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Kinematic frames and "active longitudes": does the Sun have a face?
Context: .It has recently been claimed that analysis of Greenwichsunspot data over 120 years reveals that sunspot activity clustersaround two longitudes separated by 180° ("active longitudes") withclearly defined differential rotation during activity cycles. Inprevious work we demonstrated that such effects can be observed insynthetic data without such features, as an artefact of the method ofanalysis. Aims: .In the present work we extend this criticalexamination of methodology to the actual Greenwich sunspot data and alsoconsider newly proposed methods of analysis claiming to confirm theoriginal identification of active longitudes. Methods: .Weperformed fits of different kinematic frames onto the actual sunspotdata. Firstly, a cell-counting statistic was used to analyse a comovingsystem of frames and show that such frames extract useful informationfrom the data. Secondly, to check the claim of century-scale persistentactive longitudes in a contramoving frame system, we made acomprehensive exploration of parameter space following the originalmethodology as closely as possible. Results: .Our analysisrevealed that values obtained for the parameters of differentialrotation are not stable across different methods of analysis proposed totrack persistent active longitudes. Also, despite a very thorough searchin parameter space, we were unable to reproduce results claiming toreveal the century-persistent active longitudes. Previous parameterspace exploration has been restricted to frames whose latitudinalprofile is opposite to solar surface differential rotation. Relaxingthis restriction we found that the highest values of nonaxisymmetryoccur for frames comoving with the solar surface flow. Further analysisindicates that even these solutions are the result of purely statisticalfluctuations. Conclusions: .We can therefore say that strong andwell substantiated evidence for an essential and century-scalepersistent nonaxisymmetry in the sunspot distribution does not exist.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin
A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on theultrafast rotating yellow giant FK Comae Berenices (HD 117555; vsini~163km s-1) recorded emission profiles of C III λ977(T~8×104 K) and O VI λ1031(T~3×105 K) that are exceptionally broad andasymmetric, but nearly identical in shape, aside from a bluewardabsorption component in the latter (identified as interstellar O I,rather than, say, a C III outflow feature). The FWHMs exceed 500 kms-1, twice the broadest far-UV line shape of any normallate-type star observed to date, but similar to the Hα profiles ofFK Com, and following the trend of other fast spinning early G giantsthat often display ``superrotational'' broadening of their UV ``hot''lines. Although the red-asymmetric O VI λ1031 profile issuggestive of an outflow at ~3×105 K, the weaker memberof the doublet, λ1037, does not display the differentialabsorption pattern expected from a warm wind. Furthermore, at times thechromospheric Mg II λ2796 + λ2803 composite profile, froma collection of International Ultraviolet Explorer (IUE) echellegramsobtained two decades earlier, is nearly identical in shape tored-asymmetric O VI λ1031. A contemporaneous optical Doppler mapplaces the photospheric dark spots mainly in the polar regions of theapproaching hemisphere. The dominantly redward biased profiles of C IIIand O VI could be explained if the associated emission zones wereleading the starspots in phase and partially rooted in lower latitudes.
| Study of FK Comae Berenices. VI. Spot motions, phase jumps and a flip-flop from time-series modelling
Aims.Time-series spot modelling was used to follow the longitude changesof active regions responsible for the light variability of FK Combetween 1987-2004. Methods: .The photometric data are analysed inthe time-series mode of a spot modelling code. A scenario of one polarand two low-latitude active regions (hereafter spots, for simplicity)depicts the light variations very well. The role of the polar spotremains unclear because photometry in general does not provide directlatitudinal surface resolution, however, Doppler imaging results of FKCom also show very high latitude or even polar spots besides thelow-latitude ones. We also used a light-curve inversion method toconfirm some of the results. Results: .The two low-latitude spotsslowly migrate around 90° and 270° longitudes with quasiperiodsof 5.8 and 5.2 years. The spots prefer to stay alternately on one or theother, but on the same hemisphere of the star, with a separation oftypically 90-140°. We monitored a flip-flop in the light curve of FKComae in 1999. The two low-latitude spots, being ≈140-180° fromeach other during the season, gradually decreased until they bothpractically vanished. Shortly thereafter, two new spots appeared andstarted to grow. One of the new spots was near the location of the oldone, whereas the other turned up 90° shifted in longitude;consequently, the activity as a whole was shifted to the otherhemisphere of the star. We followed a phase jump in 1997, when the twolow-latitude spots got closer in longitude and finally merged, or elseone of them vanished. A new spot appeared soon, shifted by 100° inlongitude, but the activity remained on the same hemisphere.Conclusions: .The difference between flip-flops and phase jumps isdemonstrated. The derived longitude changes of activity centres mayallow us to better constrain the theoretical modelling on thetime-behaviour of stellar magnetic activity.
| A Non-axisymmetric Spherical α2-Dynamo
Using the Chebyshev-tau method, the generation of oscillatorynon-axisymmetric stellar magnetic fields by theα2-dynamo is studied in spherical geometry. Followingthe boundary conditions given by Schubert & Zhang, the sphericalα2-dynamo consists of a fully convective sphericalshell with inner radius ri and outer radius ro. Acomparison of the critical dynamo numbers of axisymmetric andphi-dependent modes for different thicknesses of the convective shelland different α-profiles leads to the following qualitativeresults: (i) when the angular factor of α-profile issinnθ cos θ (n = 1,2,4) the solutions of theα2-dynamo are oscillatory and non-axisymmetric, (ii)the thinner the convective shell, the more easily is thenon-axisymmetric mode excited and the higher is the latitudinal wavenumber, (iii) the thickness of the outer convective shell has an effecton the symmetries of the magnetic fields.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): sample selection and the cluster logN-logS
In this paper we serendipitously identify X-ray cluster candidates usingXMM-Newton archival observations complemented by five-band opticalphotometric follow-up observations (r~ 23 mag) as part of the X-rayIdentification (XID) programme. Our sample covers an area of ~2.1deg2 (15 XMM-Newton fields) and comprises a total of 21 (19serendipitous + two target) extended X-ray sources to the limitfx (0.5-2 keV) ~ 6 × 10-15 ergs-1 cm-2, with a high probability (>99.9 percent) of being extended on the XMM-Newton images. Of the 21 X-rayclusters, 14 are detected for the first time while seven arespectroscopically confirmed in the literature. Exploiting the opticaldata available for these fields we discover that >~68 per cent of theX-ray cluster candidates are associated with optical galaxyoverdensities. We also attempt to constrain the redshifts of our clustercandidates using photometric methods. We thus construct the photometricredshift distribution of galaxies in the vicinity of each X-ray selectedcluster candidate and search for statistically significant redshiftpeaks against that of the background distribution of field galaxies.Most of our clusters have photometric or spectroscopic redshifts in therange 0.4 < z < 0.6. Comparison of photometric with spectroscopicredshift estimates for the confirmed clusters suggests that our simplemethod is robust out to z~ 0.5. For clusters at higher z, deeper opticaldata are required to estimate reliable photometric redshifts. Using thesample of the 19 serendipitous X-ray selected cluster candidates, weestimate their surface density down to fx (0.5-2 keV) ~ 6× 10-15 erg s-1 cm-2 and find itto be in fair agreement with previous and recent studies.
| Active longitudes, nonaxisymmetric dynamos and phase mixing
We discuss the problem of solar active longitudes from the viewpoint ofdynamo theory. We start from a recent observational analysis of theproblem undertaken by Berdyugina & Usoskin (2003, A&A, 405,1121) and Usoskin et al. (2005, A&A, 441, 347) who demonstrated froma study of sunspot data that solar active longitudes rotatedifferentially, with a small but significant asynchrony between northernand southern hemispheres. We suggest two concepts by which theunderlying magnetic structure could lead to the observed phenomenology -the true differential rotation of a nonaxisymmetric magnetic structureand a stroboscopic effect. In the latter case, a solid body rotation ofnonaxisymmetric magnetic structure is illuminated by an activity wavepropagating from middle latitudes to the solar equator, and so mimics adifferential rotation. We then discuss several mechanisms which could inprinciple lead to the excitation of active longitudes. In particular, weconsider dynamo excitation of nonaxisymmetric magnetic modes,nonaxisymmetric structures as a manifestation of a relic magnetic fieldin the solar core, nonaxisymmetric solar hydrodynamics and nonlinearinstabilities that lack axial symmetry. We conclude that thesemechanisms all provide ways to explain the phenomenology, provided thestroboscopic interpretation is accepted. Of course, a quantitativeexplanation in the framework of any scenario requires ultimately adetailed numerical simulation. The interpretation of the availableobservations as a true differential rotation appears to provide a muchmore severe challenge for theorists. We are unable to suggest aplausible mechanism of this kind; however we can not exclude inprinciple such an explanation. We relate the phenomenon of solar activelongitudes to the information available concerning stellar activelongitudes, and also consider evidence from other tracers of solaractivity.
| The Effects of Binary Stars on AGB Nucleosynthesis . in particular the Consequences for Type Ia Supernovae
At least 60% of stars appear to binary and about half of these are closeenough to interact. Because of the enormous expansion on the AGB, manyof these interactions involve an AGB star and a relatively compactcompanion, anything from a low-mass main-sequence star to a degenerateremnant. Mass loss plays the dominant role in determining the lifetimeand the extent of nuclear processing of the AGB phase. In most casesbinary interaction increases the mass loss from the AGB star andcurtails its evolution, either through Roche-lobe overflow,common-envelope evolution or the driving of an enhanced stellar wind butit can also expose parts of stars that would otherwise have remaineddeep within. Type Ia supernovae are exploding CO white dwarfs whichformed as the cores of AGB stars. The source of the supernovas light isthe radioactive decay of 56Ni created in the thermonuclearrunaway and expelled in the explosion. Much of the core reaches nuclearstatistical equilibrium so that the amount of 56Ni createdfalls off with increasing excess of neutrons at the onset ofthermonuclear runaway. Neutron-rich 22Ne is a majorcontributor to this excess. AGB stars produce primary 22Ne byprocessing dredged-up carbon through hydrogen and then helium burningbefore depositing it in the CO core. So any variation in the AGBevolution of the progenitor star can ultimately lead to variation in thepeak luminosity of the supernova.
| 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.
| Long-term spot activity variation in FK Comae Berenices.
Not Available
| 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.
| Structure and evolution of low-mass W Ursae Majoris type systems - III. The effects of the spins of the stars
In a previous paper, using Eggleton's stellar evolution code, we havediscussed the structure and evolution of low-mass W Ursae Majoris (WUMa) type contact binaries with angular momentum loss owing togravitational radiation or magnetic braking. We find that gravitationalradiation is almost insignificant for cyclic evolution of low-mass W UMatype systems, and it is possible for angular momentum to be lost from WUMa systems in a magnetic stellar wind. The weaker magnetic activityshown by observations in W UMa systems is likely caused by the lowermass of the convective envelopes in these systems than in similar butnon-contact binaries. The spin angular momentum cannot be neglected atany time for W UMa type systems, especially for those with extreme massratios. The spin angular momenta of both components are included in thispaper and they are found to have a significant influence on the cyclicevolution of W UMa systems. We investigate the influence of the energytransfer on the common convective envelopes of both components indetail. We find that the mass of the convective envelope of the primaryin contact evolution is slightly more than that in poor thermal contactevolution, and that the mass of the convective envelope of the secondaryin contact evolution is much less than that in poor thermal contactevolution. Meanwhile, the rate of angular momentum loss of W UMa typesystems is much lower than that of poor thermal contact systems. This isindeed caused by the lower masses of the convective envelopes of thecomponents in W UMa type systems. Although the models with angularmomentum loss for W UMa systems exhibit cyclic evolution, they seem toshow that a W UMa system cannot continue this type of cyclic evolutionindefinitely, and it might coalesce into a fast-rotating star afterabout 1200 cycles of evolution (about 7.0 × 109 yr).
| 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.
| Stellar dynamos - theoretical aspects
I review some dynamo models and constraints for stars other than theSun. The models include mean-field dynamo models for RS Cvn binarysystems and global magnetoconvection simulations for supergiants and forfully convective stars. In the latter case, we find generation ofmixed-parity large-scale magnetic fields once the dynamo reachessaturation. In addition to the cool, narrow downdrafts known from localsimulations, these models also show a hot convective plume stretchingfrom the centre towards the surface.
| Hα observations of the star FK Com
High-resolution spectroscopic observations around the Hα line ofFK Com are presented. The analysis of the spectrareveals that: (a) the intensity of the R peak of the mean Hαprofile is larger than that of the V peak; (b) the intensities of the Vand R emission peaks changes almost in anti-phase; (c) the ratio V/Rvaries in phase with V; (d) the total Hα emission has a maximum atphase 0.75. The similarity of the Hα profile of FK Com with thoseof the disk-like stars leads us to the conclusion that its broadtwo-peaked emission Hα line originates from an extended disk. Itis supposed that the disk produces the mean Hα line. Thevariability of the Hα profile is explained by the presence ofadditional sources of absorption and emission at diametrically oppositeregions of the disk. The asymmetry of the mean Hα profile requiresthe disk to be half-illuminated. It is supposed that a source ofillumination is a low-mass hot secondary orbiting the disk. A model ofthe Hα emitting configuration of FK Com is proposed.
| X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectralproperties of a large sample of sources detected serendipitously withthe XMM-Newton observatory in 25 selected fields, for which opticalidentification is in progress. The survey covers a total solid angle of~3.5 deg2 and contains 1137 sources with~10-151043~erg s-1, and thereforeclassified as type 2 AGNs) is significantly higher (40%), with a hint ofmoderately higher columns. After correcting for absorption, we do notfind evidence for a redshift evolution of the underlying power law indexof BLAGNs, which stays roughly constant at Γ 1.9, withintrinsic dispersion of 0.4. A small fraction (~7%) of BLAGNs and NELGsrequire the presence of a soft excess, that we model as a black bodywith temperature ranging from 0.1 to 0.3 keV. Comparing our results onabsorption to popular X-ray background synthesis models, we findabsorption in only ~40% of the sources expected. This is due to adeficiency of heavily absorbed sources (with NH1022-1024~cm-2) in our sample incomparison with the models. We therefore conclude that the synthesismodels require some revision in their specific parameters.
| Chromospherically Active Stars. XXIV. The Giant, Single-lined Binaries HD 37824, HD 181809, and HD 217188
We have obtained spectroscopy and photometry of three chromosphericallyactive, single-lined spectroscopic binaries, HD 37824 (V1149 Ori), HD181809 (V4138 Sgr), and HD 217188 (AZ Psc). HD 37824 has a circularorbit with a period of 53.57 days. Its primary is a K0 III star, whilethe secondary is likely a G or K dwarf. HD 181809 has an orbit with aperiod of 13.04667 days and a low eccentricity of 0.040. The primary hasa spectral type of K0 III-IV, and its secondary is probably an M dwarf.The orbit of HD 217188 has a period of 47.1209 days and a moderatelyhigh eccentricity of 0.470. The spectral type of the primary is K0 III,while the secondary is likely an M dwarf. All three systems areestimated to have near solar iron abundances. Photometric observationsspanning 15-16 years for all three stars yield mean photometric periodsof 53.12, 59.85, and 90.89 days for HD 37824, HD 181809, and HD 217188,respectively. Thus, HD 37824 is rotating synchronously with the orbitalperiod, while HD 181809 and HD 217188 are both rotating considerablyslower than synchronously. All three stars show long-term variations inmean brightness and photometric amplitude, but no correlations areobserved between the seasonal mean brightness, photometric amplitude,and seasonal photometric period in any of the stars. No clear evidencefor long-term periodic variations in any of these parameters is present.The circular orbit of HD 37824 and the synchronous rotation of its Kgiant argue that the star is in the core helium-burning phase of itsevolution. The giant components of HD 181809 and HD 217188 areasynchronous rotators, and both systems have eccentric orbits. Thus,those two stars are likely first-ascent giants.
| Nonaxisymmetric magnetic field generation in rapidly rotating late-type stars
Recently the discovery of the “flip-flop” phenomenon, bywhich a predominant “active” longitude jumps by about 180°, on several rapidly rotating late-type stars has directedattention to the issues surrounding the presence of large-scalenonaxisymmetric magnetic fields in these objects. Here we studynonlinear mean field dynamo models with quasi-cylindrical rotation laws,acting in spherical shells. Stable nonaxisymmetric magnetic fields arefound to be generated for a range of parameters and, for certain ofthese models, jumps in a measure plausibly related to the position ofactive longitudes are found to occur. In general, nonaxisymmetric fieldsappear to be easier to generate in thinner dynamo-active regions.However, flip-flops appear to be stronger for intermediate thicknessesof the dynamo-active shells.
| 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.
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| 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.
| FK Comae: King of Spin
Fast-rotating early-G giants often display ultraviolet profiles of ``hotlines,'' like O VI (3×105 K), up to twice as broad asanticipated from the photospheric υsin {i}. This peculiar behaviorhas been attributed to highly extended coronal outer atmospheres.FUSE recently has contributed fundamentally to the exploration of these``super-rotational'' effects by observing the fascinating object FKComae Berenices (G5 III). It is prototype of a class of rapidly rotatingsingle giants which display spectacular emission activity from X-rays toradio. FK Com has a spin period of only 2.4 d, and a remarkableυsin {i} of 160 km s-1. The origins of suchultra-fast--rotating stars are controversial. They might result from acoalesced contact binary, cannibalism of a ``hot Jupiter,'' or simplyrepresent the top of the natural spin distribution of moderate-massHertzsprung gap giants.In February 2004, FUSE obtained a 13 ks exposure of FK Com in the920--1180 Å range, the first UV observation of this unusual objectsince the IUE era a decade ago, and by far the best quality spectrum todate. The FUV emissions of O VI λ 1031 and C III λ 977 areenormously broad, asymmetric, but nearly identical in shape, aside froma blue-shifted absorption component in the latter. The FHWM's are aremarkable 600 km s-1, about twice the broadest FUV profileof any late-type star observed up to that point.The blueshifted C III feature might represent a wind at 3×104 K, or alternatively a scattering structure in thehighly extended coronal envelope, something like the ``prominences''seen already in Hα . The asymmetric O VI profile might indicate anoutflow at higher 3× 105 K temperatures; orsimply reflects a skewed distribution of high-altitude activity in theequatorial zones of FK Com. The relationship between the hot lines andphotospheric active regions---deduced from contemporaneous opticalDoppler mapping---also will be discussed.This work was supported by FUSE Guest Investigator grant NNG04GH25G.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| Dynamo models and the flip-flop phenomenon in late-type stars
The `flip-flop' phenomenon has now been found to occur on severallate-type stars: active longitudes, associated with dark regions of thesurface, are seen to jump by 180°. A simple non-linearnon-axisymmetric mean field dynamo model is described, in which for thefirst time magnetic behaviour is found that is akin to the observedflip-flop.
| 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.
| 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).
| Stellar surface imaging: mapping brightness and magnetic fields
Rapidly rotating late-type stars typically display signs of magneticactivity that exceed those seen on the Sun by over two orders ofmagnitude. The techniques of Doppler imaging and Zeeman Doppler imaginghave been instrumental in unveiling magnetic activity patterns at thephotospheres of these active stars. Essentially, these techniques workby inverting time-series of high resolution spectra to producetemperature, brightness and/or magnetic field maps at the surfaces ofstars. I will describe how these techniques work and review what theyhave taught us about the nature of magnetic activity in rapid rotatorsover the last 20 years. Finally, I will conclude by outlining thecapabilities of these techniques in light of new instrumentation that isnow becoming available.
| The Rapidly Rotating, Hydrogen-deficient, Hot Post-Asymptotic Giant Branch Star ZNG 1 in the Globular Cluster M5
We report observations of the hot post-asymptotic giant branch star ZNG1 in the globular cluster M5 (NGC 5904) with the Far UltravioletSpectroscopic Explorer. From the resulting spectrum, we derive aneffective temperature Teff = 44300+/-300 K, a surface gravitylogg = 4.3+/-0.1, a rotational velocity vsini = 170+/-20 kms-1, and a luminosity log(L/Lsolar)=3.52+/-0.04.The atmosphere is helium-rich (Y=0.93), with enhanced carbon (2.6% bymass), nitrogen (0.51%), and oxygen (0.37%) abundances. The spectrumshows evidence of a wind with a terminal velocity near 1000 kms-1 and an expanding shell of carbon- and nitrogen-richmaterial around the star. The abundance pattern of ZNG 1 is suggestiveof the ``born-again'' scenario, whereby a star on the white dwarfcooling curve undergoes a very late shell flash and returns to theasymptotic giant branch, but the star's rapid rotation is more easilyexplained by a previous interaction with a binary companion.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer, which is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
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