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New Times of Minima of Some Eclipsing Binary 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.
| The analysis of indexed astronomical time series - X. Significance testing of O-C data
It is assumed that O-C (`observed minus calculated') values of periodicvariable stars are determined by three processes, namely measurementerrors, random cycle-to-cycle jitter in the period, and possiblylong-term changes in the mean period. By modelling the latter as arandom walk, the covariances of all O-C values can be calculated. Thecovariances can then be used to estimate unknown model parameters, andto choose between alternative models. Pseudo-residuals which could beused in model fit assessment are also defined. The theory is illustratedby four applications to spotted stars in eclipsing binaries.
| New Times of Minima of Some Eclipsing Binary Systems
We present 42 photoelectric minima observations of 24 eclipsingbinaries.
| 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.
| Spectroscopic and Photometric Observations of the Short-Period RS CVn-Type Star UV Piscium
High-resolution spectroscopic observations around the Hα line andlong-term BVRI photometry of the eclipsing short-period RS CVn star UVPsc are presented. The simultaneous solution of our radial velocitycurves and light curves yields the following values for the globalparameters of the components: M1=1.1 Msolar,M2=0.81 Msolar, R1=1.14Rsolar, and R2=0.85 Rsolar. Themeasured rotational broadening of the spectral lines corresponds toequatorial velocities V1=68.3 and V2=53.3 kms-1. Our spectral data reveal high activity of the twostellar components and very fast variability of the Hα line in thecenter of the primary eclipse. Modeling our photometric data showstrends in the starspot behavior. The trend toward active longitude beltscan have occasional exceptions. The observed secular luminosity decreasein 1999 may signal the onset of a new magnetic activity cycle.
| Catalogue of the orbital elements, masses, and luminosities for short-periodic RS CVn-type eclipsing systems
New data on the orbital elements, masses, and luminosities werecollected for 31 pre-contact binary systems of short-periodic {RS}{CVn}-type. We treat the catalogued data statistically in order toaccurately define the properties and evolutionary status of each classsystem. The ages of pre-contact systems were estimated by the isochronemethod. Numerous comments and bibliographic references to the catalogueare also included.
| 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.
| CCD Times of Minima of Selected Eclipsing Binaries
682 CCD minima observations of 259 eclipsing binaries made mainly byauthor are presented. The observed stars were chosen mainly fromcatalogue BRKA of observing programme of BRNO-Variable Star Section ofCAS.
| An Assessment of Dynamical Mass Constraints on Pre-Main-Sequence Evolutionary Tracks
We have assembled a database of stars having both masses determined frommeasured orbital dynamics and sufficient spectral and photometricinformation for their placement on a theoretical H-R diagram. Our sampleconsists of 115 low-mass (M<2.0 Msolar) stars, 27pre-main-sequence and 88 main-sequence. We use a variety of availablepre-main-sequence evolutionary calculations to test the consistency ofpredicted stellar masses with dynamically determined masses. Despitesubstantial improvements in model physics over the past decade, largesystematic discrepancies still exist between empirical and theoreticallyderived masses. For main-sequence stars, all models considered predictmasses consistent with dynamical values above 1.2 Msolar andsome models predict consistent masses at solar or slightly lower masses,but no models predict consistent masses below 0.5 Msolar,with all models systematically underpredicting such low masses by5%-20%. The failure at low masses stems from the poor match of mostmodels to the empirical main sequence below temperatures of 3800 K, atwhich molecules become the dominant source of opacity and convection isthe dominant mode of energy transport. For the pre-main-sequence samplewe find similar trends. There is generally good agreement betweenpredicted and dynamical masses above 1.2 Msolar for allmodels. Below 1.2 Msolar and down to 0.3 Msolar(the lowest mass testable), most evolutionary models systematicallyunderpredict the dynamically determined masses by 10%-30%, on average,with the Lyon group models predicting marginally consistent masses inthe mean, although with large scatter. Over all mass ranges, theusefulness of dynamical mass constraints for pre-main-sequence stars isin many cases limited by the random errors caused by poorly determinedluminosities and especially temperatures of young stars. Adopting awarmer-than-dwarf temperature scale would help reconcile the systematicpre-main-sequence offset at the lowest masses, but the case for this isnot compelling, given the similar warm offset at older ages between mostsets of tracks and the empirical main sequence. Over all age ranges, thesystematic discrepancies between track-predicted and dynamicallydetermined masses appear to be dominated by inaccuracies in thetreatment of convection and in the adopted opacities.
| Spectroscopic and photometric observations of the short-period RS CVn-type star BH Virginis
High-resolution spectroscopic observations around theHα line and BVRI photometry from 1993 to 2003 of theeclipsing short-period RS CVn star BH Vir are presented. Thesimultaneous solution of our radial velocity curves and light curvesyielded the following values for global parameters of the components:M1= 1.173 ± 0.006 Mȯ; M2=1.046 ± 0.005 Mȯ; R1= 1.22 ±0.05 Rȯ; R2= 1.11 ± 0.04Rȯ; i= 87.5° ±0.8°. The measured rotational broadening of the spectrallines corresponds to equatorial velocities V1 = 79.8 kms-1 and V2= 68.4 km s-1. Our datareveal considerable Hα emission excess of the twostellar components. We modelled the photometric data to find the sizeand location of the starspots for each year. The established decreasingtrend of the spot latitudes may indicate a latitudinal cycle of at leasta decade.Based on spectral observations collected at the National AstronomicalObservatory at Rozhen and photometric observations collected at Mt.Laguna Observatory operated by San Diego State University, research wassupported in part by the NATO Linkage grant No. PST.CLG.978810, grantNo. 8/2003 of the Shoumen University, a Cottrell College Science Awardof the Research Corporation, and grants from NASA and the GaposchkinFund administered by the AAS Small Grants Program.Photometric and spectroscopic data are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/424/993Figures \ref{Fig2}-\ref{Fig7} and Figs. \ref{Fig9}-\ref{Fig16} are onlyavailable in electronic form at http://www.edpsciences.org
| The role of the time step and overshooting in the modelling of PMS evolution: The case of EK Cephei
EK Cephei (HD 206821) is a unique candidate to testpredictions based on stellar evolutionary models. It is a double-lineddetached eclipsing binary system with accurate absolute dimensionsavailable and a precise determination of the metallicity. Mostimportantly for our work, its low mass (1.12 Msun) componentappears to be in the pre-main sequence (PMS) phase.We have produced detailed evolutionary models of the binary EKCep using the CESAM stellar evolution code (Morel\cite{morel97}). A χ2-minimisation was performed toderive the most reliable set of modelling parameters (age,αA, αB and Yi). We havefound that an evolutionary age of about 26.8 Myr fits both components inthe same isochrone. The positions of EK Cep A and B in the HR diagramare consistent (within the observational uncertainties) with ourresults.Our revised calibration shows clearly that EK Cep Ais in the beginning of the main sequence, while EK CepB is indeed a PMS star. Such a combination allows for aprecise age determination of the binary, and provides a strict test ofthe modelling. In particular we have found that the definition of thetime step in calculating the PMS evolution is crucial to reproduce theobservations. A discussion of the optimal time step for calculating PMSevolution is presented.The fitting to the radii of both components is a more difficult task;although we managed to do it for EK Cep B, EK Cep A has a lower radiusthan our best models.We further studied the effect of the inclusion of a moderate convectiveovershooting; the calibration of the binary is not significantlyaltered, but the effect of the inclusion of overshooting can be dramaticin the approach to the main sequence of stars with masses high enough toburn hydrogen through the CNO cycle on the main sequence.
| A period investigation of two chromospherically active binary stars: RT Coronae Borealis and PW Herculis
Orbital period variations of two chromospherically active binarysystems, RT CrBand PW Her, arepresented. It is shown that the orbital period of RT CrB undergoes acyclic oscillation with a period of 53.9years. For PW Her, an alternatechange, with a period of 42.7years, is found to superimpose on a rapidsecular increase (dP/dt=+3.53×10-6 days/year). If theperiod oscillations of those two systems are caused by the light-timeeffect of a third body, the analysis for RT CrB indicatesthat the third body would be a low-mass main-sequence star, while, forPW Her, the massof the third body should be no less than 7.8 Msolar. Since nospectral lines of the third body were seen in PW Her from thespectroscopic study by Popper [AJ 100 (1990) 247], if there is a thirdbody in the system, it can only be a black hole. However, as bothcomponents in the two binary stars were showing strong chromosphericactivity, the alternate period variations are more plausibly explainedas the result of magnetic activity cycles. No secular period changes ofRT CrB are found,which is in agreement with the detached evolved configuration of thesystem. The long-term period increase of PW Her may indicatethat it is on an active phase of mass transfer(dm/dt=2.17×10-6 Msolar/year).
| Minimum Times of Several Eclipsing Binaries
We present 26 minima times of 11 eclipsing binaries, observed between1996 and 1999.
| CCD Minima for Selected Eclipsing Binaries in 2002
Not Available
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| Disentangling discrepancies between stellar evolution theory and sub-solar mass stars. The influence of the mixing length parameter for the UV Psc binary
Serious discrepancies have recently been observed between predictions ofstellar evolution models in the 0.7-1.1 Msun mass range andaccurately measured properties of binary stars with components in thismass range. We study one of these objects, the eclipsing binary UVPiscium, which is particularly interesting because Popper(\cite{popper1997}) derived age estimates for each component thatdiffered by more than a factor of two. In an attempt to solve thissignificant discrepancy (a difference in age of 11 Gyr), we compute alarge grid of stellar evolution models with the CESAM code for eachcomponent. By fixing the masses to their accurately determined values(relative error smaller than 1% for both stars), we consider a widerange of possible metallicities Z (0.01 to 0.05), and helium content Y(0.25 to 0.34) uncorrelated to Z. In addition, the mixing lengthparameter alphaMLT is left as another free parameter. Weobtain a best fit in the Teff-radius diagram for a commonchemical composition (Z, Y) = (0.012, 0.31), but a different MLTparameter alphaMLT a = 0.95 +/-0.12(statistical)+0.30(systematic) and alphaMLT b = 0.65 +/-0.07(stat)+0.10(syst). The apparent age discrepancy found by Popper(\cite{popper1997}) disappears with this solution, the components beingcoeval to within 1%. This suggests that fixing alphaMLT toits solar value ( ~ 1.6), a common hypothesis assumed in most stellarevolutionary models, may not be correct. Secondly, sincealphaMLT is smaller for the less massive component, thissuggests that the alphaMLT parameter may decrease withstellar mass, showing yet another shortcoming of the mixing lengththeory to explain stellar convection. This trend needs furtherconfirmation with other binary stars with accurate data.
| 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.
| Studies of Intermediate-Mass Stellar Models Using Eclipsing Binaries
Evolutionary computations for intermediate-mass stars are analyzed usingobserved parameters for eclipsing SB2 binaries and theoreticalparameters based on evolutionary tracks. Modern observations cannot beused to distinguish between models with and without convectiveovershooting for stars in the vicinity of the main sequence.Statistically significant discrepancies between the observed andcomputed stellar parameters are associated with systematic errors inphotometric effective temperatures. After taking into account systematiceffects, the theoretical computations fit the observational datauniformly well throughout the entire mass interval studied. Empiricaland semiempirical (i.e., reduced to the ZAMS and with solar elementalabundances) formulas for the mass-luminosity, mass-effectivetemperature, and mass-radius relations are proposed.
| Absolute Dimensions of the M-Type Eclipsing Binary YY Geminorum (Castor C): A Challenge to Evolutionary Models in the Lower Main Sequence
We present new spectroscopic observations of the detached late-typedouble-lined eclipsing binary YY Geminorum (P=0.814 day), a member ofthe Castor sextuple system and one of the benchmarks for the comparisonbetween observations and stellar evolution theory in the lower mainsequence. In addition, we have reanalyzed existing light curves inseveral passbands using modern techniques that account for theconspicuous presence of spots. This, combined with the spectroscopy, hasyielded a very precise determination of the absolute dimensions of thecomponents, which are virtually identical to each other. We obtain forthe mean mass, radius, and effective temperature the valuesM=0.5992+/-0.0047 Msolar, R=0.6191+/-0.0057Rsolar, and Teff=3820+/-100 K, respectively. Boththe mass and the radius determinations are good to better than 1%, whichin the case of the radius represents a fourfold improvement overprevious results and significantly enhances the value of this quantityfor testing the models. We discuss the importance of systematic effectsin these measurements by comparison with another high-precisiondetermination of the mass by Ségransan and coworkers. Areanalysis of the Hipparcos transit data for Castor AB that accounts forthe relative motion of the pair in its 467 yr period orbit has yieldedan improved parallax for the system of 66.90+/-0.63 mas. With this, wehave estimated the age (~370 Myr) and metal abundance ([Fe/H]~0.0) of YYGem from isochrone fits to Castor A and B under the assumption of acommon origin. This, along with the other physical properties, allow foran unusually stringent test of the models for low-mass stars. We havecompared the observations of YY Gem with a large number of recenttheoretical calculations, and we show that all models underestimate theradius by up to 20% and that most overestimate the effective temperatureby 150 K or more. Both of these trends are confirmed by observations ofanother similar system in the Hyades (V818 Tau). Consequently,theoretical ages for relatively low mass objects such as T Tauri starsderived by placing them on the H-R diagram may be considerably biased.If the radius is used directly as a measure of evolution, ages could beunderestimated by as much as a factor of 10 in this mass regime. In viewof these discrepancies, absolute ages from essentially all currentmodels for the lower main sequence must be viewed with at least somemeasure of skepticism. Finally, we derive a new and very accurateephemeris based on all available times of eclipse, and we lay to restprevious claims of sudden changes in the orbital period of the binary,which we show to be spurious. Some of the observations reported herewere obtained with the Multiple Mirror Telescope, a joint facility ofthe Smithsonian Institution and the University of Arizona.
| 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.
| Long-Term Study of the Starspot Activity on the Eclipsing Short-Period RS Canum Venaticorum Star UV Piscium
We present optical photometry of the short period eclipsing RS CVnsystem, UV Piscium for the years 1966-1984. After removing the spoteffects from the light curves of Vivekananda Rao and Sarma (1981), weanalyzed the cleaned data to obtain system parameters. For each lightcurve, we model the distortion waves in order to study the behaviour ofstarspots in this system.
| YY Geminorum: A Very Late Type Close Binary with Possible Magnetic Stellar Wind
The O-C curve of the very late type close binary (dM1e+dM1e) YYGeminorum is formed and analyzed based on all available times of minimumlight. It is found that the orbital period shows a secular decrease withrate dP/dt=-1.08×10-8 days yr-1. There isweak evidence indicating that a small-amplitude period oscillation issuperposed on the period decrease. YY Gem is a well-detached eclipsingbinary containing two very active components. The secular perioddecrease may suggest that the system is undergoing secular mass andangular momentum loss via a magnetic stellar wind. If thesmall-amplitude oscillation is real, it can be explained either bymagnetic activity cycles or by the presence of a giant planet or browndwarf.
| Polarimetry of evolved stars. I. RS CVn and Mira variables
We present broadband optical polarimetry of 3 RS CVn stars and 3 Miravariables, including the symbiotic star R Aqr, which contains a Miracomponent. Polarimetric variability has been studied on time-scales fromhours to years. Our programme objects at the time of our observationsshowed different forms of P(lambda ) dependence, P∝~λ-4 for most of the RS CVn-type stars, and P∝~λ-2 for the RS CVn-type star UV Psc and the MiraCeti-type variables, and a significant increase of polarization to thered for the Mira R Cet. Combining our data with previously publisheddata, we conclude that most of RS CVn-type and Mira Ceti-type objectsshow evidence of large polarimetric variability at wavelengths shorterthan 0.5 mu m, whereas the level of polarization is more stable in thered. This behaviour is consistent with episodic mass ejection andformation of small dust particles in the circumstellar environment.Although all targets showed polarimetric variability on different timescales, only for IM Peg might these variations be possibly linked withthe photometric period.
| Observational evidence and analysis of the coupling between the dynamical and the thermodynamic processes for close binaries.
Analytical methods of the orbital period, the types of its variations,the mechanisms causing the changes in the orbital period, and problemsand progresses in the orbital period study are introduced. Theproperties of the variations in te orbital period and in the lightcurve, and the connections between the changes of the orbital period andthe light curve in different types of close binaries are also reviewed.The changes of the orbital period, the variations of the light curve andtheir connections in some sample stars are studied. The statisticalrelation of the parameters in contact binaries is also investigated.
| Long-term study of the starspot activity on the eclipsing short-period RS Canum Venaticorum binary UV Piscium
We present optical photometry of the short-period eclipsing RS CVnsystem, UV Piscium for the years 1966-1984. For each light curve, wemodel the distortion waves in order to study the behaviour of starspotsin this system. After removing the spot effects from the light curves,we model the cleaned data to obtain system parameters. We also notechanges in the luminosity of the primary star that are not explained bythe spot variation.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Age and Metallicity Estimates for Moderate-Mass Stars in Eclipsing Binaries
We estimate the ages and metallicities for the components of 43 binarysystems using a compilation of accurate observational data on eclipsingbinaries for which lines of both components are visible in theirspectra, together with two independent modern sets of stellar evolutionmodels computed for a wide range of masses and chemical abundances. Theuncertainties of the resulting values are computed, and their stabilityis demonstrated. The ages and metallicity are compared with thosederived in other studies using different methods, as well as withindependent estimates from photometric observations and observations ofclusters. These comparisons con firm the reliability of our ageestimates. The resulting metallicities depend significantly on thechoice of theoretical model. Comparison with independent estimatesfavors the estimates based on the evolutionary tracks of the Genevagroup.
| Star-Spots Studies for Three Short Period RS CVn-Type Binaries
Light curve analysis for three short-period RS CVn-type binaries wereperformed, using: a) Fourier analysis techniques for light curve changesin the frequency domain; b) photometric curve fit by means of numericalquadratures to develop theoretical light curves appropriate to RS CVnstars. The two methods were applied to the systems ER Vul, BH Vir, andUV Psc. Improved physical and geometrical parameters are found by takingthe average of the results measured by the above two methods.
| Period Changes in Four Short-Period Spotted Binaries: UV Piscium, YY Geminorum, CG Cygni, and XY Ursae Majoris
Recently acquired times of minimum for three RS CVn systems and one BYDraconis binary, all with periods under a day, are presented. Times ofminimum for UV Psc, YY Gem, CG Cyg, and XY UMa have been measured fordecades. These observations have been collected, and the O-C curves foreach system have been examined for period changes. The O-C curves for UVPsc and YY Gem showed a linear deviation, and improved periods have beencomputed. CG Cyg and XY UMa, though, appear to have experienced truechanges in their periods. An additional 11.0 yr periodic variation hasbeen found in the O-C data of XY UMa. The mechanism for the periodvariations remains uncertain.
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