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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.
| LO Pegasi: an investigation of multiband optical polarization
We present BVR polarimetric study of the cool active star LO Pegasi (LOPeg) for the first time. LO Peg was found to be highly polarized amongthe cool active stars. Our observations yield average values ofpolarization in LO Peg: PB = 0.387 +/- 0.004 per cent,θB = 88° +/- 1° PV = 0.351 +/-0.004 per cent, θV = 91° +/- 1° andPR = 0.335 +/- 0.003 per cent, θR = 91°+/- 1°. Both the degree of polarization and the position angle arefound to be variable. The semi-amplitude of the polarization variabilityin B, V and R bands is found to be 0.18 +/- 0.02, 0.13 +/- 0.01 and 0.10+/- 0.02 per cent, respectively. We suggest that the levels ofpolarization observed in LO Peg could be the result of scattering of ananisotropic stellar radiation field by an optically thin circumstellarenvelope or scattering of the stellar radiation by prominence-likestructures.
| Grain Growth and Global Structure of the Protoplanetary Disk Associated with the Mature Classical T Tauri Star, PDS 66
We present Australia Telescope Compact Array interferometricobservations of the old (13 Myr), nearby (86 pc) classical TTauri star (cTTS), PDS 66. Unresolved 3 and 12 mm continuumemission is detected toward PDS 66, and upper limits are derivedfor the 3 and 6 cm flux densities. The millimeter-wave data show aspectral slope flatter than that expected for interstellar medium-sizeddust particles, which is evidence of grain growth. We also presentHST/NICMOS 1.1 μm point-spread-function-subtracted coronagraphicimaging observations of the circumstellar environment of PDS 66.The Hubble Space Telescope observations reveal a bilaterally symmetriccircumstellar region of dust scattering ~0.32% of the central starlight,declining in surface brightness as r –4.53. Thelight-scattering disk of material is inclined 32° ± 5°from a face-on viewing geometry, and extends to a radius of 170 AU.These data are combined with published optical and longer wavelengthobservations to make qualitative comparisons between the median Taurusand PDS 66 spectral energy distributions. By comparing thenear-infrared emission to a simple model, we determine that the locationof the inner disk radius is consistent with the dust sublimation radius(~1400 K at 0.1 AU). We place constraints on the total disk mass using aflat-disk model and find that it is probably too low to form gas giantplanets according to current models. Despite the fact that PDS 66is much older than a typical cTTS (<=5 Myr), its physicalproperties are not much different.
| High levels of surface differential rotation on the young G0 dwarf HD171488
We present high-resolution images of the young, rapidly rotating G0dwarf HD171488, using both Stokes I and Stokes V data. The observationswere secured with the MuSiCoS spectropolarimeter at Telescope BernardLyot from 2005 May 31 to June 10. The photospheric surface brightnessdistributions show a strong and slightly decentred polar cap thatdominates over weak high- and low-latitude spot features. Thelarge-scale magnetic field topology shows a strong ring of anticlockwiseazimuthal field with a latitudinal dependence on polarity and largeregions of radial field with negative polarity at all latitudes. Usingthe good phase coverage of our data, we measure the differentialrotation on HD171488. The results indicate that the equator laps thepole every 12 days for brightness data and 13 days for magnetic data,which is the highest measurement of differential rotation obtained usingZeeman-Doppler imaging techniques.Spectropolarimetric observations were obtained, from 2005 May 31 to June10, with the MuSiCoS echelle spectropolarimeter at the Telescope BernardLyot (Observatoire du Pic du Midi, France).E-mail: s.v.jeffers@uu.nl
| Differential rotation on both components of the pre-main-sequence binary system HD 155555
We present the first measurements of surface differential rotation on apre-main-sequence binary system. Using intensity (Stokes I) andcircularly polarized (Stokes V) time-series spectra, taken over 11nights at the Anglo-Australian Telescope (AAT), we incorporate asolar-like differential rotation law into the surface imaging process.We find that both components of the young, 18 Myr, HD 155555 (V824 Ara,G5IV + K0IV) binary system show significant differential rotation. Theequator-pole lap times as determined from the intensity spectra are 80 dfor the primary star and 163 d for the secondary. Similarly, for themagnetic spectra we obtain equator-pole lap times of 44 and 71 d,respectively, showing that the shearing time-scale of magnetic regionsis approximately half of that found for stellar spots. Both componentsare therefore found to have rates of differential rotation similar tothose of the same spectral-type main-sequence single stars. The resultsfor HD 155555 are therefore in contrast to those found in other, moreevolved, binary systems where negligible or weak differential rotationhas been discovered. We discuss two possible explanations for this:first that at the age of HD 155555 binary tidal forces have not yet hadtime to suppress differential rotation and secondly that the weakdifferential rotation previously observed on evolved binaries is aconsequence of their large convection zone depths. We suggest that thelatter is the more likely solution and show that both temperature andconvection zone depth (from evolutionary models) are good predictors ofdifferential rotation strength. Finally, we also examine the possibleconsequences of the measured differential rotation on the interaction ofbinary star coronae.
| The first magnetic maps of a pre-main-sequence binary star system - HD155555
We present the first maps of the surface magnetic fields of apre-main-sequence binary system. Spectropolarimetric observations of theyoung, 18Myr, HD155555 (V824Ara, G5IV+K0IV) system were obtained at theAnglo-Australian Telescope in 2004 and 2007. Both data sets are analysedusing a new binary Zeeman-Doppler imaging (ZDI) code. This allows us tosimultaneously model the contribution of each component to the observedcircularly polarized spectra. Stellar brightness maps are also producedfor HD155555 and compared to previous Doppler images.Our radial magnetic maps reveal a complex surface magnetic topology withmixed polarities at all latitudes. We find rings of azimuthal field onboth stars, most of which are found to be non-axisymmetric with thestellar rotational axis. We also examine the field strength and therelative fraction of magnetic energy stored in the radial and azimuthalfield components at both epochs. A marked weakening of the fieldstrength of the secondary star is observed between the 2004 and 2007epochs. This is accompanied by an apparent shift in the location ofmagnetic energy from the azimuthal to radial field. We suggest that thiscould be indicative of a magnetic activity cycle. We use the radialmagnetic maps to extrapolate the coronal field (by assuming a potentialfield) for each star individually - at present ignoring any possibleinteraction. The secondary star is found to exhibit an extreme tilt(~75°) of its large-scale magnetic field to that of its rotationaxis for both epochs. The field complexity that is apparent in thesurface maps persists out to a significant fraction of the binaryseparation. Any interaction between the fields of the two stars istherefore likely to be complex also. Modelling this would require a fullbinary field extrapolation.
| Chromospheric features of LQ Hydrae from Hα line profiles
We analyze the Hα spectral variability of the rapidly-rotatingK1-dwarf LQ Hya using high-resolution Hα spectra recorded duringApril-May 2000. Chromospheric parameters were computed from the Hαprofile as a function of rotational phase. We find that all theseparameters vary in phase, with a higher chromospheric electron densitycoinciding with the maximum Hα emission. We find a clearrotational modulation of the Hα emission that is better emphasizedby subtracting a reference photospheric template built up with aspectrum of a non-active star of the same spectral type. A geometricalplage model applied to the Hα variation curve allows us to derivethe location of the active regions that come out to be close inlongitude to the most pronounced photospheric spots found with Dopplerimaging applied to the photospheric lines in the same spectra. Ouranalysis suggests that the Hα features observed in LQ Hya in 2000are a scaled-up version of the solar plages as regards dimensions and/orflux contrast. No clear indication of chromospheric mass motionsemerges.
| Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
We examine the implications for the distribution of extrasolar planetsbased on the null results from two of the largest direct imaging surveyspublished to date. Combining the measured contrast curves from 22 of thestars observed with the VLT NACO adaptive optics system by Masciadri andcoworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDIdevices by Biller and coworkers (for a total of 60 unique stars), weconsider what distributions of planet masses and semimajor axes can beruled out by these data, based on Monte Carlo simulations of planetpopulations. We can set the following upper limit with 95% confidence:the fraction of stars with planets with semimajor axis between 20 and100 AU, and mass above 4 MJup, is 20% or less. Also, with adistribution of planet mass of dN/dM~M-1.16 in the range of0.5-13 MJup, we can rule out a power-law distribution forsemimajor axis (dN/da~aα) with index 0 and upper cutoffof 18 AU, and index -0.5 with an upper cutoff of 48 AU. For thedistribution suggested by Cumming et al., a power-law of index -0.61, wecan place an upper limit of 75 AU on the semimajor axis distribution. Ingeneral, we find that even null results from direct imaging surveys arevery powerful in constraining the distributions of giant planets (0.5-13MJup) at large separations, but more work needs to be done toclose the gap between planets that can be detected by direct imaging,and those to which the radial velocity method is sensitive.
| 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).
| New Variable Star in the Field of the Seyfert Galaxy Mrk 290
CCD BVRI phase light curves of the new variable are almost sinusoidaland have a period of 1.51785 days. This period and the relationshipbetween V and color indices indicate that this star may belong to theclass of spotted variable stars.
| The Structure of the Local Interstellar Medium. IV. Dynamics, Morphology, Physical Properties, and Implications of Cloud-Cloud Interactions
We present an empirical dynamical model of the LISM based on 270 radialvelocity measurements for 157 sight lines toward nearby stars. Physicalparameter measurements (i.e., temperature, turbulent velocity,depletions) are available for 90 components, or one-third of the sample,enabling initial characterizations of the physical properties of LISMclouds. The model includes 15 warm clouds located within 15 pc of theSun, each with a different velocity vector. We derive projectedmorphologies of all clouds and estimate the volume filling factor ofwarm partially ionized material in the LISM to be between ~5.5% and 19%.Relative velocities of potentially interacting clouds are oftensupersonic, consistent with heating, turbulent, and metal depletionproperties. Cloud-cloud collisions may be responsible for thefilamentary morphologies found in ~1/3 of LISM clouds, the distributionof clouds along the boundaries of the two nearest clouds (LIC and G),the detailed shape and heating of the Mic Cloud, the location of nearbyradio scintillation screens, and the location of an LISM cold cloud.Contrary to previous claims, the Sun appears to be located in thetransition zone between the LIC and G Cloud.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with programs 9525 and 10236.
| The Gemini Deep Planet Survey
We present the results of the Gemini Deep Planet Survey, a near-infraredadaptive optics search for giant planets and brown dwarfs around 85nearby young stars. The observations were obtained with the Altairadaptive optics system at the Gemini North telescope, and angulardifferential imaging was used to suppress the speckle noise of thecentral star. Typically, the observations are sensitive to angularseparations beyond 0.5" with 5 σ contrast sensitivities inmagnitude difference at 1.6 μm of 9.5 at 0.5", 12.9 at 1", 15.0 at2", and 16.5 at 5". These sensitivities are sufficient to detect planetsmore massive than 2 MJ with a projected separation in therange 40-200 AU around a typical target. Second-epoch observations of 48stars with candidates (out of 54) have confirmed that all candidates areunrelated background stars. A detailed statistical analysis of thesurvey results is presented. Assuming a planet mass distributiondn/dm~m-1.2 and a semimajor-axis distributiondn/da~a-1, the 95% credible upper limits on the fraction ofstars with at least one planet of mass 0.5-13 MJ are 0.28 forthe range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; thisresult is weakly dependent on the semimajor-axis distribution power-lawindex. The 95% credible interval for the fraction of stars with at leastone brown dwarf companion having a semimajor axis in the range 25-250 AUis 0.019+0.083-0.015, irrespective of anyassumption on the mass and semimajor-axis distributions. Theobservations made as part of this survey have resolved the stars HD14802, HD 166181, and HD 213845 into binaries for the first time.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
| An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| Photometric periods of LQ Hydrae between 1992 and 2000
Aims. Stellar spots produce periodic variations in photometric data. Weaim to determine the observed periods with a high accuracy to improvemeasurements of stellar differential rotation. Methods: We applied themethod of three-stage weighted multichannel period analysis (MPA) tophotometric observations of LQ Hydrae obtained in 1992-2000. Results:We detected two periods in 10/1993-5/1994, two periods in10/1998-6/1999, and three periods in 10/1999-3/2000. The primary periodwas found to be present for all observing seasons with smallfluctuations within 1.60°-1.61°, while including the additionalsecondary periods, the fluctuations become three times larger within1.585°-1.625°. We also found that the amplitudes of the modelcurves of the primary period varied and reached their maxima in10/1993-5/1994 and their minima in 10/1997-6/1998, while the amplitudesof the additional periods were always below the amplitudes of theprimary period in the same season. Conclusions: We conclude that thestellar spots responsible for the photometric variations of the primaryperiod are long-lived and change their positions between two latitudescorresponding to two different rotation periods of 1.06° and of1.61° (or have slightly varying longitude migration rates). Theconstancy of the primary period in the individual seasons means aconstancy of latitude over the observation time intervals. We interpretthe appearance of the additional periods to be caused by the formationof new surface structures at different latitudes. The surface structureis thus temporary. Based on our detected period range, the correspondinglatitudinal differential rotation rate ΔΩ / Ω = 0.025is small compared with that of the Sun (ΔΩ / Ω = 0.2).
| 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.
| Angular momentum conservation and torsional oscillations in the Sun and solar-like stars
Context: The solar torsional oscillations, i.e., the perturbations ofthe angular velocity of rotation associated with the eleven-yearactivity cycle, are a manifestation of the interaction among theinterior magnetic fields, amplified and modulated by the solar dynamo,and rotation, meridional flow and turbulent thermal transport.Therefore, they can be used, at least in principle, to put constraintson this interaction. Similar phenomena are expected to be observed insolar-like stars and can be modelled to shed light on analogousinteractions in different environments. Aims: The source of torsionaloscillations is investigated by means of a model for the angularmomentum transport within the convection zone. Methods: A descriptionof the torsional oscillations is introduced, based on an analyticalsolution of the angular momentum equation in the mean-field approach. Itprovides information on the intensity and location of the torquesproducing the redistribution of the angular momentum within theconvection zone of the Sun along the activity cycle. The method can beextended to solar-like stars for which some information on thetime-dependence of the differential rotation is becoming available.Results: Illustrative applications to the Sun and solar-like stars arepresented. Under the hypothesis that the solar torsional oscillationsare due to the mean-field Lorentz force, an amplitude of the Maxwellstresses |B_rBφ| ⪆ 8 × 103G2 at a depth of ~0.85 Rȯ at low latitude isestimated. Moreover, the phase relationship between Br andBφ can be estimated, suggesting that BrBφ > 0 below ~0.85 Rȯ andBr Bφ < 0 above. Conclusions: Suchpreliminary results show the capability of the proposed approach toconstrain the amplitude, phase and location of the perturbations leadingto the observed torsional oscillations.
| 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.
| Why are accreting T Tauri stars observed to be less luminous in X-rays than non-accretors?
Accreting T Tauri stars are observed to be less luminous in X-rays thannon-accretors, an effect that has been detected in various star-formingregions. To explain this we have combined, for the first time, aradiative transfer code with an accretion model that considers magneticfields extrapolated from surface magnetograms obtained fromZeeman-Doppler imaging. Such fields consist of compact magnetic regionsclose to the stellar surface, with extended field lines interacting withthe disc. We study the propagation of coronal X-rays through themagnetosphere and demonstrate that they are strongly absorbed by thedense gas in accretion columns. The reduction in the observed X-rayemission depends on the field geometry, which may explain why accretingT Tauri stars show a larger scatter in their observed X-ray luminositycompared with non-accreting stars.
| The coronal structure of AB Doradus determined from contemporaneous Doppler imaging and X-ray spectroscopy
We obtain contemporaneous observations of the surface and corona of ABDoradus (AB Dor), a young single cool star, using ground-basedcircularly polarized spectra from the Anglo-Australian Telescope andX-ray light curves and spectra from the Chandra satellite. Theground-based data are used to construct surface magnetic field maps,which are extrapolated to produce detailed models of the quiescentcorona. The X-ray data serve as a new test for the validity of thesecoronal models.We find that AB Dor's X-ray corona must be concentrated close to itssurface, with a height, H ~ 0.3-0.4 R* this height isdetermined by the high coronal density and complex multipolar magneticfield from the surface maps. There is also significant correlationbetween the positions of surface and coronal active longitudes asdetermined from the surface spot and magnetic field maps and the X-raylight curve. At this epoch (2002 December) AB Dor appears to possess onevery large active longitude region, covering almost half the star;displaying enhanced activity in the form of large dark spots, strongmagnetic fields and chromospheric emission. This is unusual as previoussurface maps of AB Dor typically display more active regions that span awider range of longitudes.Finally, the level of rotational modulation and shape of the X-ray lightcurve depend on the distribution of magnetic field in the obscuredhemisphere (AB Dor is inclined by 60°). The models that bestreproduce the rotational modulation observed in the contemporaneousChandra X-ray light curve and spectra require the magnetic field in theobscured hemisphere to be of the same polarity as that in the observedhemisphere. The Sun shows different behaviour, with the leading polarityreversed in the opposite hemisphere. The X-ray observations provide aunique constraint on the magnetic structure in the obscured hemisphere.
| High-Dispersion Optical Spectra of Nearby Stars Younger Than the Sun
We present high-dispersion (R~16,000) optical (3900-8700 Å)spectra of 390 stars obtained with the Palomar 60 inch telescope. Themajority of stars observed are part of the Spitzer Legacy ScienceProgram ``The Formation and Evolution of Planetary Systems.'' Throughdetailed analysis we determine stellar properties for this sample,including radial and rotational velocities, Li I λ6708 andHα equivalent widths, the chromospheric activity indexR'HK, and temperature- and gravity-sensitive lineratios. Several spectroscopic binaries are also identified. From ourtabulations, we illustrate basic age- and rotation-related correlationsamong measured indices. One novel result is that Ca II chromosphericemission appears to saturate at vsini values above ~30 kms-1, similar to the well-established saturation of X-raysthat originate in the spatially separate coronal region.
| Spotting in stars with a low level of activity, close to solar activity
Data on the variability of the continuum optical emission are used forthe first time to estimate the degree of spotting in stars with activitylevels similar to that of the sun. It is shown that the amount ofspotting increases gradually from the sun to the highly spotted starsfor which Alekseev and Gershberg constructed the zonal model for thedistribution of spots. A close relationship is found between spottingand the power of the x-ray emission from stars with widely varyinglevels of activity.
| Magnetic activity on AB Doradus: temporal evolution of star-spots and differential rotation from 1988 to 1994.
Surface brightness maps for the young K0 dwarf AB Doradus arereconstructed from archival data sets for epochs spanning 1988 to 1994.By using the signal-to-noise enhancement technique of Least-SquaresDeconvolution, our results show a greatly increased resolution of spotfeatures than obtained in previously published surface brightnessreconstructions. These images show that for the exception of epoch1988.96, the starspot distributions are dominated by a long-lived polarcap, and short-lived low to high latitude features. The fragmented polarcap at epoch 1988.96 could indicate a change in the nature of the dynamoin the star. For the first time we measure differential rotation forepochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). Thesemeasurements show variations on a timescale of at least one year, withthe strongest surface differential rotation ever measured for AB Doroccurring in 1994.86. In conjunction with previous investigations, ourresults represent the first long-term analysis of the temporal evolutionof differential rotation on active stars.
| Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog
We derive detailed theoretical models for 1074 nearby stars from theSPOCS (Spectroscopic Properties of Cool Stars) Catalog. The Californiaand Carnegie Planet Search has obtained high-quality (R~=70,000-90,000,S/N~=300-500) echelle spectra of over 1000 nearby stars taken with theHamilton spectrograph at Lick Observatory, the HIRES spectrograph atKeck, and UCLES at the Anglo Australian Observatory. A uniform analysisof the high-resolution spectra has yielded precise stellar parameters(Teff, logg, vsini, [M/H], and individual elementalabundances for Fe, Ni, Si, Na, and Ti), enabling systematic erroranalyses and accurate theoretical stellar modeling. We have created alarge database of theoretical stellar evolution tracks using the YaleStellar Evolution Code (YREC) to match the observed parameters of theSPOCS stars. Our very dense grids of evolutionary tracks eliminate theneed for interpolation between stellar evolutionary tracks and allowprecise determinations of physical stellar parameters (mass, age,radius, size and mass of the convective zone, surface gravity, etc.).Combining our stellar models with the observed stellar atmosphericparameters and uncertainties, we compute the likelihood for each set ofstellar model parameters separated by uniform time steps along thestellar evolutionary tracks. The computed likelihoods are used for aBayesian analysis to derive posterior probability distribution functionsfor the physical stellar parameters of interest. We provide a catalog ofphysical parameters for 1074 stars that are based on a uniform set ofhigh-quality spectral observations, a uniform spectral reductionprocedure, and a uniform set of stellar evolutionary models. We explorethis catalog for various possible correlations between stellar andplanetary properties, which may help constrain the formation anddynamical histories of other planetary systems.
| Rotational modulation of X-ray emission from T Tauri stars.
We have modeled the rotational modulation of X-ray emission from T Tauristars assuming that they have isothermal, magnetically confined coronae.By extrapolating surface magnetograms we find that T Tauri coronae arecompact and clumpy, such that rotational modulation arises from X-rayemitting regions being eclipsed as the star rotates. Emitting regionsare close to the stellar surface and inhomogeneously distributed aboutthe star. However some regions of the stellar surface, which containwind bearing open field lines, are dark in X-rays. From simulated X-raylight curves, obtained using stellar parameters from the Chandra OrionUltradeep Project, we calculate X-ray periods and make comparisons withoptically determined rotation periods. We find that X-ray periods aretypically equal to, or are half of, the optical periods. Further, wefind that X-ray periods are dependent upon the stellar inclination, butthat the ratio of X-ray to optical period is independent of stellar massand radius.
| Surface velocity network with anti-solar differential rotation on the active K-giant σ Geminorum
We demonstrate the power of the local correlation tracking technique onstellar data for the first time. We recover the spot migration patternof the long-period RS CVn-type binary σ Gem from a set of sixDoppler images from 3.6 consecutive rotation cycles. The resultingsurface flow map suggests a weak anti-solar differential rotation with{α≈-0.0022±0.0016}, and a coherent poleward spotmigration with an average velocity of 220±10 m s-1.This result agrees with our recent findings from another study and couldalso be confirmed theoretically.
| Anti-solar differential rotation and surface flow pattern on UZ Libræ
We re-investigate UZ Libræ spectra obtained at KPNO in 1998 and 2000.From the 1998 data we compose 11 consecutive Doppler images using the CaI-6439, Fe I-6393 and Fe I-6411 lines. Applying the method of averagecross-correlation of contiguous Doppler images we find anti-solardifferential rotation with a surface shear of α≈-0.03. The pilotapplication of the local correlation tracking technique for the samedata qualitatively confirms this result and indicates complex flowpattern on the stellar surface. From the cross-correlation of the twoavailable Doppler images in 2000 we also get anti-solar differentialrotation but with a much weaker shear of α≈-0.004.
| 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.
| Modelling the time variation of the surface differential rotation in AB Doradus and LQ Hydrae
Sequences of Doppler images of the young, rapidly rotating late-typestars AB Dor and LQ Hya show that their equatorial angular velocity andthe amplitude of their surface differential rotation vary versus time.Such variations can be modelled to obtain information on the intensityof the azimuthal magnetic stresses within stellar convection zones. Weintroduce a simple model in the framework of the mean-field theory anddiscuss briefly the results of its application to those solar-likestars.
| Differential rotation on rapidly rotating stars
Theories of meridional circulation and differential rotation instellar convective zones predict trends in surface flow patterns onmain-sequence stars that are amenable to direct observational testing.Here I summarise progress made in the last few years in determiningsurface differential rotation patterns on rapidly-rotating youngmain-sequence stars of spectral types F, G, K and M. Differentialrotation increases strongly with increasing effective temperature alongthe main sequence. The shear rate appears to increase with depth in thesub-photospheric layers. Tidal locking in close binaries appears tosuppress differential rotation, but better statistics are needed beforethis conclusion can be trusted.
| Photospheric and chromospheric activity on EY Dra
Magnetic activity in the photosphere and chromosphere of the M dwarf EYDra is studied and possible correlations between the two areinvestigated using photometric observations in the V and R bands andoptical and near infrared spectroscopy. The longitudinal spotconfiguration in the photosphere is obtained from the V band photometry,and the chromospheric structures are investigated using variations inthe Hα line profile and observations of the Paschen β line. Theshape of the V band light-curve indicates two active regions on thestellar surface, about 0.4 in phase apart. The spectroscopicobservations show enhanced Hα emission observed close to the phasesof the photometrically detected starspots. This could indicatechromospheric plages associated with the photospheric starspots. Someindications of prominence structures are also seen. The chromosphericpressure is limited to log {m}_TR < -4 based on the non-detection ofemission in the Paschen β wavelength region.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
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