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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.
| EUV activity in late-type stars during the ROSAT WFC All-Sky Survey - I. Techniques and initial results
We study 127 active late-type stars, including the well-known flarestars, in the extreme ultraviolet (EUV) region of the electromagneticspectrum, using the ROSAT Wide Field Camera (WFC) survey observations.Our analysis includes photometry for all the stars in the sample, timinganalysis of the observed photon events, and a statistical analysis forpossible small-scale, low-level variability. 49 stars in our sample weredetected in one or both EUV wavebands. A total of 35 flares were seen,from 23 stars, not all of them well-known flare stars. In addition,about half of the detections exhibited low-level variability. Furtheranalysis reveals that the observed low-level variability was not due torotational or orbital modulation, but rather was related to small-scaleactivity on the stars, possibly in the form of small-amplitude,flare-like events which were too faint to be recognized as individualflares at the sensitivity of the WFC. We call this low-level activity`milliflaring'. We also give estimated upper-limit count rates for thenon-detections.
| 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.
| Galaxy evolution in distant, x-ray selected clusters of galaxies, 2: CL 0303+1658 and CL 0303+1717
We present a photometric and spectroscopic survey of two clusters ofgalaxies at z = 0.42. These are members of a sample of five distant,high x-ray luminosity clusters of galaxies, which have been selectedfrom the Einstein Medium Sensitivity Survey Catalog (EMSS). Ourobjective is to compare the galaxy population in these x-ray selectedclusters to the populations in optically selected clusters, which showstrong evolution. We have obtained R and I photometry complete to Rapproximately 22.3 in a 5.7 min x 6.5 min field centered on eachcluster. In addition, we have obtained the spectra of 69 galaxies in thedirection of the two clusters; 53 of these are cluster members. Thex-ray luminosities of Cl 0303+1658 and Cl 0302+1717 (0.2-4.5 keV) areapproximately 5 x 1044 and approximately 4 x 1044ergs, and their velocity dispersions are 921 +192/-123 and 821 +137/-94km/s, respectively. The evolution of the galaxy population observed inoptically selected clusters at z approximately 0.4 (as compared withclusters at the present epoch) is also found in these x-ray selectedclusters. This confirms the results that we presented in a previouspaper for Cl 1358+6245 at z = 0.33. In both optically and x-ray selectedclusters at z approximately 0.4, 20-30% of the galaxies have spectrawith strong Balmer absorption lines or emission lines, consistent withrecent star formation activity. We identify one possible differencebetween the optically and x-ray selected clusters; galaxies with verystrong Balmer absorption lines, EW between Hdelta +Hgamma but greater than 8 A, are so far absent in our x-rayselected sample, yet they constitute approximately 25% of thespectroscopically active population in Dressler and Gunn's opticallyselected sample.
| 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.
| Activity in X-ray-selected late-type stars
A spectroscopic study has been conducted of nine X-ray bright late-typestars selected from two Einstein X-ray surveys: the ColumbiaAstrophysical Laboratory Survey (five stars) and the CFA MediumSensitivity Survey (MSS; four stars). Spectral classes were determinedand radial and V sin(i) velocities were measured for the stars. Four ofthe Columbia Survey stars were found to be new RS CVn-type binaries. Thefifth Columbia survey star was found to be an active G dwarf starwithout evidence for binarity. None of the four MSS stars were found tobe either binaries or optically active stars. Activity in these starswas assessed by measuring the excess emission in H-alpha and the Ca IIIRT (8498, 8542) lines in comparison with inactive stars of similarspectral types. A correlation was found between X-ray luminosity and Vsin(i) and H-alpha line excess. The measured excess line emission inH-alpha was also correlated with V sin(i) but not with the IRT lineexcess.
| Spectroscopic Observations of X-Ray-Selected Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987AJ.....93..869S&db_key=AST
| 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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Наблюдательные данные и астрометрия
| Созвездие: | Овен |
| Прямое восхождение: | 03h06m42.61s |
| Склонение: | +17°28'40.8" |
| Видимая звёздная величина: | 8.944 |
| Собственное движение RA: | 100.2 |
| Собственное движение Dec: | 21.6 |
| B-T magnitude: | 9.508 |
| V-T magnitude: | 8.991 |
Каталоги и обозначения:
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