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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.
| 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.
| Chromospheric activity on the RS Canum Venaticorum binary SZ Piscium
Aims. We present the new high-resolution echelle spectra of SZ Psc,obtained in Nov. 2004 and Sep.-Dec. 2006, and study its chromosphericactivity. Methods: By means of the spectral subtraction technique, weanalyze our spectroscopic observations including several opticalchromospheric activity indicators (the He I D{3}, Na I D{1}, D{2},Hα, and Ca II infrared triplet lines). Results: All indicatorsshow that the chromospheric activity of the system is associated withthe cooler component. We find that the values ofEW8542/EW8498 are in the range 1-3, whichindicates optically thick emission in plage-like regions. The 2006 datasuggest the presence of active longitude phenomena. For the Ca II 8542and 8662 and the Hα lines, it seems that the excess emission isstronger near the two quadratures of system. This may be anti-correlatedwith the behavior of the Na I D{1} line. The absorption features aredetected in the subtracted Hα lines, which could be explained byprominence-like extended material seen on the stellar disk or by masstransfer from the cooler component to the hotter one.
| Spots, plages, and flares on λ Andromedae and II Pegasi
Aims.We present the results of a contemporaneous photometric andspectroscopic monitoring of two RS CVn binaries, namelyλ And and II Peg. The aimof this work is to investigate the behavior of surface inhomogeneitiesin the atmospheres of the active components of these systems that havenearly the same temperatures but different gravities. Methods: Thelight curves and the modulation of the surface temperature, as recoveredfrom line-depth ratios (LDRs), were used to map the photospheric spots,while the Hα emission was used as an indicator of chromosphericinhomogeneities. The spot temperatures and sizes were derived from aspot model applied to the contemporaneous light and temperature curves. Results: We find larger and cooler spots on II Peg (T_sp ≃ 3600 K)than on λ And (T_sp ≃ 3900 K); this could be the result ofboth the difference in gravity and the higher activity level of theformer. Moreover, we find a clear anti-correlation between the Hαemission and the photospheric diagnostics (temperature and lightcurves). We have detected a modulation in the intensity of the He I D3line with the star rotation, suggesting surface features also in theupper chromosphere of these stars. A rough reconstruction of the 3Dstructure of their atmospheres was also performed by applying aspot/plage model to the light and temperature curves and to the Hαflux modulation. In addition, a strong flare affecting the Hα, theHe I D3, and the cores of Na I D{1,2} lines has been observed on II Peg. Conclusions: The spot/plage configuration has been reconstructed in thevisible component of λ And and II Peg, which have nearly the sametemperature but very different gravities and rotation periods. A closespatial association of photospheric and chromospheric active regions, atthe time of our observations, was found in both stars. Larger and coolerspots were found on II Peg, the system with the active component ofhigher gravity and a higher activity level. The area ratio of plages tospots seems to decrease when the spots get bigger. Moreover, with boththis and literature data, a correlation between the temperaturedifference Δ T = T_ph-T_sp and the surface gravity is alsosuggested.Based on observations collected at Catania Astrophysical Observatory(Italy) and Ege University Observatory (İzmir, Turkey).
| Anti-solar differential rotation on the active K-giant σ Geminorum
The active K1 giant σ Gem and its differential surface rotation isrevisited. We refine our previous inconclusive result by recovering thespot migration pattern of this long-period RS CVn-type binary throughapplication of the technique of “average cross-correlation ofcontiguous Doppler images” to a set of six Doppler images from 3.6consecutive rotation cycles. We find an anti-solar differential rotationlaw with a shear of α≈ -0.022±0.006. We also findevidence of a poleward migration trend of spots with an average velocityof ≈300 m s-1.
| The rotation-activity correlation among G and K giants in binary systems
Aims.The present study aims (i) to test the existence of a correlationbetween magnetic activity and rotation among G and K giants in binarysystems and (ii) to test whether parameters other than rotation play arole in determining the X-ray emission level of intermediate-massgiants. Methods: The method consists in testing the existence ofcorrelations between measured stellar parameters including the X-raysurface flux, rotation period, Rossby number and surface gravity of asample of G and K giants with masses included between 1.5 M_ȯ and3.8 M_ȯ. Results: I found evidence that the X-ray surface fluxFX of intermediate-mass G and K giants is correlated withtheir rotation period P as previously observed on single G giants.Confidence in the degree of correlation is not higher when the Rossbynumber is used in place of the rotation period, but it significantlyimproves when stellar gravity g is taken into account. The empiricalrelation given by log (F_X) = -0.73 × log (P) + 0.64 × log(g/gȯ)+ 7.9 differs from the power-law dependence withan index of about -2 between X-ray to bolometric luminosity ratio andthe rotation period that is observed for main-sequence stars. The X-raysurface flux of single G giants and of intermediate-mass G and K giantsin close binary systems, such as RS CVn systems, also depends on thestellar gravity. This dependence could result from the effect of gravityon the electron density and emission measure of the X-ray emittingplasmas, as well as on the characteristic sizes of coronal magneticloops. The measured X-ray surface-flux dependence on gravity is,however, not as steep as the one predicted by simple models ofhydrostatic loops that assume a fixed ratio between the coronal energylosses by thermal conduction and by radiation. Conclusions: .I concludethat (i) a relation exists between the rotation and X-ray activity levelin giants, (ii) that this relation is not directly dependent on thepresence of a companion and applies to all intermediate-mass giants witheither G or K spectral type, and (iii) that gravity is an importantstellar parameter in determining the X-ray surface flux ofintermediate-mass giants.
| Differential rotation of giant stars
From a set of high-resolution spectral observations of late type giantstars we used Doppler imaging to derive time-series temperature maps ofthe stellar surfaces. Using these temperature maps, it is possible totrack the temporal changes of the spot features and derive estimates ofthe strength and sign of the differential surface rotation of thesestars. Looking into the latitudinal changes of the surface maps, it isalso possible to derive meridional flows on these stars. But due to thelower accuracy of the latitudes of the reconstructed spot features, thedata requirements are higher than for the detection of differentialrotation. Nevertheless, a correlation between the differential rotationand meridional flow estimates is suggested.
| 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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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 RS CVn binary HK Lacertae: long-term photometry from Sonneberg sky-patrol plates
Long-term photographic photometry of the active long-period RS CVnbinary HK Lac (HD 209813) was obtained from more than 2000 SonnebergSky-Patrol plates taken between 1956 and 1996. We achieve an internalaccuracy of 0.07 m. The correspondence with contemporaneoushigh-precision photoelectric photometry from automatic telescopes isstriking and successfully demonstrates the feasibility of our approach.Based on a Bayesian time series analysis, we improve the previouslypublished cycle period to 13.37± 0.08 years, and present evidenceof an additional period of 9.48± 0.13 years. This establishes themulti-periodicity of dynamo action in these overactive stars as comparedto the Sun. The already known 6.7-years cycle turns out to be anovertone of the dominating 13.4-years cycle. Our long-term photographicphotometry even allowed the detection of the star's mean rotationalperiod of 24.35 days.
| Photospheric and chromospheric active regions on three single-lined RS CVn binaries
A monitoring of three active RS CVn binaries has been performed withmedium resolution spectroscopy with the aim of investigating thebehavior of chromospheric and photospheric inhomogeneities. Surfacetemperature, as recovered from line-depth ratios (LDRs), allowed us tomap the photospheric spots, while the Hα emission has been used asan indicator of chromospheric inhomogeneities. We have found that therotational modulation of the Hα emission is always in anti-phasewith the temperature wave, i.e. at the time of our observations activeregions at chromospheric and photospheric levels are closely spatiallyassociated in these active stars. The residual Hα profiles,obtained as the difference between the observed spectra and non-activetemplates, are well reproduced by a two Gaussian fitting. The broademission component, responsible for the wide emission wings in near allthe spectra, is often blue-shifted with respect to the center of thestellar disk. The narrow Hα emission displays a phase-dependentvariation in all stars and is anti-correlated with the photosphericdiagnostics, while the broad one displays no or little rotationalmodulation. We suggest that the broad emission component is mainlyrelated to physical phenomena, like micro-flaring or strongchromospheric velocity fields, occurring all over the star disk, whilethe central narrow emission is more affected by chromospheric plages. Wehave also detected a modulation of the intensity of the He I D3 linewith the star rotation, suggesting surface features also in the upperchromosphere of these stars.
| Photometric observations from theoretical flip-flop models
Some active stars show a so-called flip-flop phenomenon in which themain spot activity periodically switches between two active longitudesthat are 180° apart. In this paper we study the flip-flop phenomenonby converting results from dynamo calculations into long-term syntheticphotometric observations, which are then compared to the real stellarobservations. We show that similar activity patterns as obtained fromflip-flop dynamo calculations, can also be seen in the observations. Thelong-term light-curve behaviour seen in the synthesised data can be usedfor finding new stars exhibiting the flip-flop phenomenon.
| 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.
| Indications for anti-solar differential rotation of giant stars
Observational evidence of anti-solar differential rotation of K-typegiant stars is presented. Time-series Doppler imaging based on 70 nightsof spectroscopic data was used to derive the spot evolution of thestellar surfaces. The relative differential rotation parameters(α) of the binary stars IM Peg, HD 208472, and HK Lac wereobtained using two techniques, cross-correlation analysis and thesheared-image method. Additionally, two previously published singlegiant stars are revisited and qualitatively compared to recenttheoretical models.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| 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.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
| 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.
| 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.
| 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
| 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.
| Measuring starspot temperature from line depth ratios. I. The method
Gray and collaborators have recently demonstrated that line-depth ratiosare a powerful tool for temperature discrimination, able to resolvedifferences <=10 K. The method has been applied to detect temperaturevariations in the 5-15 K range due to activity cycles (e.g. Gray et al.\cite{Gray96a}, \cite{Gray96b}) or to rotation modulation produced bylarge surface features, called ``star-patches", like that detected in xiBoo A by Toner & Gray (\cite{Toner88}). Cool starspots of a fewtenths of the stellar surface produce bumps in a line profile, whichmigrate through the line profile allowing Doppler-imaging in fastrotating stars. In the hypothesis that in slowly-rotating stars thepassage of dark spots produces modulation of the center line depth ofdifferent amount in lines of different sensitivity to temperature, wehave made test observations on three active binaries of the RS CVn type.Based on observations made at the Catania Astrophysical Observatory at aresolution R = 14 000, we show that line-depth ratios can be effectivelyused to determine spot temperatures of active binary systems. Using, onaverage, ten line pairs, selected in the 6100-6300 Å wavelengthrange, with the help of observations of 30 main sequence and giantstars, we have derived a calibration relation of line-depth ratios (LDR)in an absolute temperature scale, taking into account the gravity effectin the calibration relation. Single LDRs converted to temperaturethrough the calibration relations have led to clear rotationalmodulation of the average surface temperature with amplitudes of 177 K,119 K, and 127 K for VY Ari, IM Peg and HK Lac, with average estimatederrors of about 10 K. We show that the observed temperature variationamplitude allows us to define a minimum fractional spotted area coverageas a function of spot-photosphere temperature ratio. Adopting themaximum value of average temperature, determined from the LDRs, as thatof the unspotted photosphere, we computed the average spot temperaturecorresponding to the minimum spot coverage. Although not univocallyconstrained, the temperature difference (Delta T =Tph-Tsp) obtained for the three systems, Delta T =890 K for VY Ari, Delta T = 750 K for IM Peg, and Delta T = 810 K for HKLac, are in good agreement with values derived with other methods. Basedon observations collected at Catania Astrophysical Observatory, Italy.}
| The Rotation of Binary Systems with Evolved Components
In the present study we analyze the behavior of the rotational velocity,vsini, for a large sample of 134 spectroscopic binary systems with agiant star component of luminosity class III, along the spectral regionfrom middle F to middle K. The distribution of vsini as a function ofcolor index B-V seems to follow the same behavior as their singlecounterparts, with a sudden decline around G0 III. Blueward of thisspectral type, namely, for binary systems with a giant F-type component,one sees a trend for a large spread in the rotational velocities, from afew to at least 40 km s-1. Along the G and K spectral regionsthere are a considerable number of binary systems with moderate tomoderately high rotation rates. This reflects the effects ofsynchronization between rotation and orbital motions. These rotatorshave orbital periods shorter than about 250 days and circular or nearlycircular orbits. Except for these synchronized systems, the largemajority of binary systems with a giant component of spectral type laterthan G0 III are composed of slow rotators.
| 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.
| Starspot lifetimes
Photometry and Doppler imaging are both powerful techniques that can beused to evaluate the timescales of surface activity phenomena on activerapidly rotating objects. Active longitudes are most easily detectedthrough photometry. These are found to have lifetimes of between 4-8years. Many RSCVn binary systems and single stars show the ``flip-flop''effect, where dominant spotted regions switch back and forth by180deg longitude over a set number of years. Doppler imagingis most effective at evaluating the presence of polar spots and smallerscale spots in the mid to low latitude regions (up to 3degresolution at the equator). This technique enables the monitoring ofspot group lifetimes with greater accuracy than with photometry alone.Polar spots are found to have lifetimes of over a decade in RSCVn binarysystems (V711 Tau & EI Eri) and in single MS stars, (AB Dor). In ABDor, long-term photometry and Doppler imaging show that when the starwas at its most spotted, there was no polar spot. Recent resultsindicate that surface shear was also suppressed in AB Dor at the sameepoch. This implies that spot lifetimes can also be affected by changingsurface shear rates over the course of an activity cycle. Mid to lowlatitude spots on single MS stars are found to have lifetimes of under 1month. Spots in active components of RSCVn binaries show less modulationover a month compared to single MS rapid rotators. This indicates thateither less flux is injected into the stellar surface over one month, orelse that flux emergence is confined to small preferred regions intidally locked systems. More long-term monitoring of these and otherrapidly rotating systems using Doppler imaging, photometry and molecularband mapping using TiO and OH will enable us to evaluate whether or notthese initial trends are representative of active cool stars.
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