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Keck Interferometer Observations of FU Orionis Objects
We present new K-band long-baseline interferometer observations of threeyoung stellar objects of the FU Orionis class, namely, V1057 Cyg, V1515Cyg, and Z CMa-SE, obtained at the Keck Interferometer during itscommissioning science period. The interferometer clearly resolves thesource of near-infrared emission in all three objects. Using simplegeometric models, we derive size scales (0.5-4.5 AU) for this emission.All three objects appear significantly more resolved than expected fromsimple models of accretion disks tuned to fit the broadband optical andinfrared spectrophotometry. We explore variations in the key parametersthat are able to lower the predicted visibility amplitudes to themeasured levels and conclude that accretion disks alone do not reproducethe spectral energy distributions and K-band visibilitiessimultaneously. We conclude that either disk models are inadequate todescribe the near-infrared emission or additional source components areneeded. We hypothesize that large-scale emission (tens of AU) in theinterferometer field of view is responsible for the surprisingly lowvisibilities. This emission may arise in scattering by large envelopesbelieved to surround these objects.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| A search for magnetic fields in the variable HgMn star α Andromedae
Context: .The chemically peculiar HgMn stars are a class of Bp starswhich have historically been found to be both non-magnetic andnon-variable. Remarkably, it has recently been demonstrated that thebright, well-studied HgMn star α And exhibits clear Hg ii lineprofile variations indicative of a non-uniform surface distribution ofthis element. Aims: .With this work, we have conducted anextensive search for magnetic fields in the photosphere of αAnd. Methods: .We have acquired new circular polarisation spectrawith the MuSiCoS and ESPaDOnS spectropolarimeters. We have also obtainedFORS1 circular polarisation spectra from the ESO Archive, and consideredall previously published magnetic data. This extensive dataset has beenused to systematically test for the presence of magnetic fields in thephotosphere of α And. We have also examined the high-resolutionspectra for line profile variability. Results: .The polarimetricand magnetic data provide no convincing evidence for photosphericmagnetic fields. The highest-S/N phase- and velocity-resolved Stokes Vprofiles, obtained with ESPaDOnS, allow us to place a 3σ upperlimit of about 100 G on the possible presence of any undetected puredipolar, quadrupolar or octupolar surface magnetic fields (and just 50 Gfor fields with significant obliquity). We also consider and dismiss thepossible existence of more complex fossil and dynamo-generated fields,and discuss the implications of these results for explaining thenon-uniform surface distribution of Hg. The very high-quality ESPaDOnSspectra have allowed us to confidently detect variability of Hg iiλ 6149, λ 5425 and λ 5677. The profile variabilityof the Hg ii lines is strong, and similar to that of the Hg ii λ3984 line. On the other hand, variability of other lines (e.g. Mn, Fe)is much weaker, and appears to be attributable to orbital modulation,continuum normalisation differences and weak, variable fringing.
| The Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disksizes using the 85 m baseline Keck Interferometer. Targets were chosento span the maximum range of stellar properties to probe the disk sizedependence on luminosity and effective temperature. For most targets,the measured near-infrared sizes (ranging from 0.2 to 4 AU) support asimple disk model possessing a central optically thin (dust-free)cavity, ringed by hot dust emitting at the expected sublimationtemperatures (Ts~1000-1500 K). Furthermore, we find a tightcorrelation of disk size with source luminosity R~L1/2 for Aeand late Be systems (valid over more than two decades in luminosity),confirming earlier suggestions based on lower quality data.Interestingly, the inferred dust-free inner cavities of the highestluminosity sources (Herbig B0-B3 stars) are undersized compared topredictions of the ``optically thin cavity'' model, likely because ofoptically thick gas within the inner AU.
| Flip-flop phenomenon: observations and theory
In many active stars the spots concentrate on two permanent activelongitudes which are 180 ° apart. In some of these stars thedominant part of the spot activity changes the longitude every fewyears. This so-called flip-flop phenomenon has up to now been reportedin 11 stars, both single and binary alike, and including also the Sun.To explain this phenomenon, a non-axisymmetric dynamo mode, giving riseto two permanent active longitudes at opposite stellar hemispheres, isneeded together with an oscillating axisymmetric magnetic field. Here wediscuss the observed characteristics of the flip-flop phenomenon andpresent a dynamo solution to explain them.
| The relation between X-ray activity and rotation in intermediate-mass G giants
I study the relation between X-ray activity and rotation amongintermediate-mass single G giants. The results show evidence that thequiescent coronal activity of these stars, as measured by their X-raysurface flux, increases linearly with the angular rotation velocity andwith the inverse of the Rossby number. Even the most rapidly rotating Ggiants do not reach the canonical log(L_X/Lbol) ≈ -3saturation level. The effect of rapid rotation on these stars couldresult mainly in an increased coverage of their surface with magneticclose loop structures. The empirical activity-rotation relationshipaccounts for the occurrence of a maximum of magnetic activity in theatmosphere of intermediate-mass stars as they evolve off themain-sequence near the bottom of the red giant branch. Remarkably, therelation between X-ray to bolometric luminosity ratio and the Rossbynumber or rotation period for G giants differs from the power lawdependence with an index of about -2 that is observed for main-sequencestars. Possible implications for the dynamo generation of magneticfields on giants are discussed.
| The X-ray activity of the slowly rotating G giant δ CrB
δ CrB is a single G giant whose distinctive characteristicsinclude an X-ray luminosity exceptionally high for a slowly rotatingstar. δ CrB was observed in March 2003 by the {XMM{-}Newton}observatory. The X-ray spectra of δ CrB are described by a MEKALplasma model with two components at 6.5 ×106 K and107 K. Series of lines of highly ionized Fe and several linesof the Ly series are visible in RGS spectra, most notably from O and Ne.The oxygen abundance is similar to the average abundance of the otherelements but the Ne/O ratio found for δ CrB seems higher than inthe solar photosphere, reminiscent of a similar anomaly observed in asubset of solar flares and in active stellar coronae. The spectralfitting of the EPIC and RGS spectra of δ CrB suggests a coronaconfiguration with little contribution from quiet regions similar to theSun. On the contrary the temperature T ≈ 6.5 × 106 Kof the “cool” plasma component is reminiscent of solar typeactive regions, while the hot (T ≈ 107 K) component may becaused by disruptions of magnetic fields associated with a permanentflaring activity. The analysis results of the {XMM{-}Newton} observationof δ CrB were compared with those of other single G giants withsimilar spectral type, mass and evolutionary status but with higherrotation rates. The comparison suggests that rapid rotation (P < 9days) could increase the surface coverage with active regions and theflaring rate on G giants as expected from classical, helicity related,dynamo-driven activity. We argue that the X-ray emission of δ CrBand slowly rotating giants could be related to the existence of magneticfields induced by turbulent motion.
| Photospheric magnetic field and surface differential rotation of the FK Com star HD 199178
We present spectropolarimetric observations of the FK Com star HD 199178obtained between 1998 December and 2003 August at the TélescopeBernard Lyot (Observatoire du Pic du Midi, France). We report thedetection of a photospheric magnetic field and reconstruct itsdistribution by means of Zeeman-Doppler imaging. We observe largeregions where the magnetic field is mainly azimuthal, suggesting thatthe dynamo processes generating the magnetic activity of HD 199178 maybe active very close to the stellar surface. We investigate the rapidevolution of surface brightness and magnetic structures from acontinuous monitoring of the star over several weeks in 2002 and 2003.We report that significant changes occur in the distribution ofcool-spots and magnetic regions on typical time-scales of the order oftwo weeks. Our spectropolarimetric observations also suggest that thesurface of HD 199178 is sheared by differential rotation, with adifference in rotation rate between equatorial and polar regions of theorder of 1.5 times that of the Sun.
| Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR1099 (V711 Tau) secured from 1998 February to 2002 January with thespectropolarimeter MuSiCoS at the Télescope Bernard Lyot(Observatoire du Pic du Midi, France). We apply Zeeman-Doppler imagingand reconstruct surface brightness and magnetic topologies of the K1primary subgiant of the system, at five different epochs. We confirm thepresence of large, axisymmetric regions where the magnetic field ismainly azimuthal, providing further support to the hypothesis thatdynamo processes may be distributed throughout the whole convective zonein this star.We study the short-term evolution of surface structures from acomparison of our images with observations secured at close-by epochs byDonati et al. at the Anglo-Australian Telescope. We conclude that thesmall-scale brightness and magnetic patterns undergo major changeswithin a time-scale of 4-6 weeks, while the largest structures remainstable over several years.We report the detection of a weak surface differential rotation (bothfrom brightness and magnetic tracers) indicating that the equatorrotates faster than the pole with a difference in rotation rate betweenthe pole and the equator about four times smaller than that of the Sun.This result suggests that tidal forces also affect the global dynamicequilibrium of convective zones in cool active stars.
| Spot sizes on Sun-like stars
The total area coverage by starspots is of interest for a variety ofreasons, but direct techniques only provide estimates of this importantquantity. Sunspot areas exhibit a lognormal size distributionirrespective of the phase of the activity cycle, implying that mostsunspots are small. Here we explore the consequences if starspot areaswere similarly distributed. The solar data allow for an increase in thefraction of larger sunspots with increasing activity. Taking thisdifference between the size distribution at sunspot maximum and minimum,we extrapolate to higher activity levels, assuming different dependencesof the parameters of the lognormal distribution on total spot coverage.We find that, even for very heavily spotted (hypothetical) stars, alarge fraction of the spots are smaller than the current resolutionlimit of Doppler images and hence might be missed on traditional Dopplermaps.
| Some Like It Hot: The X-Ray Emission of the Giant Star YY Mensae
We present an analysis of the X-ray emission of the rapidly rotatinggiant star YY Mensae observed by Chandra HETGS and XMM-Newton. Thehigh-resolution spectra display numerous emission lines of highlyionized species; Fe XVII to Fe XXV lines are detected, together withH-like and He-like transitions of lower Z elements. Although no obviousflare was detected, the X-ray luminosity changed by a factor of 2between the XMM-Newton and Chandra observations taken 4 months apart(from logLX~32.2 to 32.5 ergs s-1, respectively).The coronal abundances and the emission measure distribution have beenderived from three different methods using optically thin collisionalionization equilibrium models, which is justified by the absence ofopacity effects in YY Men as measured from line ratios of Fe XVIItransitions. The abundances show a distinct pattern as a function of thefirst ionization potential (FIP), suggestive of an inverse FIP effect asseen in several active RS CVn binaries. The low-FIP elements (<10 eV)are depleted relative to the high-FIP elements; when compared to itsphotospheric abundance, the coronal Fe abundance also appears depleted.We find a high N abundance in YY Men's corona, which we interpret as asignature of material processed in the CNO cycle and dredged up in thegiant phase. The corona is dominated by a very high temperature (20-40MK) plasma, which places YY Men among the magnetically active stars withthe hottest coronae. Lower temperature plasma also coexists, albeit withmuch lower emission measure. Line broadening is reported in some lines,with a particularly strong significance in Ne X Lyα. We interpretsuch broadening as Doppler thermal broadening, although rotationalbroadening due to X-ray-emitting material high above the surface couldbe present as well. We use two different formalisms to discuss the shapeof the emission measure distribution. The first one infers theproperties of coronal loops, whereas the second formalism uses flares asa statistical ensemble. We find that most of the loops in the corona ofYY Men have their maximum temperature equal to or slightly larger thanabout 30 MK. We also find that small flares could contributesignificantly to the coronal heating in YY Men. Although there is noevidence of flare variability in the X-ray light curves, we argue thatYY Men's distance and X-ray brightness do not allow us to detect flareswith peak luminosities LX<=1031 ergss-1 with current detectors.
| Anti-solar differential rotation
The differential rotation of anti-solar type detected by observationsfor several stars may result from a fast meridional flow. Thesufficiently intensive meridional circulation may be caused bylarge-scale thermal inhomogeneities or, perhaps, by tidal forcing from acompanion star. First results of simulations of the anti-solar rotationof a giant star with magnetically induced thermal inhomogeneities arepresented. Perspectives for observational check of the theory arediscussed.
| Differential rotation of cool active stars
The surface differential rotation of active solar-type stars can beinvestigated by means of Doppler and Zeeman-Doppler Imaging, bothtechniques enabling one to estimate the short-term temporal evolution ofphotospheric structures (cools spots or magnetic regions). Afterdescribing the main modeling tools recently developed to guarantee aprecise analysis of differential rotation in this framework, we detailthe main results obtained for a small number of active G and K fastrotating stars. We evoke in particular some preliminary trends that canbe derived from this sample, bearing the promise that major advances inthis field will be achieved with the new generation ofspectropolarimeters (ESPaDOnS/CFHT, NARVAL/TBL).
| The corona of HD 199178 (V 1794 Cygni)
HD 199178 (V 1794 Cygni), a chromospherically active late-type giantwith a high X-ray luminosity, was observed by the XMM-Newton spaceobservatory. Series of lines of highly ionized Fe and several Lymanlines of hydrogen-like ions are visible in the reflection gratingspectra, most notably from O and Ne. Analysis results suggest a scenariowhere the corona of HD 199178 is dominated by large magnetic structuressimilar in size to interconnecting loops between solar active regionsbut significantly hotter. The surface area coverage of these activeregions may approach up to a 90%. An hypothesis is that the interactionof these structures themselves induces a flaring activity in a smallscale not visible in the EPIC light curves that is responsible forheating HD 199178 plasma to coronal temperatures of T≥107 K. The intense X-ray activity of HD 199178 isrelated to its evolutionary position at the bottom of the red giantbranch. It is anticipated that its rotation will spin-down in the futurewith the effect of decreasing its helicity related, dynamo drivenactivity and suppressing large scale magnetic structures in its corona.
| Temporal fluctuations in the differential rotation of cool active stars
This paper reports positive detections of surface differential rotationon two rapidly rotating cool stars at several epochs, using stellarsurface features (both cool spots and magnetic regions) as tracers ofthe large-scale latitudinal shear that distorts the convective envelopein this type of star. We also report definite evidence that thisdifferential rotation is different when estimated from cool spots ormagnetic regions, and that it undergoes temporal fluctuations ofpotentially large amplitude on a time-scale of a few years.We consider these results as further evidence that the dynamo processesoperating in these stars are distributed throughout the convective zonerather than being confined at its base, as in the Sun. By comparing ourobservations with two very simple models of the differential rotationwithin the convective zone, we obtain evidence that the internalrotation velocity field of the stars we investigated is not like that ofthe Sun, and may resemble what we expect for rapid rotators. Wespeculate that the changes in differential rotation result from thedynamo processes (and from the underlying magnetic cycle) thatperiodically converts magnetic energy into kinetic energy and viceversa.We emphasize that the technique outlined in this paper corresponds tothe first practical method for investigating the large-scale rotationvelocity field within convective zones of cool active stars, and offersseveral advantages over asteroseismology for this particular purpose andthis specific stellar class.
| High-speed photometry of the recurrent nova IM Normae
The recurrent nova IM Nor is found to have an orbital period of 2.462 h,shown by periodic dips in brightness. This is only the second recurrentnova known to have such a short period. We interpret the light curve aslargely produced by a reflection effect from the heated face of thesecondary star, probably with the addition of a partial eclipse of theaccretion disc.
| Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine the astrometric positions of 19radio stars in the International Celestial Reference Frame (ICRF). Thepositions of these stars were directly linked to the positions ofdistant quasars through phase-referencing observations. The positions ofthe ICRF quasars are known to 0.25 mas, thus providing an absolutereference at the angular resolution of our radio observations. Averagevalues for the errors in our derived positions for all sources were 13and 16 mas in αcosδ and δ, respectively, withaccuracies approaching 1-2 mas for some of the stars observed.Differences between the ICRF positions of the 38 quasars and thosemeasured from our observations showed no systematic offsets, with meanvalues of -0.3 mas in αcosδ and -1.0 mas in δ.Standard deviations of the quasar position differences of 17 and 11 masin αcosδ and δ, respectively, are consistent with themean position errors determined for the stars. Our measured positionswere combined with previous Very Large Array measurements taken from1978 to 1995 to determine the proper motions of 15 of the stars in ourlist. With mean errors of ~1.6 mas yr-1, the accuracies ofour proper motions approach those derived from Hipparcos and, for a fewof the stars in our program, are better than the Hipparcos values.Comparing the positions of our radio stars with the Hipparcos Catalogue,we find that at the epoch of our observations, the two frames arealigned to within formal errors of approximately 3 mas. This resultconfirms that the Hipparcos frame is inertial at the expected level.
| 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
| 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.
| Solar spots as prototypes for stellar spots
What is the nature of what we commonly refer to as starspots? The answerto that question determines what we can learn from the study ofstarspots about stars, their internal dynamics, and their magneticactivity. Observations of the Sun, the only cool star that we canobserve in detail, naturally led to the hypothesis that starspots inother cool stars are magneto-convective phenomena in which a magneticfield substantially affects the structure of a stellar atmosphere.Spectral line features, eclipse light curves, and intensity modulationsare consistent with that hypothesis. Yet for almost all stars for whichstarspots are reported, the surface coverage is very much higher thanfor the Sun, while many stars manifest spots at high latitudes where theSun has never been seen to do so. In this review, I address how wellstarspots compare to sunspots, discuss some differences expected whenmoving away from the Sun in the HR diagram or in level of activity, andidentify resources to further deepen our understanding.
| Study of FK Comae Berenices. IV. Active longitudes and the ``flip-flop'' phenomenon
Doppler imaging techniques have earlier been used to study the starspotsand their evolution over a four year period in a single, late-type starFK Com. In the present work we publish new photometric observations ofFK Com for the year 2001 and analyse them together with the previouslypublished photometry obtained since 1966. These observations enable usto study the spot configuration on the stellar surface over much longertime period than the Doppler imaging alone permits, and so to look forpossible activity cycles. The longitudinal spot configuration isrecovered from the spot filling factor maps obtained with light curveinversion method. From the maps it is clear that the shape of the lightcurve is usually caused by one active region, which is often extended,and only occasionally by two regions. The spots tend to occur at twoactive longitudes which are 180 degr apart. These active longitudes areperiodically active, i.e. the dominant part of the spot activityabruptly changes the longitude after about 3 years, indicating the``flip-flop'' event. The full activity cycle is estimated to be 6.4years. There is also clear evidence for migration of the activelongitudes with at least three different rates. These rates correspondto the rotational periods of 2fd40038 +/- 0fd00009 (for the years1979-1993), 2fd4030 +/- 0fd0003 (1994-1997) and 2fd3960 +/- 0fd0004(1997-2001). These periods are confirmed by using a more traditionaltime series analysis. The different migration rates of the activelongitudes can be explained by weak solar-type differential rotation.Based on the observations obtained at Phoenix 10, Arizona, USA; Wolfgangand Amadeus, Arizona, USA; Mount Maidanak Observatory, Uzbekistan; LaPalma KVA 0.6 m Cassegrain telescope, La Palma, Spain. Table 1 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb-u-strasbg.fr/cgi-bin/qcat?J/A+A/390/179
| Polarimetry of evolved stars. I. RS CVn and Mira variables
We present broadband optical polarimetry of 3 RS CVn stars and 3 Miravariables, including the symbiotic star R Aqr, which contains a Miracomponent. Polarimetric variability has been studied on time-scales fromhours to years. Our programme objects at the time of our observationsshowed different forms of P(lambda ) dependence, P∝~λ-4 for most of the RS CVn-type stars, and P∝~λ-2 for the RS CVn-type star UV Psc and the MiraCeti-type variables, and a significant increase of polarization to thered for the Mira R Cet. Combining our data with previously publisheddata, we conclude that most of RS CVn-type and Mira Ceti-type objectsshow evidence of large polarimetric variability at wavelengths shorterthan 0.5 mu m, whereas the level of polarization is more stable in thered. This behaviour is consistent with episodic mass ejection andformation of small dust particles in the circumstellar environment.Although all targets showed polarimetric variability on different timescales, only for IM Peg might these variations be possibly linked withthe photometric period.
| Time series analysis of V511 Lyrae photometry
Our time series analysis of sixteen BV light curves of thechromospherically active binary V511 Lyr confirmed the 2fd7 rotationperiod uniquely. The seasonal periodicity changes of 3.8% indicated thepresence of detectable surface differential rotation. The significant2fd67455 periodicity in the light curve minimum epochs suggested thatthe strongest starspot formation in V511 Lyr was concentrated on onestable active longitude. Table 2 is only available in electronic form atthe CDS via anonymous ftp to cdarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A&A/383/197. Thephotometric data are also available athttp://schwab.tsuniv.edu/t3/v511lyr/v511lyr.html
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| Surface imaging of HD 199178 (V1794 Cygni)
We present surface temperature maps for the FK Comae-type starHD 199178 (V1794 Cygni) calculated from highresolution spectra obtained in 1994 and 1995. The spot pattern evolves,but all maps reveal a large cool spot remaining nearly at the same highlatitude. The main spot is 1200-1600 K cooler than the mean surfacetemperature. The observed slightly flat bottomed absorption lines wouldusually be interpreted as evidence for a large cool polar spot. We arguethat antisolar surface differential rotation offers a better explanationfor the box like shape of the line profiles. However, we do not findconclusive evidence for antisolar differential rotation and note thatthere are still other possible explanations for the slightly flatbottomed line profiles. Based on observations made with the 2 mtelescope of the National Astronomical Observatory, Rozhen, Bulgaria,and the Nordic Optical Telescope, operated on the island of La Palmajointly by Denmark, Finland, Iceland, Norway, and Sweden, in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| Doppler imaging of stellar surface structure. XV. A possible detection of differential rotation and local meridional flows on the rapidly-rotating giant HD 218153 = KU Pegasi
Time-resolved Doppler images of the rapidly rotating, but long-period(25 days), giant KU Pegasi show several cool low-to-medium latitudespots as well as an asymmetric polar feature. The average spottemperature is about 700 K below the photospheric temperature of 4700 K.KU Peg is one of the most massive, and currently the most evolved,late-type star with a Doppler image. We obtained two independent imagesfrom two consecutive stellar rotations covering 50 nights with a totalof 43 spectra. From a cross-correlation analysis of the two maps, wedetect systematic longitudinal and latitudinal shifts that wetentatively interpret as latitude-dependent differential rotation andlocal meridional flows, respectively. The differential-rotation patternis more complex than on the Sun, but on average in the sense that thepoles rotate slower than the stellar equator, i.e. in the same directionand also of the same order than on the Sun. The latitudinal shifts areof the order of 0.4° day-1 towards the stellar pole andoccur at longitudes of around 40° and 330°. The residual{Hα } profiles show a stationary emission component at restwavelength and a blue-shifted absorption. The latter suggests an outwardpointed velocity field with a flow velocity of approximately 35 kms-1.
| Time series analysis of V815 Herculis photometry between 1984 and 1998
As a case study of the solar-stellar connection, we have analysed aprolonged time series of BV photometry of the chromospherically activebinary V815 Her . The surface differential rotation in the rapidlyrotating G5{v} primary caused changes of 4.6% in the seasonalphotometric rotation periods. This would imply a differential rotationcoefficient of k = 0.184, if the rotation of the starspots follows thesolar law of differential rotation and the activity is confined withinthe same latitudinal range as in the Sun, having k = 0.189 and thespectral-type of G2{v}. Our analysis of the primary and secondary minimaof the seasonal light curves indicated that the regions of strongeractivity have concentrated on one active longitude, which has maintaineda constant rotation period of 1.d79244 for about 14 years. Noregular activity cycle was detected in the mean brightness changes ofV815 Her. Table~1 is available only in electronic form at CDS viaanonymous ftp to edarc.u-strasbg.fr (130.79.1285) or via http://cdsweb.u-strasbg.fr/Abstract.html. These data are also available athttp: //schwab.tsuniv.edu/t3/v815her/v815her.html
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