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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.
| 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}
| The Program to Link the HIPPARCOS Reference Frame to an Extragalactic Reference System Using the Fine Guidance Sensors of the Hubble Space Telescope
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2796H&db_key=AST
| 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}
| Photometric and spectroscopic studies of cool stars discovered in EXOSAT X-ray images. III. Photometric properties.
We present high-precision UBV(RI)_c_ photometry for a sample of activestars detected serendipitously by the EXOSAT satellite. Fourteenobjects, out of 47 likely optical counterparts, turned out to be clearlyvariable, with periods in the range 1-8 days, including a newlydiscovered eclipsing binary. For most of them the optical variability isconsistent with the presence of photospheric cool spots. We have usedour multicolour photometry to estimate spectral classifications anddistances. The derived distances indicate that some of the observedstars are previously unidentified nearby (d<=25pc) M dwarfs. Whencombined with the results of high resolution spectroscopy, ourphotometric observations allow us to investigate the stellar content ofthe selected X-ray sample. Young stars and RS CVn-type binaries appearto constitute a large fraction of the selected sample.
| 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
| 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.
| Investigation of the Praesepe cluster. I - Identification of halo members
Coravel radial velocities and UBVRI (Kron) photometry of 117 starsselected by their proper motion in the region surrounding Praesepe (NGC2632, C 0837 + 201) have permitted identification of 48 members of thecluster halo, up to a distance of 4 deg from the cluster center. The 43(F5-K0) new members represent 51 percent of the number of nucleusmembers known in this spectral interval. Ten spectroscopic binaries havebeen discovered among the F5-F0 members, resulting in a frequency of 23percent, which rises to 33 percent four probable binaries are taken intoaccount. Six orbits have been obtained, with periods ranging from 1.2 to143 d. Four additional binaries have been detected among the non-memberstars.
| 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.
| 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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| Takýmyýldýz: | Yengec |
| Sað Açýklýk: | 08h54m00.50s |
| Yükselim: | +20°13'49.4" |
| Görünürdeki Parlaklýk: | 8.62 |
| Uzaklýk: | 99.305 parsek |
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| B-T magnitude: | 9.172 |
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