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Gamma-ray constraints on the infrared background excess
Motivated by the idea that the recently detected near-infrared (1.2 4μm) excess over the contribution of known galaxies is due toredshifted light from the first cosmic stars [MNRAS 339 (2003) 973], wehave used the effect caused by photon photon absorption on gamma-rayspectra of blazars to put constraints on extragalactic background light(EBL) from the optical to the far-IR bands. Our analysis is mainly basedon the blazar H 1426+428, for which we assume a power-law unabsorbedspectrum. We find that an EBL model with no excess over known galaxiesin the near-infrared background (NIRB) is in agreement with all theconsidered blazars; however, it implies a very peculiar intrinsicspectrum for H 1426+428. Additional data on the blazars 1ES1101-232, H2356-309 and PKS 2155-304 exclude the existence of a strong NIRB excessconsistent with Kelsall’s model of zodiacal light subtraction(ZL); the COBE/DIRBE measurements, after Wright’s model ZLsubtraction, represent a firm NIRB upper limit. The constraints on theoptical EBL are weaker, due to the fact that predictions from differentoptical EBL models are often comparable to the experimental errors. Inthe mid-infrared the SPITZER measurement of νIν = 2.7nW m‑2 sr‑1 at 24 μm gives a goodfit for all the considered blazars.
| 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.
| Tertiary companions to close spectroscopic binaries
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB)with periods from 1 to 30 days for higher-order multiplicity. Asubsample of 62 targets were observed with the NACO adaptive opticssystem and 13 new physical tertiary companions were detected. Anadditional 12 new wide companions (5 still tentative) were found usingthe 2MASS all-sky survey. The binaries belong to 161 stellar systems; ofthese 64 are triple, 11 quadruple and 7 quintuple. After correction forincompleteness, the fraction of SBs with additional companions is foundto be 63% ± 5%. We find that this fraction is a strong functionof the SB period P, reaching 96% for P<3d and dropping to34% for P>12^d. Period distributions of SBs with and withouttertiaries are significantly different, but their mass ratiodistributions are identical. The statistical data on the multiplicity ofclose SBs presented in this paper indicates that the periods and massratios of SBs were established very early, but the periods of SB systemswith triples were further shortened by angular momentum exchange withcompanions.
| 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.
| Spurious `active longitudes' in parametric models of heavily spotted eclipsing binaries
In this paper, the size distributions of starspots extrapolated from thecase of the Sun are modelled on the eclipsing binary SV Cam tosynthesize images of stellar photospheres with high spot fillingfactors. These spot distributions pepper the primary's surface withspots, many of which are below the resolution capabilities ofeclipse-mapping and Doppler-imaging techniques. The light curvesresulting from these modelled distributions are used to determine thelimitations of image reconstruction from photometric data. Surfacebrightness distributions reconstructed from these light curves showdistinctive spots on the primary star at its quadrature points. It isconcluded that two-spot modelling or chi-squared minimization techniquesare more susceptible to spurious structures being generated bysystematic errors, arising from incorrect assumptions about photosphericsurface brightness, than simple Fourier analysis of the light curves.
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| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| 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.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
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| The Chromospherically Active Contact Binary CE Leonis
We present complete VRI light curves of the contact binary CE Leo andprovide detailed photometric solutions, as well as an analysis of itsperiod variation. A total of 73 times of minima over 50 years, includingour observations, were used for the period study. The complex periodvariation can be sorted into a linear period improvement, a period ofP=0.30342771 days, a secular period increase at the rate ofP/P=+3.05×10-7 days yr-1, and a 22.6+/-0.5yr periodic component. The ~22.6 yr periodic variation in the O-Cresiduals most likely arises from the light-travel time effect from alow-mass (m3~0.3Msolarsini) dM tertiary companionmoving in an eccentric (e'=0.61+/-0.04) orbit. However, it isalso possible that this periodic variation arises from the effects of amagnetic activity cycle, known as the Applegate mechanism. The lightcurves show asymmetries in the two different maxima. The 1998 lightcurves show that primary maximunm was 0.042, 0.038, and 0.038 magbrighter than secondary maximum in V, R, and I, respectively. Thedurations of totality for three epochs were 18.3 minutes in 1989, 19.7minutes in 1998, and 18.8 minutes in 2001. The relative depth of primaryminimum was found to vary between 1.20 and 1.32 mag. We have analyzedthe light curves from the three epochs using the 1993 version of theWilson-Devinney differential corrections computer code to find a uniquesolution for CE Leo. The corresponding spectral type of the secondarystar from the colors and effective temperature is ~dK2. The asymmetriclight curves can be explained by the effects of starspots. We find thatthe light curves are best fitted by employing a cool spot on the cooler,larger component of the system. The spot-effect parameter SE=0.05 foundfor CE Leo is relatively large for W UMa binaries.
| 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
| 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
| VLA and MERLIN observations of RZ Cassiopeiae
We present radio interferometric observations of the Algol-type binarysystem RZ Cassiopeiae made with the VLA and MERLIN arrays at 6 cm overan incomplete orbital cycle of the system (1.195 d). We detected RZ Caswith both instruments. The images were unresolved in both cases, withangular extents comparable to the synthesized beams. The peak fluxdensity in the VLA image was 1.14 mJy beam-1 and in theMERLIN image it was 0.93 mJy beam-1. The derived brightnesstemperatures are 4.02 × 108 and 4.35 ×108 K and the effective electron energies are 0.347 and 0.346MeV for the MERLIN and VLA data respectively. The radio light curveshows an interesting modulation centred close to the primary eclipsewhich seems to correlate with ASCA SIS observations of the system. Theresults can be interpreted as an emitting region on the outer hemisphereof the cool component aligned along the centroid axis of the binarysystem.
| Chromospheric activity and unique solution of SZ Psc
We have confirmed that the MgII h and k emission lines of SZ Psc aremainly enhanced by the K subgiant component of the system, using theradial velocities reduced from the emission lines. The intensityvariation of the emission line is correlated with the orbital phase ofSZ Psc. The shape of light curves of SZ Psc were changing every season.The three representative light curves of three different epochs, 1978,1979 and 1981, were selected to find a unique solution of SZ Psc whichcan explain the variation of light curves. The binary parameters,including inclination, mass ratio, temperatures and potentials of bothcomponents for the three epochs coincide, although the shapes of lightcurves for three epochs are different from each other. Changing onlyspot parameters, the model light curves fit to the observed light curvesfor three epochs. This means that the variation of spot location andsize is the main reason for changing the shape of light curves.
| WW Dra - ein heller BAV-Programmstern mit vielen Problemen.
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| 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.
| 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.
| A study of the Mg II 2796.34 Å emission line in late-type normal and RS CVn stars
We carry out an analysis of the Mg II 2796.34 Å emission line inRS CVn stars and make a comparison with the normal stars studied in aprevious paper (Paper I). The sample of RS CVn stars consists of 34objects with known HIPPARCOS parallaxes and observed at high resolutionwith IUE. We confirm that RS CVn stars tend to possess wider Mg II linesthan normal stars having the same absolute visual magnitude. However, wecould not find any correlation between the logarithmic line width logWdeg and the absolute visual magnitude MV (theWilson-Bappu relationship) for these active stars, contrary to the caseof normal stars addressed in Paper I. On the contrary, we find that astrong correlation exists in the (MV, log LMg II)plane (LMg II is the absolute flux in the line). In thisplane, normal and RS CVn stars are distributed along two nearly parallelstraight lines with RS CVn stars being systematically brighter by ~1dex. Such a diagram provides an interesting tool to discriminate activefrom normal stars. We finally analyse the distribution of RS CVn and ofnormal stars in the (log LMg II, log Wdeg) plane,and find a strong linear correlation for normal stars, which can be usedfor distance determinations.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Simultaneous optical and X-ray observations of flares and rotational modulation on the RS CVn binary HR 1099 (V711 Tau) from the MUSICOS 1998 campaign
We present simultaneous and continuous observations of the Hα,Hβ, He I D3, Na I D1, D2 doubletand the Ca II H & K lines for the RS CVn system HR 1099. Thespectroscopic observations were obtained during the MUSICOS 1998campaign involving several observatories and instruments, both echelleand long-slit spectrographs. During this campaign, HR 1099 was observedalmost continuously for more than 8 orbits of 2.8 deg. Two large opticalflares were observed, both showing an increase in the emission ofHα, Ca II H & K, Hβ and He I D3 and a strongfilling-in of the Na I D1, D2 doublet.Contemporary photometric observations were carried out with the robotictelescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories.Maps of the distribution of the spotted regions on the photosphere ofthe binary components were derived using the Maximum Entropy andTikhonov photometric regularization criteria. Rotational modulation wasobserved in Hα and He I D3 in anti-correlation with thephotometric light curves. Both flares occurred at the same binary phase(0.85), suggesting that these events took place in the same activeregion. Simultaneous X-ray observations, performed by ASM on board RXTE,show several flare-like events, some of which correlate well with theobserved optical flares. Rotational modulation in the X-ray light curvehas been detected with minimum flux when the less active G5 V star wasin front. A possible periodicity in the X-ray flare-like events was alsofound.Based on observations obtained during the MUSICOS 98 MUlti-SIteCOntinuous Spectroscopic campaign from Observatoire de Haute-Provence,France, Kitt Peak National Observatory, USA, ESO La Silla, Chile, Mt.Stromlo Observatory, Australia, Xinglong National Observatory, China,Isaac Newton Telescope, Spain, Laboratório Nacional deAstrofísica, Brazil, and South African Astronomical Observatory,South Africa. Contemporaneous observations from Catania, Italy andFairborn Observatories, USA, and on data obtained with the Rossi X-rayTiming Explorer.
| 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.
| Time series photometric spot modelling V. Phase coherence of spots on UZ Librae
We present spot models for nine years of continuous VI_C photometry ofUZ Lib from 1993-2001. The relatively stable double-humped light curveshape suggests extreme phase coherence. From the spot-modellinganalysis, we found that the major spots or spot groups are alwayslocated on the hemisphere facing the secondary star and exactly in theopposite hemisphere anti-facing the secondary. Several single-humpedlight curves and our suggested binary scenario rule out a pureellipsoidal variability as the cause of the double-humped light curveshape. We try to explain this preferred spot pattern with amagnetic-field structure that connects the two components, as suggestedearlier for RS CVn stars in general. A possible 4.8 years spot cycle isfound from the long-term brightness variations but needs confirmation.We rediscuss the basic astrophysical data of UZ Lib. The Hipparcosparallax is likely wrong, a possible reason could be that UZ Lib ise.g., a triple system.
| Gravitational quadrupole-moment variations in active binaries
The principal characteristics of the observed orbital period variationin magnetically active close binaries are briefly reviewed and thetheoretical models proposed to interpret them are presented. Inparticular, we focus on the models proposed by Applegate (1992) andLanza, Rodonò& Rosner (1998a) to explain the short-termmodulation of the orbital period, as a consequence of the changes of thegravitational quadrupole moment of the active component driven by acyclic hydromagnetic dynamo. Recent observational results support ourinterpretation and the constraints on the intensity of the internalmagnetic fields that are required by the proposed mechanism, arediscussed. A novel, stringent test of our model based on futureasteroseismic space observations of oscillation mode splitting is alsobriefly presented.
| Solar spots as prototypes for stellar spots
What is the nature of what we commonly refer to as starspots? The answerto that question determines what we can learn from the study ofstarspots about stars, their internal dynamics, and their magneticactivity. Observations of the Sun, the only cool star that we canobserve in detail, naturally led to the hypothesis that starspots inother cool stars are magneto-convective phenomena in which a magneticfield substantially affects the structure of a stellar atmosphere.Spectral line features, eclipse light curves, and intensity modulationsare consistent with that hypothesis. Yet for almost all stars for whichstarspots are reported, the surface coverage is very much higher thanfor the Sun, while many stars manifest spots at high latitudes where theSun has never been seen to do so. In this review, I address how wellstarspots compare to sunspots, discuss some differences expected whenmoving away from the Sun in the HR diagram or in level of activity, andidentify resources to further deepen our understanding.
| 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.
| Polarimetry of evolved stars. I. RS CVn and Mira variables
We present broadband optical polarimetry of 3 RS CVn stars and 3 Miravariables, including the symbiotic star R Aqr, which contains a Miracomponent. Polarimetric variability has been studied on time-scales fromhours to years. Our programme objects at the time of our observationsshowed different forms of P(lambda ) dependence, P∝~λ-4 for most of the RS CVn-type stars, and P∝~λ-2 for the RS CVn-type star UV Psc and the MiraCeti-type variables, and a significant increase of polarization to thered for the Mira R Cet. Combining our data with previously publisheddata, we conclude that most of RS CVn-type and Mira Ceti-type objectsshow evidence of large polarimetric variability at wavelengths shorterthan 0.5 mu m, whereas the level of polarization is more stable in thered. This behaviour is consistent with episodic mass ejection andformation of small dust particles in the circumstellar environment.Although all targets showed polarimetric variability on different timescales, only for IM Peg might these variations be possibly linked withthe photometric period.
| Long-term starspot evolution, activity cycle and orbital period variation of RT Lacertae
A sequence of V-band light curves of the active close binary RT Lacertae(G5+G9 IV), extending from 1965 to 2000, is presented and analysed toderive the spot distribution and evolution on the component stars. Inour modelling approach, the Roche geometry and Kurucz's atmosphericmodels were adopted. The resulting maps of the spot surface distributionwere regularized by means of the Maximum Entropy and Tikhonov criteriato take full advantage of the increased geometrical resolution duringeclipses. By comparing the maps obtained with these two criteria, it waspossible to discriminate between surface features actually required bythe data and artifacts introduced by the regularization process.Satisfactory fits were obtained assuming spots on both components andthe unspotted V-band luminosity ratio: LG5/LG9 IV= 0.65 +/- 0.05. The more massive G5 primary appears to be the mostactive star in the system and its spotted areas are mainly responsiblefor the light curve distortions. The yearly spot distributions on bothcomponents indicate that their spot patterns consist of two components,one uniformly and the other non-uniformly distributed in longitude, thelatter suggesting the presence of preferential longitudes. Inparticular, spots are concentrated around the substellar points andtheir antipodes on both stars. The eclipse scanning reveals spots withdiameters of ~ 40deg, or possibly smaller, on the hemisphereof the primary star being occulted. The primary shows clear evidence fora short-term activity cycle with a period of ~ 8.5 yr and a possiblelong-term cycle with a period of approximately 35 yr. The variation ofthe spot migration rate may be related with surface differentialrotation, with a lower limit of Delta Omega /Omega ~ 3.2x10-3. The G9 IV secondary does not show evidence for anactivity cycle, its spot coverage appearing rather constant at ~ 15-20%of its surface. The relative amplitude of its surface differentialrotation, as indicated by the variation of the spot migration rate, isDelta Omega /Omega ~ 2.7x 10-3. The variation of the orbitalperiod shows a correlation with the activity level of the primarycomponent. Specifically, the decreases of the orbital period appear tobe associated with minimum spottedness and sizeable changes of thesurface spot distribution that may be related to increases of therotation rate of the spot pattern. Conversely, an episode of increase ofthe orbital period was related to an increase of the spotted area on theprimary star. Such results support the recently proposed models thatconnect the perturbations of the orbital dynamics with the variation ofthe figure of equilibrium of the active components, due to the operationof non-linear hydromagnetic dynamos in their extended convectiveenvelopes.
| Optical positions of 55 radio stars from astrolabe observations from the Yunnan Observatory
The observations by the photoelectric astrolabe at Yunnan Observatoryrelative to the Hipparcos Catalogue and the optical positions of 55radio stars were obtained from observations between 1991 and 2000. Theyall resulted from processing the photon counts obtained by means of theastrolabe after the automation of the instrument. There are 46 stars incommon with the Hipparcos Catalogue. Tables \ref{t1} and \ref{t2} arealso available in electronic form at the CDS via anonymous ftpcdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/1062.
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