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Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
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| New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
We present optical lightcurves of 428 periodic variable stars coincidentwith ROSAT X-ray sources, detected using the first run of the SuperWASPphotometric survey. Only 68 of these were previously recognised asperiodic variables. A further 30 of these objects are previously knownpre-main sequence stars, for which we detect a modulation period for thefirst time. Amongst the newly identified periodic variables, many appearto be close eclipsing binaries, their X-ray emission is presumably theresult of RS CVn type behaviour. Others are probably BY Dra stars,pre-main sequence stars and other rapid rotators displaying enhancedcoronal activity. A number of previously catalogued pulsating variables(RR Lyr stars and Cepheids) coincident with X-ray sources are also seen,but we show that these are likely to be misclassifications. We identifyfour objects which are probable low mass eclipsing binary stars, basedon their very red colour and light curve morphology.Tables 1 and 2 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/J/A+A/467/785
| 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.
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
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| 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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
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| 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.
| 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.
| 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.
| Spin and orbital angular momentum exchange in binary star systems. II. Ascending the giant branch: a new path to FK Comae stars
Using the model by Keppens (1997), we investigate the angular momentum(AM) evolution in asymmetric binary star systems from Zero-Age MainSequence times until at least one component has ascended the giantbranch. We concentrate on stars ranging in mass from 0.9 Msun- 1.7 Msun, in almost synchronous, short period systems(P_orb<9 days). We address synchronization and circularization bytidal interaction, allowing for structural evolution and stellar winds.A Weber-Davis prescription is used to quantify the wind influence,thereby accounting for changes in its acceleration mechanism from theinterplay of the evolving thermal-magneto-centrifugal effects. Weidentify a scenario for fast in-spiraling components with d ln P_orb/dt=~ -{cal O}(10-8) which is primarily driven by faststructural evolution as the heaviest component ascends the giant branch.This leads to the formation of contact systems, which ultimatelycoalesce and form FK Comae-like objects on relatively short timescalesdue to the continuing expansion of the primary. The obtained mass lossrates and orbital period variations d ln P_orb/dt are confronted withtheir observed ranges. The predicted mass loss rates agree with thesolar value on the main sequence and with the Reimers relation in thegiant phase. Observations of period evolution in close, active binariessuggest, however, that other influences than those considered here mustplay an important role. Finally, we point out how the mass asymmetry ofthe binary system can be a crucial ingredient in the angular momentumevolution: while the primary dictates the spin-orbital AM exchange inthe system, the slowly evolving lighter component can develop anefficient magneto-centrifugally driven wind and thereby drain the AMfrom the system.
| 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.
| Optical spectroscopy of X-ray sources in the old open cluster M 67
We have obtained optical spectra of seven stars in the old galacticcluster M 67 that are unusual sources of X-rays, andinvestigate whether the X-ray emission is due to magnetic activity or tomass transfer. The two binaries below the giant branch S 1063 and S1113, the giant with the white dwarf companion S 1040 and the eccentricbinary on the subgiant branch S 1242 show magnetic activity in the formof Ca Ii H&K emission and Hα emission, suggesting that theirX-rays are coronal. The reason for the enhanced activity level in S 1040is not clear. The two wide, eccentric binaries S 1072 and S 1237 and theblue straggler S 1082 do not show evidence for Ca Ii H&K emission. Asecond spectral component is found in the spectrum of S 1082, mostclearly in the variable Hα absorption profile. We interpret thisas a signature of the proposed hot subluminous companion. Based onobservations made with the William Herschel Telescope operated on theisland of La Palma by the Isaac Newton Group in the Spanish Observatoriodel Roque de los Muchachos of the Instituto de Astrofisica de Canarias
| Observations of radio stars at the Valinhos CCD Meridian Circle
We present the astrometric results from the optical observation of 16stars from the list of \cite[Wendker (1995)]{wend95}, with detected orsuspected radio emission. The results are given on the Hipparcos/Tychoreference frame. The stars are evenly distributed in right ascension andthe declinations range from -15.6(deg) to +28.6(deg) . Thisdistribution, also allowed to obtain positions relative to the TwinAstrographic Catalogue (TAC 1.0). The observations were carried out withthe Valinhos CCD Meridian Circle, in Brazil, operating in drift scanningmode. The average positional precision is at 40 mas and generally below2 mas/yr for proper motions, obtained with the new reductions of theAstrographic Catalogue - AC2000, as first epoch. In the strips takenfrom the observation of the 16 radio stars, 573 Tycho stars, 545 ACTstars and 566 TAC stars were found. The analysis of the independentreductions made relatively to those frames and the large number ofcommon stars enables to focus on the equatorial zone, as represented bythe three catalogues. As a result, the quality of Valinhos observationswith regard to ACT and TAC internal accuracies is verified. Table 1 ofthis paper, plus 16 additional tables with observed positions for theACT reference stars, are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The FIRST Unbiased Survey for Radio Stars
Comparison of the VLA FIRST survey with various catalogs of bright starsallows an examination of the prevalence of stellar radio emissionindependent of optical selection criteria. This FIRST unbiased surveyfor radio stars covers nearly 5000 deg^2 of the northern sky to a fluxdensity limit of 0.7 mJy at 20 cm. Using astrometric catalogs thatinclude proper-motion information, we have detected 26 stellar radiosources, doubling the number of such objects previously known in thisregion of high-latitude sky. We also show that, in the absence of goodproper motions, even the 1" precision of the FIRST positions isinsufficient to avoid crippling chance coincidence rates. We calculatethe fraction of radio detections as a function of stellar magnitude andshow that, when proper motions from the Guide Star Catalog II becomeavailable, the number of stellar radio source detections should increasefourfold.
| The ROSAT All-Sky Survey of Active Binary Coronae. III. Quiescent Coronal Properties for the BY Draconis--Type Binaries
We present X-ray observations of 35 active late-type BY Draconis dwarfbinary systems and 28 evolved binary systems, similar in nature to theRS Canum Venaticorum systems, obtained with the Position SensitiveProportional Counter (PSPC) during the ROSAT All-Sky Survey phase of themission. Of this sample, 52 targets were detected in exposures ofroughly 600 s or less. When these new data are combined with the earlierresults from Dempsey et al. (1993b), this survey represents the largestsample of active binary systems observed to date at any wavelength,including X-rays. We expand our investigation of how coronal properties(e.g., surface flux, luminosity, etc.) correlate with stellar parameters(e.g., rotation period, color, etc.) and confirm the conclusions ofDempsey et al. (1993b). Rotation period provides the best correlationwith X-ray surface flux with F_{{X}}~P^{-0.59+/-0.10}_{{rot}} for theentire sample. We find no evidence for a "basal" or nonmagnetic X-rayflux component. We model the low-resolution pulse-height spectra for 12systems with two-temperature thermal plasmas. The derived temperaturesfor the BY Dra systems are identical to those previously derived foractive evolved giants and subgiants in close binaries (Dempsey et al.1993c). We also show that the dependence of temperature and emissionmeasures on rotation period is the same for the dwarf, subgiant, andgiant binaries.
| EUV Emission from RS Canum Venaticorum binaries.
We performed a study of 104 RS CVn systems in the extreme ultraviolet(EUV) using the all-sky survey data obtained by the Extreme UltravioletExplorer (EUVE). The present sample includes several new RS CVndetections; 11 more than in the published EUVE catalogs, and 8 more thanin the ROSAT Wide Field Camera catalog. The ratio of detections tonon-detections remained constant throughout the sky, implying that ourdetections are not limited by the exposure time but are most likelylimited by absorption from the interstellar medium. A general trend ofincreasing Lex/B (50-180Å) flux with decreasing rotational periodis clear. The dwarf systems exhibit a leveling-off for the fasterrotators. In contrast, the evolved systems exhibit no such effect. Forthe RS CVn systems the losses in the EUV represent a smaller fraction ofthe coronal radiative losses, as compared to active late-type dwarfs.
| Photoelectric Minima of Selected Eclipsing Binaries
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| RS CVn Versus Algol-Type Binaries: A Comparative Study of Their X-Ray Emission
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111.2415S&db_key=AST
| A new H-alpha survey of chromospherically active binaries.
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| The emission of the RS CVn binaries in the IRAS passbands.
In the literature, there is an ambiguity pertaining to the existence ofa far-IR excess in RS CVn systems. In the current paper we undertook astudy of the behaviour of 103 such systems in the IRAS passbands. Wefound 72 acceptable detections in the 12μm band, and 40 in the25μm band (50% more than the IRAS Point Source Catalog). Although ourfindings may be interpreted as indicating towards the existence of anexcess beyond 12μm for some systems, the evidence is not conclusivein all but two cases. These are systems GX Lib and HR 7428, with Capellabeing the only system where the IRAS fluxes in all four bands originatefrom the stellar photosphere. Given the accuracy of the data we did notfind 12μm excess for any system. We argue that the IRAS data alonecannot settle the issue, as their uncertainty is, in many cases, higherthan what the Signal-to-Noise ratio of each observation implies.Furthermore, at the higher wavelength bands the IRAS angular resolutiondrops from 0.5' at 12μm to 2' at 100μm and the background becomesvery complex, so one cannot be certain about the origin of the observedflux. The existence of IR excess in the RS CVn stars is important as faras the evolutionary scenarios for these systems are concerned. Futuremissions such as the Infrared Space Observatory will return moresensitive and accurate measurements and the ambiguity can be removed.The fluxes we quote will be helpful when planning these futureobservations, as we provide more accurate photometry and for a largerselection of sources than the IRAS Point Source Catalog.
| Are Rotational Axes Perpendicular to Orbital Planes in Binary Systems. III.Main Sequence and Short-Period RS CVn Stars.
Inclinations of the rotational axes, irot, are determined for46 main sequence binaries of F, G, K and M spectral type and shortperiod RS CVn systems. Seven binaries are asynchronous. The inclinationsirot are then compared with the orbital inclinations,iorb, to test the alignment between the equatorial andorbital planes. In all 39 cases of synchronous rotators irotis equal or nearly equal to iorb. In a sample of sevenasynchronous systems, at least six, and perhaps all, are non-coplanar.
| On the Role of Mass Transfer in X-Ray Emission of RS CVN Systems
Abstract image available at:http://adsabs.harvard.edu/abs/1995AJ....109.2187W
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| The age-mass relation for chromospherically active binaries. I. The evolutionary status
In this paper we present a study of the evolutionary status of a sampleof chromospherically active binary systems for which accuratedeterminations of their stellar parameters are available. Stellar ageshave been obtained by using evolutionary tracks. The agreement betweenthe estimates of ages for the two components of a given system is verygood, which proves the reliability of our method. It has been possibleto separate the chromospherically active binaries in three groups,according to the mass of the primary component: evolved stars withmasses in the range 2.5-5Msun_, evolved stars (subgiants)with M=~1.4Msun_ and main-sequence stars withM=~1.1Msun_. We have found a relationship between stellarmasses and ages of the formLog(Age)=9.883(+/-0.022)-2.965(+/-0.122)Log(Mass/Msun_). Thisrelationship is very close to that for stars on the TAMS. The relationcan be understood, in the framework of the evolution of the componentsand orbital elements of binary systems, as an effect of the increase ofthe stellar radius as the components evolve off the main sequence, andthe decrease of the rotation period due to tidal effects which leads toenhanced chromospheric emission levels, several times higher than thatof the Sun. The relationship has a dependence on rotation, due to thefact that for a given range of masses, younger stars rotate faster. Wehave also found that the more evolved stars are the more active, for agiven interval of rotation periods.
| Eclipse observations of RS CVn binaries. 2: A parametric model of extended matter
We have surveyed ten RS CVn binary systems and have detected largeregions of extended, prominencelike material which we have shown to be acommon feature in these systems. These extended regions are apparent asanomalous absorption features in the spectra when they obscure the diskof the eclipsed star and as weak emission features when they appear withno disk behind. We have constructed a first-generation model to verifythe general prominencelike nature of the material and to estimate itsextent and its most basic physical characteristics. In this paper wepresent our results of our modeling on four of the systems in our surveythat showed clear signatures of extended material: AW Her, MM Her, SSBoo, and UV Psc. We find that these stellar prominencelike structuresare very much larger than their solar counterparts, though their grossphysical characteristics (i.e., their electron density and temperatureand their general appearance as tenuous, vertically oriented,magnetically confined structures) are similar to those seen in solarprominences. These features appear to evolve fairly rapidly, sincemoderate- to high-velocity flows ranging from several tens of km/s to170 km/s are apparent. However, in at least one case (SS Boo) they arerecurrent over a time scale of at least a year. We conclude bydiscussing the implications of this work regarding the nature of closebinary star active structures and their evolution.
| CA II H and K and H alpha emissions in chromospherically active binary systems (RS Canum Venaticorum and BY Draconis)
We report high-resolution observations of the Ca II H and K and Balmer Halpha lines in 53 chromospherically active binary systems including theRS Canum Venaticorum binaries (RS CVn) and BY Draconis (BY Dra) classes.The rotational periods of the sample cover a range from 0.8 to 83.2days. H and K emission-line surface fluxes have been calculated for allthe stars in the sample. In spectra where the H eta line appears inemission, absolute surface fluxes were also determined. The Ca II lineprofiles corresponding to different seasons and orbital phases areanalyzed in order to determine the contribution of each component and tostudy the chromospheric activity variations. We have measured the Halpha equivalent width of the program stars and the H alpha coreemission was determined by subtraction of the equivalent width of apresumably inactive star of the same spectral type and luminosity class.Three different types of H alpha line profiles have been found: strongemission line, weak emission line with strong superposed absorptionprofile, and filled-in absorption line profile.
| The active dynamo stars: RS CVn, BY Dra, FK Com, Algol, W UMa, and T Tau
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| A catalog of chromospherically active binary stars (second edition)
The catalog contains 206 spectroscopic binary systems with at least onelate-type component that shows Ca II H and K emission in its spectrum.These systems include the classical RS CVn binaries and BY Dra binaries.The catalog summarizes information on the photometric, spectroscopic,orbital, and physical properties of the systems as well as space motionsand positions. Up to 42 'parameters' for each stellar system are listedfollowed by the appropriate reference to direct interested colleagues tothe original papers. A comprehensive selection of further informationfor each star is given in the individual notes. In addition, the catalogcontains a candidate list of 138 stars that have similar characteristicsbut are not definitely known binaries or have not had Ca II H and Kemission observed.
| The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems
One hundred and thirty-six RS CV(n) active binary systems were observedwith the ROSAT Position Sensitive Proportional Counter (PSPC) during theAll-Sky Survey component of the mission. The entire sky was surveyed,which represents the largest sample of RS CV(n) systems observed to dateat any wavelength, including X-rays. X-ray surface fluxes for the RSCV(n) systems are found to lie in the range 10 exp 4 to 10 exp 8 ergs/sqcm seconds. Surface flux as a function of (B - V) color is reported. Adecrease in surface flux with increasing rotation period for the entiresample is observed. The rotation period provides the best stellar ororbital parameter to predict the X-ray surface flux level. The absenceof correlation of F(x) or L(x) with Gamma is noted due to the fact thatthe coronal heating mechanism for these active stars must be magnetic incharacter, and the magnetic field depends on the interaction betweenconvection and differential rotation inside the star. X-ray propertiesof the RS CV(n) systems with 6 cm radio and C IV UV emission systems iscompared.
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