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Starspots
Starspots are created by local magnetic fields on the surfaces of stars,just as sunspots. Their fields are strong enough to suppress theoverturning convective motion and thus block or redirect the flow ofenergy from the stellar interior outwards to the surface andconsequently appear as locally cool and therefore dark regions againstan otherwise bright photosphere (Biermann in Astronomische Nachrichten264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots areobservable tracers of the yet unknown internal dynamo activity and allowa glimpse into the complex internal stellar magnetic field structure.Starspots also enable the precise measurement of stellar rotation whichis among the key ingredients for the expected internal magnetictopology. But whether starspots are just blown-up sunspot analogs, we donot know yet. This article is an attempt to review our current knowledgeof starspots. A comparison of a white-light image of the Sun (G2V, 5Gyr) with a Doppler image of a young solar-like star (EK Draconis;G1.5V, age 100 Myr, rotation 10 × ? Sun) and witha mean-field dynamo simulation suggests that starspots can be ofsignificantly different appearance and cannot be explained with ascaling of the solar model, even for a star of same mass and effectivetemperature. Starspots, their surface location and migration pattern,and their link with the stellar dynamo and its internal energytransport, may have far reaching impact also for our understanding oflow-mass stellar evolution and formation. Emphasis is given in thisreview to their importance as activity tracers in particular in thelight of more and more precise exoplanet detections around solar-like,and therefore likely spotted, host stars.
| Multiple and changing cycles of active stars. II. Results
Aims. We study the time variations in the cycles of 20 active starsbased on decade-long photometric or spectroscopic observations. Methods:A method of time-frequency analysis, as discussed in a companion paper,is applied to the data. Results: Fifteen stars definitely show multiplecycles, but the records of the rest are too short to verify a timescalefor a second cycle. The cycles typically show systematic changes. Forthree stars, we found two cycles in each of them that are not harmonicsand vary in parallel, indicating a common physical mechanism arisingfrom a dynamo construct. The positive relation between the rotationaland cycle periods is confirmed for the inhomogeneous set of activestars. Conclusions: Stellar activity cycles are generally multiple andvariable.
| The chromospherically active binary star EI Eridani: I. Absolute dimensions
We present a detailed determination of the astrophysical parameters ofthe chromospherically active binary star EI Eridani. Our new radialvelocities allow to improve the set of orbital elements and reveallong-term variations of the barycentric velocity. A possible third-bodyorbit with a period of {? 19} years is presented. Absoluteparameters are determined in combination with the {Hipparcos} parallax.EI Eri's inclination angle of the rotational axis is confined to 56.0 °± 4.5 °, its luminosity class {IV} is confirmed by its radius of{2.37 ± 0.12} R_{?}. A comparison to theoreticalstellar evolutionary tracks suggests a mass of 1.09 ± 0.05M_{?} and an age of ? 6.15 Gyr. The presentinvestigation is the basis of our long-term Doppler imaging study of itsstellar surface.
| 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.
| Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries(hierarchy 2 + 2), is studied in comparison with triple stars, withrespective sample sizes of 81 and 724. Seven representative quadruplesystems are discussed in greater detail. The main conclusions are asfollows. (i) Quadruple systems of ? Lyr type with similar massesand inner periods are common, in 42 per cent of the sample the outermass ratio is above 0.5 and the inner periods differ by less than 10times. (ii) The distributions of the inner periods in triple andquadruple stars are similar and bimodal. The inner mass ratios do notcorrelate with the inner periods. (iii) The statistics of outer periodsand mass ratios in triples and quadruples are different. The medianouter mass ratio in triples is 0.39 independently of the outer period,which has a smooth distribution. In contrast, the outer periods of 25per cent quadruples concentrate in the narrow range from 10 to 100yr,the outer mass ratios of these tight quadruples are above 0.6 and theirtwo inner periods are similar to each other. (iv) The outer and innermass ratios in triple and quadruple stars are not mutually correlated.In 13 per cent of quadruples both inner mass ratios are above 0.85(double twins). (v) The inner and outer orbital angular momenta andperiods in triple and quadruple systems with inner periods above 30dshow some correlation, the ratio of outer-to-inner periods is mostlycomprised between 5 and 104. In the systems with small periodratios the directions of the orbital spins are correlated, while in thesystems with large ratios they are not. The properties of multiple starsdo not correspond to the products of dynamical decay of small clusters,hence the N-body dynamics is not the dominant process of theirformation. On the other hand, rotationally driven (cascade)fragmentation possibly followed by migration of inner and/or outerorbits to shorter periods is a promising scenario to explain the originof triple and quadruple stars.
| Long-term magnetic activity in close binary systems. I. Patterns of color variations
Aims.This is the first of a series of papers in which we present theresults of a long-term photometric monitoring project carried out atCatania Astrophysical Observatory aimed at studying magnetic activity inlate-type components of close binary systems, its dependence on globalstellar parameters, and its evolution on different time scales from daysto years. In this first paper, we present the complete observationdataset and new results of an investigation into the origin ofbrightness and color variations observed in the well-known magneticallyactive close binary stars: AR Psc, VYAri, UX Ari, V711 Tau,EI Eri, V1149 Ori, DHLeo, HU Vir, RS CVn,V775 Her, AR Lac, SZPsc, II Peg and BY Dra Methods: About 38 000 high-precision photoelectric nightly observationsin the U, B and V filters are analysed. Correlation and regressionanalyses of the V magnitude vs. U-B and B-V color variations are carriedout and a comparison with model variations for a grid of active regiontemperature and filling factor values is also performed. Results: Wefind the existence of two different patterns of color variation. Eightstars in our sample: BY Dra, VYAri, V775 Her, II Peg,V1149 Ori, HU Vir, EIEri and DH Leo become redder when theybecome fainter, as is expected from the presence of active regionsconsisting of cool spots. The other six stars show the oppositebehaviour, i.e. they become bluer when they become fainter. ForV711 Tau this behaviour could be explained by theincreased relative U- and B-flux contribution by the earlier-typecomponent of the binary system when the cooler component becomesfainter. On the other hand, for AR Psc, UXAri, RS CVn, SZ Psc andAR Lac the existence of hot photospheric faculae mustbe invoked. We also found that in single-lined and double-lined binarystars in which the fainter component is inactive or much less active theV magnitude is correlated to B-V and U-B color variations in more than60% of observation seasons. The correlation is found in less than 40% ofobservation seasons when the fainter component has a non-negligiblelevel of activity and/or hot faculae are present but they are eitherspatially or temporally uncorrelated to spots.I dedicate this paper to the memory of the P.I. of this project, Prof.Marcello Rodonò, who suddenly passed away on October 23, 2005. Tohim my sincere estimation and deepest gratitude.Based onobservations collected at INAF-Catania Astrophysical Observatory, Italy.
| 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.
| Changing stellar activity cycles
We investigated continuous long-term photometric datasets of thirteenactive stars, Ca II variability of one single main-sequence star, and10.7cm radio data of the Sun, with simple Fourier- and time-frequencyanalysis. The data reflect the strength of the activity manifested inmagnetic spots. All studied stars show multiple (2 to 4) cycles ofdifferent lengths. The time-frequency analysis reveals, that in severalcases of the sample one or two of the cycles exhibit continuous changes(increase or decrease). For four stars (V711 Tau, IL Hya, HK Lac, HD100180) and for the Sun we find that the cycle length changes arestrong, amounting to 10-50% during the observed time intervals. Thecycle lengths are generally longer for stars with longer rotationalperiods.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| Zonal Model of Starspots. Application to RS CVn Systems
Model results on starspots in 15 chromospherically active type RS CVnbinary systems are presented. The dependences of the parameters of thespots on the principal characteristics of the stars (spectral class,luminosity class, rotation, Rossby number) are examined. Latitudinaldrift of the spots, cycles in the spot activity, and differentialrotation are found in 9 of the stars.
| Long-Term Starspot Activity of the Eclipsing Variable System CG Cyg
Photometric observations of the eclipsing variable system CG Cyg (G9+K3,P=0 d .63), which belongs to a group of short-period, chromosphericallyactive RS CVn type stars with the same type of activity as the sun, havebeen made over two seasons in 2003 and 2004. The spotting of thestar’s surface is modelled using the original observationstogether with photometric data published since 1965. In every season thespots were concentrated near the equator and covered up to 18% of thestar’s entire surface, while the temperature difference betweenthe quiescent photosphere and the spotted regions was 2100 K. Cyclicalvariations in the total area and average latitude of the spots, as wellas flip-flop of the active longitudes, are suspected. Latitudinal driftof the spots during a cycle and differential rotation of the star arediscovered. The following orbital elements are obtained: M1 = 0.93M?, M2 = 0.81 M?, R1 = 1.01 R?, and R2 = 0.82 R?.
| Direct Evidence for a Polar Spot on SV Camelopardalis
We have used spectrophotometric data from the Hubble Space Telescope(HST) to eclipse-map the primary component of the RS CVn binary SV Camover nine HST orbits. We find from these observations and the Hipparcosparallax that the surface flux in the eclipsed low-latitude region ofthe primary is about 30% lower than that computed from a PHOENIX modelatmosphere at the effective temperature that best fits the spectralenergy distribution of the eclipsed flux. This can only be accounted forif about a third of the primary's surface is covered with unresolveddark starspots. Extending this to the full surface of the primary, wefind that, even taking into account this spot-filling factor, there isan additional flux deficit on the primary star. This can only beexplained if there is a large polar spot on the primary star extendingfrom the pole to latitude 48deg+/-6deg.
| Measuring starspot temperature from line-depth ratios. II. Simultaneous modeling of light and temperature curves
We present and apply to VY Ari, IM Peg and HK Lac a new method todetermine spot temperatures (Tsp) and areas (Arel)from the analysis of simultaneous light curves and temperaturemodulations deduced from line-depth ratios. A spot model, developed byus, has been applied to light and temperature curves. Grids of solutionswith comparable χ-square have been found for a wide range of spottemperatures. The behavior of the solution grids for temperature andlight curves in the Tsp-Arel plane is verydifferent and a rather small and unique intersection area can be found.In our spot-model we used spectral energy distributions (SEDs) based onthe Planck law and on model atmospheres to evaluate the flux ratiobetween spots and unspotted photosphere and we found higher spottemperatures with SEDs based on model atmospheres than on the Plancklaw.Based on observations collected at Catania Astrophysical Observatory,Italy. Appendix A is only available in electronic form athttp://www.edpsciences.org
| 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.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| 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.
| 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.
| Magnetic activity in HD 111456, a young F5-6 main-sequence star
We present spectroscopic and photometric observations of the young F5-6V field star HD 111456 which exhibits emission coresin the Ca Ii H and K lines and a filling in the H\alpha line. Absorptionlines of He I D_3 and Li I are clearly detected and we give a new valueof v\sin i=41.5 km s-1. Although the limit of magneticactivity detectability at optical wavelengths occurs around F5-6spectral types, HD 111456 shows unusual chromosphericactivity. High UV fluxes in the C Ii, C Iv, Si Ii and Si Iv lines and astrong X-ray emission confirm that HD 111456 is oneof the most active F5-6 V star. The Ca Ii and H\alpha chromosphericemission fluxes do not show any detectable variation on time scales offew days, as well as uvby photometry. A possible explanation in terms ofa very high level of magnetic activity and a homogeneous distribution ofactive regions, both at chromospheric and photospheric level, or a verylow inclination of the rotation axis with respect to the line of sightis proposed. From 1998 to 2001 the radial velocity varies indicating apossible binary system, but the spectral distribution of HD 111456 isbetter reproduced by a single normal F5 V star, and composite spectrasimulations (Ca Ii, H\alpha) with a normal inactive F star and a veryactive low-mass K-M star are not able to reproduce the observedemissions.Based on observations collected at the Observatoire de Haute Provence(CNRS), France, and at the Osservatorio Astrofisico di Catania, Italy.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Doppler imaging of stellar surface structure. XX. The rapidly-rotating single K2-giant HD 31993 = V1192 Orionis
We present two Doppler images from two consecutive stellar rotations ofthe single K2-giant HD 31993. Each Doppler image is reconstructed fromspectra obtained within a single stellar rotation. With its 25-dayrotational period and a radius of ~18 solar radii, HD 31993 isconsidered a very rapidly rotating star and thus allows the applicationof the Doppler-imaging technique, despite the unusually long period. Allmaps reveal 7 isolated, predominantly low-latitude spots with atemperature difference, photosphere minus spot, of just ~200 K. No polarspot or high-latitude activity above, say, +60° is seen. A largewarm feature is detected at high latitude and is believed to be real.These spots act as tracers for a cross correlation analysis and yield aclear signature of anti-solar differential surface rotation, i.e. thepolar regions rotating faster than the equator, with alpha =0.125+/-0.05 corresponding to a lap time of ~200 days. A detailed parameterstudy is carried out to verify the reality of the HD 31993 maps.
| 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.
| 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.
| Reprocessing the Hipparcos Intermediate Astrometric Data of spectroscopic binaries. II. Systems with a giant component
By reanalyzing the Hipparcos Intermediate Astrometric Data of a largesample of spectroscopic binaries containing a giant, we obtain a sampleof 29 systems fulfilling a carefully derived set of constraints andhence for which we can derive an accurate orbital solution. Of these,one is a double-lined spectroscopic binary and six were not listed inthe DMSA/O section of the catalogue. Using our solutions, we derive themasses of the components in these systems and statistically analyzethem. We also briefly discuss each system individually.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997) and on data collected with theSimbad database.
| The photospheric abundances of active binaries. I. Detailed analysis of HD 113816 (IS Vir) and HD 119285 (V851 Cen)
The high-resolution optical spectra of the two X-ray active binaries RSCVn stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed andtheir Na, Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined, in theframework of a larger program of chemical analysis of RS CVn stellaratmosphere. The analysis of IS Vir and V851 Cen is performed with threedifferent LTE methods. In the first one, abundances are derived for alarge set of transitions (among which 28 Fe I lines, spanning a broadinterval in excitation potential and equivalent width, and 6 Fe IItransitions) using measured equivalent widths and Kurucz LTE modelatmospheres as input for the MOOG software package. The inputatmospheric parameters and abundances are iteratively modified until (i)the Fe I abundances exhibit no trend with excitation potential orequivalent width, (ii) Fe I and Fe II average abundances are the sameand (iii) Fe and Alpha elements average abundances are consistent withthe input values. The second method follows a similar approach, but usesa restricted line list (without the Fe I ``low excitation potential''transitions) and relies on the B-V and V-I colour indices to determinethe temperature. The third method uses the same restricted line list asthe second method and relies on fitting the 6162 Å Ca I line wingprofiles to derive the surface gravity. The reliability of these methodsis investigated in the context of single line RS CVn stars. It is shownthat the V-I photometric index gives, on a broader sample of stars,significantly cooler estimates of the effective temperature than the B-Vindex. All approaches give results in good agreement with each other,except the V-I based method. The analysis of IS Vir and V851 Cen resultsin both cases in their primaries being giant stars of near-solarmetallicity. Their parameters as derived with the first method arerespectively T_eff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and T_eff =4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen thederived iron content is significantly higher than a previousdetermination in the literature. Both stars exhibit relativeoverabundances of several elements (e.g. Ca) with respect to the solarmix. Table 2 is only available in electronic form athttp://www.edpsciences.org
| 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.
| Starspot cycles from long-term photometry
We continue investigating the photometric cycle lengths of some activestars. Using datasets now up to 34 years long we confirm previouslyderived activity cycles for the shorter period systems of our sample (LQHya, V833 Tau, EI Eri, V711 Tau, HU Vir and IL Hya), but find differentcycle lengths for the two long-period giants HK Lac and IM Peg. We addUZ Lib to our previous sample. The connection between the rotationalperiod and cycle length seems evident for the shortest derived cyclelengths. A similar connection between the rotational period and thelonger cycle lengths is not as clear, taking into account recent resultson the time variability of the longer term solar cycles. While the timebase of the observations puts an upper limit to the detectable lengthsof the longer cycles, a general dependence of the cycle period on therotation rate is maintained. The length of the shortest cycle that wefound for LQ Hya agrees with the cycle period derived from dynamomodelling of this star.
| Starspots as tracers of differential surface rotation
Differential rotation is a key ingredient in theories of stellarmagnetic field generation. The solar surface differential rotation wasinitially discovered via the simple method of tracking the rotationrates of individual starspots at different latitudes. Today, the sametechnique can be applied to rapidly rotating stars, using sequences ofDoppler images spanning several stellar rotations. Early results suggestthat solar-like differential rotation patterns prevail on rapidlyrotating dwarf stars, but much remains to be done in tracing global flowpatterns on pre-main sequence stars, giants, and tidally-locked binarycomponents. I outline the relative merits of the three main methods thathave so far been used to track the latitude dependence of starspotrotation rates, and discuss the validity of the physical assumptionsthat underpin them.
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| Doppler imaging of stellar surfaces - techniques and issues
The development of Doppler imaging has allowed us to observe stellaractivity on the surface of stars other than the Sun for over a decade.We are now in a position to compare activity on rapidly rotating TTauri, RS CVn and young main-sequence stars and to compare the activityon those, in turn, with that of the Sun. The images produced show somestartling differences between stellar and solar activity. The strengthsand weakness of the Doppler imaging technique must be reviewed regularlyto remind us of what observed features are reliable and of when weshould have doubts. This review is a general survey of the techniqueemphasizing the issue of testing and of potential artifacts withoutattempting excessive detail on variations in application or results. Thedifficulties faced as the technique is extended from images of surfacetemperature or abundance to images that include magnetic information arebriefly surveyed.
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