atargeted exoplanet search with the transit technique by preselecting asubset of candidates from large catalogs of stars. Assuming spin-orbitalignment, this can be achieved by considering stars that have a higherprobability to be oriented nearly equator-on (inclination close to90°). Methods: We used activity-rotation velocity relationsfor low-mass stars with a convective envelope to study the dependence ofthe position in the activity-vsini diagram on the stellar axisinclination. We composed a catalog of G-, K-, M-type main-sequencesimulated stars using isochrones, an isotropic inclination distributionand empirical relations to obtain their rotation periods and activityindexes. Then the activity-vsini diagram was completed and statisticswere applied to trace the areas containing the higher ratio of starswith inclinations above 80°. A similar statistics was applied tostars from real catalogs with log(R'HK) and vsini data tofind their probability of being oriented equator-on. Results: Wepresent our method to generate the simulated star catalog and thesubsequent statistics to find the highly inclined stars from realcatalogs using the activity-vsini diagram. Several catalogs from theliterature are analyzed and a subsample of stars with the highestprobability of being equator-on is presented. Conclusions:Assuming spin-orbit alignment, the efficiency of an exoplanet transitsearch in the resulting subsample of probably highly inclined stars isestimated to be two to three times higher than with a general searchwithout preselection.Table 4 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/537/A147
| Effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-fluxrelationships using a sample of 298 late-type dwarf active stars withspectral types F to M. The chromospheric lines were observedsimultaneously in each star to avoid spread as a result of long-termvariability. Unlike other works, we subtract the basal chromosphericcontribution in all the spectral lines studied. For the first time, wequantify the departure of dMe stars from the general relations. We showthat dK and dKe stars also deviate from the general trend. Studying theflux-colour diagrams, we demonstrate that the stars deviating from thegeneral relations are those with saturated X-ray emission and we showthat these stars also present saturation in the H? line. Usingseveral age spectral indicators, we show that these are younger starsthan those following the general relationships. The non-universality offlux-flux relationships found in this work should be taken into accountwhen converting between fluxes in different chromospheric activityindicators.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| A high-resolution spectroscopic survey of late-type stars: chromospheric activity, rotation, kinematics, and age
Aims: We present a compilation of spectroscopic data from asurvey of 144 chromospherically active young stars in the solarneighborhood, which may be used to investigate different aspects of itsformation and evolution in terms of kinematics and stellar formationhistory. The data have already been used by us in several studies. Withthis paper, we make all these data accessible to the scientificcommunity for future studies on different topics. Methods: Weperformed spectroscopic observations with echelle spectrographs to coverthe entirety of the optical spectral range simultaneously. Standard datareduction was performed with the IRAF echelle package. We applied thespectral subtraction technique to reveal chromospheric emission in thestars of the sample. The equivalent width of chromospheric emissionlines was measured in the subtracted spectra and then converted tofluxes using equivalent width-flux relationships. Radial and rotationalvelocities were determined by the cross-correlation technique.Kinematics, equivalent widths of the lithium line ?6707.8 Åand spectral types were also determined. Results: A catalog ofspectroscopic data is compiled: radial and rotational velocities, spacemotion, equivalent widths of optical chromospheric activity indicatorsfrom Ca II H & K to the calcium infrared triplet and the lithiumline in ?6708 Å. Fluxes in the chromospheric emission linesand R'_HK are also determined for each observation of a star in thesample. We used these data to investigate the emission levels of ourstars. The study of the H? emission line revealed two differentpopulations of chromospheric emitters in the sample, clearly separatedin the logFH?/Fbol - (V-J) diagram. Thedichotomy may be associated with the age of the stars.Based on observations made with the 2.2 m telescope of theGerman-Spanish Astronomical Centre, Calar Alto (Almería, Spain),operated jointly by the Max-Planck-Institute for Astronomy, Heidelberg,and the Spanish National Commission for Astronomy; the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway and Sweden, in the Spanish Observatorio delRoque de Los Muchachos of the Instituto de Astrofísica deCanarias; the Isaac Newton Telescope (INT) operated on the island of LaPalma by the Isaac Newton Group in the Spanish Observatorio del Roque deLos Muchachos of the Instituto de Astrofísica de Canarias; withthe Italian Telescopio Nazionale Galileo (TNG) operated on the island ofLa Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionaledi Astrofisica) at the Spanish Observatorio del Roque de Los Muchachosof the Instituto de Astrofísica de Canarias; and with theHobby-Eberly Telescope (HET) operated by McDonald Observatory on behalfof The University of Texas at Austin, the Pennsylvania State University,Stanford University, Ludwig-Maximilians-Universität München,and Georg-August-Universität Göttingen. This research has madeuse of the SIMBAD database and VizieR catalog access tool, operated atCDS, Strasbourg, France.Tables A.1-A.4 and reduced spectra are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A97
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| BD -22 5866: A Low-Mass, Quadruple-lined Spectroscopic and Eclipsing Binary
We report our discovery of an extremely rare, low-mass, quadruple-linedspectroscopic binary BD -22 5866 (=NLTT 53279, integrated spectral type= M0 V), found during an ongoing search for the youngest M dwarfs in thesolar neighborhood. From the cross-correlation function, we are able tomeasure relative flux levels, estimate the spectral types of thecomponents, and set upper limits on the orbital periods and separations.The resulting system is hierarchical, composed of a K7 + K7 binary andan M1 + M2 binary with semimajor axes ofaAsiniA<=0.06 andaBsiniB<=0.30 AU. A subsequent search of theSuperWASP photometric database revealed that the K7 + K7 binary iseclipsing with a period of 2.21 days and at an inclination angle of85°. Within uncertainties of 5%, the masses and radii of bothcomponents appear to be equal (0.59 Msolar, 0.61Rsolar). These two tightly orbiting stars (a=0.035 AU) are insynchronous rotation, causing the observed excess Ca II, Hα,X-ray, and UV emission. The fact that the system was unresolved withpublished adaptive optics imaging, limits the projected physicalseparation of the two binaries at the time of the observation todAB<~4.1 AU at the photometric distance of 51 pc. Themaximum observed radial velocity difference between the A and B binarieslimits the orbit to aABsiniAB<=6.1 AU. As thistight configuration is difficult to reproduce with current formationmodels of multiple systems, we speculate that an early dynamical processreduced the size of the system, such as the interaction of the twobinaries with a circumquadruple disk. Intensive photometric,spectroscopic, and interferometric monitoring, as well as a parallaxmeasurement of this rare quadruple system, is certainly warranted.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope (CFHT). The Keck Observatory is operatedas a scientific partnership between the California Institute ofTechnology, the University of California, and NASA, and was madepossible by the generous financial support of the W. M. Keck Foundation.The CFHT is operated by the National Research Council of Canada, theCentre National de la Recherche Scientifique of France, and theUniversity of Hawaii.
| The effect of activity on stellar temperatures and radii
Context: Recent analyses of low-mass eclipsing binary stars haveunveiled a significant disagreement between the observations andpredictions of stellar structure models. Results show that theoreticalmodels underestimate the radii and overestimate the effectivetemperatures of low-mass stars but yield luminosities that accord withobservations. A hypothesis based upon the effects of stellar activitywas put forward to explain the discrepancies. Aims: In this paper westudy the existence of the same trend in single active stars and providea consistent scenario to explain systematic differences between activeand inactive stars in the H-R diagram reported earlier. Methods: Theanalysis is done using single field stars of spectral types late-K and Mand computing their bolometric magnitudes and temperatures throughinfrared colours and spectral indices. The properties of the stars insamples of active and inactive stars are compared statistically toreveal systematic differences. Results: After accounting for a numberof possible bias effects, active stars are shown to be cooler thaninactive stars of similar luminosity therefore implying a larger radiusas well, in proportions that are in excellent agreement with those foundfrom eclipsing binaries. Conclusions: The present results generalisethe existence of strong radius and temperature dependences on stellaractivity to the entire population of low-mass stars, regardless of theirmembership in close binary systems.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/478/507
| Spotting in stars with a low level of activity, close to solar activity
Data on the variability of the continuum optical emission are used forthe first time to estimate the degree of spotting in stars with activitylevels similar to that of the sun. It is shown that the amount ofspotting increases gradually from the sun to the highly spotted starsfor which Alekseev and Gershberg constructed the zonal model for thedistribution of spots. A close relationship is found between spottingand the power of the x-ray emission from stars with widely varyinglevels of activity.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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).
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| 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.
| 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.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is 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/397/997
| The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| 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.
| The Flux Deficits in Star Spots
The bolometric flux deficits of the photospheres of spotted stars arederived for the first time in the framework of zonal spottedness modelsfor red dwarfs computed at the Crimean Astrophysical Observatory. Theresulting flux deficits are compared to the estimated radiative lossesfrom the chromospheres and coronas measured during quasi-simultaneousobservations. A linear correlation is found between the logarithms ofthese quantities, with the Sun fitting these relations. Radiative lossesfrom the outer stellar atmospheres in quiescence and during individualsporadic flares are significantly lower than the bolometric deficits ofthe spotted photospheres of active stars. This suggests that the fluxdeficit due to spots leads to global reconstruction of the atmospheresof red dwarfs, analogous to the local atmospheric reconstruction thatoccurs during solar and stellar flares. This process may be realized viathe superposition of a large number of weak impulsive flares and otherdynamic events, which develop on these stars and heat their coronas(i.e., in this view, microflaring is favored as the principal coronalheating mechanism for these stars). A brief analysis of the long-termvariations in the chromospheric and photospheric radiation of F-K starsfrom the HK project and of the Sun suggests that such dynamicalreconstruction of the outer atmosphere by energy associated with theflux deficit of the spotted photosphere occurs at times of increasedsurface activity in all F-M stars.
| Peculiarities of the Plasma Mechanism of Radio Emission from Late-Type Stars
The plasma mechanism of radio emission in the coronas of late-type starsis shown to be considerably more efficient than that in the solar coronabecause of the high plasma temperature in their magnetic arches. This isattributable to an increase in the length of plasma-wave conversion intoelectromagnetic waves and a decrease in the optical depth of collisionalwave absorption. Magnetic-arch filamentation results in a decrease inthe intensity of the fundamental-tone radio emission and in the relativedominance of the second-harmonic radio emission. The efficiency of thefundamental-tone radio emission increases with plasma density in acoronal arch. The plasma mechanism accounts for the high brightnesstemperature of the flare radio emission from stars (more than1014 K).
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Catalogue and bibliography of the UV Cet-type flare stars and related objects in the solar vicinity
This new catalogue of flare stars includes 463 objects. It containsastrometric, spectral and photometric data as well as information on theinfrared, radio and X-ray properties and general stellar parameters.From the total reference list of about 3400 articles, partial listsselected by objects, authors, key words and by any pairs of thesecriteria can be obtained Tables 1, 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.
| The ROSAT Wide Field Camera Extragalactic Survey
We report the results of a new analysis of the ROSAT Wide Field Camera(WFC) all-sky survey data, designed to detect extragalactic sources ofextreme ultraviolet (EUV) radiation in regions of low Galactic N_H. Weidentify a total of 19 active galactic nuclei (AGN), more than doublethe number of confirmed AGN in the published WFC (2RE) survey. Oursample contains eight narrow-line Seyfert 1 galaxies, making this thefirst reasonably sized, complete sample of these extreme AGN, along withsix broad-line Seyfert 1s and five BL Lacertae objects. TheseEUV-selected sources typically have steep soft X-ray spectra with a meanpower-law energy index alpha_X ~ -2. The derived AGN luminosity functionis rather flat and appears to cut off above a luminosity of 10^46 ergs^-1 keV^-1, measured monochromatically at 200 eV. Narrow-line Seyfert1s account for roughly half of the local (z ~ 0) volume emissivity inthe EUV band.
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