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WZ Cephei: A Close Binary at the Beginning of Contact Phase
Photometric photoelectric data of the short-period close binary system,WZ Cephei, are presented. A new photometric analysis with the 2003version of the Wilson-Devinney method confirmed that it is a shallowcontact binary (f ~ 13.3%) with a high level of spot activity on theprimary component. Combining new determined times of light minimum withthe others published in the literature, the period change of the binarystar is investigated. A periodic variation, with a period of 34.2 yearsand an amplitude of 0fd013, was discovered to be superimposed on along-term period decrease (dP/dt = –8.8 ×10–8 days year–1). Both the secularperiod decrease and the shallow contact configuration suggest that thisbinary system is at the beginning of contact phase. It is on the way toevolving into a normal overcontact phase via secular angular momentumloss and/or mass transfer from the more massive component to the lessmassive one. The period oscillation can be explained either by thelight-time effect due to the presence of a third body or by magneticactivity. On the one hand, if it is caused by the light-time effect, themass and the orbital radius of the additional body should be m3 = 0.17 M sun and a 3 = 26.6 AU,respectively, when this body is coplanar to the orbit of the eclipsingpair. On the other hand, since the system shows high levels of spotactivity, the period oscillation may be explained as a consequence ofmagnetic cycles. In this case, the required variation of the quadrupolemoment is calculated to be 9.2 × 1049 g cm2.
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| Estimating Stellar Rotation from Starspot Detection during Planetary Transits
A new method for determining the stellar rotation period is proposedhere, based on the detection of starspots during transits of anextrasolar planet orbiting its host star. As the planet eclipses thestar, it may pass in front of a starspot which will then make itselfknown through small flux variations in the transit light curve. If weare lucky enough to catch the same spot on two consecutive transits, itis possible to estimate the stellar rotational period. This method issuccessfully tested on transit simulations on the Sun yielding thecorrect value for the solar period. By detecting two starspots on morethan one transit of HD 209458 observed by the Hubble Space Telescope, itwas possible to estimate a period of either 9.9 or 11.4 days for thestar, depending on which spot is responsible for the signature in thelight curve a few transits later. Comparison with period estimates of HD209458 reported in the literature indicates that 11.4 days is the mostlikely stellar rotation period.
| On the Period Variations of BH Virginis
In the present work, 17 new times of the light minimum for BHVir werederived from observations by Kjurkchieva etal. (2004, A&A, 424,993). Combining the new determined eclipse times with others compiledfrom the literature, the behavior of their O-C variation wasinvestigated. It has been found that the orbital period of BHVir showssome cyclic variations with three different periods: a long-periodvariation of 51.7years, and two short-period variations of 9.2years and11.8years, respectively. The mechanisms that could explain the periodchanges of the system are discussed.
| Detection of 6-cm radio-continuum emission from an EB (?-Lyrae type) variable star - HIP 68718
We report the 6-cm radio-continuum detection of a previously unknownradio star, HIP 68718, identified by the Hipparcos satellite as an EB or?-Lyrae type optical variable. Only five prior radio detections ofthis type are reported in the literature. The radio source (VLAJ140402-002145) was detected at the 6? level, with an associated<1arcsec position error. The radio-optical separation is 0.2arcsecand is well within the position errors. A ROSAT X-ray source (RXJ1404.0-0021) is located 4.5 arcsec from the radio position. Based on thecomponent spectral types (F5V + G5V), 1.52 d period, variations in thepeak of the light curve and the X-ray and 6-cm radio luminosities, weconclude the star is a candidate RS CVn. The binary is in a near-contactphase based on the asymmetric light curve (O'Connell effect). Evidenceof two faint possible companion objects was also found in the J- andK-band Two-Micron All-Sky Survey images.
| A Mechanism for Orbital Period Modulation and Irregular Orbital Period Variations in Close Binaries
Orbital period modulation is observed in many magnetically active closebinaries. It can be explained by magnetic connection between two stars.Magnetic connection produces weak force between the two stars. As themagnetic field varies periodically, the orbital period also showscyclical variations. The mechanism can also be used to explain irregularorbital period variations and orbital period jumps. The mean surfacemagnetic strength is calculated by using the Radia package, which isdedicated to 3D magnetostatics computation. On the basis of the results,a practical equation is given to calculate surface magnetic strength.
| Multiwavelength optical observations of chromospherically active binary systems. V. FF UMa (2RE J0933+624): a system with orbital period variation
Context: This is the fifth paper in a series aimed at studying thechromospheres of active binary systems using several opticalspectroscopic indicators to obtain or improve orbital solution andfundamental stellar parameters. Aims: We present here the study ofFF UMa (2RE J0933+624), a recently discovered,X-ray/EUV selected, active binary with strong Hα emission. Theobjectives of this work are, to find orbital solutions and definestellar parameters from precise radial velocities and carry out anextensive study of the optical indicators of chromospheric activity. Methods: We obtained high resolution echelle spectroscopic observationsduring five observing runs from 1998 to 2004. We found radial velocitiesby cross correlation with radial velocity standard stars to achieve thebest orbital solution. We also measured rotational velocity bycross-correlation techniques and have studied the kinematic by galacticspace-velocity components (U, V, W) and Eggen criteria. Finally, we havedetermined the chromospheric contribution in optical spectroscopicindicators, from Ca ii H & K to Ca ii IRT lines, using the spectralsubtraction technique. Results: We have found that this system presentsan orbital period variation, higher than previously detected in other RSCVn systems. We determined an improved orbital solution, finding acircular orbit with a period of 3.274 days. We derived the stellarparameters, confirming the subgiant nature of the primary component(MP = 1.67 Mȯ and R sin{i}_P=2.17Rȯ) and obtained rotational velocities (v sin{i}), of33.57 ± 0.45 km s-1 and 32.38 ± 0.75 kms-1 for the primary and secondary components respectively.From our kinematic study, we can deduce its membership to the Castormoving group. Finally, the activity study has given us a betterunderstanding of the possible mechanisms that produce the orbital periodvariation.Based on observations collected with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto(Almería, Spain), operated jointly by the Max-Planck Institutfür Astronomie and the Instituto de Astrofísica deAndalucía (CSIC); with the Nordic Optical Telescope (NOT),operated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway and Sweden, in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias; with the2.1 m Otto Struve Telescope at McDonald Observatory of the University ofTexas at Austin (USA) and with Hobby-Eberly Telescope, which is a jointproject of the University of Texas at Austin, the Pennsylvania StateUniversity, Stanford University, Ludwig-Maximilians-UniversitätMünchen, and Georg-August-Universität Göttingen. Tables 7and 8 are only available in electronic form at http://www.aanda.org
| The role of stellar envelope convection in gravity-darkening and its relation to observational data
We study the role of convection in the surface heat flow of low massstellar envelopes with the aid of Paczynski’s public domainprogram program GOB. This role is important, for example, for theanalysis of light curves of close binary stars. We have consideredatmospheric models for a range of masses similar to the components ofcontact or near-contact binaries between 0.4 and 1.1Mȯand examined the effects of changing luminosity, surface temperature andmixing length for stars of given mass. Our presentation includes checksof the modeling against published standard stellar envelopes. A slightprogram modification allowed gravity darkening coefficients for selectedmodels to be calculated directly. If the procedure proposed by Lucy isfollowed, similar values of the index β (˜0.06 0.1) areobtained for a fairly wide range of masses, luminosities and effectivetemperatures of cool stars. There also appears no strong dependence onthe mixing-length parameter α. There are, however, physicaldifferences between the conditions that apply to this derivation of theindex and those of a photosphere distorted by rotation and tides, buthaving net dynamical stability. Thus, the dependency of Te ong was argued to come from the ratio of partial derivatives of theadiabatic constant K for the layer where convection starts; but asubconvective equipotential surface, where K would be constant, cannotcoincide with such a layer. The adopted procedure is thereforeinappropriate, and would involve different heat transfer regimes indifferent radial directions (say, polar or equatorial). A correspondingevaluation for the layer in which the convective flux becomes maximalshows less sensitivity to Te. The corresponding index is thenmuch closer to, or even greater than, the von Zeipel value. Otherarguments are also considered, including those of Anderson and Shu[Anderson, L., Shu, F.H., 1977. ApJ 214, 798] about the independence ofthe convective flux to local gravity, as well as the role ofsuperphotospheric circulation effects, which could reduce the gravitydarkening index to less than its subphotospheric value. Observationalevaluations of gravity darkening for close binary systems, in general,are still inconclusive, due to the strong correlation between β andother parameters characterizing the light curve shape, particularly inthe scale photometric ‘ellipticity’ effects. It is possiblethat very precise light curves that may be obtained in the future willallow this situation to be improved. The general surface distribution ofemergent flux in the far infra-red of Jupiter may be a pointer in thedirection of future studies of the subject.
| Very Large Array Plus Pie Town Astrometry of 46 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine astrometric positions of 46 radiostars in the International Celestial Reference Frame (ICRF). Positionswere obtained in the ICRF directly through phase referencing of thestars to nearby ICRF quasars whose positions are accurate at the 0.25mas level. Radio star positions are estimated to be accurate at the 10mas level, with position errors approaching a few milliarcseconds forsome of the stars observed. Our measured positions were combined withprevious measurements taken from as early as 1978 to obtainproper-motion estimates for all 46 stars with average uncertainties of~1.7 mas yr-1. We compared our radio star positions andproper motions with the Hipparcos Catalogue data and found consistencyin the reference frames produced by each data set on the 1 σlevel, with errors of ~2.7 mas per axis for the reference frameorientation angles at our mean epoch of 2003.78. No significant spin isfound between our radio data frame and the Hipparcos Celestial ReferenceFrame, with the largest rotation rates of +0.55 and -0.41 masyr-1 around the x- and z-axes, respectively, with 1 σerrors of 0.36 mas yr-1. Thus, our results are consistentwith a nonrotating Hipparcos frame with respect to the ICRF.
| Dynamical evolution of active detached binaries on the logJo-logM diagram and contact binary formation
Orbital angular momentum (OAM, Jo), systemic mass (M) andorbital period (P) distributions of chromospherically active binaries(CAB) and W Ursae Majoris (W UMa) systems were investigated. Thediagrams of and logJo-logM were formed from 119 CAB and 102 WUMa stars. The logJo-logM diagram is found to be mostmeaningful in demonstrating dynamical evolution of binary star orbits. Aslightly curved borderline (contact border) separating the detached andthe contact systems was discovered on the logJo-logM diagram.Since the orbital size (a) and period (P) of binaries are determined bytheir current Jo, M and mass ratio, q, the rates of OAM loss(dlogJo/dt) and mass loss (dlogM/dt) are primary parametersto determine the direction and the speed of the dynamical evolution. Adetached system becomes a contact system if its own dynamical evolutionenables it to pass the contact border on the logJo-logMdiagram. The evolution of q for a mass-losing detached system is unknownunless the mass-loss rate for each component is known. Assuming q isconstant in the first approximation and using the mean decreasing ratesof Jo and M from the kinematical ages of CAB stars, it hasbeen predicted that 11, 23 and 39 per cent of current CAB stars wouldtransform to W UMa systems if their nuclear evolution permits them tolive 2, 4 and 6 Gyr, respectively.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Optical Photometry and Spectroscopy of the Suspected ``Cool Algol'' AV Delphini: Determination of the Physical Properties
We present new spectroscopic and BVRI photometric observations of thedouble-lined eclipsing binary AV Del (period=3.85 days) conducted oversix observing seasons. A detailed radial velocity and light-curveanalysis of the optical data shows the system to be most likelysemidetached, with the less massive and cooler star filling its Rochelobe. The system is probably a member of the rare class of ``coolAlgol'' systems, which are distinguished from the ``classical'' Algolsystems in that the mass-gaining component is also a late-type starrather than a B- or A-type star. By combining the spectroscopic andphotometric analyses, we derive accurate absolute masses for thecomponents of M1=1.453+/-0.028Msolar andM2=0.705+/-0.014Msolar and radii ofR1=2.632+/-0.030Rsolar andR2=4.233+/-0.060Rsolar, as well as effectivetemperatures of 6000+/-200 and 4275+/-150 K for the primary andsecondary, respectively. There are no obvious signs of activity(spottedness) in the optical light curve of the binary.
| The Eclipsing Binary System AR Monocerotis
New differential UBV photoelectric photometry and echelle spectroscopyfor the eclipsing binary AR Mon are presented. A total of 46 radialvelocities for each component are obtained using the TODCOR procedure.We solve the new and previously published multicolor light curvessimultaneously with the new radial velocities using the latest versionof the Wilson-Devinney program. We confirm that AR Mon is a semidetachedbinary consisting of two evolved giant stars and is a member of the rareclass of ``cool Algols.'' The size of the primary, more massivecomponent is about 1/3 that of its limiting lobe, and the secondary staris a lobe filler. We present two solutions, one without third light, theother including amounts of third light of 8% in V, 6% in B, and 5% in U.We determine magnitudes and colors for AR Mon's two components, as wellas for the third star. If real, the latter is estimated to be a dwarfforeground star. The mass and radius of the primary(2.480+/-0.014Msolar, 8.002+/-0.089Rsolar) placethe star very close to the evolutionary track of a 2.5Msolarsingle star of solar chemical composition, indicating that the star mayhave adjusted its structure to mass gained from its companion.Synchronous rotation rates for both stars are in excellent agreementwith vsini values determined from line profiles.
| Flip-flop phenomenon: observations and theory
In many active stars the spots concentrate on two permanent activelongitudes which are 180 ° apart. In some of these stars thedominant part of the spot activity changes the longitude every fewyears. This so-called flip-flop phenomenon has up to now been reportedin 11 stars, both single and binary alike, and including also the Sun.To explain this phenomenon, a non-axisymmetric dynamo mode, giving riseto two permanent active longitudes at opposite stellar hemispheres, isneeded together with an oscillating axisymmetric magnetic field. Here wediscuss the observed characteristics of the flip-flop phenomenon andpresent a dynamo solution to explain them.
| Search for Nanosecond Optical Pulses from Nearby Solar-Type Stars
With ``Earth 2000'' technology we could generate a directed laser pulsethat outshines the broadband visible light of the Sun by 4 orders ofmagnitude. This is a conservative lower bound for the technicalcapability of a communicating civilization; optical interstellarcommunication is thus technically plausible. We have built a pair ofsystems to detect nanosecond pulsed optical signals from a target listthat includes some 13,000 Sun-like stars, and we have made some 16,000observations totaling nearly 2400 hr during five years of operation. Abeam splitter-fed pair of hybrid avalanche photodetectors at the 1.5 mWyeth Telescope at the Harvard/Smithsonian Oak Ridge Observatory(Agassiz Station) triggers on a coincident pulse pair, initiatingmeasurement of pulse width and intensity at subnanosecond resolution. Anidentical system at the 0.9 m Cassegrain at Princeton's Fitz-RandolphObservatory performs synchronized observations with 0.1 μs eventtiming, permitting unambiguous identification of even a solitary pulse.Among the 11,600 artifact-free observations at Harvard, the distributionof 274 observed events shows no pattern of repetition, and is consistentwith a model with uniform event rate, independent of target. With onepossible exception (HIP 107395), no valid event has been seensimultaneously at the two observatories. We describe the search andcandidate events and set limits on the prevalence of civilizationstransmitting intense optical pulses.
| Stellar activity and the long-term use of robotic telescopes
A number of automated and robotic telescopes are nowadays devoted to thesystematic monitoring of magnetically active stars and binary systems atseveral astronomical institutions, all over the world, and their numberis steadily increasing. Standard equipments include wide- andnarrow-band photometers and, more recently, spectroscopic capabilities.The long-term time series that those telescopes are providing turn outto be of paramount importance in order to significantly progress in ourunderstanding of solar-like stellar activity of magnetic origin, thatseemingly affect most of late-type dwarfs and subgiants. Our principalaim is to illustrate which key parameters, that can be derived from suchlong-term time series, determine the appearance and evolution of stellaractivity phenomena in different astrophysical environments other thansolar, and their role in determining the physical characteristics ofstarspots, their surface distribution, filling factor, migration inlatitude and longitude, and evolution in time. By using spots as tracersof stellar rotation, reliable data on stellar differential rotation, theprime motor of magnetic activity, can be derived. Moreover, the activitycycle is the additional fundamental parameter that can be provided bylong-term time series. In order to properly address the study of stellaractivity, an internationally coordinated network of 1-2 m class robotictelescopes dedicated to multi-wavelength systematic observations shouldbe established.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Up-to-Date Linear Elements of Eclipsing Binaries
About 1800 O-C diagrams of eclipsing binaries were analyzed and up-todate linear elements were computed. The regularly updated ephemerides(as a continuation of SAC) are available only in electronic form at theInternet address: http://www.as.ap.krakow.pl/ephem/.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Catalogue of Algol type binary stars
A catalogue of (411) Algol-type (semi-detached) binary stars ispresented in the form of five separate tables of information. Thecatalogue has developed from an earlier version by including more recentinformation and an improved layout. A sixth table lists (1872) candidateAlgols, about which fewer details are known at present. Some issuesrelating to the classification and interpretation of Algol-like binariesare also discussed.Catalogue is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/263
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| Long-term photometric behaviour of the RS CVn binary RT Lacertae
A sequence of the seasonal light curves of RT Lac, covering the period1978-2000, is analysed in the framework of the starspot hypothesis todefine the spot distribution, based on the interpretation of the B-bandobservations. The analysis of the corresponding light curves is madeusing Djurašević's inverse-problem method. To explain thelight-curve variations we modelled the binary system using a Roche modelthat involved regions containing spots on both components. Satisfactoryfits were obtained assuming spots on both components. The more-massiveG5 primary appears to be the most active star in the system and itsspotted areas are mainly responsible for the light-curve distortions.Spots are concentrated around longitudes 45degr -170degr and at highlatitudes (above 45degr ). Our analysis indicates two spots withdiameters of ~ 10degr -50degr on both hemispheres of the primary.However, the less-massive cool component seems to have only one spotwhich covers a relatively small area. Total spotted area of themore-massive primary component indicates clear evidence for a short-termactivity cycle with a period of 8.4 yr, and a possible long-term cyclewith a period of 33.5 yr. The G9IV secondary does not show any evidencefor an activity cycle, its spot coverage appearing rather constant atabout 10% of its surface. The variation of the orbital period seems tobe correlated with the total activity level of the system. Inparticular, the decrement of the orbital period appears to be associatedwith minimum spottedness and sizeable changes of the surface spotpattern distribution on the surface of each star. This result, ifconfirmed by the future observations, can provide further support forrecently proposed models for connection between the magnetic activityand orbital period variations.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| Optical Photometry and X-Ray Monitoring of the ``Cool Algol'' BD +05°706: Determination of the Physical Properties
We present new photometric observations in the BVRI bands of thedouble-lined eclipsing binary BD +05°706 conducted over threeobserving seasons, as well as new X-ray observations obtained with ROSATcovering a full orbital cycle (P=18.9 days). A detailed light-curveanalysis of the optical data shows the system to be semidetached,confirming indications from an earlier analysis by Torres et al.(published in 1998), with the less massive and cooler star filling itsRoche lobe. The system is a member of the rare class of cool Algolsystems, which are different from the ``classical'' Algol systems inthat the mass-gaining component is also a late-type star rather than aB- or A-type star. By combining the new photometry with a reanalysis ofthe spectroscopic observations reported by Torres et al., we deriveaccurate absolute masses for the components ofM1=2.633+/-0.028 Msolar andM2=0.5412+/-0.0093 Msolar, radii ofR1=7.55+/-0.20 Rsolar andR2=11.02+/-0.21 Rsolar, as well as effectivetemperatures of 5000+/-100 and 4640+/-150 K, for the primary andsecondary, respectively. There are obvious signs of activity(spottedness) in the optical light curve of the binary. Our X-ray lightcurve clearly shows the primary eclipse but not the secondary eclipse,suggesting that the primary star is the dominant source of the activityin the system. The depth and duration of the eclipse allow us to infersome of the properties of the X-ray-emitting region around that star.
| Archive of Photometric Plates Obtained at Cluj Astronomical Observatory
The archive of photometric plates obtained at the AstronomicalObservatory of Cluj in the period 1952-1974 is presented. The plateinventory is made within the framework of the Wide-Field Plate Databaseproject. The plates were taken in the regions around 110 variable stars(eclipsing variables and RR Lyrae-type stars), as well as in somecomparison stars regions.
| Time-series photometric spot modelling VI. A new computer code and its application to 23 years of photometry of the active giant IM Pegasi
We present and apply a new computer program named SpotModeL to analyzesingle and multiple bandpass photometric data of spotted stars. It isbased on the standard analytical formulae from Budding and Dorren. Theprogram determines the position, size, and temperature of up to threespots by minimizing the fit residuals with the help of theMarquardt-Levenberg non-linear least-squares algorithm. We also expandthis procedure to full time-series analysis of differential data, justas real observations would deliver. If multi-bandpass data areavailable, all bandpasses can be treated simultaneously and thus thespot temperature is solved for implicitly. The program may be downloadedand used by anyone. In this paper, we apply our code to an ~23 year longphotometric dataset of the spotted RS CVn giant IM Peg. We extracted andmodelled 33 individual light curves, additionally, we fitted the entireV dataset in one run. The resulting spot parameters reflect the longterm light variability and reveal two active longitudes on thesubstellar point and on the antipode. The radius and longitude of thedominant spot show variations with 29.8 and 10.4 years period,respectively. Our multicolour data suggests that the spot temperature isincreasing with the brightening of the star. The average spottemperature from V,I_C is 3550+/- 150 K or approximately 900 K below theeffective temperature of the star.
| Orbital period modulation in close binaries due to cyclic alpha 2-dynamo activity
The question is answered whether dynamo-generated magnetic fields areable to produce such quadrupole terms in the gravity potential which canexplain the observed cyclic orbital variation of RS CVn stars. We startwith spherical dynamo models with outer convection zones but without anydifferential rotation, i.e. with alpha 2-dynamos which areknown as nonoscillating. With the known anisotropic alpha -tensor ofrapidly rotating stars the magnetic modes with the lowest dynamo numbersare nonaxisymmetric with a slow azimuthal drift. We also find, however,stable (i.e. with the lowest dynamo number) axisymmetric oscillatingmodes but only for a very special, highly inhomogeneous alpha -tensor.The dynamo model is a linear one with an arbitrary field amplitude whichcan be scaled in order to reproduce the observations. The star proves tobe as prolate during the maximum of the toroidal field energy and itproves to be as oblate during the maximum of the poloidal field energy.In the time average the influence of the toroidal field dominates andthe star is slightly prolate. From the computed temporal variations ofthe gravitation quadrupole moment a magnetic field of more than105 G is found in order to produce the period modulation oforder 10-5 which has been observed.
| The Evolution of Cool Algols
We apply a model of dynamo-driven mass loss, magnetic braking, and tidalfriction to the evolution of stars with cool convective envelopes; inparticular, we apply it to binary stars where the combination ofmagnetic braking and tidal friction can cause angular momentum loss fromthe orbit. For the present we consider the simplification that only onecomponent of a binary is subject to these nonconservative effects, butwe emphasize the need in some circumstances to permit such effects inboth components.The model is applied to examples of (1) the Sun, (2) BYDra binaries, (3) Am binaries, (4) RS CVn binaries, (5) Algols, and (6)post-Algols. A number of problems regarding some of these systems appearto find a natural explanation in our model. There are indications fromother systems that some coefficients in our model may vary by a factorof 2 or so from system to system; this may be a result of the chaoticnature of dynamo activity.
| Determination of the Ages of Close Binary Stars on the Main Sequence from Evolutionary Model Stars of Claret and Gimenez
A grid of isochrones, covering a wide range of stellar ages from thezero-age main sequence to 10 billion years, is calculated in the presentwork on the basis of the model stars of Claret and Gimenez withallowance for convective overshoot and mass loss by the components. Theages of 88 eclipsing variables on the main sequence from Andersen'scatalog and 100 chromospherically active stars from Strassmeier'scatalog are calculated with a description of the method of optimuminterpolation. Comparisons with age determinations by other authors aregiven and good agreement is established.
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