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Hipparcos astrometry for 257 stars using Tycho-2 data
We present improved Hipparcos astrometry for 257 Hipparcos stars,resolved into 342 components. For 64 of the stars no astrometry wasobtained in the Hipparcos Catalogue, while for the remaining starsadditional components have been added by this solution or the positionshave been revised considerably. We have used the published Hipparcostransit data for the new solutions, together with results from thesecond reduction of the Tycho data for defining better initial values.Based on observations made with the ESA Hipparcos astrometry satellite.Table 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Stroemgren four-color photometry of X-ray active late-type stars: Evidence for activity-induced deficiency in the m_1_ index.
We present the results of a uvby-β photometric study of a sample ofactive late-type stars (F-K) selected from the Einstein Extended MediumSensitivity Survey. Our work shows the presence in the sample of a starpopulation with the photometric index c_1_ typical of main sequencestars and an unexpected deficiency in m_1_ index. Stars with moreanomalous values of m_1_ have also very high values of f_ X_/f_V_ andX-ray surface fluxes, near the "saturation" limit observed in the mostactive stars and similar to the flux observed in solar active regions.We discuss these results in the light of similar results found in theSun comparing m_1_ indices in quiet and active regions and in othersamples of active stars.
| 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.
| 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 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 medium sensitivity survey - A new sample of X-ray sources with optical identifications and the revised extragalactic log N-log S
Maccacaro et al. (1982) surveyed approximately 50 sq deg in differentregions of the sky at medium sensitivity flux levels intermediatebetween the Uhuru/Ariel V limit and the Einstein deep survey limit. This'first' medium sensitivity survey (MSS 1) was primarily aimed at theanalysis of the source-count relation for extragalactic X-ray sources.The present investigation is concerned with an extension of the MSS 1 onthe basis of an analysis of new fields obtained with the aid of theImaging Proportional Counter (IPC) on board the Einstein Observatory. Aparallel optical program to identify all the new sources was alsoundertaken. X-ray and optical data are presented for this 'second'complete sample of medium sensitivity survey X-ray sources (MSS 2). Thelog N-log S relation is derived and discussed.
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