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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.
| V781 Tauri: a W Ursae Majoris binary with decreasing period
We analyze light curves of the W UMa type eclipsing binary V781 Taurifrom three epochs and radial velocity curves from two epochssimultaneously, including previously unpublished B and V data. Theoverall time span is from 1983 to 2000 and the solution is donecoherently in time (not phase) with five light curves and two sets ofprimary and secondary velocity curves. Minor systematic differencesamong the individual light curves are not large enough to undermine thevalue of a coherent solution that represents 18 years of observations.Times of minima confirm a period of 0.34491d and the general solutionfinds a small period change, dP/P, of(5.08±{04})×10-11 that represents recentbehavior. The eclipse timings cover the last half-century and find dP/Pabout four times smaller, corresponding to a period change time scale,P/(dP/dt) of about 6 million years. The system is over-contact with afilling factor of 0.205. The solution produces a temperature differenceof about 260 K between the components, an inclination of 65.9dg, and amass ratio M_2/M_1=2.47. Separate solutions of the several light curvesthat incorporate dark spots find parameters that differ little fromcurve to curve. Absolute masses, luminosities, radii and the distanceare derived, with luminosities and distance based on star 1 being oftype G0V. The orbital angular momentum is compared with those of other WUMa type binaries and is normal. The star to star mass flow that one caninfer from dP/dt is opposite to that expected from TRO (ThermalRelaxation Oscillator) theory, but pertains to a time span that is veryshort compared to the time scale of TRO oscillations.
| 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.
| 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.
| Photometric Investigations of Three Short-Period Binary Systems: GSC 0763-0572, RR Centauri, and ɛ Coronae Australis
CCD photometric observations of the short-period eclipsing binary GSC0763-0572 were carried out in the B and V bands at Yunnan Observatory inChina. Three light minimum times were determined from the observationsand the orbital period of this system was revised. Photometric solutionsfor three W UMa-type stars (i.e., GSC 0763-0572, RR Cen, and ɛCrA) were deduced by the 2003 version of the Wilson-Devinney code withand without third lights. All three systems are found to be A-typeeclipsing binaries with low mass ratios. Combining with theradial-velocity curves for RR Cen (King, Hilditch 1984, MNRAS, 209, 645)and ɛ CrA (Goecking, Duerbeck 1993, A&A, 278, 463), theabsolute parameters for those two binaries were redetermined. Based onall minimum times of RR Cen, it is found that the orbital period shows along-term increase superimposed on a cyclic variation. The secularchange with a rate of dP/dt = +1.21 × 10-7 dyr-1 may suggest that RR Cen is undergoing mass transfer fromthe secondary component to the primary one with dm/dt = +3.12×10-8Modot yr-1. The cyclicoscillation with a period of 65.1(±0.4)yr and an amplitude of0.0124(±0.0007) d may be explained by the presence of a thirdbody, which can be identified by the revised photometric solution. It isbelieved that if the orbital period increases for those systems is true,this kind of binary may evolve into a rapid-rotating single star.
| 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.
| Structure and evolution of low-mass W Ursae Majoris type systems - II. With angular momentum loss
In a previous paper, using Eggleton's stellar evolution code we havediscussed the structure and evolution of low-mass W Ursae Majoris (WUMa) type contact binaries without angular momentum loss (AML). Themodels exhibit cyclic behaviour about a state of marginal contact on athermal time-scale. Part of the time of each cycle is spent in contactand part in a semidetached state. According to observations, W UMasystems suffer AML. We present the models of low-mass contact binarieswith AML due to gravitational wave radiation or magnetic stellar wind(MSW). We find that gravitational radiation cannot prevent the cyclicevolution of W UMa systems, and the effect of gravitational radiation onthe cyclic behaviour of contact binary evolution is almost negligible.We also find that the most likely AML mechanism for W UMa systems ismagnetic braking, and that magnetic braking effects can increase theperiod of the cyclic evolution and shorten the fraction of the timespent in the poor thermal contact state exhibiting EB light curve. If WUMa stars do not undergo cyclic evolution, and their AML is causedsimultaneously by MSW of both components, we find that the value of theparameter, λ, should be taken as about 3.8 for W UMa systems,which is larger than the largest value of similar single stars derivedfrom observations. This indicates that the AML efficiency in W UMasystems may be lowered in comparison with non-contact stars because ofless mass contained in the convective envelopes of the components in WUMa systems or some feedback mechanism which may have an effect on W UMasystems. If W UMa systems lose their angular momentum at a constantrate, an angular momentum rate of d lnJ/dt~ 1.6 × 10-9yr-1 can prevent the cyclic behaviour of the model, and themodel can keep in good contact with an essentially constant depth ofcontact.
| 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
| A W Roberts: the observations (paper 2).
Not Available
| A CCD photometric study of the W UMa contact binary Y Sextantis
A V light curve of the W UMa contact binary Y Sex is presented inthis paper. From the observations, two times of minimum light wasdetermined and from the present times of minimum light and thosecollected from the references, the change in the orbital period of thesystem was analyzed with the method of Kalimeris et al. [A&A 282(1994) 775]. The result reveals that the orbital period of the systemoscillates with a cycle of about 50 years and an amplitude of1.1×10-6 days. The light curve was analyzed by means ofthe latest version of the Wilson-Devinney code, which was also used tocorrect photometric effects on the radial-velocity curve obtained byMcLean and Hilditch [MNRAS 203 (1983) 1]. The results suggest that Y Sex is an A-subtypecontact binary with a mass ratio of q=0.180(2). In the new photometricsolution, a positive value of the third light of the system suggeststhat the sinusoidal variation of the period could be caused by a thirdcomponent in the system. The absolute dimensions of Y Sex are found tobe: M1=1.21(18) Msolar, M2=0.22(3)Msolar, R1=1.50(2) Rsolar,R2=0.75(1) Rsolar, L1=3.00(44)Lsolar, L2=0.69(10) Lsolar, A=2.66(11)Rsolar.
| Are overcontact binaries undergoing thermal relaxation oscillation with variable angular momentum loss?
Orbital period variations of five W-type overcontact binaries, GW Cep,VY Cet, V700 Cyg, EM Lac and AW Vir, are presented based on the analysisof all available times of light minimum. It is discovered that theperiod of GW Cep is decreasing at a rate of dP/dt=-6.62×10-8 d yr-1. For VY Cet and V700 Cyg, acyclic oscillation is found superimposed on a secular period increase,which can be explained either by the light-time effect of an assumedthird body or by magnetic activity cycles. For the other two, EM Lac andAW Vir, the periods show a secular increase. GW Cep is a low mass ratiosystem with q= 0.37, while the others are high mass ratio systems (q=0.67, 0.65, 0.63 and 0.76, respectively). The period changes of the fivesample stars are in good agreement with Qian's conclusion that low massratio overcontact binaries usually show a decreasing period, while theperiods of high mass ratio systems are increasing.Based on the period variations of 59 overcontact binaries, a statisticalinvestigation of period change is given. It is confirmed that the periodchange of a W UMa-type binary star is correlated with the mass ratio (q)and with the mass of the primary component (M1). Meanwhile,some statistical relations (M1-P,Js-M1, Js-M2 andJs-P) for overcontact binaries are presented using theabsolute parameters of 78 systems. From these relations, the followingresults may be drawn: (i) free mass transfer in both directions existsbetween the components, which is assumed by thermal relaxationoscillation (TRO) theory; (ii) angular momentum loss (AML) can make a WUMa-type star maintain shallow overcontact and not evolve fromovercontact to semidetached configurations as proposed by Rahunen; (iii)the evolution of the W UMa-type systems may be oscillation around acritical mass ratio, while the critical mass ratio varies with the massof the primary component. These results can be plausibly explained bythe combination of the TRO and the variable AML via a change of depth ofthe overcontact, which is consistent with the X-ray and IUEobservations.
| Catalogue of the field contact binary stars
A catalogue of 361 galactic contact binaries is presented. Listedcontact binaries are divided into five groups according to the type andquality of the available observations and parameters. For all systemsthe ephemeris for the primary minimum, minimum and maximum visualbrightness and equatorial coordinates are given. If available,photometric elements, (m1+m2)sin3i,spectral type, parallax and magnitude of the O'Connell effect are alsogiven. Photometric data for several systems are augmented by newobservations. The quality of the available data is assessed and systemsrequiring modern light-curve solutions are selected. Selectedstatistical properties of the collected data are discussed.
| The 7.5 Magnitude Limit Sample of Bright Short-Period Binary Stars. I. How Many Contact Binaries Are There?
A sample of bright contact binary stars (W UMa type or EW, and related:with β Lyr light curves, EB, and ellipsoidal, ELL-in effect, allbut the detached, EA) to the limit of Vmax=7.5 mag is deemedto include all discoverable short-period (P<1 day) binaries withphotometric variation larger than about 0.05 mag. Of the 32 systems inthe final sample, 11 systems have been discovered by the Hipparcossatellite. The combined spatial density is evaluated at(1.02+/-0.24)×10-5 pc-3. The relativefrequency of occurrence (RFO), defined in relation to the main-sequencestars, depends on the luminosity. An assumption of RFO~=1/500 forMV>+1.5 is consistent with the data, although the numberstatistics is poor with the resulting uncertainty in the spatial densityand the RFO by a factor of about 2. The RFO rapidly decreases forbrighter binaries to a level of 1/5000 for MV<+1.5 and to1/30,000 for MV<+0.5. The high RFO of 1/130, previouslydetermined from the deep OGLE-I sample of disk population W UMa typesystems toward Baade's window, is inconsistent with and unconfirmed bythe new results. Possible reasons for the large discrepancy arediscussed. They include several observational effects but also apossibility of a genuine increase in the contact-binary density in thecentral parts of the Galaxy. Based on data from the Hipparcos satellitemission and from the David Dunlap Observatory, University of Toronto.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphere
Within the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105
| Orbital period changes of contact binary systems: direct evidence for thermal relaxation oscillation theory
Orbital period changes of ten contact binary systems (S Ant, ɛCrA, EF Dra, UZ Leo, XZ Leo, TY Men, V566 Oph, TY Pup, RZ Tau and AGVir) are studied based on the analysis of their O-C curves. It isdiscovered that the periods of the six systems, S Ant, ɛ CrA, EFDra, XZ Leo, TY Men and TY Pup, show secular increases. For UZ Leo, itssecular period increase rate is revised. For the three systems, V566Oph, RZ Tau and AG Vir, weak evidence is presented that a periodicoscillation (with periods of 20.4, 28.5 and 40.9yr respectively) issuperimposed on a secular period increase. The cyclic period changes canbe explained by the presence of an unseen third body in the threesystems. All the sample stars studied are contact binaries withM1>=1.35Msolar. Furthermore, orbital periodchanges of 27 hot contact binaries have been checked. It is found that,apart from AW UMa with the lowest mass ratio (q=0.072), none shows anorbital period decrease. The relatively weak magnetic activity in thehotter contact binaries means little angular momentum loss (AML) fromthe systems via magnetic stellar winds. The period increases of these WUMa binaries can be explained by mass transfer from the secondary to theprimary components, which is in agreement with the prediction of thethermal relaxation oscillation (TRO) models. This suggests that theevolution of a hotter W UMa star is mainly controlled by TRO. On theother hand, for a cooler W UMa star(M1<=1.35Msolar), its evolution may be TRO plusAML, which coincides with the recent results of Qian.
| A CCD Photometric Study of the Contact Binary V396 Monocerotis
Complete BV light curves of the W Ursae Majoris binary V396 Mon arepresented. The present CCD photometric observations reveal that thelight curves of the system are obviously asymmetric, with the primarymaximum brighter than the secondary maximum (the ``O'Connell effect'').The light curves are analyzed by means of the latest version of theWilson-Devinney code. The results show that V396 Mon is a W-subtype WUMa contact binary with a mass ratio of 0.402. The asymmetry of thelight curves is explained by a cool spot on the secondary component. Thenature of the overluminosity of the secondary of a W UMa-type system isanalyzed. It is shown that the overluminosity of the secondary isclearly related to the mass of the primary and that, for a W UMa system,the higher the mass of the primary, the greater the overluminosity ofthe secondary. In addition, the overluminosity of the secondary is alsorelated to its own density: the lower the density of the secondary, thegreater its overluminosity.
| UY Ursae Majoris: A W-subtype W UMa system with a small mass ratio
We present light curves and photometric solutions of the contact binaryUY UMa in this paper. The light curves appear to exhibit a typicalO'Connell effect, with Maximum I being 0.034 mag(V) and 0.030 mag(B)brighter than Maximum II, respectively, and Maximum I shifting to phase0.26. The light curves are analyzed by means of the latest version ofthe WD program. The results show that UY UMa is a W-subtype contactbinary with a small mass ratio q=0.134. The asymmetry of the lightcurves is explained by star spot models. From the collected data of 9UMa contact systems with a smaller mass ratio than 0.20, the nature ofthe secondaries is analyzed. The results show that the luminosities anddensities of the secondaries could be controlled by the primaries andthat the smaller the mass ratio, the stronger the control.
| ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation
From ROSAT all-sky survey (RASS) data we obtained X-ray fluxes for 57 WUMa type contact systems. In our sample we detected three stars whichare the shortest period main sequence binaries ever found as X-raysources. For stars with (B-V)_0 < 0.6 the normalized X-ray fluxdecreases with a decreasing color index but for (B-V)_0 > 0.6 aplateau is reached, similar to the saturation level observed for single,rapidly rotating stars. The X-ray flux of W UMa stars is about 4-5 timesweaker than that of the fastest rotating single stars. Because earlytype, low activity variables have longer periods, an apparentperiod-activity relation is seen among our stars, while cool stars with(B-V)_0 > 0.6 and rotation periods between 0.23 and 0.45 days do notshow any such relation. The lower X-ray emission of the single, ultrafast rotators (UFRs) and W UMa stars is interpreted as the result of adecreased coronal filling factor. The physical mechanisms responsiblefor the decreased surface coverage differs for UFRs and W UMa systems.For UFRs we propose strong polar updrafts within a convection zone,driven by nonuniform heating from below. The updrafts should beaccompanied by large scale poleward flows near the bottom of theconvective layer and equatorward flows in the surface layers. The flowsdrag dynamo generated fields toward the poles and create a field-freeequatorial region with a width depending on the stellar rotation rate.For W UMa stars we propose that a large scale horizontal flow embracingboth stars will prevent the magnetic field from producing long-livedstructures filled with hot X-ray emitting plasma. The decreased activityof the fastest rotating UFRs increases the angular momentum loss timescale of stars in a supersaturated state. Thus the existence of a periodcutoff and a limiting mass of W UMa stars can be naturally explained.
| Photometric Study of An A-Type Contact Binary: AP Aurigae
We present here a new photometric light curve analysis of the eclipsingbinary AP Aur. The CCD photometry, performed at the Yunnan Observatoryusing the 1.06 m telescope, has given 179 individual data points in Band V bandpass filters. From these data we have obtained two minimumtimes. Based on these two minima, together with some other photoelectricminima collected from the literature, we have determined a new epoch andthe rate of the period variation (p=1.268×10-9 dayscycle-1) of the system. The Wilson-Devinney model (firstdescribed in 1971) was used to derive the photometric solutions. AP Auris a W UMa-type contact binary system. The mass ratio(q=m2/m1=0.246) suggests that the system has anA-type W UMa binary configuration. We find that the asymmetry of thelight curve is most likely caused by mass transfer from the secondary tothe primary or a circulation effect in the common envelope. Theevolutionary status is considered, and we find that AP Aur has justevolved away from the main sequence. Finally the evolutionary future ofthe system is discussed.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| 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.
| CCD photometric study of the contact binary FG Hydrae
A new light curve of the contact binary FG Hya is presented in thispaper. The new light curve and those obtained by other authors from 1955to 1985 show the change in the shape of the light curves of the system.The difference in the depths between the primary and secondary eclipsesincreased yearly from 1955 to 1985, but the present light curve seems toexhibit the same shape as the light curve of Smith in 1955. Thelong-term decrease of the orbital period of the system may be clear ifMahdy et al.'s (1985) timings in question were counted out. The presentlight curve and those obtained by Binnendijk in 1962 and Yang et al. in1982 are all analyzed by means of the latest version of the WD program.The results show that FG Hya is an over-contact binary with a small massratio. Although the light curves of the system changed considerably, thesolutions of the light curves obtained in 1955, 1982 and 1999 are inagreement. Table 4 is only available in electronic form athttp://www.edpsciences.org
| Radial velocities of HIPPARCOS southern B8-F2 type stars
Radial velocities have been determined for a sample of B8-F2 type starsobserved by the Hipparcos satellite. Observations were obtained withinthe framework of an ESO key-program. Radial velocities have beenmeasured using a cross-correlation method, the templates being a grid ofsynthetic spectra. The obtained precision depends on effectivetemperature and projected rotational velocity of the star as well as ona possible asymmetry of the correlation peak generally due to secondarycomponents. New spectroscopic binaries have been detected from theseasymmetries and the variability of the measured radial velocity.Simulations of binary and triple systems have been performed. Forbinaries our results have been compared with Hipparcos binary data.Adding the variable radial velocities, the minimum binary fraction hasbeen found 60% for physical systems. Radial velocities have beendetermined for 581 B8-F2 stars, 159 being new. Taking into accountpublished radial velocities, 39% south A-type stars with V magnitudelower than 7.5 have a radial velocity. Based on observations obtained atthe European Southern Observatory (ESO, La Silla, Chile) and on datafrom the ESA Hipparcos astrometry satellite.}\fnmsep \thanks{Tables 7, 8and 9 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Contact Binaries of the Galactic Disk: Comparison of the Baade's Window and Open Cluster Samples
The paper attempts to integrate the available data for contact binariesof the disk population in a deep Galactic field and in old openclusters. The two basic data sets consist of 98 systems in thevolume-limited 3 kpc subsample of contact binaries detected by the OGLEmicrolensing project toward Baade's window (BW_3) and of 63 members of11 old open clusters (CL). Supplementary data on the intrinsicallybright, but spatially rare, long-period binaries are provided by 238systems in the BW sample to the distance of 5 kpc (BW_5). The basic BW_3sample and the CL sample are remarkably similar in the period, color,luminosity, and variability-amplitude distributions, in spite of verydifferent selections, for BW_3-as a volume-limited subsample of allcontact systems discovered by the OGLE project, and for CL-as acollection of contact systems discovered in open clusters that had beensubject to searches differing in limiting magnitudes, cluster areacoverage, and photometric errors. The contact systems are found in thecolor interval 0.3 < (B-V)_0 < 1.2, where the turn-off points(TOP) of the considered clusters are located; however, they are notconcentrated at the respective TOP locations, but, once the TOP happensto fall in the above color interval, they can appear anywhere within it.The luminosity function for the BW sample appears to be very similar inshape to that for the solar neighborhood main-sequence (MS) stars whencorrections for the Galactic disk structure are applied, which implies aflat apparent frequency-of-occurrence distribution. In the accessibleinterval 2.5 < M_V < 7.5, the frequency of contact binariesrelative to MS stars equals about 1/130 for the exponential disk lengthscale h_R = 2.5 kpc and about 1/100 for h_R = 3.5 kpc. The highfrequency cannot continue for M_V < 2.5 as the predicted numbers ofbright systems would then become inconsistent with the numbers of knownsystems to V_lim = 7.5 in the sky sample. The previous estimate of thefrequency from the BW sample of 1/250-1/300 did not correctly relate thenumbers of the contact binaries to the numbers of MS stars. Themagnitude limit of the OGLE survey limits the accuracy of the currentluminosity function determination for M_V > 5.5, but the availabledata are consistent with a continuation of the high apparent frequencybeyond M_V = 7.5, i.e., past the current short-period, low-luminosityend, delineated by the shortest period field system CC Com at M_V = 6.7.The current data indicate that the sky-field sample of contact binariesstarts showing discovery-selection effects at a level as high as V ~=10-11.
| An observational approach for the determination of gravity darkening in contact binaries of W UMa type
A new method for the determination of the gravity darkening exponents inclose binaries of W UMa type is presented. The method is based onKopal's method of Fourier analysis of the light changes of eclipsingvariables in the Frequency Domain. In the analysis, the exponents forboth components of a W UMa system are assumed to be the same. First, themethod was successfully applied to two theoretical (test) light curves,belonging to two systems with radiative and convective envelopes,respectively. Then the method was applied to 36 W UMa systems for whichgeometric and photometric elements have been derived by the mostpowerful techniques. Systems showing large light curve anomalies wereexcluded from the analysis, since the effectiveness of the methoddepends strongly on the quality of the observations. Our results confirmthe previous results with the assumed, same geometrical parameters.Thederived values of the gravity darkening exponents are very close to thepredicted ones by the existing theory of radiative transfer orconvective equilibrium.
| Absolute Magnitude Calibration for the W UMa-Type Systems Based on HIPPARCOS Data
Hipparcos parallax data for 40 contact binary stars of the W UMa-type(with epsilon M_V < 0.5) are used to derive a new, (B-V)-basedabsolute-magnitude calibration of the form M_V = M_V(log P,B-V). Thecalibration covers the ranges 0.26 < (B-V)_0 < 1.14, 0.24 < P< 1.15 day, and 1.4 < M_V < 6.1; it is based on a solutionweighted by relative errors in the parallaxes (2.7% to 24%). Previouscalibrations have not been based on such a wide period and color space,and while they have been able to predict M_V with sufficient accuracyfor systems closely following the well-known period-color relation, thenew calibration should be able to give also good predictions for moreexotic ``outlying'' contact binary systems. The main limitations of thiscalibration are the inadequate quality of the ground-based photometricdata, and the restriction to the (B-V) index, which is more sensitive tometallicity effects than the (V-I) index; metallicities are, however,basically unknown for the local W UMa-type systems. (SECTION: Stars)
| On the nature of the AM phenomenon or on a stabilization and the tidal mixing in binaries. I. Orbital periods and rotation.
The paper casts a questioning eye on the unique role of the diffusiveparticle transport mechanism in explaining the Am phenomenon and arguesthat the so-called tidal effects might be of great importance incontrolling diffusion processes. A short period cutoff at =~1.2d as wellas a 180-800d gap were found in the orbital period distribution (OPD) ofAm binaries. The existence of the former can be ascribed to the state ofthe primaries with the almost-filled Roche lobes. The latter couldresult from the combined effects of the diffusion, tidal mixing andstabilization processes. Because the tidal mixing might surpassdiffusion in the binaries with the orbital periods P_orb_ less thanseveral hundred days and might thus sustain the He convection zone,which would otherwise disappear, no Am stars should lie below thisboundary. The fact that they are nevertheless seen there implies theexistence of some stabilization mechanism (as, e.g., that recentlyproposed by Tassoul & Tassoul 1992) for the binaries with orbitalperiods less than 180d. Further evidence is given to the fact that theOPD for the Am and the normal binaries with an A4-F1 primary arecomplementary to each other, from which it stems that Am stars are closeto the main sequence. There are, however, indications that they haveslightly larger radii (2.1-3 Rsun_) than expected for theirspectral type. The generally accepted rotational velocity cutoff at=~100km/s is shown to be of little value when applied on Am binaries ashere it is not a single quantity but, in fact, a function of P_orb_whose shape is strikingly similar to that of the curves of constantmetallicity as ascertained from observations. This also leads to thewell known overlap in rotational velocities of the normal and Am starsfor 402.5d.We have exploited this empirical cutoff function to calibrate thecorresponding turbulent diffusion coefficient associated with tidalmixing, having found out that the computed form of the lines of constantturbulence fits qualitatively the empirical shape of the curves ofconstant metallicity. As for larger orbital periods(20d55km/s found by Burkhart(1979) would then be nothing but a manifestation of insufficientlypopulated corresponding area of larger P_orb_.
| The properties of W Ursae Majoris contact binaries: new results and old problems.
The physical properties of W UMa binary systems are revisited on thebasis of the observational data published in the last decade and of therecent theoretical studies on angular-momentum-loss-driven secularevolution. The absolute elements (masses, radii, luminosities) arederived by an inference method and a calibration based on the availablehigh quality spectroscopic orbits. The derived age (8Gy) agrees with theestimate of Guinan and Bradstreet from space motions. The analysis ofthe resulting physical parameters shows little correlation between thestandard classification in A and W subtype (first proposed by Binnendijk(1970) and only related to the light curve morphology) and theevolutionary status and origin of the systems. Most A-subtype systemsseem to have no evolutionary link with W-subtype ones. The relationbetween total mass and mass ratio for the "bona fide" sample alsosuggests that mass loss from the system may play an important role.
| Influence of a third body on the luminosity of a contact binary system
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