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XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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 Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365
PDS 365 is a newly detected, rapidly rotating (vsini=20 kms-1), single, low-mass giant star that with HD 233517 and HD219025 forms a remarkable ensemble of single K giants with the uniqueproperties of rapid rotation, very strong Li lines, an asymmetricalHα profile, and a large far-infrared excess. Their vsini valuesare between 18 and 23 km s-1, and their LTE Li abundances,logɛ(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365reveals it to be a ~1 Msolar giant with a value of12C/13C approximately equal to 12. A clearrelation between high rotational velocities and very high Li abundancesfor K giant stars is found only when asymmetrical Hα profiles andlarge far-infrared excesses are present. If we consider single K giants,we find that among rapid (vsini>=8 km s-1) rotators, avery large proportion (~50%) are Li-rich giants. This proportion is incontrast with a very low proportion (~2%) of Li-rich stars among themuch more common slowly rotating K giants. This striking difference isdiscussed in terms of proposed mechanisms for Li enrichment.
| The accretion of brown dwarfs and planets by giant stars - II. Solar-mass stars on the red giant branch
This paper extends our previous study of planet/brown dwarf accretion bygiant stars to solar-mass stars located on the red giant branch. Themodel assumes that the planet is dissipated at the bottom of theconvective envelope of the giant star. The evolution of the giant isthen followed in detail. We analyse the effects of different accretionrates and different initial conditions. The computations indicate thatthe accretion process is accompanied by a substantial expansion of thestar, and, in the case of high accretion rates, hot bottom burning canbe activated. The possible observational signatures that accompany theengulfing of a planet are also extensively investigated. They includethe ejection of a shell and a subsequent phase of IR emission, anincrease in the ^7Li surface abundance and a potential stellarmetallicity enrichment, spin-up of the star because of the deposition oforbital angular momentum, the possible generation of magnetic fields andthe related X-ray activity caused by the development of shear at thebase of the convective envelope, and the effects on the morphology ofthe horizontal branch in globular clusters. We propose that the IRexcess and high Li abundance observed in 4-8per cent of the G and Kgiants originate from the accretion of a giant planet, a brown dwarf ora very low-mass star.
| Study of FK Comae Berenices. I. Surface images for 1994 and 1995
We present new surface images of FK Com for August 1994 and July 1995.For the 1995 images two different inversion methods, Tikhonovregularization and the Occamian approach, are used to check thedependence on the formal assumptions. The images are found to be verysimilar when the same local line profiles, models, stellar parametersand observations are used as inputs for both methods. The validity ofthe maps and their temperature scales are independently checked withphotometric observations. The maps for both years show active regions ofvery similar substructures and latitudes. It seems that the same spotgroup has survived on the surface of FK Com for the 11 months betweenthe observations; however, during that time it has moved about 0.2 inphase. The movement and evolution of the spot groups seem to cause thephotometrically observed ``flip-flop'' phenomenon, which is noticed tobe repeated with an average period of 6.5 years, similar to some RSCVn-stars. Based on the observations obtained at the Nordic OpticalTelescope, Observatorio Roque de los Muchachos, La Palma, CanaryIslands, Spain.
| The accretion of brown dwarfs and planets by giant stars - I. Asymptotic giant branch stars
We study the response of the structure of an asymptotic giant branch(AGB) star to the accretion of a brown dwarf or planet in its interior.In particular, we examine the case in which the brown dwarf spirals-in,and the accreted matter is deposited at the base of the convectiveenvelope and in the thin radiative shell surrounding the hydrogenburning shell. In our spherically symmetric simulations, we explore theeffects of different accretion rates and we follow two scenarios inwhich the amounts of injected mass are equal to ~ 0.01 and ~ 0.1M_solar. The calculations show that for high accretion rates(solarM_acc= 10^-4 M_solaryr ^-1), the considerable release of accretionenergy produces a substantial expansion of the star and gives rise tohot bottom burning at the base of the convective envelope. For somewhatlower accretion rates (solar M_acc= 10^-5 M_solar yr ^-1), the accretionluminosity represents only a small fraction of the stellar luminosity,and as a result of the increase in mass (and concomitantly of thegravitational force), the star contracts. Our simulations also indicatethat the triggering of thermal pulses is delayed (accelerated) if massis injected at a slower (faster) rate. We analyse the effects of thisaccretion process on the surface chemical abundances and show thatchemical modifications are mainly the result of deposition of freshmaterial rather than of active nucleosynthesis. Finally, we suggest thatthe accretion of brown dwarfs and planets can induce the ejection ofshells around giant stars, increase their surface lithium abundance andlead to significant spin-up. The combination of these features isfrequently observed among G and K giant stars.
| 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.
| Rotational Velocities of Late-Type Stars
A calibration based on the results of Gray has been used to determineprojected rotational velocities for 133 bright stars with spectral typesof F, G, or K, most of which appear in {\it The Bright Star Catalogue}.The vast majority have {\it v} sin {\it i} $\leq$ 10 km s$^{-1}$ and,thus, are slow rotators. With the new calibration, projected rotationalvelocities have been determined for a sample of 111 late-type stars,most of which are chromospherically active. Some of the stars have hadtheir rotational velocities measured for the first time. (SECTION:Stars)
| The Extremely Active Single Giant 1E 1751+7046 = ET Draconis: Revised Properties and a Reevaluation of Its Evolutionary Status
New spectroscopic observations have resulted in significant revisions toseveral of the orginally published properties of the X-ray bright,chromospherically active star 1E 1751+7046: the spectral classificationhas been revised from K5 IV to K0 III, v sin i from 30--40 km s-1 to 23km s-1, and the reported nondetection of the Li I 6707 A line to asignificant log epsilon (Li) = 1.8. Chromospheric and transition regionsurface fluxes from IUE observations and the coronal surface flux fromearlier Einstein data are close to saturation levels, and comparableonly to very active binaries, pre--main-sequence stars, and FK Comaeitself. IUE observations also recorded a flare. Fifteen new radialvelocity measurements show no evidence for a companion and areconsistent with intermediate or young disk membership. On the otherhand, we show the star to be located about 250 pc above the galacticplane, suggesting an intermediate or old disk object. The new spectralclass (Teff) and limits on the luminosity indicate that 1E 1751+7046 isa low-mass star on its first ascent of the giant branch. Photometry fromthe Four College Consortium Automatic Photometry Telescope is consistentwith the recently published photometric period of 13.98 days, and thelight curve is well fitted by a model consisting of two large spots atlatitudes of ~30 deg and ~50 deg. There are currently only two possibleevolutionary scenarios for this anomalous star: (a) coalescence from aprogenitor W UMa--type contact binary; or (b) the dredge-up of bothangular momentum and nuclear processed material in a low-mass (~1--2.5Mȯ) giant. A space motion, obtainable once a parallax and propermotion are available from Hipparcos, may resolve the age (old disk-youngdisk) uncertainty: young disk motions would favor the angular momentumtransfer scenario. There is no current theory that can account for theobserved lithium abundance.
| An Automated Search for Variability in Chromospherically Active Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2926H&db_key=AST
| 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 72nd Name-List of Variable Stars
Not Available
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| 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
| Lithium and rapid rotation in chromospherically active single giants
The rotational velocities presently obtained via spectroscopicobservations of a group of moderately rapidly rotating, chomosphericallyactive single giants indicate that Gray's (1989) rotostat hypothesisrequires modification. Their rapid rotation appears to be due to high Liabundances, and results in increased chromospheric activity. A scenariois projected in which the surface convection zone reaches the rapidlyrotating core just as a star begins its first ascent of the giantbranch, and dredges both high angular momentum material and freshlysynthesized Li to the surface.
| The FK Comae-type candidate 1E 1751 + 7046 - UBVRI photometry
New UBVRI photometry of the FK Comae-type candidate 1E 1751 + 7046 ispresented and the photometric rotation period 13.9820 +/- 0.0078 d isderived. The peak-to-peak amplitude of the rotational modulation ofbrightness (nearly 0.3 m in V) is extremely high. The previouslyobserved, apparently constant, radial velocity may vary with a period of13.912 +/- 0.025 d.
| 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 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.
| Life After Coalescence: Spun-Down FK Comae Stars: 1E1751 + 7046 and NGC 188 I-1
Not Available
| Nonthermal radio emission from the X-ray-bright K5 IV star 1E 1751 + 7046
VLA observations are analyzed of the K5 IV star 1E 1751 + 7046, which isknown to be an X-ray source on the basis of Einstein observations andwhich shows some similarity to stars of the FK Comae type. Radiocontinuum emission has been detected at 3.6, 6, and 20 cm, and thespectral energy distribution is nearly flat. There is no evidence forcircular polarization, and upper limits are given. The 6-cm flux isroughly one order of magnitude less than reported in a previous VLAobservation, indicating that this star may flare at radio wavelengths.The inferred radio variability and the nearly flat spectral energydistribution suggest that the emission is of nonthermal origin. Avariety of possible emission mechanisms is examined, and it is concludedthat the emission is best explained in terms of agyrosynchrotron/synchrotron model with the electrons following apower-law energy distribution. A brief comparison of the radioproperties of 1E 1751 + 7046 with FK Comae-like stars and RS CVn systemsis given.
| An astrometric catalogue of radio stars
The first part is presented of a radio star catalog encompassing 186objects whose selection was guided by the priority criteria of theHipparchos Input Catalogue Consortium. Since these criteria are wellsuited to the need for linkage of ground-based optical systems to radioreference frames, this first selection is also considered a suitablebase for the catalog. Seventeen categories of stellar parameters arefurnished for each of the stars, including optical and radio positions,optical and radio parallax, radial velocity, type of variability, andoptical structure.
| 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.
| Photometry and UV Spectroscopy of the New FK Comae Candidate 1E1751+7046
Not Available
| The common envelope phase in the evolution of binary stars
The common envelope phase in the evolution of binary systems isexamined. Three parameters are identified which characterize theefficiency of energy deposition, the importance of three-dimensionaleffects, and the efficiency of spin-up of the envelope. It isdemonstrated that the efficiency of deposition of orbital energy intoenvelope ejection can be quite low. It is found that significant spin-upof the envelope can be expected to occur in relatively early stages,when the spiralling-in occurs inside evolved supergiant envelopes. Innormal giants spin-up can occur only in the final stages of thespiralling-in process. The results of a simplified three-dimensionalnumerical calculation of the common envelope phase are presented, andthe implications of the results for the formation of planetary nebulaewith binary nuclei, double white dwarf systems, and FK Com stars arediscussed.
| 1E 1751 + 7046 - A new FK comae candidate
1E 1751 + 7046 is a star that was serendipitously detected by theEinstein Observatory Medium Sensitivity Survey. X-ray, optical, andradio observations of this star show it to have characteristics similarto the RS CVn binaries. However, repeated high-resolution spectroscopicobservations fail to detect any significant radial-velocity variations.The possibility of the star being pre-main-sequence is dismissed becauseof its location and lack of photospheric Li I. This makes 1E 1751 + 7046an attractive candidate for the recently proposed class of FK Comaestars. Although there is disagreement over the evolutionary status ofthese evolved stars, they are nonetheless interesting as exhibitors ofextreme chromospheric and coronal activity.
| 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.
| 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
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