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Photometric and spectroscopic analysis of YY CrB
The eclipsing binary YY Coronae Borealis is a short-period contactsystem of the W UMa type (sub-type A) and one of the systems with agreat potential for providing a combined photometric and spectroscopicsolution. Using WD and FOTEL codes we reanalyzed the system, and applieda spotted model to treat the asymmetry of the light curve. The resultsshow that the more massive component is cooler than the less massivewith ΔT˜100K. The system is an overcontact one with fill-outfactor approximately equal to 64% and distance to the system 88±7pc.
| The Minimum Mass Ratio for Contact Close Binary Systems of W Ursae Majoris-type
Extreme mass ratio close binaries of W UMa-type represent an interestingclass of objects in which ˜1M main-sequence star is in contactwith a significantly less massive companion (M˜0.1M). Earliertheoretical investigations of these systems found that there is aminimum mass ratio q=M/M≈0.085-0.095 (obtained for n=3polytrope-fully radiative primary) above which these systems are stable.If the mass ratio is lower than minimum, a tidal instability develops(Darwin's instability). This instability, which is secular, growing on aviscous dissipation timescale, eventually forces the stars to merge intoa single, rapidly rotating object (such as FK Com-type stars or bluestragglers), implying that such systems would not be observed. Thereappear to be, however, some W UMa-type binaries with empiricallyobtained q values below the theoretical limit for stability. The aim ofthis dissertation was to try to resolve the discrepancy between theoryand observations by considering rotating polytropes. Other candidatesystems for stellar mergers such as AM CVn-type stars have also beendiscussed in the dissertation.
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| An Orbital Period Investigation of the Solar-Type Overcontact Binary V700 Cygni
Not Available
| Possible solution to the problem of the extreme mass ratio W UMa-type binaries
When the total angular momentum of a binary system is at a critical(minimum) value, a tidal instability occurs (Darwin's instability),eventually forcing the stars to merge into a single, rapidly rotatingobject. The instability sets in at some critical separation which in thecase of contact binaries corresponds to a minimum mass ratio dependingon dimensionless gyration radius k1. If one considers n = 3polytrope (fully radiative primary with Γ1 = 4/3),k21 = 0.075 and qmin ~ 0.085-0.095.There appears to be, however, some W UMa-type binaries with q valuesvery close, if not below these theoretical limits, implying that primaryin these systems is probably more centrally condensed. We try to solvethe discrepancy between theory and observations by considering rotatingpolytropes. We show by deriving and solving a modified Lane-Emdenequation for n = 3 polytrope that including the effects of rotation doesincrease the central concentration and could reduce qmin toas low as 0.070-0.074, more consistent with the observed population.
| On Period Variations of the K-Type Eclipsing Binary BM Ursae Majoris
From new CCD observations, 11 light minimum times are presented in thispaper. The orbital period change of a W UMa-type eclipsing binary, BMUrase Majoris, was investigated, based on all available times of lightminimum. It was discovered that the orbital period of the binary showscyclic oscillation, superimposed on a secular period decrease. Theamplitude and the period of the cyclic variation are A =0.0101(±0.0004)d and P3 = 30.8(±0.2)yr,respectively. This kind of cyclic variation may be due to a light-timeeffect via an unseen third body. If the existence of an additional bodyis true, it may remove a great amount of angular momentum from thecentral system, and may play an important role in the formation of closebinary. The orbital period continuously decreases at a rate of dP/dt =‑7.49(±0.16) × 10-8d yr-1,which may be due to mass transfer from the primary to the secondary,accompanying mass and angular-momentum loss from the binary system. Theorbital period decrease, accompanying mass and angular momentum loss,would cause the contact degree to increase. Therefore, this binary willevolve into a deep contact configuration.
| New absolute magnitude calibrations for W Ursa Majoris type binaries
Parallaxes of W UMa stars in the Hipparcos catalogue have been analyzed.31 W UMa stars, which have the most accurate parallaxes(σπ/π<0.15) which are neither associated with aphotometric tertiary nor with evidence of a visual companion, wereselected for re-calibrating the Period-Luminosity-Color (PLC) relationof W UMa stars. Using the Lutz-Kelker (LK) bias corrected (mostprobable) parallaxes, periods ({0.26< P< 0.87}, P in days), andcolors ({0.04<(B-V)0<1.28}) of the 31 selected W UMa,the PLC relation have been revised and re-calibrated. The differencebetween the old (revised but not bias corrected) and the new (LK biascorrected) relations are almost negligible in predicting the distancesof W UMa stars up to about 100 pc. But, it increases and may becomeintolerable as distances of stars increase. Additionally, using(J-H)0 and (H-K_s)0 colors from 2MASS (Two MicronAll Sky Survey) data, a PLC relation working with infrared data wasderived. It can be used with infrared colors in the range-0.01<(J-H)0<0.58, and{-0.10<(H-K_s)0<0.18}. Despite of the fact that the2MASS data refer to single epoch observations which are not guaranteedto be taken at maximum brightness of the W UMa stars, the establishedrelation has been found surprisingly consistent and reliable inpredicting LK corrected distances of W UMa stars.
| A New Photometric Investigation of the W UMa-Type Binary BI CVn
New photometric observations and their investigation of the W UMa-typebinary, BI CVn, are presented. The variations of the orbital period wereanalyzed based on 12 new determined times of light minimum together withthe others compiled from the literature. It is discovered that theperiod of BI CVn shows a long-term period decrease at a rate of\dot{P}=-1.51(\pm 0.12)\times {10^{-7}} daysyear-1 while it undergoes a cyclic variation with aperiod of 27.0 years and an amplitude of 0fd0151. Photometricsolutions determined with the Wilson-Devinney method suggest that BI CVnis a contact binary with a degree of contact of 18.0(±1.7)%. Theasymmetry of the light curves was interpreted by the presence of darkspots on both components, and absolute parameters were determined bycombining the photometric elements with the spectroscopic solutionsgiven by Lu. The observed period decrease can be plausibly explained bya combination of the mass transfer from the primary to the secondary andangular momentum loss via magnetic braking. The cyclic periodoscillation suggests that BI CVn is a triple system containing atertiary component with a mass no less than 0.58 M sun in a27.0 year orbit. As in the cases of the other contact binaries (e.g., AHCnc, AP Leo, AD Cnc, and UX Eri), it is possible that this tertiarycompanion played an important role for the formation and evolution ofthe contact system by removing angular momentum from the central systemvia Kozai oscillation or a combination of Kozai cycle and tidalfriction, which causes the eclipsing pair to have a short initialorbital period (e.g., P < 5d). In that case, can theinitially detached system evolve into the present contact configurationvia a combination of magnetic torques from stellar winds and a case Amass transfer?
| Angular momentum and mass evolution of contact binaries
Various scenarios of contact binary evolution have been proposed in thepast, giving hints of (sometimes contradictory) evolutionary sequencesconnecting A- and W-type systems. As the components of close detachedbinaries approach each other and contact binaries are formed, followingevolutionary paths transforms them into systems of two categories:A-type and W-type. The systems evolve in a similar way but underslightly different circumstances. The mass/energy transfer rate isdifferent, leading to quite different evolutionary results. Analternative scenario of evolution in contact is presented and discussed,based on the observational data of over one hundred low-temperaturecontact binaries. It results from the observed correlations amongcontact binary physical and orbital parameters. Theoretical tracks arecomputed assuming angular momentum loss from a system via stellar wind,accompanied by mass transfer from an advanced evolutionary secondary tothe main-sequence primary. A good agreement is seen between the tracksand the observed graphs. Independently of details of the evolution incontact and a relation between A- and W-type systems, the ultimate fateof contact binaries involves the coalescence of both components into asingle fast rotating star.
| High Fill-Out, Extreme Mass Ratio Overcontact Binary Systems. VIII. EM Piscium
CCD photometric observations of the newly discovered close binary, EMPiscium, obtained from 2006 December 4 to 2008 January 7, are presented.The light curves are symmetric and show complete eclipses with aneclipse duration of 54 minutes. When comparing the present light curveswith those published by González-Rojas et al., it is found thatthe depths of the two minima of the light curve have been interchanged,and the positive O'Connell effect has disappeared. The symmetric lightcurves in R and I bands were analyzed with the 2003 version of the W-Dcode. It is found that EM Piscium is a high fill-out overcontact binarysystem (f = 95.3 ± 2.7%) with an extreme mass ratio of q =0.1487, suggesting that it is on the late evolutionary stage oflate-type tidal-locked binaries. Based on the nine instances of lightminimum that we determined and those published by previousinvestigators, it is discovered that the orbital period shows a cyclicperiod variation with a period of 3.3 years, while it undergoes acontinuously rapid increase at a rate of dP/dt = +3.97 ×10-6 days year-1. The cyclic periodreveals the presence of a tertiary companion, which may play animportant role for the formation and evolution of the overcontact binaryby drawing angular momentum from the central system via Kozaioscillation or a combination of Kozai cycle and tidal friction. The highfill-out, the extreme mass ratio, and the rapid period increase maysuggest that the binary system is quickly evolving into a rapid-rotatingsingle star.
| DN Bootis: A low mass-ratio W UMa-type contact binary
New photoelectric BVR light curves and radial velocity curves wereobtained for the HIPPARCOS discovery DN Boo at theTÜBİTAKTÜBİTAK: The Scientific and TechnicalResearch Council of Turkey.1 National Observatory of Turkey(TUG) and Dominion Astrophysical Observatory (DAO), respectively, todetermine physical nature of the variable. The character of the obtainedlight curves and double-lined spectroscopic structures in the obtainedspectra are revealed that DN Boo is a genuine EW type eclipsing binary.During the analysis of our new observations a simultaneous solution werederived for the photometric and spectroscopic data by using the WilsonDevinney code and orbital parameters with absolute dimensions of thesystem were determined for the first time. Finally, the importance ofvery low mass-ratio contact binaries in the late stages of close binaryevolution was discussed.
| The evolutionary status of W Ursae Majoris-type systems
Well-determined physical parameters of 130 W Ursae Majoris (W UMa)systems were collected from the literature. Based on these data, theevolutionary status and dynamical evolution of W UMa systems areinvestigated. It is found that there is no evolutionary differencebetween W- and A-type systems in the M-J diagram, which is consistentwith the results derived from the analysis of observed spectral type andof M-R and M-L diagrams of W UMa systems. M-R and M-L diagrams of W- andA-type systems indicate that a large amount of energy should betransferred from the more massive to the less massive component, so thatthey are not in thermal equilibrium and undergo thermal relaxationoscillation. Moreover, the distribution of angular momentum, togetherwith the distribution of the mass ratio, suggests that the mass ratio ofthe observed W UMa systems decreases with decreasing total mass. Thiscould be the result of the dynamical evolution of W UMa systems, whichsuffer angular momentum loss and mass loss as a result of the magneticstellar wind. Consequently, the tidal instability forces these systemstowards lower q values and finally to rapidly rotating single stars.
| The First Light Curve Analysis of Two Overcontact Binaries: EY Cas and NO Vul
Not Available
| AD Cancri: A Shallow Contact Solar-Type Eclipsing Binary and Evidence for a Dwarf Third Component and a 16 Year Magnetic Cycle
CCD photometric observations of AD Cancri obtained from 2000 March 7 to2004 December 20 are presented. Variations of the light levels at theprimary minimum and both maxima are found. Uniform solutions of foursets of photometric data were derived by using the Wilson-Devinneymethod. The solutions suggest that AD Cancri is a shallow W-type contactbinary (f=8.3%+/-1.3%) with a high mass ratio of 1/q=0.770+/-0.002. Thelong-term variation of the light curve is explained by variabledark-spot models of the more massive component star with a possible 17yr cycle. Our 13 times of light minimum over 5 years, including otherscollected from the literature, have been used for the period study. Thecomplex period changes can be sorted into a long-term period increase atrate of dP/dt=+(4.94+/-0.16)×10-7 days yr-1,a 16.2 yr periodic component (A3=0.0155 days), and a verysmall amplitude period oscillation (A4=0.0051 days,P4=6.6 yr). The existence of third light may indicate thatthere is a tertiary component in the binary system. Solving thefour-band light curves of Samec & Bookmyer, it is found that thecontribution of the tertiary component to the total light of the triplesystem increases with wavelength, which suggests that it is very cooland may be a very red main-sequence star. The small-amplitude periodoscillation may be caused by the light-time effect of the cool tertiarycomponent (M3~0.41 Msolar). The 16.2 yr periodiccomponent in the orbital period and the 17 yr cyclic activity of thedark spot on the more massive component both may reveal that the moremassive component displays solar-type magnetic activity with a cyclelength of about 16 yr.
| Contact Binaries with Additional Components. III. A Search Using Adaptive Optics
We present results of the Canada-France-Hawaii Telescope adaptive optics(AO) search for companions of a homogeneous group of contact binarystars, as a contribution to our attempts to prove the hypothesis thatthese binaries require a third star to become as close as observed. Inaddition to directly discovering companions at separations of>=1″, we introduced a new method of AO image analysis utilizingdistortions of the AO diffraction ring pattern at separations of0.07″-1″. Very close companions, with separations in thelatter range, were discovered in the systems HV Aqr, OO Aql, CK Boo, XYLeo, BE Scl, and RZ Tau. More distant companions were detected in V402Aur, AO Cam, and V2082 Cyg. Our results provide a contribution to themounting evidence that the presence of close companions is a very commonphenomenon for very close binaries with orbital periods <1 day.Based on observations obtained at the Canada-France-Hawaii Telescope,which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.
| Deep, Low Mass Ratio Overcontact Binary Systems. VII. QX Andromedae in the Intermediate-Age Open Cluster NGC 752
QX Andromedae is a short-period eclipsing binary in the intermediate-ageopen cluster NGC 752. Charge-coupled device photometric observations ofthe close binary system obtained from 2004 November 13 to 2006 November18 are presented. It is confirmed that the light curves show partialeclipses, and night-to-night intrinsic variations are seen. As in thecase of AH Cancri in the old open cluster M67, the light curve of QX Andseems to change between A and W types. Both the short- and long-termlight variations suggest that QX And shows strong magnetic activity,which is in agreement with its X-ray observations. The symmetric lightcurves in B and V bands obtained the night of 2004 November 13 wereanalyzed with the new version of the W-D code. It is found that QX Andis a deep overcontact binary system with a high degree of overcontact off = 55.9% and a low mass ratio of q = 0.2327, suggesting that it is inthe late stage of overcontact evolution. Based on our 23 times of lightminimum, including four recently published eclipse times, the orbitalperiod of the eclipsing binary was revised. It was discovered that theorbital period shows a continuous period increase at a rate of dP/dt =+2.48 × 10-7 days yr-1, which can beinterpreted as a mass transfer from the less massive component to themore massive one. As the period increases, the mass ratio of the systemwill decrease. It may finally evolve into a rapid-rotating single starwhen it meets the more familiar criterion that the orbital angularmomentum is less than 3 times the total spin angular momentum. Theexistence of QX And in the late evolutionary stage of an overcontactbinary in the intermediate-age open cluster NGC 752 indicates that itmay undergo strong cluster stellar interaction. This means it had a veryshort initial orbital period and could have evolved into the presentevolutionary state within the cluster age. By comparing with theevolutionary state of TX Cnc in M44, it is estimated that the lifetimeof overcontact binaries may be no less than 1 Gyr.
| The minimum mass ratio of W UMa-type binary systems
When the total angular momentum of a binary system Jtot =Jorb + Jspin is at a certain critical (minimum)value, a tidal instability occurs which eventually forces the stars tomerge into a single, rapidly rotating object. The instability occurswhen Jorb = 3Jspin, which in the case of contactbinaries corresponds to a minimum mass ratio qmin ~0.071-0.078. The minimum mass ratio is obtained under the assumptionthat stellar radii are fixed and independent. This is not the case withcontact binaries where, according to the Roche model, we haveR2 = R2(R1, a, q). By finding a newcriterion for contact binaries, which arises from dJtot = 0,and assuming k21 ≠ k22for the component's dimensionless gyration radii, a theoretical lowerlimit qmin = 0.094-0.109 for overcontact degree f = 0-1 isobtained.
| Formation and Evolution of W Ursae Majoris Contact Binaries
The origin and evolution of W UMa systems are discussed based on All SkyAutomated Survey (ASAS) data and the mean kinematic ages of foursubgroups of 97 field contact binaries (FCBs). The period distributionof eclipsing binaries discovered by ASAS suggests that a period limit totidal locking for the close binaries is about 2.24 days, so that most WUMa systems might be formed from detached binaries with periodsP<~2.24 days, and a maximum advanced time from a detached system to aW UMa is about 3.23 Gyr. Moreover, the secular evolution of the angularmomentum (AM), the system mass, and the orbital period of 97 FCBs wereinvestigated according to the mean kinematic ages, which were setaccording to AM bins. AMs, systemic masses, and orbital periods wereshown to be decreasing with kinematic age. Their first-order decreasingrates have been determined as J˙/J=1.86×10-10yr-1, M˙/M=0.95×10-10 yr-1,and P˙/P=1.24×10-10 yr-1, and theaverage amplification (A¯=dlnJ/dlnM) is derived to be 1.96. Theseare lower than those derived from detached chromospherically activebinaries (CABs). This suggests that the magnetic activity level of FCBsis indeed weaker than that of CABs. Meanwhile, the decreasing rate of AMof FCBs is found to be equal to an average value in a cycle of a cyclicmodel of contact binaries. This might suggest that the evolution of FCBsundergoes thermal relaxation oscillation (TRO) and that the coalescenceof W UMa systems is a very long process, which is also indicated by thedynamical evolution of FCBs.
| The N2K Consortium. VII. Atmospheric Parameters of 1907 Metal-rich Stars: Finding Planet-Search Targets
We report high-precision atmospheric parameters for 1907 stars in theN2K low-resolution spectroscopic survey, designed to identify metal-richFGK dwarfs likely to harbor detectable planets. Of these stars, 284 arein the ideal temperature range for planet searches,Teff<=6000 K, and have a 10% or greater probability ofhosting planets based on their metallicities. The stars in thelow-resolution spectroscopic survey should eventually yield >60 newplanets, including 8-9 hot Jupiters. Short-period planets have alreadybeen discovered orbiting the survey targets HIP 14810 and HD 149143.
| Orbital period investigations of two short-period early-type overcontact binaries BH Cen and V701 Sco in two extremely young galactic clusters IC 2944 and NGC 6383
Both V701 Sco and BH Cen are two early-type short-period overcontactsystems (P = 0.d762 and P = 0.d792, respectively).V701 Sco is a member of the young galactic cluster NGC 6383, while BHCen is a component of a younger galactic cluster IC 2944 where starformation is in process. They provide good opportunity to understand theformation and evolution of binary stars. In the present paper, orbitalperiod changes of the two binaries are investigated. It is discoveredthat the orbital period of BH Cen shows a long-term increase with a rateof dP/dt = +1.70(±0.39) × 10‑7 days/yearwhile it undergoes a cyclic oscillation with a period of 44.6 years andan amplitude of A3 = 0.d0216. For V701 Sco, itsO-C curve reveals a periodic change with a period of 41.2 years andamplitude of A3 = 0.d0158. The mass ratio of BHCen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a massratio of unit. The continuous period increase of BH Cen is caused by themass transfer from the less massive component to the more massive one ata rate of dM2/dt = 3.5 × 10‑6days/year. The cyclic period changes of both systems can be plausiblyexplained as the results of light-travel time effects suggesting thatthey are triple systems. The astrophysical parameters of the unseentertiary components in the two systems have been determined. We thinkthat the invisible tertiary components in both binaries played animportant role in the formations and evolutions of the overcontactconfigurations by bringing angular momentum out from the centralsystems. For BH Cen, this process created the initial short period andwill support its evolution into an overcontact configuration via a CaseA mass transfer within the life time of the extremely young cluster IC2944. For V701 Sco, two identical zero-age main-sequence components inan overcontact configuration suggest that it may have been formed byfission, possibly by the fission of the third body. The fact that nolong-term continuous period variations were found for V701 Sco maysuggest that an overcontact binary with the mass ratio of unity can bein an equilibrium revealing that the original configuration of thebinary was overcontact as is its present state. It has been reportedthat faint stars in the two extremely young clusters are relativelyscare. From the present study, it is shown that faint stars in youngclusters are usually formed as companions of OB stars (includingbinaries). It is very difficult to detect them because of their lowluminosity when compared with the more luminous OB stars.
| Radial Velocity Studies of Southern Close Binary Stars. I. Winter Systems
Radial velocity measurements and sine-curve fits to the orbital velocityvariations are presented for nine contact binaries: V1464 Aql, V759 Cen,DE Oct, MW Pav, BQ Phe, EL Aqr, SX Crv, VZ Lib, and GR Vir. For thefirst five of these, our observations are the first available radialvelocity data. For the three remaining radial velocity variables, CE Hyiis a known visual binary, while CL Cet and V1084 Sco are suspected to bemultiple systems in which the contact binary is spectrally dominated byits companion (which itself is a binary in V1084 Sco). Five additionalvariable stars, V872 Ara, BD Cap, HIP 69300, BX Ind, and V388 Pav, areof unknown type, but most are pulsating stars; we give their mean radialvelocities and Vsini.Based on data obtained at the European Southern Observatory.
| Contact Binaries with Additional Components. II. A Spectroscopic Search for Faint Tertiaries
It is unclear how very close binary stars form, given that during thepre-main-sequence phase the component stars would have been inside eachother. One hypothesis is that they formed farther apart but were broughtin closer after formation by gravitational interaction with a thirdmember of the system. If so, all close binaries should be members oftriple (or higher order) systems. As a test of this prediction, wepresent a search for the signature of third components in archivalspectra of close binaries. In our sample of 75 objects, 23 show evidencefor the presence of a third component, down to a detection limit oftertiary flux contributions of about 0.8% at 5200 Å (consideringonly contact and semidetached binaries, we find 20 out of 66). In ahomogeneous subset of 59 contact binaries, we are fairly confident thatthe 15 tertiaries we have detected are all tertiaries present with massratios 0.28<~M3/M12<~0.75 and implied outerperiods P<~106 days. We find that if the frequency oftertiaries were the same as that of binary companions to solar-typestars, one would expect to detect about 12 tertiaries. In contrast, ifall contact binaries were in triple systems, one would expect about 20.Thus, our results are not conclusive but are sufficiently suggestive towarrant further studies.
| Masses and angular momenta of contact binary stars
Results are presented on component masses and system angular momenta forover 100 low-temperature contact binaries. It is found that thesecondary components in close binary systems are very similar in mass.Our observational evidence strongly supports the argument that theevolutionary process goes from near-contact binaries to A-type contactbinaries, without any need of mass loss from the system. Furthermore,the evolutionary direction of A-type into W-type systems with asimultaneous mass and angular momentum loss is also discussed. Theopposite direction of evolution seems to be unlikely, since it requiresan increase of the total mass and the angular momentum of the system.
| Deep, Low Mass Ratio Overcontact Binary Systems. VI. AH Cancri in the Old Open Cluster M67
CCD photometric light curves in the B and V bands obtained in 2001 andin the V band obtained in 2002 of AH Cnc in the old open cluster M67 arepresented. It is shown that AH Cnc is a total-eclipsing binary and itslight curves correspond to a typical A type according to Binnendijk'sclassification. The variations of the light curve around the primaryminimum and second maximum were found. Our nine epochs of light minimummonitored from 2001 to 2005, including others collected from theliterature, were used to create the first study of the period changes ofthe binary system. A cyclic oscillation with a period of 36.5 yr and anamplitude of 0.0237 days was discovered to be superposed on a continuousperiod increase (dP/dt=3.99×10-7 days yr-1).Weak evidence indicates that there exists another small-amplitude periodoscillation (A4=0.0035 days, P4=7.75 yr). Thesymmetric light curves in the B and V bands obtained in 2001 wereanalyzed with the 2003 version of the Wilson-Devinney code. It isconfirmed that AH Cnc is a deep overcontact binary system with a highdegree of overcontact f=58.5%+/-4.5% and a low mass ratio ofq=0.1682+/-0.0012. The existence of the third light and the cyclicperiod oscillation both may suggest that AH Cnc is a triple systemcontaining an unseen third body. The tertiary component may have playedan important role in the origin of the overcontact binary star byremoving angular momentum from the central system, which would cause itto have a short initial orbital period and thus evolve into anovercontact configuration by angular momentum loss. The long-term periodincrease can be interpreted as a mass transfer from the less massivecomponent to the more massive one. As the orbital period increases, thedecrease of the mass ratio will cause it finally to evolve into a singlerapid-rotating star when the system meets the more familiar criterionthat the orbital angular momentum be less than 3 times the total spinangular momentum. Therefore, AH Cnc may be a progenitor of the bluestraggler stars in M67.
| 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.
| 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.
| Deep, Low Mass Ratio Overcontact Binary Systems. IV. V410 Aurigae and XY Bootis
The complete charge-coupled device (CCD) light curves in the V, R, and Ibands for the eclipsing binary V410 Aur were observed at the YunnanObservatory in China. Four new light minimum times of V410 Aur werederived from new observations. The photometric solution for V410 Aur wasobtained for the first time, while the orbital elements for XY Boo(observed by Binnendijk in 1971) were reanalyzed by using the latestWilson-Devinney code. The third light contributions to the total lightsof the two binaries are obtained. The results reveal that both systemsare A-subtype W UMa binaries with low mass ratios and deep degrees ofovercontact (i.e., qph=0.1428 and f=52.4% for V410 Aur, andqph=0.1855 and f=55.9% for XY Boo). The photometric massratios of the two binaries are very consistent with the spectroscopicmass ratios. By combining the spectroscopic elements with ourphotometric solutions, the absolute photometric parameters for V410 Aurand XY Boo are (re)determined. Analyzing the orbital periods of the twobinaries, it is discovered that their orbital periods show continuousperiod increases, suggesting that the systems are undergoing masstransfer from the less massive component to the more massive one throughthe inner Lagrangian point L1. This kind of binary, with alow mass ratio, a deep degree of overcontact, and a continuous periodincrease, may coalesce into a rapidly rotating single star due to tidalinstability.
| Deep, Low Mass Ratio Overcontact Binary Systems. V. The Lowest Mass Ratio Binary V857 Herculis
Charge-coupled device (CCD) photometric light curves in the B, V, and Rbands of the complete eclipsing binary star V857 Her are presented. Itis shown that the light curves of the W UMa-type binary are symmetricand of A type according to Binnendijk's classification. Our four epochsof light minimum along with others compiled from the literature wereused to revise the period and study the period change. Weak evidenceindicates that the orbital period of V857 Her may show a continuousincrease at a rate of dP/dt=+2.90×10-7 daysyr-1. The photometric parameters of the system weredetermined with the 2003 version of the Wilson-Devinney code. It isshown that V857 Her is a deep overcontact binary system withf=83.8%+/-5.1%. The derived mass ratio of q=0.06532+/-0.0002 suggeststhat it has the lowest mass ratio among overcontact binary systems. Asthe orbital period increases, the decrease of the mass ratio will causeit to evolve into a single rapidly rotating star when it meets the morefamiliar criterion that the orbital angular momentum be less than 3times the total spin angular momentum. To understand the evolutionarystate of the system, long-term photometric monitoring and spectroscopicobservations will be required.
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Observation and Astrometry data
| Constellation: | Virgo |
| Right ascension: | 14h45m20.27s |
| Declination: | -06°44'04.1" |
| Apparent magnitude: | 8.011 |
| Distance: | 53.107 parsecs |
| Proper motion RA: | -82.5 |
| Proper motion Dec: | 47.9 |
| B-T magnitude: | 8.65 |
| V-T magnitude: | 8.064 |
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