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Nonthermal Hard X-Ray Emission and Iron Kα Emission from a Superflare on II Pegasi
We report on an X-ray flare detected on the active binary system IIPegasi with the Swift telescope. The event triggered the Burst AlertTelescope (BAT) in the hard X-ray band on 2005 December 16 at 11:21:52UT with a 10-200 keV luminosity of 2.2×1032 ergss-1-a superflare, by comparison with energies of typicalstellar flares on active binary systems. The trigger spectrum indicatesa hot thermal plasma with T~180×106 K. X-ray spectralanalysis from 0.8 to 200 keV with the X-Ray Telescope and BAT in thenext two orbits reveals evidence for a thermal component(T>80×106 K) and Fe K 6.4 keV emission. A tail ofemission out to 200 keV can be fit with either an extremely hightemperature thermal plasma (T~3×108 K) or power-lawemission. Based on analogies with solar flares, we attribute the excesscontinuum emission to nonthermal thick-target bremsstrahlung emissionfrom a population of accelerated electrons. We estimate the radiatedenergy from 0.01 to 200 keV to be ~6×1036 ergs, thetotal radiated energy over all wavelengths ~10 38 ergs, theenergy in nonthermal electrons above 20 keV ~3×1040ergs, and conducted energy <5×1043 ergs. Thenonthermal interpretation gives a reasonable value for the total energyin electrons >20 keV when compared to the upper and lower bounds onthe thermal energy content of the flare. This marks the first occasionin which evidence exists for nonthermal hard X-ray emission from astellar flare. We investigate the emission mechanism responsible forproducing the 6.4 keV feature, and find that collisional ionization fromnonthermal electrons appears to be more plausible than thephotoionization mechanism usually invoked on the Sun andpre-main-sequence stars.
| 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.
| Synchrotron emission from the T Tauri binary system V773 Tauri A
The pre-main sequence binary system V773 Tau A showsremarkable flaring activity around periastron passage. Here, we presentthe observation of such a flare at a wavelength of 3 mm (90 GHz)performed with the Plateau de Bure Interferometer. We examine differentpossible causes for the energy losses responsible for the e-folding timeof 2.31 ± 0.19 h of that flare. We exclude synchrotron,collisional, and inverse Compton losses because they are not consistentwith observational constraints, and we propose that the fading of theemission is due to the leakage of electrons themselves at eachreflection between the two mirror points of the magnetic structurepartially trapping them. The magnetic structure compatible with both ourleakage model and previous observations is that of a helmet streamerthat, as in the solar case, can occur at the top of the X-ray-emitting,stellar-sized coronal loops of one of the stars. The streamer may extendup to ~20 R* and interact with the corona of the other starat periastron passage, causing recurring flares. The inferred magneticfield strength at the two mirror points of the helmet streamer is in therange 0.12-125 G, and the corresponding Lorentz factor, γ, of thepartially trapped electrons is in the range 20 < γ < 632. Wetherefore rule out that the emission could be of gyro-synchrotronnature: the derived high Lorentz factor proves that the nature of theemission at 90 GHz from this pre-main binary system is synchrotronradiation.
| Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| Tertiary companions to close spectroscopic binaries
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB)with periods from 1 to 30 days for higher-order multiplicity. Asubsample of 62 targets were observed with the NACO adaptive opticssystem and 13 new physical tertiary companions were detected. Anadditional 12 new wide companions (5 still tentative) were found usingthe 2MASS all-sky survey. The binaries belong to 161 stellar systems; ofthese 64 are triple, 11 quadruple and 7 quintuple. After correction forincompleteness, the fraction of SBs with additional companions is foundto be 63% ± 5%. We find that this fraction is a strong functionof the SB period P, reaching 96% for P<3d and dropping to34% for P>12^d. Period distributions of SBs with and withouttertiaries are significantly different, but their mass ratiodistributions are identical. The statistical data on the multiplicity ofclose SBs presented in this paper indicates that the periods and massratios of SBs were established very early, but the periods of SB systemswith triples were further shortened by angular momentum exchange withcompanions.
| 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.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| A Large Millimeter Flare on the RS CVn Binary σ Geminorum
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
| 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.
| Photometric evidence for two-temperature photospheric inhomogeneities on magnetically active K dwarf stars
We derive the properties of spotted regions in late-type active starsfrom V- and B-band photometric data. Specifically, we compare theamplitudes of spot-induced light and color variations with modelamplitudes, which depend on the area, temperature and distribution ofspotted regions over the stellar surface. Our analysis is applied tothree well-known chromospherically active K dwarf stars: DXLeonis, AB Doradus and LQHydrae, and indicates that all three stars show significantvariations of the average spot temperature from epoch to epoch. Theobserved temporal variations of the starspot temperatures are bestexplained as arising from the contemporary presence on the stellarphotosphere of two-component (two-temperature) active regions. Theseconsist of either dark spots and hot faculae and/or dark umbrae andpenumbrae and the variations over time arise from change of their arealratio.
| Photospheric and chromospheric active regions on three single-lined RS CVn binaries
A monitoring of three active RS CVn binaries has been performed withmedium resolution spectroscopy with the aim of investigating thebehavior of chromospheric and photospheric inhomogeneities. Surfacetemperature, as recovered from line-depth ratios (LDRs), allowed us tomap the photospheric spots, while the Hα emission has been used asan indicator of chromospheric inhomogeneities. We have found that therotational modulation of the Hα emission is always in anti-phasewith the temperature wave, i.e. at the time of our observations activeregions at chromospheric and photospheric levels are closely spatiallyassociated in these active stars. The residual Hα profiles,obtained as the difference between the observed spectra and non-activetemplates, are well reproduced by a two Gaussian fitting. The broademission component, responsible for the wide emission wings in near allthe spectra, is often blue-shifted with respect to the center of thestellar disk. The narrow Hα emission displays a phase-dependentvariation in all stars and is anti-correlated with the photosphericdiagnostics, while the broad one displays no or little rotationalmodulation. We suggest that the broad emission component is mainlyrelated to physical phenomena, like micro-flaring or strongchromospheric velocity fields, occurring all over the star disk, whilethe central narrow emission is more affected by chromospheric plages. Wehave also detected a modulation of the intensity of the He I D3 linewith the star rotation, suggesting surface features also in the upperchromosphere of these stars.
| Speckle interferometry of nearby multiple stars. III.
Not Available
| XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5
We present results of X-ray spectroscopy for TWA 5, amember of the young TW Hydrae association, observed with XMM-Newton.TWA 5 is a multiple system which shows Hαemission, a signature typical of classical T Tauri stars, but noinfrared excess. From this analysis of the RGS and EPIC spectra, we havederived the emission measure distribution vs. temperature of the X-rayemitting plasma, its abundances, and the electron density. Thecharacteristic temperature and density of the plasma suggest a coronasimilar to that of weak-line T Tauri stars and active late-type mainsequence stars. TWA 5 also shows low iron abundance(~0.1 times the solar photospheric one) and a pattern of increasingabundances for elements with increasing first ionization potentialreminiscent of the inverse FIP effect observed in highly active stars.The especially high ratio Ne/Fe10 is similar to that of theclassical T Tauri star TW Hya, where the accretingmaterial has been held responsible for the X-ray emission. We discussthe possible role of an accretion process in this scenario. Since all TTauri stars in the TW Hydrae association studied so far have very highNe/Fe ratios, we also propose that environmental conditions may causethis effect.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Recovering facular areas through Doppler imaging
Similar to the Sun, stellar active regions appear to consist of darkspots in combination with surrounding bright facular regions.The present study uses the Doppler imaging technique to recover thestellar surface structures from simulated spectral observationsobtained from a given theoretical stellar surface.The objective of the study is to investigate how the recoveredsurface structure depends on observing parameters such as rotationalphase coverage, choice of spectral lines, signal-to-noise ratio, andprojected equatorial rotational velocity.The inversions demonstrate that those stellar surface activeregions with high temperature contrast in both longitudinal andlatitudinal directions can be satisfactorily reproduced usingDoppler imaging.Observations must, however, fulfill such strict conditions thata complete reconstruction is somewhat unrealistic.Nevertheless, in a more realistic example, the hot facular areasresult in quite distinct and easily recognizable features on thestellar surface map, even though they are not completelyreconstructed.
| Discovery of Solar Rieger periodicities in another star
The Rieger periods are solar cycles with a time scale of months, whichare present in both flaring activity and sunspot occurrence. Theseshort-term periodicities, tentatively explained by equatorially trappedRossby-type waves modulating the emergence of magneticflux at the surface, are considered a peculiar and not yet fullyunderstood solar phenomenon. We chose a stellar system with solarcharacteristics, UX Arietis, and performed a timing analysis of two9-year datasets of radio and optical observations. The analysis revealsa 294-day cycle. When the two 9-year datasets are folded with thisperiod, a synchronization of the peak of the optical light curve (i.e.,the minimum spot coverage) with the minimum radio flaring activity isobserved. This close relationship between two completly independentcurves makes it very likely that the 294-day cycle is real. We concludethat the process invoked for the Sun of a periodical emergence ofmagnetic fluxmay also be applied to UX Arietis and can explain the cyclic flaringactivity triggered by interactions between successive cyclic emergencesof magnetic flux.
| Modelling the colour-brightness relation of chromospherically active stars
The photometric flux of chromospherically active, spotted starsgenerally becomes redder and fainter when large starspots rotate intoview on the stellar disc. However, some of the most active RS CVnsystems show a bluer flux as they get fainter. Modelling of one suchsystem (UX Ari) has shown that hot, bright, photospheric facular regionsaccompanying the cool, dark spots on the cooler, more active componentare a possible explanation. The bluer flux of the hotter, less activecomponent does not appear to be sufficient to explain the observedbehaviour. We have begun a search for additional chromosphericallyactive stars with a similar relation between colour and brightness, toinvestigate whether these relations can be explained in the same way.Our results for V711 Tau and RS CVn are presented here, and we concludethat the photospheric faculae remain the most probable explanation forthe observed behaviour.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| Evidence for Superhumps in the Radio Light Curve of Algol and a New Model for Magnetic Activity in Algol Systems
Extensive radio data of two Algol systems and two RS CVn binaries werereanalyzed. We found evidence for a new periodicity that we interpret asa superhump in β Per (Algol), for which it may have been expectedaccording to its semidetached nature and low binary mass ratio. Theconcluded presence of an accretion disk (or an annulus) is consistentwith previous studies of optical data and numerical simulations ofβ Per. In our model, the 50 day period, previously found in theradio data of β Per, is explained as the apsidal precession of theelliptical accretion disk or annulus. If our interpretation is correct,this is the first detection of the superhump phenomenon in the radio andthe first observation of superhumps in Algol systems. According to ourresult, the accretion disk or annulus in β Per precesses in spiteof its non-Keplerian nature, and therefore, this phenomenon is notrestricted to the classical Keplerian accretion disks in compactbinaries. We propose that in Algol systems with short orbital periods,which have accretion disks or annuli, the disk is magnetically active inaddition to the cool secondary star. The magnetic field in the diskoriginates from amplification of the seed field in the magnetizedmaterial transferred from the secondary. The disk and stellar fieldsinteract with each other, with reconnection of the field lines causingflares and particle acceleration. Relativistic particles are trapped inthe field and directed toward the polar regions of the secondary starbecause of the dipole structure of its magnetic field. These systemsare, therefore, somewhat analogous to the RS CVn systems, which have twomagnetically interacting stars. Our proposed model for the magneticactivity in Algol systems provides a simple explanation for the observedproperties of β Per in the radio wavelengths, and in particular,for the presence of quiescent gyrosynchrotron emission near the polarregion of the secondary star, where electrons are difficult to confineif the field lines are open as in normal, single, magnetic stars. Itfurther explains the recent discovery that the Doppler shifts of theemission lines in the X-ray data of β Per are somewhat lower thanwhat is expected from the orbital motion of the secondary star. Wepropose that the superhump variation in the radio is generated byenhanced reconnection when the elongated side of the elliptic accretiondisk is the closest to the cool star. This leads to flares andenhancement in particle acceleration and is manifested as strongergyrosynchrotron radiation. The observed superhump period, at3.037+/-0.013 days (~6% longer than the orbital period), was used todeduce a binary mass ratio of ~0.23 in β Per, which is consistentwith previous studies. Our finding opens the possibility to extend thesuperhump phenomenon to Algol systems and to test the theories ofprecessing accretion disks in various types of interacting binaries. Itprovides a new method to estimate the mass ratios in these binaries. Italso offers new insights and improves our understanding of thecomplicated magnetic interaction and feedback between mass transferdynamics, time-dependent disk accretion, and induced magnetic activityin the Algol and related systems.
| Differential photometry of speckle-interferometric binary and multiple stars
A method for differential photometry of speckle-interferometric binaryand multiple stars is presented. Both the accuracy and sources ofsystematic errors of the method are analysed. The photometric accuracyranges between 0.02 m and 0.20 m, depending on the atmospheric seeing,the brightness and the separation of the system components. A comparisonbetween our magnitude differences and those of other authors ispresented.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| Plage and flare Activity of the RS CVn-type Star UX Arietis during 2001-2002
The very active RS CVn-type star UX Ari was observed usinghigh-resolution echelle spectrograph attached to the 2.16m telescope ofXinglong station in Nov.-Dec. 2001 and Dec. 2002. By means of syntheticspectral subtraction method, the information about chromosphericactivity of the system was obtained through several chromosphericactivity indicators HeI D3, NaI D1D2,Hα, and CaII IRT lines. Based on the analysis for theseactivity indicators, we found that the chromospheric activity of UX Arishowed obvious orbital modulation phenomenon, and the favorite activelongitudes were around the quadratures of the binary system. During thetwo observing runs, hot plage and very strong optical flare events weredetected, which were always happened around the favorite activelongitudes of the system. Moreover, they were linked with thephotospheric starspots in spatial structure, and appeared just above themain starspots.
| The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 activestars observed with the High Energy Transmission Grating Spectrometer onChandra in order to investigate their coronal plasma density. Densitieswere investigated using the lines of the He-like ions O VII, Mg XI, andSi XIII. Si XIII lines in all stars of the sample are compatible withthe low-density limit (i.e., ne<~1013cm-3), casting some doubt on results based on lowerresolution Extreme Ultraviolet Explorer (EUVE) spectra finding densitiesne>1013 cm-3. Mg XI lines betray thepresence of high plasma densities up to a few times 1012cm-3 for most of the sources with higher X-ray luminosity(>~1030 ergs s-1) stars with higherLX and LX/Lbol tend to have higherdensities at high temperatures. Ratios of O VII lines yield much lowerdensities of a few times 1010 cm-3, indicatingthat the ``hot'' and ``cool'' plasma resides in physically differentstructures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TYPyx, and IM Peg, our results represent the first spectroscopic estimatesof coronal density. No trends in density-sensitive line ratios withstellar parameters effective temperature and surface gravity were found,indicating that plasma densities are remarkably similar for stars withpressure scale heights differing by up to 3 orders of magnitude. Ourfindings imply remarkably compact coronal structures, especially for thehotter (~7 MK) plasma emitting the Mg XI lines characterized by thecoronal surface filling factor, fMgXI, ranging from10-4 to 10-1, while we find fOVIIvalues from a few times 10-3 up to ~1 for the cooler (~2 MK)plasma emitting the O VII lines. We find that fOVIIapproaches unity at the same stellar surface X-ray flux level ascharacterizes solar active regions, suggesting that these stars becomecompletely covered by active regions. At the same surface flux level,fMgXI is seen to increase more sharply with increasingsurface flux. These results appear to support earlier suggestions thathot 107 K plasma in active coronae arises from flaringactivity and that this flaring activity increases markedly once thestellar surface becomes covered with active regions. Comparison of ourmeasured line fluxes with theoretical models suggests that significantresidual model inaccuracies might be present and, in particular, thatcascade contributions to forbidden and intercombination lines resultingfrom dielectronic recombination might be to blame.
| On the sizes of stellar X-ray coronae
Spatial information from stellar X-ray coronae cannot be assesseddirectly, but scaling laws from the solar corona make it possible toestimate sizes of stellar coronae from the physical parameterstemperature and density. While coronal plasma temperatures have longbeen available, we concentrate on the newly available densitymeasurements from line fluxes of X-ray lines measured for a large sampleof stellar coronae with the Chandra and XMM-Newton gratings. We compileda set of 64 grating spectra of 42 stellar coronae. Line counts of strongH-like and He-like ions and Fe XXI lines were measured with the CORAsingle-purpose line fitting tool by \cite{newi02}. Densities areestimated from He-like f/i flux ratios of O VII and Ne IX representingthe cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5-11 from the O VII triplet and between logne ≈ 10.5-12 from the Ne IX triplet, but we caution thatthe latter triplet may be biased by contamination from Fe XIX and Fe XXIlines. We find that low-activity stars (as parameterized by thecharacteristic temperature derived from H- and He-like line flux ratios)tend to show densities derived from O VII of no more than a few times1010 cm-3, whereas no definitive trend is foundfor the more active stars. Investigating the densities of the hotterplasma with various Fe XXI line ratios, we found that none of thespectra consistently indicates the presence of very high densities. Weargue that our measurements are compatible with the low-density limitfor the respective ratios (≈ 5× 1012cm-3). These upper limits are in line with constant pressurein the emitting active regions. We focus on the commonly used \cite{rtv}scaling law to derive loop lengths from temperatures and densitiesassuming loop-like structures as identical building blocks. We derivethe emitting volumes from direct measurements of ion-specific emissionmeasures and densities. Available volumes are calculated from theloop-lengths and stellar radii, and are compared with the emittingvolumes to infer filling factors. For all stages of activity we findsimilar filling factors up to 0.1.Appendix A is only available in electronic form athttp://www.edpsciences.org
| A Unified Picture of the First Ionization Potential and Inverse First Ionization Potential Effects
We discuss models for coronal abundance anomalies observed in thecoronae of the sun and other late-type stars following a scenario firstintroduced by Schwadron, Fisk, & Zurbuchen of the interaction ofwaves at loop footpoints with the partially neutral gas. Instead ofconsidering wave heating of ions in this location, we explore theeffects on the upper chromospheric plasma of the wave pondermotiveforces. These can arise when upward-propagating waves from thechromosphere transmit or reflect upon reaching the chromosphere-coronaboundary, and are in large part determined by the properties of thecoronal loop above. Our scenario has the advantage that for realisticwave energy densities both positive and negative changes in theabundance of ionized species compared to neutrals can result, allowingboth first ionization potential (FIP) and inverse FIP effects to comeout of the model. We discuss how variations in model parameters canaccount for essentially all of the abundance anomalies observed in solarspectra. Expected variations with stellar spectral type are alsoqualitatively consistent with observations of the FIP effect in stellarcoronae.
| Rotational Brightness Modulation and Starspots on the RS CVn-type Stars IN Com, IL Com, UX Ari, and V711 Tau
UBVRI photometric observations and models of spotting are presented forfour noneclipsing RS CVn systems: IN Com (G5III/IV), IL Com (F8V+F8V),UX Ari (K01V+G5V), and V711 Tau (K1IV+G5V). A low amplitude variabilitycaused by cold (T=1700K)spots which can occupy up to 19% of the star''ssurface is confirmed for the little-studied star IL Com. Long-term lightcurves are constructed and the stellar magnitudes and color indices ofthe unspotted photospheres are estimated for IN com, UX Ari, and V711Tau. It is shown that UX Ari becomes bluer with decreasing brightness,so its variability cannot be fully explained in terms of cold spots.Models of spotting on In Com and V711 Tau are constructed from the fullset of available photometric observations. The spots on both of thesevariables lie at middle latitudes and occupy up to 22% (In Com) and 33%(V711 Tau) of the stars'' surfaces. Both stars manifest a tendency forthe width of the spots to decrease as their area increases. This is acrude analog of the Maunder butterflies. These stars experience cyclicalspot activity and have a differential rotation of the type found on thesun.
| 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.
| Speckle interferometry of nearby multiple stars. II.
This paper is a continuation of diffraction-limited speckleinterferometry of binary and multiple stars carried out at the 6-mtelescope of the Special Astrophysical Observatory in Zelenchuk. Theprogram has concentrated on nearby (π>10 mas) close binariesdiscovered or measured during the Hipparcos mission. Here, we present132 measurements of relative positions and magnitude differences for 99pairs and 8 measurements for 6 triple systems. 54 entries in the paperare new Hipparcos binaries. New triple systems with late-type dwarfcomponents, discovered in the course of observations, are HIP 8533 andHIP 25354.Based on data collected at the Special Astrophysical Observatory,Russia.{Tables 1-3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/422/627
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
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