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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
| The Gemini Deep Planet Survey
We present the results of the Gemini Deep Planet Survey, a near-infraredadaptive optics search for giant planets and brown dwarfs around 85nearby young stars. The observations were obtained with the Altairadaptive optics system at the Gemini North telescope, and angulardifferential imaging was used to suppress the speckle noise of thecentral star. Typically, the observations are sensitive to angularseparations beyond 0.5" with 5 σ contrast sensitivities inmagnitude difference at 1.6 μm of 9.5 at 0.5", 12.9 at 1", 15.0 at2", and 16.5 at 5". These sensitivities are sufficient to detect planetsmore massive than 2 MJ with a projected separation in therange 40-200 AU around a typical target. Second-epoch observations of 48stars with candidates (out of 54) have confirmed that all candidates areunrelated background stars. A detailed statistical analysis of thesurvey results is presented. Assuming a planet mass distributiondn/dm~m-1.2 and a semimajor-axis distributiondn/da~a-1, the 95% credible upper limits on the fraction ofstars with at least one planet of mass 0.5-13 MJ are 0.28 forthe range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; thisresult is weakly dependent on the semimajor-axis distribution power-lawindex. The 95% credible interval for the fraction of stars with at leastone brown dwarf companion having a semimajor axis in the range 25-250 AUis 0.019+0.083-0.015, irrespective of anyassumption on the mass and semimajor-axis distributions. Theobservations made as part of this survey have resolved the stars HD14802, HD 166181, and HD 213845 into binaries for the first time.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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 ROSAT North Ecliptic Pole Survey: the Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All SkySurvey have been used to construct a contiguous area survey consistingof a sample of 445 individual X-ray sources above a flux of~2×10-14 ergs cm-2 s-1 in the0.5-2.0 keV energy band. The NEP survey is centered atα2000=18h00m,δ2000=+66deg33' and covers aregion of 80.7 deg2 at a moderate Galactic latitude ofb=29.8d. Hence, the NEP survey is as deep and covers a comparable solidangle to the ROSAT serendipitous surveys but is also contiguous. We haveidentified 99.6% of the sources and determined redshifts for theextragalactic objects. In this paper we present the opticalidentifications of the NEP catalog of X-ray sources including basicX-ray data and properties of the sources. We also describe with somedetail the optical identification procedure. The classification of theoptical counterparts to the NEP sources is very similar to that ofprevious surveys, in particular the Einstein Extended Medium SensitivitySurvey (EMSS). The main constituents of the catalog are active galacticnuclei (AGNs) (~49%), either type 1 or type 2 according to the broadnessof their permitted emission lines. Stellar counterparts are the secondmost common identification class (~34%). Clusters and groups of galaxiescomprise 14%, and BL Lacertae objects 2%. One non-AGN galaxy and oneplanetary nebula have also been found. The NEP catalog of X-ray sourcesis a homogeneous sample of astronomical objects featuring completeoptical identification.
| The 77th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1152 variable stars recently designated in the system ofthe General Catalogue ofVariable Stars. New designations are alsointroduced for two ``old'' variables whose corrected coordinates bringthem to other constellations.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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.
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| 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 ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. IV. The northern hemisphere sample
We present high-precision photometry, high- and medium-resolutionspectroscopy for a sample of 32 stars likely to be the opticalcounterparts of X-ray sources serendipitously detected by the EXOSATsatellite. Using also recent results from the Hipparcos satellite, weinfer spectral types, compute X-ray luminosities and Li abundances andinvestigate the single or binary nature of the sample stars. We foundeleven new variable stars, whose photometric periods fall in the1.2-27.5 day range, for most of which the optical variability isconsistent with the presence of photospheric cool spots. For our sampleof X-ray selected stars we confirm the existence of a strong correlationbetween the stellar rotation rate and the level of activity, and alsobetween the X-ray and bolometric luminosities. Two stars in our sampleare likely to be pre-main sequence objects, one is likely to be apreviously unknown M-type star within 25 pc. Based on data collected atCatania Astrophysical Observatory (Mt. Etna, Italy), at ObservatorioAstronomico Nacional (UNAM, S. Pedro Martir, BC, Mexico) and at KittPeak National Observatory (Kitt Peak, AZ, USA) Table 1 is available inthe on-line version; Table 3 and the complete data set are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr orvia http://cdsweb.u-strasbg.fr/Abstract.html}
| A New Technique for Performing Two-Dimensional Spectroscopy of Objects of Large Dynamic Range: The Binary System HD 167605 A+B
We present a new technique for performing two-dimensional(integral-field) spectroscopy of objects with a large range inintensity. It is based on the capability of optical fibers to discretizethe intensity distribution at the telescope focal plane. This allows notonly reformatting but also the equalization of fiber output intensities,matching them to the dynamic range of the detector and reducing theoptical cross talk inside the spectrograph. This technique, which wehave called equalized integral field spectroscopy, permits spectralinformation to be obtained as well as accurate relative photometry andastrometry. Here we present the concept and a practical realization.Using this technique, we have observed the environment of the K2 V starHD 167605, for which recent Hubble Space Telescope/Near-Infrared Cameraand Multiobject Spectrometer observations in coronagraphic mode haverevealed the presence of a faint companion. We have also detected thisstar, whose spectral type was determined (M4 V), together with itsrelative distance ( Delta alpha =+0 ."80 and Delta delta =+0 ."86) andphotometry ( Delta R=3.8+/-0.1 and Delta I=3.1+/-0.1 mag) with respectto the primary, confirming that these two stars form a binary system.For the companion we have also found the presence of H alpha inemission, which indicates that this binary system is relatively young.The present technique may be of interest for Next Generation SpaceTelescope instrumentation.
| 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.
| Absolute proper motions for the calibration of the HIPPARCOS proper motion system
Absolute proper motions of 48 Hipparcos stars in nine fields distributedover the northern hemisphere are presented. The proper motions have beenrecently used as part of the Bonn program for the extragalactic link ofthe Hipparcos proper motion system. We describe methods which have ledto the absolute proper motions of the major part of the Bonn fields forthe link. For six fields the extragalactic calibration was achievedusing the bright quasar in the centre of the field. In addition, data ofthree fields of globular clusters were included. For these fields thelink to an extragalactic reference system was performed by stars, whoseabsolute proper motions with respect to galaxies were determined onplates of the Lick astrograph. In addition, we used for the link in onefield plates from the ESO Schmidt telescope in combination withmeasurements from glass copies of the Palomar Sky Survey. The accuracyof a single proper motion is of the order of 2 to 3 mas/a for eachcoordinate. Table 2 is also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html}
| 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
| Lithium, X-ray activity and rotation in an X-ray selected sample of solar-type stars
We present an analysis of the connection between X-ray activity level,photospheric abundance of lithium and surface rotation in late typeactive main sequence stars (G and K), using the ratio between opticaland X-ray luminosity fx/fv as an uniform activity indicator. We performthis analysis for a sample of X-ray selected sources from Einstein-basedsurveys compared to stars from the Pleiades open cluster and to a sampleof active binary stars. We show that these parameters show differentdegrees of statistical correlation in the three samples. In particular,the Pleiades sample shows a significant correlation between all threequantities, while in the X-ray selected sample lithium and rotation aresignificantly correlated with each other but neither is correlated withthe activity level. No correlation is evident for the three quantitiesstudied in the active binary sample. We show how the behavior of theX-ray selected sample can be used to discriminate among differenthypothesis about the nature of the so-called `yellow star excess'observed in X-ray flux-limited surveys, showing that this is composed bya population of young, near Zero-Age Main Sequence (ZAMS) stars withcharacteristics similar to the Pleiades.
| The general catalogue of trigonometric [stellar] paralaxes
Not Available
| A Search for Yellow Young Disk Population Stars among EMSS Stellar X-Ray Sources by Means of Lithium Abundance Determination
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...277..428F&db_key=AST
| The Einstein Slew Survey
A catalog of 819 sources detected in the Einstein IPC Slew Survey of theX-ray sky is presented; 313 of the sources were not previously known asX-ray sources. Typical count rates are 0.1 IPC count/s, roughlyequivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources havepositional uncertainties of 1.2 arcmin (90 percent confidence) radius,based on a subset of 452 sources identified with previously knownpointlike X-ray sources (i.e., extent less than 3 arcmin).Identifications based on a number of existing catalogs of X-ray andoptical objects are proposed for 637 of the sources, 78 percent of thesurvey (within a 3-arcmin error radius) including 133 identifications ofnew X-ray sources. A public identification data base for the Slew Surveysources will be maintained at CfA, and contributions to this data baseare invited.
| The EXOSAT high Galactic latitude survey
This study presents a survey of serendipitous sources performed in thevery soft X-ray band (0.05-2.0 keV) using the Exosat imaging telescopes.It covers 783 sq deg of high Galactic latitude sky and includes 210serendipitous sources which define a complete (flux-limited) sample. Twohundred of the 210 detected sources are identified via extensive opticaland radio observations together with cross-correlations with catalogs ofknown objects. The log N-log S relation was found to be consistent withthat of the Einstein extended Medium-Sensitivity Survey (EMSS) and withthe 'Euclidean' value of 1.5. The normalization of the relation is astrong function of the assumed spectral slope of AGN. It is inferredfrom the consistency with the EMSS results that the average (energy)slope of extragalactic sources in the soft X-ray band is very steep(approximately 1.5). An analysis of the association between AGNdetection and Galactic N(H) also shows that the average slope is steepand inconsistent with the canonical value of 0.7.
| 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 Einstein Observatory Extended Medium-Sensitivity Survey. I - X-ray data and analysis
This paper presents the results of the analysis of the X-ray data andthe optical identification for the Einstein Observatory ExtendedMedium-Sensitivity Survey (EMSS). The survey consists of 835serendipitous sources detected at or above 4 times the rms level in 1435imaging proportional counter fields with centers located away from theGalactic plane. Their limiting sensitivities are about (5-300) x 10 tothe -14th ergs/sq cm sec in the 0.3-3.5-keV energy band. A total area of778 square deg of the high-Galactic-latitude sky has been covered. Thedata have been analyzed using the REV1 processing system, which takesinto account the nonuniformities of the detector. The resulting EMSScatalog of X-ray sources is a flux-limited and homogeneous sample ofastronomical objects that can be used for statistical studies.
| The relation between X-ray emission and rotation in late-type stars from the perspective of X-ray selection
An X-ray-selected sample of 128 late-type (F-M) stars analyzed. Thesestars were identified as optical counterparts to serendipitous X-raydetections made by the Einstein Observatory Extended Medium SensitivitySurvey. Once identified as X-ray sources, the sample was followed up byan extensive program of optical observations, including high-resolutionand low-resolution spectroscopy and photometry. Spectral types,luminosity classes, distances, X-ray luminosities, projected rotationrates (v sin i), radial velocities, and binary status have beendetermined for the sample.
| A systematic search for long-term variability in a large sample of X-ray sources
A method which uses the correct measurement error distribution (Poisson)and which naturally takes into account detections as well as upperlimits has been developed to search for flux variability in X-raysources. The method is applied to 256 sources observed at least twicewith the Einstein Observatory Imaging Proportional Counter. Thesesources have, in the very large majority of cases, an X-ray flux in therange 1 x 10 to the -13th to 6 x 10 to the -12th erg/sq cm/s (0.2-3.5keV). Thirty-one sources show flux variations with amplitude rangingfrom about 20 percent to a factor of 6. Flux variations of smallamplitude seem to be fairly common, but can be detected only in brightsources or in sources observed with very long exposures. Variations of afactor of 2 or more are seen in about 15 percent of the sources forwhich variations of such an amplitude are detectable.
| X-ray-selected M dwarfs and the diffuse X-ray background
A systematic optical identification program was carried out forserendipitous X-ray sources discovered by the Einstein Observatory. Atotal of 104 were detected above the threshold flux of 3 x 10 to the-13th ergs/sq cm per sec in the 114 sq deg survey region; 11 of these104 sources are dM stars. Including X-ray-selected M dwarfs from the LMCsurvey, the Medium Sensitivity Survey, and the Einstein Deep Survey, adwarf M star composite X-ray spectrum was constructed; the best-fitthermal model yields kT = 1.3 + or - 0.3 keV, consistent with typicalactive star spectra. An M dwarf X-ray luminosity function was alsoconstructed and it is concluded that the integrated contribution to thediffuse soft (0.28-1.0 keV) X-ray background from such stars is lessthan 10 percent, a factor of more than about 2 lower than previousestimates. This result is consistent with a limit on the dwarf M starcontribution derived from a spectral decomposition of the observed X-raybackground spectrum.
| Results from an extensive Einstein stellar survey
The preliminary results of the Einstein Observatory stellar X-ray surveyare presented. To date, 143 soft X-ray sources have been identified withstellar counterparts, leaving no doubt that stars in general constitutea pervasive class of low-luminosity galactic X-ray sources. Stars alongthe entire main sequence, of all luminosity classes, pre-main sequencestars as well as very evolved stars have been detected. Early type OBstars have X-ray luminosities in the range 10 to the 31st to 10 to the34th ergs/s; late type stars show a somewhat lower range of X-rayemission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late typemain-sequence stars show little dependence of X-ray emission levels uponstellar effective temperature; similarly, the observations suggest weak,if any, dependence of X-ray luminosity upon effective gravity. Instead,the data show a broad range of emission levels (about three orders ofmagnitude) throughout the main sequence later than F0.
| Meridian observations made in Brorfelde (Copenhagen University Observatory) 1975-76.
This catalogue presents positions for selected faint stars mainly fromAGK3 observed with the 7" transit circle at Brorfelde. The stars aredistributed in 43 selected areas around radio sources and are suitableas reference stars for measuring the optical counterparts to the radiosources. The obscr 'itioi cre carried out from 1975.16 to 1976.04, andevery star was observed at least two times giving a m.s.e. of "13 forthe catalogue positions given in the FK4 system. These positions usedtogether with AGK3 proper motions will in the period 1975 to 1980 have am.s.e. at the epoch ranging from `:13 to `:15, which is less than halfof what is expected when using AGK3 positions. Key words: transit circle- catalogue of positions
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