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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.
| Evolutionary Status of Late-Type Contact Binaries
The old model of an unevolved, cool contact binary, in which thesecondary component is strongly oversized due to energy transfer fromthe primary, and the whole system is out of thermal equilibrium,encounters serious problems.I present a new scenario for evolution of contact binaries, which solvesthe problem of thermal nonequilibrium by assuming that contact binariesare past mass exchange with a mass ratio reversal. The scenario isdivided into three phases. In PhaseI loss of angular momentum (AM) dueto magnetized wind of a detached binary is followed until the primarycomponent fills its critical Roche lobe. In Phase II mass transfer takesplace until mass ratio reversal. Arguments are given in favor of such aprocess in pre-contact binaries. In PhaseIII an approximate evolutionarypath of the contact binary is followed until a possible coalescence. AMloss, evolutionary effects of the components and mass transfer to theprimary are taken into account.It is concluded that WUMa type binaries are old objects with secondariesin an advanced evolutionary stage, possibly with small helium cores.Both components fulfill the mass-radius relation for contact binarieswhile being in thermal equilibrium.
| 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binary V781 Taurifrom three epochs and radial velocity curves from two epochssimultaneously, including previously unpublished B and V data.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.
| 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 young binary brown dwarf in the R-CrA star formation region
We present imaging and spectroscopic observations with HST (WFPC2,ACS/HRC and STIS), VLT (FORS2) and Keck (HIRES) of the dM 8 ultra-cooldwarf DENIS-P J185950.9-370632, located in the R-CrAregion. The presence of lithium absorption at 670.8 nm and the strongHα emission indicate a young age and a sub-stellar mass. Ourdiffraction-limited images resolve a companion at the separation limitof HST/ACS (0.06 arcsec). The 2.1 mJy flux in the LW2 filter(5.0-8.5 μm) of the Infrared Space Observatory\citep{1999A&A...350..883} likely corresponds to an infrared excess,suggesting the presence of circumstellar material. Proper motion andphotometric measurements, as well as the Hα activity, confirmmembership in the R-CrA star forming region. If confirmed by furtherobservations, DENIS-P J185950.9-370632 would be the first accretingsub-stellar multiple system observed to date.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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.
| Erdgebundene gegenuber Satelliten-Beobachtung.
Not Available
| Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F andlater and derived ``overall'' broadening functions in selectedwavelength regions utilizing a Least Squares Deconvolution (LSD)procedure. Precision values of the projected rotational velocity v \siniwere derived from the first zero of the Fourier transformed profiles andthe shapes of the profiles were analyzed for effects of differentialrotation. The broadening profiles of 70 stars rotating faster than v\sini = 45 km s-1 show no indications of multiplicity nor ofspottedness. In those profiles we used the ratio of the first two zerosof the Fourier transform q_2/q_1 to search for deviations from rigidrotation. In the vast majority the profiles were found to be consistentwith rigid rotation. Five stars were found to have flat profilesprobably due to cool polar caps, in three stars cuspy profiles werefound. Two out of those three cases may be due to extremely rapidrotation seen pole on, only in one case (v \sini = 52 km s-1)is solar-like differential rotation the most plausible explanation forthe observed profile. These results indicate that the strength ofdifferential rotation diminishes in stars rotating as rapidly as v \sini>~ 50 km s-1.Table A.1 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/813Based on observations collected at the European Southern Observatory, LaSilla, 69.D-0015(B).
| On the performance of GAIA on photometry of eclipsing binaries: The case of four near-contact and contact systems
The light curves of four eclipsing binaries (two near-contact and twocontact) are analysed by means of light curve synthesis techniques toderive the geometric and photometric elements and the physicalparameters of the systems. For the analysis we used ground basedphotometric observations and Hipparcos/Tycho photometric data, whichmimic the photometric observations that should be obtained by GAIA, theapproved Cornerstone 6 mission by ESA. The results are compared and theachievable precision of the basic stellar parameters derived by GAIAphotometry is 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.
| AD Cancri: A Contact Binary with Components in Poor Thermal Contact
We present the light curve and photometric solutions of the contactbinary AD Cnc. The light curve appears to exhibit a typical O'Connelleffect, with Maximum I brighter than Maximum II by 0.010 mag. in V. From1987 to 2000, the light curve showed changes of shape: the depth of theprimary eclipse increased by about 0.056m while that of thesecondary eclipse decreased by about 0.032m, so thedifference between the primary and the secondary eclipses increased byabout 0.088m, while there was no obvious variation in theO'Connell effect. Using the present and past times of minimum light, thechanges in the orbital period of the system are analyzed. The resultreveals that the orbital period of AD Cnc has continuously increased ata rate of {d p}/{d t}=4.4× 10-7 day yr-1.The light curve is analyzed by means of the latest version of theWilson-Devinney code. The results show that AD Cnc is a W-subtypecontact binary with a small mass ratio of 0.267 and the two componentsare in poor thermal contact. AD Cnc has a component temperaturedifference exceeding 500K, and exhibits a shallow contact of 3.6%. Theasymmetry of the light curves is explained by the star spot model. Wepresent the equivalent widths of 15 extrasolar-planet host stars.Thesedata were based on the high-resolution, high signal-to-noise ratiospectra obtained with the 2.16m telescope at Xinglong station. The errorin the Xinglong equivalent width is estimated by a comparison of thesedata with those given in previous studies of common stars.
| V432 Persei: A Contact Binary with Components in Poor Thermal Contact
CCD photometric observation of the short-period eclipsing binary V432Persei was carried out in the B and V bands at the Yunnan Observatory ofChina. The light curves of the system are obviously asymmetrical, withthe primary maximum brighter than the secondary maximum, which is knownas the O'Connell effect. The presented light curves are analyzed bymeans of the latest version of the Wilson-Devinney program. A grid ofsolutions for several fixed values of the mass ratio was calculated. Thebest fitting possible is for a mass ratio of 0.269 and a low degree ofcontact. The results show that V432 Per is seen to be a W-subtype WUrsae Majoris contact binary in poor thermal contact. The differencebetween the mean temperatures of the components is about 850 K. Theasymmetry of the light curves is explained by a cool spot on thesecondary component. The nature of the overluminosity of the secondaryof the system suggests that there should be very great energy transferfrom the primary to the secondary.
| 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.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| 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.
| On the Variability of F1-F9 Luminosity Class III-V Stars
Hipparcos Satellite photometry of F1-F9 luminosity class III-V starsindicates that most are not particularly variable. A few stars for whichfurther study is desirable are identified.
| Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud
We present the ROSAT All-Sky Survey data in a 126 deg2 areain and around the CrA star forming region. With low-resolutionspectroscopy of unidentified ROSAT sources we could find 19 new pre-mainsequence stars, two of which are classical T Tauri stars, the othersbeing weak-lined. The spectral types of these new T Tauri stars rangefrom F7 to M6. The two new classical T Tauri stars are located towardstwo small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram withisochrones for an assumed distance of 130 pc, the distance of the mainCrA dark cloud. The new off-cloud weak-line T Tauri stars may haveformed in similar cloudlets, which have dispersed recently.High-resolution spectra of our new T Tauri stars show that they havesignificantly more lithium absorption than zero-age main-sequence starsof the same spectral type, so that they are indeed young. From thosespectra we also obtained rotational and radial velocities. For somestars we found the proper motion in published catalogs. The directionand velocity of the 3D space motion - south relative to the galaticplane - of the CrA T Tauri stars is consistent with the dark cloud beingformed originally by a high-velocity cloud impact onto the galacticplane, which triggered the star formation in CrA. We also present VRIJHKphotometry for most of the new T Tauri stars to derive theirluminosities, ages, and masses. Partly based on observations collectedat the 1.52 m and 3.5 m telescopes of the European Southern Observatory,Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and onobservations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope.
| 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
| Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases
We have investigated the variation of coronal X-ray emission duringearly post-main-sequence phases for a sample of 120 late-type starswithin 100 pc, and with estimated masses in the range 1-3Msun, based on Hipparcos parallaxes and recent evolutionarymodels. These stars were observed with the ROSAT/PSPC, and the dataprocessed with the Palermo-CfA pipeline, including detection andevaluation of X-ray fluxes (or upper limits) by means of a wavelettransform algorithm. We have studied the evolutionary history of X-rayluminosity and surface flux for stars in selected mass ranges, includingstars with inactive A-type progenitors on the main sequence and lowermass solar-type stars. Our stellar sample suggests a trend of increasingX-ray emission level with age for stars with masses M > 1.5Msun, and a decline for lower-mass stars. A similar behaviorholds for the average coronal temperature, which follows a power-lawcorrelation with the X-ray luminosity, independently of their mass andevolutionary state. We have also studied the relationship between X-rayluminosity and surface rotation rate for stars in the same mass ranges,and how this relationships departs from the Lx ~vrot2 law followed by main-sequence stars. Ourresults are interpreted in terms of a magnetic dynamo whose efficiencydepends on the stellar evolutionary state through the mass-dependentchanges of the stellar internal structure, including the properties ofenvelope convection and the internal rotation profile.
| New 3 Micron Spectra of Young Stellar Objects with H_2O Ice Bands
We present new ground-based 3 μm spectra of 14 young stellar objectswith H_2O ice absorption bands. The broad absorption feature at 3.47μm was detected toward all objects, and its optical depth iscorrelated with the optical depth of H_2O ice, strengthening an earlierfinding. The broad absorption feature at 3.25 μm was detected towardtwo more sources, and an upper limit is given for a third source. Theoptical depths of the 3.25 μm feature obtained to date are bettercorrelated with the optical depth of the refractory silicate dust thanwith that of H_2O ice. If this trend is confirmed, this would supportour proposed identification of the feature as the C-H stretch ofaromatic hydrocarbons at low temperature. An absorption feature at 3.53μm due to solid methanol was detected for the first time toward MonR2/IRS 2, as well as toward W33A and GL 2136. The wavelengths of theCH_3OH features toward W33A, GL 2136, and NGC 7538/IRS 9 can be fittedby CH_3OH-rich ices, whereas the wavelength of the feature toward MonR2/IRS 2 suggests an H_2O-rich ice environment. Solid methanolabundances toward GL 2136, NGC 7538/IRS 9, and Mon R2/IRS 2 are 3%-5%relative to H_2O ice. There is an additional narrow absorption featurenear 3.47 μm toward W33A. For the object W51/IRS 2, spatiallyresolved spectra from 2 to 4 μm indicate that the H_2O ice is locatedpredominantly in front of the eastern component and that the H_2O iceextinction is much deeper than previously estimated. For the object RNO91, spectra from 2 to 4 μm reveal stellar (or circumstellar) CO gasabsorption and deeper H_2O ice extinction than previously estimated.
| The Low-Mass Membership of the R CrA T Association
Presented here are the preliminary results of a survey which identifies38 stars as likely members of the R CrA T association. This more thandoubles the number of well-established member stars that were previouslyknown in this region. Using a combination of ROSAT imagery with newground-based VRI photometry and classification spectroscopy, the likelyoptical counterparts to 17 X-ray sources seen in the vicinity of thestar R CrA are identified. Seven of these counterparts are believed tobe previously undiscovered association members. New VRI photometry andspectroscopy were also obtained for 39 stars with uncertain membershipstatus from previous Hα/near-IR surveys of this region. Seventeenof these stars are now considered to be likely association members.
| Processing of Icy Mantles in Protostellar Envelopes
We have obtained CO absorption profiles of several young stellar objects(YSOs), spanning a range of mass and luminosity, in order to investigatetheir ice mantle composition. We present the first detection of COtoward the class I YSO L1489 IRS in the Taurus dark cloud. In general,the CO profiles for YSOs show evidence for both processed and pristineices in the same line of sight; strong indirect evidence for CO2 issuggested in R CrA IRS 7, L1489 IRS, Elias 18, and GL 961E. Toward othersources (R CrA IRS 1, IRS 2, W33A, NGC 7538 IRS 9, Mon R2 IRS 2) CO ispresent in (nearly) pure form. We propose an evolutionary scenario toexplain the chemical diversity of the icy mantles toward these objects.
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