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HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population.
We present the parallaxes, measured by Hipparcos, for a sample of X-rayselected stars. The stars belong to the stellar sample of the EinsteinExtended Medium Sensitivity Survey. They are all at galactic latitude|b|>20deg, and are generally far away from known star formingregions. Several of these stars show lithium abundance and activitylevel typical of very young stars with ages comparable to that of thePleiades. We show that the majority of our sample stars are on the mainsequence, with only =~20% being giants. We do not find a significantpresence of pre-main sequence stars in our sample, notwithstanding thefact that some of our stars have a considerable lithium abundance,showing that the stars observed are most likely young and activemain-sequence objects.
| The 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 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.
| Magnetic structure in cool stars. XVII - Minimum radiative losses from the outer atmosphere
The emissions in several chromospheric and transition region lines andin coronal soft X-rays are analyzed for a sample of cool stars. Thenature of the lower-limit flux densities is explored, and evidence isgiven for the possibility of a basal, nonmagnetic heating mechanismbeing responsible for these emission fluxes up to, and perhapsincluding, the upper transition region. It is argued that the excessflux density, derived by subtraction of the basal flux density from theobserved stellar flux, is the proper measure of magnetic activity. Thelevel of the basal flux density as a function of color is determined tobe 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sqcm/s for K-type stars.
| 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.
| An unbiased survey of field star X-ray emission
To determine the X-ray luminosity function of normal stars, 1700 starsbrighter than 10th magnitude were surveyed by the imaging proportionalcounter aboard the Einstein Observatory. Seventy star positions werefound to contain excess X-ray counting rates. The number of stars persquare degree in a number of magnitude intervals was calculated as afunction of spectral type and luminosity class, and the total number ofstars for each spectral type brighter than magnitude 9.5 derived in thismanner was compared with the 1700-star sample. The agreement is good, asis that between the surface density of soft X-ray sources and the numberof stellar emitters predicted from the field star survey. It isconcluded that stars probably do not contribute significantly to thediffuse soft X-ray background. The findings are consistent with thenotion that stellar age and/or rotation velocity are importantdeterminants of stellar X-ray emission level.
| Lunar occultation summary. II
Results of a second series of 196 two-color photoelectric lunaroccultation observations made within the interval from May 1972 toDecember 1973 are reported. Each observation has been employed toestimate the minimum magnitude difference between the observed objectand possible undetected companions. The study contains the analyses of143 disappearances and 53 reappearances. Eighteen of these eventsmanifest some degree of multiplicity. Included in the study are 18occultation events for Pleiades members.
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