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| Bluer colour of UX Arietis at fainter visual magnitudes
Aims.The RS CVn star UX Ari appears bluer when it is fainter, which isunusual for a spotted star; we aim to resolve this unusual behaviour ofthe object. Also the data available in the literature indicate that themean V magnitude of UX Ari varies by about 25 years; we want to confirmwhether this is indeed true. Methods: The most important parameter inunderstanding the colour variation is the relative brightnesses of thecomponent stars of UX Ari. We use all spectroscopic informationavailable in the literature to obtain a reasonable value for thisparameter. We also present new BV photometry of UX Ari obtained on 58nights during 2001-07. Results: The V magnitudes of UX Ari we obtainedare significantly fainter than those reported in the literature, andthey are not consistent with a 25-year period for the mean light level.The increased fractional contribution by the hotter G-type component tothe total light in the blue spectral region as the active star becomesfaint, seems to be the real cause for the bluer colour of UX Ari atfainter visual magnitudes.
| 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
| Patterns of activity in stars with cycles becoming established
Late-type stars with chromospheric and coronal activities exceedingthose of the Sun and other stars with well-defined cycles areconsidered. These rotate more rapidly than stars with well establishedcycles; for single stars, this appears to be due to their younger ages.The spots on such stars cover several per cent of the total area, whichis an order of magnitude higher than for the Sun at its activitymaximum. Our wavelet analysis of the chromospheric-emission variability,which has been observed since 1965 in the framework of the HK project,indicates that the period of the axial rotation of some of these startsvaries from year to year. This is most pronounced in two“Good” stars according to the classification of Baliunas etal., HD 149661 and HD 115404, and also in a star with a more complexvariability, HD 101501. No similar effect is exhibited by the“Excellent” cyclic-activity stars. Such variations in theperiod can be observed during epochs of appreciable rotationalmodulations of the chromospheric-emission fluxes, most likely,immediately after the maximum of a long-period wave (cycle?). This seemsto provide evidence for the existence of huge activity complexes in thechromospheres of these stars, whose longitudes remain virtually constantover several years; they drift from fairly high latitudes to the equatorat speeds close to the value typical of sunspots. The observed periodvariations are most likely due to differential rotation of the same signthat is known for the Sun. Our results provide independent confirmationof similar conclusions obtained by us previously using zonal models forhighly spotted stars. Other activity features of a selected star groupand the implications of the results for the theory of stellar and solardynamos are discussed.
| Formation and Evolution of W Ursae Majoris Contact Binaries
The origin and evolution of W UMa systems are discussed based on All SkyAutomated Survey (ASAS) data and the mean kinematic ages of foursubgroups of 97 field contact binaries (FCBs). The period distributionof eclipsing binaries discovered by ASAS suggests that a period limit totidal locking for the close binaries is about 2.24 days, so that most WUMa systems might be formed from detached binaries with periodsP<~2.24 days, and a maximum advanced time from a detached system to aW UMa is about 3.23 Gyr. Moreover, the secular evolution of the angularmomentum (AM), the system mass, and the orbital period of 97 FCBs wereinvestigated according to the mean kinematic ages, which were setaccording to AM bins. AMs, systemic masses, and orbital periods wereshown to be decreasing with kinematic age. Their first-order decreasingrates have been determined as J˙/J=1.86×10-10yr-1, M˙/M=0.95×10-10 yr-1,and P˙/P=1.24×10-10 yr-1, and theaverage amplification (A¯=dlnJ/dlnM) is derived to be 1.96. Theseare lower than those derived from detached chromospherically activebinaries (CABs). This suggests that the magnetic activity level of FCBsis indeed weaker than that of CABs. Meanwhile, the decreasing rate of AMof FCBs is found to be equal to an average value in a cycle of a cyclicmodel of contact binaries. This might suggest that the evolution of FCBsundergoes thermal relaxation oscillation (TRO) and that the coalescenceof W UMa systems is a very long process, which is also indicated by thedynamical evolution of FCBs.
| GSC 3377-0296 is a New Short-Period Eclipsing RS CVn Variable
GSC 3377-0296 is a new eclipsing RS CVn variable, which has beendiscovered in the ROTSE1 database. A very short period of 0.4224672 dayshas been determined from the available ROTSE1 data and our BVRphotometry in 2006 and 2007. Large changes in the light curve occur ontimescales of a few weeks.
| The Distribution of Activity on the RS CVn-Type Star SZ Piscium
We use contemporaneous spectra and BV light curves to derive a model ofthe distribution of spots and active regions on the cooler component ofSZ Psc for the 1993-1994 season. For that epoch, both spots and activeregions were visible at all rotational phases of this chromosphericallyactive star. The lack of large distortions of Doppler profiles ofoptical lines argues for >~15 spots with angular radii of<~8°-10°. Transition region emission was constant to within+/-12% over the orbit, nor was it eclipsed appreciably. This leads us toestimate that there are at least 20 active regions, perhaps severalhundred. Chromospheric Mg II emission seems to have been proportional tothe unspotted area of the star, not to the area of spots visible. Wedetected one strong flare in transition region lines during thisprogram. We also present new light-curve and radial velocity solutions.These solutions find the more massive, cool star very close to its Rochelobe, and the hot star rotating more slowly than synchronously, makingSZ Psc an important system for tecent he structure and evolution ofbinary stars. Changes in the radial velocities over 2 years ofsubsequent monitoring show that SZ Psc is a triple system. Hαemission seems independent of phase on the average, but it can increasemarkedly for periods of a few orbital cycles.
| Binary Life after the AGB -- Towards a Unified Picture
We attempt to construct a unified evolutionary scheme that includespost-AGB systems, barium stars, symbiotics and related systems,explaining their similarities as well as their differences. Specialattention is given to the comparison of the barium pollution andsymbiotic phenomena. Finally, we outline a `transient torus'evolutionary scenario that makes use of the various observational andtheoretical hints and aims at explaining the observed characteristics ofthe relevant systems.
| Combining astrometry with the light-time effect: The case of VW Cep, ζ Phe and HT Vir
Three eclipsing binary systems with astrometric orbit have been studied.For a detailed analysis two circular-orbit binaries (VW Cep and HT Vir)and one binary with an eccentric orbit (ζ Phe) have been chosen.Merging together astrometry and the analysis of the times of minima, oneis able to describe the orbit of such a system completely. The O-Cdiagrams and the astrometric orbits of the third bodies were analysedsimultaneously for these three systems by the least-squares method. Theintroduced algorithm is useful and powerful, but also time consuming,due to many parameters which one is trying to derive. The new orbits forthe third bodies in these systems were found with periods 30, 221, and261 yr, and eccentricities 0.63, 0.37, and 0.64 for VW Cep, ζ Phe,and HT Vir, respectively. Also an independent approach to compute thedistances to these systems was used. The use of this algorithm to VW Cepgave the distance d=(27.90 ± 0.29) pc, which is in excellentagreement with the previous Hipparcos result.Based on observations secured at the South Africa AstronomicalObservatory, Sutherland, South Africa.
| The MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| Photoelectric radial velocities, Paper XVII The orbits of 30 spectroscopic binaries in the southern Clube Selected Areas
Three of the six southern Clube Areas that were mainly observed at theEuropean Southern Observatory and whose principal results are given inPaper XVI are marginally observable from Haute-Provence. Additionalmeasurements obtained on several observing runs there enabled orbits tobe determined (in most cases rather poorly, owing to the paucity ofdata) for 30 of the newly discovered spectroscopic-binary systems; theyare presented in this paper. One object, HD 33978, seems to be of suchsignificance that special efforts have been made, largely by Drs J.Andersen and H. Lindgren, to obtain additional radial-velocitymeasurements. That system is shown to be a double-lined binary with aperiod of only 10.67 d despite one of its components being a late-typegiant. The giant star has a projected rotational velocity of almost 40km s-1 it could be expected to exhibit RS CVn-typephotometric variations, which have not yet been observed, in addition tothe `ellipsoidal variation', discovered by Hipparcos, which occasionedits designation as VV Lep.
| Catalogue of high-mass X-ray binaries in the Galaxy (4th edition)
We present a new edition of the catalogue of high-mass X-ray binaries inthe Galaxy. The catalogue contains source name(s), coordinates, findingchart, X-ray luminosity, system parameters, and stellar parameters ofthe components and other characteristic properties of 114 high-massX-ray binaries, together with a comprehensive selection of the relevantliterature. The aim of this catalogue is to provide the reader with somebasic information on the X-ray sources and their counterparts in otherwavelength ranges (γ-rays, UV, optical, IR, radio). About 60% ofthe high-mass X-ray binary candidates are known or suspected Be/X-raybinaries, while 32% are supergiant/X-ray binaries. Some sources,however, are only tentatively identified as high-mass X-ray binaries onthe basis of their X-ray properties similar to the known high-mass X-raybinaries. Further identification in other wavelength bands is needed tofinally determine the nature of these sources. In cases where there issome doubt about the high-mass nature of the X-ray binary this ismentioned. Literature published before 1 October 2005 has, as far aspossible, been taken into account.
| Detection of a Large Flare in the RS CVn Star WY Cnc
We report detection of an optical flare in the RS CVn spotted star WYCnc. The flare duration is 64 min, the amplitude is 0.134 mag and theenergy is 1 * 10^35 erg in the B band. The WY Cnc spot activity beforeand during the flare is analyzed.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Newly discovered active binaries in the RasTyc sample of stellar X-ray sources. I. Orbital and physical parameters of six new binaries
We present the first results from follow-up optical observations, bothphotometric and spectroscopic, of stellar X-ray sources, selected fromthe RasTyc sample, resulting from the cross-correlation of ROSAT All-SkySurvey (RASS) and TYCHO catalogues. In particular, we report on thediscovery of six late-type binaries, for which we obtained good radialvelocity curves and solved their orbits. We performed an automaticspectral classification of both single-lined and double-lined binarieswith codes developed by us and found two binaries composed of twomain-sequence stars and four binaries with an evolved (giant orsubgiant) component. Filled-in or pure emission Hα profilesindicative of a moderate or high level of chromospheric activity wereobserved. In nearly all the systems, we also detected a photometricmodulation ascribable to surface inhomogeneities that is correlated withthe orbital period, suggesting a synchronization between rotational andorbital periods. The position on the HR diagram of the components of thefive sources with a known parallax indicates three binaries containingonly main-sequence stars and two single-lined systems with a giantcomponent. The kinematical properties of two, or possibly four, of theobserved systems are consistent with a young disk population.
| The Nature of the Deep Lens Survey Fast Transients
The discovery and study of highly transient sources, especially thosethat rise to high brightness and then fade to obscurity, has been amajor part of modern astrophysics. Well-known examples includesupernovae and novae. A by-product of the Deep Lens Survey was thediscovery of three transients that varied on a timescale of less than anhour. All three had faint and red counterparts, the brightest of whichwas identified with an M star. However, the remaining two showed hintsof an extragalactic origin; one had a spatially extended counterpart,and the other appeared in projection on the outskirts of a brightelliptical galaxy. If these two sources were really of extragalacticorigin, then the two events represent a new class of exotic explosivetransients. We undertook spectroscopic observations with the Kecktelescope and find that the two counterparts are also late-type Galacticdwarfs. Our main conclusion is that flares from M dwarfs constitute adense foreground fog and dominate over any plausible class ofextragalactic fast transients by at least 2 orders of magnitude.Overcoming this fog will likely require dedicated surveys with carefuloptimization of target field location, filter(s), and cadence, presearchimaging to filter out late-type dwarfs, and a well-planned rapidfollow-up plan.
| Origin of the Galactic ridge X-ray emission
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructedin the 3-20 keV energy band from RXTE/PCA scan and slew observations. Weshow that the GRXE intensity follows the Galactic near-infrared surfacebrightness closely and thus traces the Galactic stellar massdistribution. The GRXE consists of two spatial components that can beidentified with the bulge/bar and the disk of the Galaxy. The parametersof these components determined from X-ray data are compatible with thosederived from near-infrared data. The inferred ratio of X-ray tonear-infrared surface brightness I_3{-20 keV}(10-11 ergs-1 cm-2 deg-2)/I3.5 μm(MJy/sr) = 0.26±0.05, and the ratio of X-ray tonear-infrared luminosity L3-20 keV/L3-4 μ m=(4.1±0.3)×10-5. The corresponding ratio of the3-20 keV luminosity to the stellar mass is L_x/M= (3.5±0.5)× 1027 erg s-1Mȯ-1, which agrees within the uncertaintieswith the cumulative emissivity per unit stellar mass of point X-raysources in the Solar neighborhood, determined in an accompanying paper(Sazonov et al.). This suggests that the bulk of the GRXE is composed ofweak X-ray sources, mostly cataclysmic variables and coronally activebinaries. The fractional contributions of these classes of sources tothe total X-ray emissivity determined from the Solar neighborhood datacan also explain the GRXE energy spectrum. Based on the luminosityfunction of local X-ray sources we predict that, in order to resolve 90%of the GRXE into discrete sources, a sensitivity limit of10-16 erg s-1 cm-2 (2-10 keV) willneed to be reached in future observations.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| A Non-axisymmetric Spherical α2-Dynamo
Using the Chebyshev-tau method, the generation of oscillatorynon-axisymmetric stellar magnetic fields by theα2-dynamo is studied in spherical geometry. Followingthe boundary conditions given by Schubert & Zhang, the sphericalα2-dynamo consists of a fully convective sphericalshell with inner radius ri and outer radius ro. Acomparison of the critical dynamo numbers of axisymmetric andphi-dependent modes for different thicknesses of the convective shelland different α-profiles leads to the following qualitativeresults: (i) when the angular factor of α-profile issinnθ cos θ (n = 1,2,4) the solutions of theα2-dynamo are oscillatory and non-axisymmetric, (ii)the thinner the convective shell, the more easily is thenon-axisymmetric mode excited and the higher is the latitudinal wavenumber, (iii) the thickness of the outer convective shell has an effecton the symmetries of the magnetic fields.
| 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.
| Periodic Changes in O-C Diagrams
The article describes basic ideas and physical background of a magneticdynamo model of quasiperiodic changes in the O-C diagram of eclipsingbinaries.
| The puzzling case of 56 Pegasi: a fast rotator seen nearly pole-on
A spectroscopic orbit has been recently found by R. Griffin for the longknown barium star 56 Peg, which is also a strong X-ray source. Thisshort-period, low mass-function orbit raises several questions regardingthe history of the system. In the present paper, we show that it is noteasy to find an evolutionary history for 56 Peg which is consistent withthe current component's masses, unless one assumes that the giant is arelatively fast rotator (a few times 10 km/s). The hypothesis of fastrotation allows us to explain some other peculiarities of this object aswell.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars
We derive the properties of spotted regions in late-type active starsfrom V- and B-band photometric data. Specifically, we compare theamplitudes of spot-induced light and color variations with modelamplitudes, which depend on the area, temperature and distribution ofspotted regions over the stellar surface. Our analysis is applied tothree well-known chromospherically active K dwarf stars: DXLeonis, AB Doradus and LQHydrae, and indicates that all three stars show significantvariations of the average spot temperature from epoch to epoch. Theobserved temporal variations of the starspot temperatures are bestexplained as arising from the contemporary presence on the stellarphotosphere of two-component (two-temperature) active regions. Theseconsist of either dark spots and hot faculae and/or dark umbrae andpenumbrae and the variations over time arise from change of their arealratio.
| 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.
| GSC 2038.0293 is a new short-period eclipsing RS CVn variable.
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| Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?
IGR J17544-2619 belongs to a distinct group of at least seven fast X-raytransients that cannot readily be associated with nearby flare stars orpre-main sequence stars and most probably are X-ray binaries with windaccretion. Sofar, the nature of the accretor has been determined in onlyone case (SAX J1819.3-2525/V4641 Sgr). We carried out a 20 ks ChandraACIS-S observation of IGR J17544-2619 which shows the source inquiescence going into outburst. The Chandra position confirms theprevious tentative identification of the optical counterpart, a blueO9Ib supergiant at 3 to 4 kpc (Pellizza, Chaty & Negueruela, inprep.). This is the first detection of a fast X-ray transient inquiescence. The quiescent spectrum is very soft. The photon index of5.9±1.2 (90% confidence error margin) is much softer than 6quiescent black hole candidates that were observed with Chandra ACIS-S(Kong et al. 2002, ApJ, 570, 277; Tomsick et al. 2003, ApJ, 599, L133).Assuming that a significant fraction of the quiescent photons comes fromthe accretor and not the donor star, we infer that the accretor probablyis a neutron star. A fit to the quiescent spectrum of the neutron staratmosphere model developed by Pavlov et al. (1994, A&A, 289, 837)and Zavlin et al. (1996, A&A, 315, 141) implies an unabsorbedquiescent 0.5-10 keV luminosity of(5.2±1.3)×1032 erg s-1. We speculateon the nature of the brief outbursts.
| Recovering facular areas through Doppler imaging
Similar to the Sun, stellar active regions appear to consist of darkspots in combination with surrounding bright facular regions.The present study uses the Doppler imaging technique to recover thestellar surface structures from simulated spectral observationsobtained from a given theoretical stellar surface.The objective of the study is to investigate how the recoveredsurface structure depends on observing parameters such as rotationalphase coverage, choice of spectral lines, signal-to-noise ratio, andprojected equatorial rotational velocity.The inversions demonstrate that those stellar surface activeregions with high temperature contrast in both longitudinal andlatitudinal directions can be satisfactorily reproduced usingDoppler imaging.Observations must, however, fulfill such strict conditions thata complete reconstruction is somewhat unrealistic.Nevertheless, in a more realistic example, the hot facular areasresult in quite distinct and easily recognizable features on thestellar surface map, even though they are not completelyreconstructed.
| Modelling the colour-brightness relation of chromospherically active stars
The photometric flux of chromospherically active, spotted starsgenerally becomes redder and fainter when large starspots rotate intoview on the stellar disc. However, some of the most active RS CVnsystems show a bluer flux as they get fainter. Modelling of one suchsystem (UX Ari) has shown that hot, bright, photospheric facular regionsaccompanying the cool, dark spots on the cooler, more active componentare a possible explanation. The bluer flux of the hotter, less activecomponent does not appear to be sufficient to explain the observedbehaviour. We have begun a search for additional chromosphericallyactive stars with a similar relation between colour and brightness, toinvestigate whether these relations can be explained in the same way.Our results for V711 Tau and RS CVn are presented here, and we concludethat the photospheric faculae remain the most probable explanation forthe observed behaviour.
| Stellar dynamos - theoretical aspects
I review some dynamo models and constraints for stars other than theSun. The models include mean-field dynamo models for RS Cvn binarysystems and global magnetoconvection simulations for supergiants and forfully convective stars. In the latter case, we find generation ofmixed-parity large-scale magnetic fields once the dynamo reachessaturation. In addition to the cool, narrow downdrafts known from localsimulations, these models also show a hot convective plume stretchingfrom the centre towards the surface.
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