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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.
| 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.
| 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.
| 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 University of New South Wales Extrasolar Planet Search: a catalogue of variable stars from fields observed between 2004 and 2007
We present a new catalogue of variable stars compiled from the datataken for the University of New South Wales Extrasolar Planet Search.From 2004 October to 2007 May, 25 target fields were each observed forone to four months, resulting in ~87000 high-precision light curves with1600-4400 data points. We have extracted a total of 850 variable lightcurves, 659 of which do not have a counterpart in the General Catalogueof Variable Stars, the New Suspected Variables catalogue or the All SkyAutomated Survey southern variable star catalogue. The catalogue isdetailed here, and includes 142 Algol-type eclipsing binaries, 23 ?Lyrae-type eclipsing binaries, 218 contact eclipsing binaries, 53 RRLyrae stars, 26 Cepheid stars, 13 rotationally variable active stars,153 uncategorized pulsating stars with periods <10 d, including? Scuti stars, and 222 long period variables with variability ontime-scales of >10 d. As a general application of variable starsdiscovered by extrasolar planet transit search projects, we discussseveral astrophysical problems which could benefit from carefullyselected samples of bright variables. These include (i) the quest forcontact binaries with the smallest mass ratio, which could be used totest theories of binary mergers; (ii) detached eclipsing binaries withpre-main-sequence components, which are important test objects forcalibrating stellar evolutionary models and (iii) RR Lyrae-typepulsating stars exhibiting the Blazhko effect, which is one of the lastgreat mysteries of pulsating star research.
| New GCVS Versions for Three Southern Constellations
We are currently working on a version of the General Catalogue ofVariable Stars (GCVS) revised taking into account the new dataaccumulated since the 4th GCVS edition. A draft new version will bereleased for each constellation as soon as the work for theconstellation is finished. It will contain all stars of the 4th GCVSedition plus a complete catalogue of the stars added to the GCVS in theName Lists of Variable Stars Nos. 67 - 78. Now we are ready for thefirst release, containing more than 1300 variable stars in theconstellations of Antlia, Ara, and Telescopium. When preparing therelease, we actively used modern data-mining possibilities to improvevariability types and light elements. This paper introduces the firstrelease of the new GCVS version and presents new results (types, lightelements), based mainly on data mining, for 213 stars.
| VSOP: the variable star one-shot project. I. Project presentation and first data release
Context: About 500 new variable stars enter the General Catalogue ofVariable Stars (GCVS) every year. Most of them however lackspectroscopic observations, which remains critical for a correctassignement of the variability type and for the understanding of theobject. Aims: The Variable Star One-shot Project (VSOP) is aimed at (1)providing the variability type and spectral type of all unstudiedvariable stars, (2) process, publish, and make the data available asautomatically as possible, and (3) generate serendipitous discoveries.This first paper describes the project itself, the acquisition of thedata, the dataflow, the spectroscopic analysis and the on-lineavailability of the fully calibrated and reduced data. We also presentthe results on the 221 stars observed during the first semester of theproject. Methods: We used the high-resolution echelle spectrographsHARPS and FEROS in the ESO La Silla Observatory (Chile) to survey knownvariable stars. Once reduced by the dedicated pipelines, the radialvelocities are determined from cross correlation with synthetic templatespectra, and the spectral types are determined by an automatic minimumdistance matching to synthetic spectra, with traditional manual spectraltyping cross-checks. The variability types are determined by manuallyevaluating the available light curves and the spectroscopy. In thefuture, a new automatic classifier, currently being developed by membersof the VSOP team, based on these spectroscopic data and on thephotometric classifier developed for the COROT and Gaia space missions,will be used. Results: We confirm or revise spectral types of 221variable stars from the GCVS. We identify 26 previously unknown multiplesystems, among them several visual binaries with spectroscopic binaryindividual components. We present new individual results for themultiple systems V349 Vel and BCGru, for the composite spectrum star V4385Sgr, for the T Tauri star V1045 Sco, andfor DM Boo which we re-classify as a BY Draconisvariable. The complete data release can be accessed via the VSOP website.Based on data obtained at the La Silla Observatory, European SouthernObservatory, under program ID 077.D-0085.
| 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.
| Physical parameters and multiplicity of five southern close eclipsing binaries
Aims.We detected tertiary components of close binaries from spectroscopyand light curve modelling, investigated the light-travel time effect andthe possibility of magnetic activity cycles, measured mass ratios forunstudied systems, and derived absolute parameters. Methods: We carriedout new photometric and spectroscopic observations of five bright (< 10.5 mag) close eclipsing binaries, predominantly in thesouthern skies. We obtained full Johnson BV light curves, which weremodelled with the Wilson-Devinney code. Radial velocities were measuredwith the cross-correlation method using IAU radial velocity standards asspectral templates. Period changes were studied with the O-C method,utilising published epochs of minimum light (XY Leo) and ASAS photometry(VZ Lib). Results: For three objects (DX Tuc, QY Hya, V870 Ara),absolute parameters have been determined for the first time. Wespectroscopically detected the tertiary components in XY Leo and VZ Liband discovered one in QY Hya. For XY Leo we updated the light-timeeffect parameters and detected a secondary periodicity of about 5100 din the O-C diagram that may hint at the existence of short-periodmagnetic cycles. A combination of recent photometric data shows that theorbital period of the tertiary star in VZ Lib is likely to be over 1500d. QY Hya is a semi-detached X-ray active binary in a triple system withK and M-type components, while V870 Ara is a contact binary with thethird smallest spectroscopic mass ratio for a W UMa star to date (q =0.082 ± 0.030). Being close to the theoretical minimum forcontact binaries, this small mass ratio suggests that V870 Ara has thepotential of constraining evolutionary scenarios of binary mergers. Theinferred distances to these systems are compatible with the Hipparcosparallaxes.Based on observations made at the Siding Spring Observatory, Australia.Light curves and radial velocity data are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/943
| 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.
| 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.
| Key parameters of W UMa-type contact binaries discovered by HIPPARCOS
A sample of W UMa-type binaries which were discovered by the HIPPARCOSsatellite was constructed with the aid of well defined selectioncriteria described in this work. The selection process showed up thatseveral systems of which the variability types have been assigned as EBin HIPPARCOS catalogue are genuine contact binaries of W UMa-type. Thelight curves of the 64 selected systems based on HIPPARCOS photometrywere analyzed with the aid of light curve synthesis method by Rucinskiand their geometric elements (namely mass ratio q, degree of contact f,and orbital inclination i) were determined. The solutions were obtainedfor the first time for many of the systems in the sample and would be agood source for their future light curve analyses based on more precisefollow-up observations.Based on observations made with the ESA HIPPARCOSastrometry satellite.
| 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.
| 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.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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Observation and Astrometry data
| Constellation: | Βωμός |
| Right ascension: | 18h08m22.68s |
| Declination: | -56°46'01.8" |
| Apparent magnitude: | 8.967 |
| Distance: | 99.9 parsecs |
| Proper motion RA: | -19.7 |
| Proper motion Dec: | 14 |
| B-T magnitude: | 9.812 |
| V-T magnitude: | 9.037 |
Catalogs and designations:
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