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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.
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| Third-Body Parameters from Whole Light and Velocity Curves
Eclipsing binaries can improve multiple system statistics via thelight-time effect and radial velocity shifts. Here an algorithm operateson data of mixed type to exploit these opportunities. Main reasons forenhanced reliability are that (1) combined light and velocity curvesgive better timewise coverage than either type alone, (2) properlyweighted solutions impersonally balance light and velocity information,and (3) the entire theory is within the computer model, so observationsare used directly without corrections. A brief history of mixedwhole-curve solutions is given and the relative importance of light-timeand radial velocity input for third-body parameters is discussed andquantified. Period sifting by power spectral analysis is essentiallyindispensable in preliminary work. Applications are to the Algol-typesystem DM Persei and the detached system VV Orionis. An assumption ofcoplanarity for DM Per's inner and outer orbits is tested and quantifiedby dynamical experiments. Derived third-body parameters for DM Per aremainly reasonable and self-consistent. For comparison with whole-curveresults, we also investigated DM Per's ephemeris in terms of eclipsetimings and found whole-curve solutions to give smaller standard errorsin reference epoch (T0), binary orbit period (P), and dP/dt,over a similar baseline in time. An astonishing outcome is lack ofevidence that can pass reasonable validity tests for VV Ori'swell-accepted third star with P~120 days. Estimates of third light doindicate a third star, but the correct period cannot now be established,so the star cannot be identified as the one heretofore recognized fromradial velocity evidence. The much cited 120 day period appears to be anartifact of the window function for VV Ori's historical velocityobservations.
| The rotation-activity correlation among G and K giants in binary systems
Aims.The present study aims (i) to test the existence of a correlationbetween magnetic activity and rotation among G and K giants in binarysystems and (ii) to test whether parameters other than rotation play arole in determining the X-ray emission level of intermediate-massgiants. Methods: The method consists in testing the existence ofcorrelations between measured stellar parameters including the X-raysurface flux, rotation period, Rossby number and surface gravity of asample of G and K giants with masses included between 1.5 M_ȯ and3.8 M_ȯ. Results: I found evidence that the X-ray surface fluxFX of intermediate-mass G and K giants is correlated withtheir rotation period P as previously observed on single G giants.Confidence in the degree of correlation is not higher when the Rossbynumber is used in place of the rotation period, but it significantlyimproves when stellar gravity g is taken into account. The empiricalrelation given by log (F_X) = -0.73 × log (P) + 0.64 × log(g/gȯ)+ 7.9 differs from the power-law dependence withan index of about -2 between X-ray to bolometric luminosity ratio andthe rotation period that is observed for main-sequence stars. The X-raysurface flux of single G giants and of intermediate-mass G and K giantsin close binary systems, such as RS CVn systems, also depends on thestellar gravity. This dependence could result from the effect of gravityon the electron density and emission measure of the X-ray emittingplasmas, as well as on the characteristic sizes of coronal magneticloops. The measured X-ray surface-flux dependence on gravity is,however, not as steep as the one predicted by simple models ofhydrostatic loops that assume a fixed ratio between the coronal energylosses by thermal conduction and by radiation. Conclusions: .I concludethat (i) a relation exists between the rotation and X-ray activity levelin giants, (ii) that this relation is not directly dependent on thepresence of a companion and applies to all intermediate-mass giants witheither G or K spectral type, and (iii) that gravity is an importantstellar parameter in determining the X-ray surface flux ofintermediate-mass giants.
| Dynamical evolution of active detached binaries on the logJo-logM diagram and contact binary formation
Orbital angular momentum (OAM, Jo), systemic mass (M) andorbital period (P) distributions of chromospherically active binaries(CAB) and W Ursae Majoris (W UMa) systems were investigated. Thediagrams of and logJo-logM were formed from 119 CAB and 102 WUMa stars. The logJo-logM diagram is found to be mostmeaningful in demonstrating dynamical evolution of binary star orbits. Aslightly curved borderline (contact border) separating the detached andthe contact systems was discovered on the logJo-logM diagram.Since the orbital size (a) and period (P) of binaries are determined bytheir current Jo, M and mass ratio, q, the rates of OAM loss(dlogJo/dt) and mass loss (dlogM/dt) are primary parametersto determine the direction and the speed of the dynamical evolution. Adetached system becomes a contact system if its own dynamical evolutionenables it to pass the contact border on the logJo-logMdiagram. The evolution of q for a mass-losing detached system is unknownunless the mass-loss rate for each component is known. Assuming q isconstant in the first approximation and using the mean decreasing ratesof Jo and M from the kinematical ages of CAB stars, it hasbeen predicted that 11, 23 and 39 per cent of current CAB stars wouldtransform to W UMa systems if their nuclear evolution permits them tolive 2, 4 and 6 Gyr, respectively.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| The new pre-cataclysmic binary PG 2200+085
Aims.We present the results of spectroscopic- and orbit-sampledphotometric observations of the faint UV-excess object PG 2200+085. Methods: .The optical CCD photometry observations of this object wereperformed by the Russian-Turkish 1.5-m telescope RTT150 at the TUBITAKNational Observatory (Turkey). The long-slit optical spectroscopyobservations with 2.6 Å resolution were carried out by 6-mtelescope BTA at the Special Astrophysical Observatory (Russia).Results: .The photometric variations over two nights are almostsinusoidal with an amplitude Δ mV = 0.04m and a periodof P = 0.3186 d. Such a light curve is typical of a detached closebinary with an illumination effect or the ellipsoidal deformation of asecondary star. The observed spectrum clearly displays a featurelessblue continuum of a hot component and a rich absorption-line andmolecular band K-star spectrum. The CaII line profiles with strongemission cores are remarkably similar to those of V471 Tau.Conclusions: .We tentatively classify PG 2200+085 as a pre-cataclysmicbinary of the V471 Tau type.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Precatalysmic binary systems.
Not Available
| The Subdwarf Database: Released
The work on the Subdwarf Database, presented at the previous meeting,has been completed, and the tool is now publicly available. The firstrelease contains data from close to 240 different literature sources,but more still awaits entry. The database interface includes advancedsearch capabilities in coordinate, magnitude and color space. Outputtables can be generated in HTML with hyperlinks to automaticallygenerated finding charts, the Aladin viewer and a detailed data sheetthat displays all registered data for each target, including physicaldata such as temperature, gravity and helium abundance, together with afinding chart. Search results can be visualized automatically asinteractive position, magnitude or color diagrams.
| Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries
We determine the possible masses and radii of the progenitors of whitedwarfs in binaries from fits to detailed stellar evolution models anduse these to reconstruct the mass-transfer phase in which the whitedwarf was formed. We confirm the earlier finding that in the first phaseof mass transfer in the binary evolution leading to a close pair ofwhite dwarfs, the standard common-envelope formalism (theα-formalism) equating the energy balance in the system (implicitlyassuming angular momentum conservation) does not work. An algorithmequating the angular momentum balance (implicitly assuming energyconservation) can explain the observations. This conclusion is now basedon 10 observed systems rather than three. With the latter algorithm (theγ-algorithm) the separation does not change much for approximatelyequal-mass binaries. Assuming constant efficiency in the standardα-formalism and a constant value of γ, we investigate theeffect of both methods on the change in separation in general andconclude that when there is observational evidence for strong shrinkageof the orbit, the γ-algorithm also leads to this. We then extendour analysis to all close binaries with at least one white dwarfcomponent and reconstruct the mass-transfer phases that lead to thesebinaries. In this way we find all possible values of the efficiency ofthe standard α-formalism and of γ that can explain theobserved binaries for different progenitor and companion masses. We findthat all observations can be explained with a single value of γ,making the γ-algorithm a useful tool to predict the outcome ofcommon-envelope evolution. We discuss the consequences of our findingsfor different binary populations in the Galaxy, including massivebinaries, for which the reconstruction method cannot be used.
| Photoelectric photometry of the unusual eclipsing binary system FF Aquarii.
FF Aqr is the first eclipsing binary (P = 9.2d) containing a hotsubdwarf (sdOB) and a chromospherically active cool giant componentclassified as a G8-K0 III star. We observed the system in U, B, V and Rfilters during 2002, 2003 and 2004 observing seasons using two differenttelescopes and detectors. We obtained a total of 1171 observationalpoints in each colour. The light curve reveals that FF Aqr has atotality in the primary eclipse, which lasts about 13h 02m. The shape ofthe light curve indicates that FF Aqr is an Algol type binary. The lightcurves display an asymmetrical wave-like distortion at outside of theeclipse which has a minimum at about 0.46P and a light amplitude of0.050m, 0.204m, 0.277m, 0.282m in the U, B, V, and R bands,respectively. This variation can not be explained by the reflectioneffect alone. We used the Wilson-Devinney code for the geometric andphysical parameters of the system. The asymmetry at outside of theeclipse light variation and the results obtained from the light curveanalysis were discussed.
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| The Nature of the Unique Precataclysmic Variable V664 Cas with Two-Peaked Balmer Lines in Its Spectrum
We analyze photometric and spectroscopic observations of the closebinary system V664 Cas. All the characteristics of its radiation areconsistent with the star being a cataclysmic variable with powerfulreflection effects. The orbital period is refined (P=0.5816475 d) andthe ephemerides of the system determined. The U, B, V, R light curves ofV664 Cas display sinusoidal variations with similar amplitudes nearΔm=1.1m. This suggests that a hot spot on the surface of thesecondary always dominates the optical radiation of the system. Thespectra contain emission lines, two-peaked hydrogen lines, and narrowlines of helium and heavy elements in high ionization states, whoseintensities vary synchronously with the brightness. The HeIIλ4686 Å line has broad absorption wings that form in theatmosphere of the O subdwarf. The mass function, f(m)=0.007M ȯ, isthe lowest among all precataclysmic variables: the mass of the secondaryexceeds the mass of the primary by more than a factor of 1.6. A full setof fundamental parameters for V664 Cas is determined based on modelingof the spectra and light curves, taking into account reflection effectsin the system. Most of the emission lines are formed under conditions ofappreciable deviations from local thermodynamic equilibrium. Thepossibility of carrying out correct modeling of the Balmer-line profilesassuming the stellar radiation is absorbed in a planetary nebula isdemonstrated.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Fluorescence in ultraviolet active binaries: the case of FF Aquarii
The eclipsing binary FF Aquarii (FF Aqr) consists of a small hot star ofuncertain type and a red giant. An Hα feature varies fromabsorption during eclipse to maximum emission during hot star transit.We have obtained simultaneous differential photometry in BVRI bands andHα spectroscopy in 1998 that covered an entire orbit with someoverlap. A binary star model was used with our light-curve, radialvelocity and Hα data to refine stellar and orbital parameters. Anew ephemeris indicates that the period has increased or is longer thanpreviously thought. A physically simple algorithm is developed thatgenerates properly blended spectral line profiles of binaries witharbitrary combinations of surface emission and absorption line regions.Although there is no radiative transfer, the algorithm can accommodatemodest geometrical intricacy. For now, all line broadening in the modelis rotational. The algorithm has now been incorporated within thegeneral Wilson-Devinney binary star model, and we used it to model FFAqr's photospheric iron profile. It also is used to generate templatesthat illustrate problems with FF Aqr's strongly time-dependent Hαemission profiles, which appear to be blends of features from thephotosphere and from above the photosphere. The dominant broadeningmechanism should be rotation for individual line components. Absorptionprofiles were generated by the model and fit to the observations toyield a red star radius ~6.9 Rsolar, which then requires i~76°. Our light-curve models have five spots with temperatures ~1000K cooler than the surrounding photosphere. Hα emission wasmodelled on the red star surface, centred at the substellar point tosimulate a fluorescent chromosphere. Additional emission is seen outsideour modelled profiles. The origins of this excess emission may becorotating prominences, winds, or coronal mass ejections, all of whichcould affect the orbit period.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| The Evolution of Cool Algols
We apply a model of dynamo-driven mass loss, magnetic braking, and tidalfriction to the evolution of stars with cool convective envelopes; inparticular, we apply it to binary stars where the combination ofmagnetic braking and tidal friction can cause angular momentum loss fromthe orbit. For the present we consider the simplification that only onecomponent of a binary is subject to these nonconservative effects, butwe emphasize the need in some circumstances to permit such effects inboth components.The model is applied to examples of (1) the Sun, (2) BYDra binaries, (3) Am binaries, (4) RS CVn binaries, (5) Algols, and (6)post-Algols. A number of problems regarding some of these systems appearto find a natural explanation in our model. There are indications fromother systems that some coefficients in our model may vary by a factorof 2 or so from system to system; this may be a result of the chaoticnature of dynamo activity.
| Determination of the Ages of Close Binary Stars on the Main Sequence from Evolutionary Model Stars of Claret and Gimenez
A grid of isochrones, covering a wide range of stellar ages from thezero-age main sequence to 10 billion years, is calculated in the presentwork on the basis of the model stars of Claret and Gimenez withallowance for convective overshoot and mass loss by the components. Theages of 88 eclipsing variables on the main sequence from Andersen'scatalog and 100 chromospherically active stars from Strassmeier'scatalog are calculated with a description of the method of optimuminterpolation. Comparisons with age determinations by other authors aregiven and good agreement is established.
| NN Ser and V664 Cas: two pre-cataclysmic binaries with large reflection effect
We present the new multicolour photometry of the two precataclysmicbinaries showing large reflection effect: NN Ser and V664 Cas.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| Post-Common-Envelope Binary Stars and the Precataclysmic Binary PG 1114+187
We present orbit-resolved spectroscopy and orbit-sampled photometry ofthe binary system PG 1114+187. Both photometry and radial velocitystudies reveal a period P=1.75992 days, which is taken to be the orbitalperiod of the binary. Strong modulation of emission-line strength withthe same period is also present. A preliminary mass ratio,M2/M1~0.7, is found from primary- andsecondary-star radial velocity amplitudes. No evidence is seen foreither an accretion disk or mass transfer, leading to the conclusionthat PG 1114+187 is not a cataclysmic variable (CV) but is in a pre-CVstate, before the initiation of mass transfer. The short orbital periodalso leads to the conclusion that the system passed through acommon-envelope phase at some time in the past. The current list ofknown post-common-envelope and precataclysmic binary stars is alsoreviewed and the general properties of this class of star are discussed.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: The Catalog and LOG N-LOG S
We describe the selection of an X-ray flux-limited sample of brightclusters of galaxies in the southern hemisphere, based on the firstanalysis of the ROSAT All-Sky Survey data (RASS1). The sample isconstructed starting from an identification of candidate clusters inRASS1, after which their X-ray fluxes are remeasured using thesteepness-ratio technique. This method is better suited than the RASS1standard algorithm to measuring flux from extended sources. The finalsample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), sothat as a result of the distribution of N_H, its effective flux limitvaries between ~3 and 4x10^-12 ergs cm^-2 s^-1 over the selected area.This covers the delta<2.5d part of the south Galactic cap region(b<-20^deg), excluding patches of low RASS1 exposure time and of theMagellanic Clouds area, for a total of 8235 deg^2. One hundred andthirty candidate sources fulfill our selection criteria for bona fideclusters of galaxies in this area. Of these, 101 are Abell/ACO clusters,while 29 do not have a counterpart in these catalogs. Of these clusters,126 (97%) have a redshift, and for these we compute an X-ray luminosity.20% of the cluster redshifts come from new observations, as part of theESO Key Program survey of RASS clusters that is currently undercompletion. Considering the intrinsic biases and incompletenessesintroduced by the flux selection and source identification processes, weestimate the overall completeness to be better than 90%. The observednumber count distribution, log N-log S, is well fitted by a power lawwith slope alpha=1.34+/-0.15 and normalization A=11.87+/-1.04 sr^-1(10^-11 ergs cm^-2 s^-1)^alpha, in good agreement with othermeasurements.
| Mutual irradiation and absorption and surface temperature variation in detached binary systems
In the paper, we calculate the absorbed flux of the mutual irradiationof the components in detached binary systems, and obtain the surfacetemperature distribution on the components. The maximum temperaturedifference over the surface is a measure of the deviation betweenisothermal and equipotential surfaces. For 33 late-type binary systemswith known elements, we evaluate this maximum difference. The resultshows that the isothermal-equipotential deviation is always larger inthe cooler than in the hotter components. This result supports Zhou andLeung's idea that the large difference between ``star-spots'' andsunspots is due to cyclones and anticyclones. The effect of limbdarkening (amounting to over 18% and always larger in the hottercomponent) is included in the consideration.
| Mutual irradiation and absorption in detached binary systems and surface temperature variation.
Not Available
| Infrared flux excesses from hot subdwarfs. II. 72 more objects
In our search, started in February, 1994, for JHK excess fluxes amongthe hot subdwarf population as an indicator for the presence of binarycompanions, results for 72 more hot objects (=63 hot subdwarfs + 1Horizontal Branch B star + 7 white dwarfs + 1 non-subdwarf object)observed with the Carlos Sanchez CVF IR photometer (in June and October,1994), are presented. The exact number of binary hot subdwarfs hasgained renewed importance after the recent discovery of pulsators withG-F companions. The total number of candidates we propose may help toset some constraints; for example, out of 41 objects with excesses, 13may have G-type binary companions. From our new sample, 14 discoveriesof binary candidates have been found: BD+25 4655, Feige 108, HD 4539, HD149382, HD 216135, KPD 2109+440, LSI+63 198, LSIV+10 9, LSV+22 38, PG0011+221, PG 0116+242, PG 0314+103, PG 2151+100 and TON 139. Besides, 2more from reanalysis of February, 1994, data - BD+37 1977 and BD+481777, may now be found to be IR excess candidates. Two suspectedbinaries, PB 8555 and SB 7, are also confirmed. By fitting \cite[Kurucz(1993)]{kur93} model spectra and assuming zero-age main sequencecompanions, we find upper limits on the subdwarf gravities. Thedistributions of upper limits on log(g), mostly between about 5.25 and6.5, are nearly identical for both sdBs and sdOs. Figure~1 is onlyavailable in the electronic version of the paper(http://www.edpsciences.com)
| Accurate Position Estimates For Known Variables
Not Available
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