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The role of stellar envelope convection in gravity-darkening and its relation to observational data
We study the role of convection in the surface heat flow of low massstellar envelopes with the aid of Paczynski’s public domainprogram program GOB. This role is important, for example, for theanalysis of light curves of close binary stars. We have consideredatmospheric models for a range of masses similar to the components ofcontact or near-contact binaries between 0.4 and 1.1Mȯand examined the effects of changing luminosity, surface temperature andmixing length for stars of given mass. Our presentation includes checksof the modeling against published standard stellar envelopes. A slightprogram modification allowed gravity darkening coefficients for selectedmodels to be calculated directly. If the procedure proposed by Lucy isfollowed, similar values of the index β (˜0.06 0.1) areobtained for a fairly wide range of masses, luminosities and effectivetemperatures of cool stars. There also appears no strong dependence onthe mixing-length parameter α. There are, however, physicaldifferences between the conditions that apply to this derivation of theindex and those of a photosphere distorted by rotation and tides, buthaving net dynamical stability. Thus, the dependency of Te ong was argued to come from the ratio of partial derivatives of theadiabatic constant K for the layer where convection starts; but asubconvective equipotential surface, where K would be constant, cannotcoincide with such a layer. The adopted procedure is thereforeinappropriate, and would involve different heat transfer regimes indifferent radial directions (say, polar or equatorial). A correspondingevaluation for the layer in which the convective flux becomes maximalshows less sensitivity to Te. The corresponding index is thenmuch closer to, or even greater than, the von Zeipel value. Otherarguments are also considered, including those of Anderson and Shu[Anderson, L., Shu, F.H., 1977. ApJ 214, 798] about the independence ofthe convective flux to local gravity, as well as the role ofsuperphotospheric circulation effects, which could reduce the gravitydarkening index to less than its subphotospheric value. Observationalevaluations of gravity darkening for close binary systems, in general,are still inconclusive, due to the strong correlation between β andother parameters characterizing the light curve shape, particularly inthe scale photometric ‘ellipticity’ effects. It is possiblethat very precise light curves that may be obtained in the future willallow this situation to be improved. The general surface distribution ofemergent flux in the far infra-red of Jupiter may be a pointer in thedirection of future studies of the subject.
| Radial Velocity Studies of Close Binary Stars. XII.
Radial velocity measurements and sine-curve fits to the orbital radialvelocity variations are presented for 10 close binary systems: OO Aql,CC Com, V345 Gem, XY Leo, AM Leo, V1010 Oph, V2612 Oph, XX Sex, W UMa,and XY UMa. Most of these binaries have been observed spectroscopicallybefore, but our data are of higher quality and consistency than in theprevious studies. While most of the studied eclipsing pairs are contactbinaries, V1010 Oph is probably a detached or semidetached double-linedbinary, and XY UMa is a detached, chromospherically active system whosebroadening functions clearly show well-defined and localized dark spotson the primary component. A particularly interesting case is XY Leo,which is a member of visually unresolved quadruple system composed of acontact binary and a detached, noneclipsing, active binary with an 0.805day orbital period. V345 Gem and AM Leo are known members of visualbinaries. We found faint visual companions at about 2"-3" from XX Sexand XY UMa.Based on data obtained at the David Dunlap Observatory, University ofToronto.
| The MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| 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.
| Variation in the orbital period of W UMa-type contact systems
The secular variation in the orbital period Porb is studiedas a function of the mass ratio q of the components in a sample of 73contact systems of class W UMa constructed from a survey of current(1991–2003) published photometric and spectroscopic data. Almostall the W UMa-systems (>93% of this sample) are found to have avariation in their orbital periods Porb which alternates insign independently of their division into A-and Wsubclasses. Astatistical study of this sample in terms of the observedcharacteristics dPorb/dt and q showed that on the average thenumbers of increases (35 systems) and decreases (33 systems) in theperiods are the same, which indicates the existence of flows directedalternately from one component to the other and illustrates the cyclicalcharacter of the thermal oscillations. An analysis of the behavior ofdPorb/dt as a function of the mass interval of the primarycomponent yields a more accurate value for the mass ratio, q ≈ 0.4÷ 0.45 at which contact binaries are separated into A-andW-subclasses. No correlations were observed between the fill-out factorfor the outer contact configuration, the total mass of the contactsystem, and the mass ratio of the components, on one hand, and the signof the secular variation in the period. The physical properties andevolutionary features of these systems are discussed.
| Contact Binaries with Additional Components. I. The Extant Data
We have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems.
| Luminosity function of contact binaries based on the All Sky Automated Survey (ASAS)
The luminosity function for contact binary stars of the W UMa type isevaluated on the basis of the All Sky Automated Survey (ASAS)photometric project covering all stars south of δ=+ 28° withina magnitude range 8 < V < 13. Lack of colour indices enforced alimitation to 3374 systems with P < 0.562 d (i.e. 73 per cent of allsystems with P < 1 d) where a simplified MV(logP)calibration could be used. The spatial density relative to themain-sequence FGK stars of 0.2 per cent, as established previously fromthe Hipparcos sample to V= 7.5, is confirmed. While the numbers ofcontact binaries in the ASAS are large and thus the statisticaluncertainties small, derivation of the luminosity function required acorrection for missed systems with small amplitudes and with orbitalperiods longer than 0.562 d; the correction, by a factor of 3, carriesan uncertainty of about 30 per cent.
| New CCD Times of Minima of Eclipsing Binary Systems
We present a total of 208 CCD timings for 103 eclipsing binaries.
| 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.
| Automatic classification of eclipsing binaries light curves using neural networks
In this work we present a system for the automatic classification of thelight curves of eclipsing binaries. This system is based on aclassification scheme that aims to separate eclipsing binary systemsaccording to their geometrical configuration in a modified version ofthe traditional classification scheme. The classification is performedby a Bayesian ensemble of neural networks trained with Hipparcos data ofseven different categories including eccentric binary systems and twotypes of pulsating light curve morphologies.
| New Minima of Selected Eclipsing Close Binaries
We present 180 CCD and photoelectric times of minima of selected closeeclipsing binaries.
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
Not Available
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
Not Available
| Evolution of Close Binary Systems
We collect data on the masses, radii, etc., of three classes of closebinary stars: low-temperature contact binaries (LTCBs), near-contactbinaries (NCBs), and detached close binaries (DCBs). We restrictourselves to systems in which (1) both components are, at leastarguably, near the main sequence, (2) the periods are less than a day,and (3) there is both spectroscopic and photometric analysis leading toreasonably reliable data. We discuss the possible evolutionaryconnections between these three classes, emphasizing the roles played bymass loss and angular momentum loss in rapidly rotating cool stars. Wedescribe a new mechanism, differential rotation as observed in the Sun,which can explain the remarkable efficiency of heat transport in theouter envelopes of contact binaries.
| CCD Observations of Times of Minima of Eclipsing Binaries
120 minima timings are reported for 39 E. B. systems observed from 2002to 2005 with the Rigel telescope at Winer Observatory. The timings weredetermined using a folded light curve analysis of light curves derivedfrom CCD images. Typical timing uncertainties were 30-60 sec.
| Photoelectric Minima of Eclipsing Binaries
Not Available
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| PSST: The Planet Search Survey Telescope
The Planet Search Survey Telescope is an automated small-aperture CCDimaging photometer designed to search for transits by extrasolar planetsacross the disks of their parent stars. It simultaneously observesthousands of stars with apparent R magnitudes between 10 and 13 in afield approximately 6deg×6deg. Stars in thisbrightness range are well within the capability of the high-precisionradial velocity systems that have successfully detected over 100extrasolar planets to date. The combination of the photometric transitdepth and radial velocity amplitude can provide both the radius of theplanet and a good estimate of its mass, since the orbit is nearlyedge-on. As a result, estimates of the planet's density and otherparameters can be obtained.
| On the properties of contact binary stars
We have compiled a catalogue of light curve solutions of contact binarystars. It contains the results of 159 light curve solutions. Theproperties of contact binary stars were studied using the cataloguedata. As is well known since Lucy's (\cite{Lucy68a},b) and Mochnacki's(\cite{Mochnacki81}) studies, primary components transfer their ownenergy to the secondary star via the common envelope around the twostars. This transfer was parameterized by a transfer parameter (ratio ofthe observed and intrinsic luminosities of the primary star). We provethat this transfer parameter is a simple function of the mass andluminosity ratios. We introduced a new type of contact binary stars: Hsubtype systems which have a large mass ratio (q>0.72). These systemsshow behaviour in the luminosity ratio- transfer parameter diagram thatis very different from that of other systems and according to ourresults the energy transfer rate is less efficient in them than in othertypes of contact binary stars. We also show that different types ofcontact binaries have well defined locations on the mass ratio -luminosity ratio diagram. Several contact binary systems do not followLucy's relation (L2/L1 =(M2/M1)0.92). No strict mass ratio -luminosity ratio relation of contact binary stars exists.Tables 2 and 3 are available in electronic form athttp://www.edpsciences.org
| The Impact of Large Databases on the Knowledge of Late-type Close Binaries
This paper presents some results of the analysis of the eclipsingbinaries samples that came out as by-products of the OGLE microlensingsurveys. These experiments monitored millions of stars in the directionof the galactic bulge (OGLE-I), and of the Small Magellanic Cloud(OGLE-II). Their completeness allowed the discovery of rare andinteresting systems. An example is a new group of long period binariesin the SMC with presumably a giant component in contact with thecritical lobe, which dominates the systemic light variation (``β-contacts"). These systems obey a period-luminosity-color relation andcould be used as an auxiliary, but independent, tool for distancedetermination. Another very interesting object, for its implications inthe studies of angular momentum loss processes by magnetic braking andof stellar activity, is the system of shortest known period with M dwarfcomponents, discovered by OGLE-I, BW3 V38, that is the target of aspectroscopic follow-up. The perspectives for close binary star researchin view of future space missions, such as COROT and Eddington arebriefly discussed.
| The BAA Observers' Workshops: Observing eclipsing variables: a beginners' guide
The General Catalogue of Variable Stars (GCVS)1 lists many differentclasses of variable star. Some of these are well known and contain manymembers; others are more obscure and contain only a few examples.However, it is also possible to split variable stars into just two basiccategories: the intrinsic variables ? stars like Mira variables, Cepheidvariables, novae and supernovae ? in which the stars themselves arevarying in brightness, and the extrinsic variables, in which theindividual stars themselves do not actually vary, and prominent amongthese are the eclipsing variables.
| Photometric solution of the eclipsing binary system EF Boötis
The EF Boo eclipsing binary system is studied in the context of opticallight curves and radial velocity curves published in the literature. Thebest-solution leads to an over-contact configuration of W-subclass of WUMa systems with a fill-out factor of 28%. Absolute parameters based onsimultaneous solution of light and radial velocity curves are presented.
| High-resolution Doppler images of the spotted contact binary AE Phe
We present Doppler images of the short period (P= 0.362 d) W UMa binaryAE Phe. In order to obtain the necessary S/N ratio and time resolutionrequired to see individual star-spot features in highly rotationallybroadened profiles, we use least-squares deconvolution, which makes useof the information content of the several thousand lines in a typicalechelle spectrum. This yields a single rotation profile (free ofsidelobes due to blending) per spectrum with a typical S/N ratio ofseveral thousand.We use radial velocity curves, generated from standard profile fittingtechniques, to measure velocity amplitudes and the mass ratio. Failureto model star-spots with this method leads to a biased set of values,and we show that an imaging code is essential if accurate systemparameters are to be derived.Images are reconstructed from four nights of data which revealstar-spots at most latitudes on both components of the common envelopesystem. Our model requires that the primary component be several hundredK cooler than the secondary in order to reproduce the profile depthchanges with phase. In a two-temperature imaging model, we interpretthis as being due to 27 per cent greater - but unresolved - spot fillingon the primary relative to the secondary component. The images revealthat dark spots are present on both stars at various latitudes andlongitudes. Star-spots are also found in the neck region of bothcomponents, which appear to be darker on the side of each star leadingin rotation phase - particularly on the secondary component. Weinvestigate the reproducibility of the images from night to night andconclude that the star-spots evolve significantly on very shorttime-scales, of the order of 1 d. This is significantly faster than theweek time-scales found on active single stars and the Sun.
| CCD Times of Minima of Some Eclipsing Binaries in 2003
We present CCD observations of minima of 31 eclipsing binaries observedby SAVS sky survey in 2003.
| Wer beobachtet mit: AH Virginis.
Not Available
| Photoelectric Minima of Selected Eclipsing Binaries
Not Available
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| 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.
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