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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.
| Aus den IBVS 5721, 5722, 5723, 5724, 5726, 5728, 5729, 5731, 5732 und 5735.
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| GALEX high time-resolution ultraviolet observations of dMe flare events
Aims.We present near ultraviolet (NUV: 1750-2800 Å) and farultraviolet (FUV: 1350-1750 Å) light-curves for flares on 4 nearbydMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5)observed with the GALEX satellite. Methods: .Taking advantage ofthe time-tagged events recorded with the GALEX photon countingdetectors, we present high temporal resolution (<0.01 s) analysis ofthese UV flare data. Results: .A statistical analysis of 700 s ofpre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5failed to reveal the presence of significant micro-flare activity intime bins of 0.2, 1 and 10 s intervals. Using an appropriatedifferential emission measure for both the quiescent and flaring state,it is possible to reproduce the observed FUV:NUV flux ratios. A majordeterminant in reproducing this flux ratio is found to be the value ofplasma electron density during the flare. We also searched the countrate data recorded during each of the four flare events for periodicityassociated with magneto-hydrodynamic oscillations in the active regioncoronal loops. Significant oscillations were detected during the flareevents observed on all 4 stars, with periodicities found in the 30 to 40s range. Flare oscillations with this periodicity can be explained asacoustic waves in a coronal loop of length of ≈109 cm foran assumed plasma temperature of 5-20 × 106 K. Thissuggests a loop length for these M-dwarf flares of less than 1/10th ofthe stellar radii. We believe that this is the first detection ofnon-solar coronal loop flare oscillations observed at ultravioletwavelengths.
| X-Ray Spectroscopy of the Contact Binary VW Cephei
Short-period binaries represent extreme cases in the generation ofstellar coronae via a rotational dynamo. Such stars are important forprobing the origin and nature of coronae in the regimes of rapidrotation and activity saturation. VW Cep (P=0.28 days) is a relativelybright, partially eclipsing, very active object. Light curves made fromChandra HETGS data show flaring and rotational modulation but noeclipses. Velocity modulation of emission lines indicates that onecomponent dominates the X-ray emission. The emission measure is highlystructured, having three peaks. Helium-like triplet lines give electrondensities of about (3-18)×1010 cm-3. Weconclude that the corona is predominantly on the polar regions of theprimary star and is compact.
| The periodic variations of a white-light flare observed with ULTRACAM
High time resolution observations of a white-light flare on the activestar EQ PegB show evidence of intensity variations with a period of≈10 s. The period drifts to longer values during the decay phase ofthe flare. If the oscillation is interpreted as an impulsively-excited,standing-acoustic wave in a flare loop, the period implies a loop lengthof ≈3.4 Mm and ≈6.8 Mm for the case of the fundamental mode andthe second harmonic, respectively. However, the small loop lengths implya very high modulation depth making the acoustic interpretationunlikely. A more realistic interpretation may be that of a fast-MHDwave, with the modulation of the emission being due to the magneticfield. Alternatively, the variations could be due to a series ofreconnection events. The periodic signature may then arise as a resultof the lateral separation of individual flare loops or current sheetswith oscillatory dynamics (i.e., periodic reconnection).
| Detection of a Large Flare in the RS CVn Star WY Cnc
We report detection of an optical flare in the RS CVn spotted star WYCnc. The flare duration is 64 min, the amplitude is 0.134 mag and theenergy is 1 * 10^35 erg in the B band. The WY Cnc spot activity beforeand during the flare is analyzed.
| Are the W Ursae Majoris-type systems EK Comae Berenices and UX Eridani surrounded by circumstellar matter?
The variations of the orbital periods of two nearly neglected W UMa-typeeclipsing binaries, EK Comae Berenices and UX Eridani, are presentedthrough a detailed analysis of the O C diagrams. It is found that theorbital period of EK Com is decreasing and the period of UX Eridani isincreasing, and several sudden jumps have occurred in the orbitalperiods of both binaries. We analyze the mechanism(s), which mightunderlie the changes of the orbital periods of both systems, and obtainsome new results. The long-term decrease of the orbital period of EKComae Berenices might be caused by the decrease of the orbital angularmomentum due to a magnetic stellar wind (MSW) or by mass transfer fromthe more massive to the less massive component. The secular increase inthe orbital period of UX Eridani might be caused by mass transfer fromthe less massive to the more massive star. The possible mechanisms,which underlie the sudden changes in the orbital periods of the closebinary systems are as the followings: (1) the variations of thestructure due to the variation of the magnetic field; (2) the rapid massexchange between the close binaries and their circumstellar matter.Finally, the evolutionary status of the systems EK Comae Berenices andUX Eridani is discussed.
| On the Use of Line Depth Ratios to Measure Starspot Properties on Magnetically Active Stars
Photometric and spectroscopic techniques have proven to be effectiveways to measure the properties of dark, cool starspots on magneticallyactive stars. Recently, a technique was introduced using atomic linedepth ratios (LDRs) to measure starspot properties. Carefullyreproducing this technique using a new set of spectroscopic observationsof active stars, we find that the LDR technique encounters difficulties,specifically by overestimating spot temperatures (because the atomiclines blend with titanium oxide absorption in cooler spots) and by nottightly constraining the filling factor of spots. While the use of LDRsfor active star studies has great promise, we believe that theseconcerns need to be addressed before the technique is more widelyapplied.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| 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.
| Activity-induced variability in SV Cam, RZ Tau and II Peg in winter 2004/2005
We report on a search for activity-induced variability in two eclipsingsystems SV Cam, RZ Tau and a single lined spectroscopic binary II Peg inthe winter season 2004/2005. The SV Cam system was modelled with twoactivity centers-spots on the primary cooler than the surroundingphotosphere with total spot coverage 20%. On the contrary, the RZTau system (its Roche lobe filling factor of order 0.42) displayssymmetric light curves and no O'Connell effect is detected. The shape ofthe II Peg light curve can not be matched easily with a single spotmodel. Instead, a two spot model (alternatively a non-uniform highlatitude spot complex) cooler than the photosphere can model theobserved light curves with spot coverage 10%.
| Roche tomography of cataclysmic variables - III. Star-spots on AE Aqr
We present a Roche tomography reconstruction of the secondary star inthe cataclysmic variable AE Aqr. The tomogram reveals several surfaceinhomogeneities that are due to the presence of large, cool star-spots.In addition to a number of lower latitude spots, the maps also show thepresence of a large, high-latitude spot similar to that seen in Dopplerimages of rapidly rotating isolated stars, and a relative paucity ofspots at a latitude of 40°. In total, we estimate that some 18 percent of the Northern hemisphere of AE Aqr is spotted.We have also applied the entropy landscape technique to determineaccurate parameters for the binary system. We obtain optimal massesM1 = 0.74Msolar, M2 =0.50Msolar, a systemic velocity γ = -63kms-1and an orbital inclination i = 66°.Given that this is the first study to successfully image star-spots onthe secondary star in a cataclysmic variable, we discuss the role thatfurther studies of this kind may play in our understanding of thesebinaries.
| 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.
| 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.
| Stellar Coronal Abundances at Intermediate-Activity Levels: ξ UMa
We present an analysis of Chandra High Energy Transmission Grating(HETG) spectra of the corona of the critical intermediate-activity,quadruple stellar system ξ UMa. Using the maximum A/B subsystemseparation in 2001, we attempted to resolve for the first time in X-raysthe two components using the HETG Medium Energy Grating (MEG). We foundthe Aa component of the system to be at least 2 orders of magnitudefainter than the Bb component. We used the Markov Chain Monte Carlomethod to reconstruct differential emission measures (DEMs) andabundances and discuss first ionization potential (FIP) issues. Thereconstructed DEMs showed two pronounced peaks at logT~6.5 and logT~7.0K, respectively, similar to brighter solar active regions. A plot ofcoronal abundances, with respect to stellar photospheric, versus FIPreveals that the FIP behavior of ξ UMa is intermediate between lessand more active stars, commensurate with its X-ray luminosity. Theabundances of Mg, Fe, and Si are just subsolar, with Fe having thelowest abundance of these. The low-FIP elements Na and Al have similarabundances, and they are both higher than the Mg, Fe, and Si group. Theabundances show a minimum at S, rising gradually to Ne, which is higherthan photospheric, as has been seen by other authors on similar stars.Some of the FIP behavior seen here is predicted by the new ponderomotiveforce model of Laming.
| Anomalous X-ray line ratios in the cTTS TW Hydrae
The cTTS TW Hya has been observed with high-resolution X-rayspectrometers. Previously found high densities inferred from He-like f/itriplets strongly suggested the detected X-ray emission to be dominatedby an accretion shock. Because of their radiation field dependenceHe-like f/i ratios do not provide unambiguous density diagnostics. Herewe present additional evidence for high densities from ratios of Fe xviilines. Key Fe xvii line ratios in TW Hya deviate from theoreticalexpectations at low densities as well as from the same measurements in alarge sample of stellar coronae. However, a quantitative assessment ofdensities is difficult because of atomic physics uncertainties. Inaddition, estimates of low optical depth in line ratios sensitive toresonance scattering effects also support a high-density emissionscenario in the X-ray emitting regions of cTTS.
| On the Binary Nature of 1RXS J162848.1-415241
We present spectroscopy of the optical counterpart to 1RXSJ162848.1-41524, also known as the microquasar candidate MCQCJ162847-4152. All the data indicate that this X-ray source is not amicroquasar and that it is a single-lined chromospherically activebinary system with a likely orbital period of 4.9 days. Our analysissupports a K3 IV spectral classification for the star, which is dominantat optical wavelengths. The unseen binary component is most likely alate-type (K7-M) dwarf or a white dwarf. Using the high-resolutionspectra, we have measured the K3 star's rotational broadening to bevsini=43+/-3 km s-1 and determined a lower limit to thebinary mass ratio of q(=M2/M1)>2.0. The highrotational broadening together with the strong Ca II H and K/Hαemission and high-amplitude photometric variations indicate that theevolved star is very chromospherically active and responsible for theX-ray/radio emission.
| 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.
| The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field
We present the first X-ray observation of an oscillation during astellar flare. The flare occurred on the active M-type dwarf ATMic and was observed with XMM-Newton. The soft X-ray lightcurve (0.2-12 keV) is investigated with wavelet analysis. The flare'sextended, flat peak shows clear evidence for a damped oscillation with aperiod of around 750 s, an exponential damping time of around 2000 s,and an initial, relative peak-to-peak amplitude of around 15%. Wesuggest that the oscillation is a standing magneto-acoustic wave tied tothe flare loop, and find that the most likely interpretation is alongitudinal, slow-mode wave, with a resulting loop length of (2.5± 0.2)×1010 cm. The local magnetic fieldstrength is found to be 105 ± 50 G. These values are consistentwith (oscillation-independent) flare cooling time models and pressurebalance scaling laws. Such a flare oscillation provides an excellentopportunity to obtain coronal properties like the size of a flare loopor the local magnetic field strength for the otherwisespatially-unresolved star.
| Spurious `active longitudes' in parametric models of heavily spotted eclipsing binaries
In this paper, the size distributions of starspots extrapolated from thecase of the Sun are modelled on the eclipsing binary SV Cam tosynthesize images of stellar photospheres with high spot fillingfactors. These spot distributions pepper the primary's surface withspots, many of which are below the resolution capabilities ofeclipse-mapping and Doppler-imaging techniques. The light curvesresulting from these modelled distributions are used to determine thelimitations of image reconstruction from photometric data. Surfacebrightness distributions reconstructed from these light curves showdistinctive spots on the primary star at its quadrature points. It isconcluded that two-spot modelling or chi-squared minimization techniquesare more susceptible to spurious structures being generated bysystematic errors, arising from incorrect assumptions about photosphericsurface brightness, than simple Fourier analysis of the light curves.
| 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 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.
| Flare X-Ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity
X-ray spectra of the late-type star AB Dor obtained with the XMM-Newtonsatellite are analyzed. AB Dor was particularly active during theobservations. An emission measure reconstruction technique is employedto analyze flare and quiescent spectra, with emphasis on the Fe XVII15-17 Å wavelength region. The Fe XVII 16.78 Å/15.01 Åline ratio increases significantly in the hotter flare plasma. Thischange in the ratio is opposite to the theoretical predictions and isattributed to the scattering of 15.01 Å line photons from the lineof sight. The escape probability technique indicates an optical depth of~0.4 for the 15.01 Å line. During the flare, the electron densityis 4.4+2.7-1.6×1010cm-3, and the fractional Fe abundance is 0.5+/-0.1 of thesolar photospheric value. Using these parameters, a path length of ~8000km is derived. There is no evidence of opacity in the quiescent X-rayspectrum of the star.
| Measuring starspot temperature from line-depth ratios. II. Simultaneous modeling of light and temperature curves
We present and apply to VY Ari, IM Peg and HK Lac a new method todetermine spot temperatures (Tsp) and areas (Arel)from the analysis of simultaneous light curves and temperaturemodulations deduced from line-depth ratios. A spot model, developed byus, has been applied to light and temperature curves. Grids of solutionswith comparable χ-square have been found for a wide range of spottemperatures. The behavior of the solution grids for temperature andlight curves in the Tsp-Arel plane is verydifferent and a rather small and unique intersection area can be found.In our spot-model we used spectral energy distributions (SEDs) based onthe Planck law and on model atmospheres to evaluate the flux ratiobetween spots and unspotted photosphere and we found higher spottemperatures with SEDs based on model atmospheres than on the Plancklaw.Based on observations collected at Catania Astrophysical Observatory,Italy. Appendix A is only available in electronic form athttp://www.edpsciences.org
| Search for activity-induced variability in AR Lac, KT Peg, KZ And, II Peg and EI Eri in autumn 1997
We report on a search for activity-induced variability in the SB2systems AR Lac, KT Peg and KZ And as well as in the SB1 systems II Pegand EI Eri in the September 1997 season. The study of Hα lineprofiles, Na I 5889Angstrom doublet, Ca II IRT lines and TiO7055Angstrom absorption band is presented. The spot properties wereestimated for the systems AR Lac and II Peg. The former possesses spotsat intermediate stellar latitudes on the primary component while thelatter shows these surface inhomogeneities at low latitudes. In bothcases, the spots cooler than the surrounding photosphere were revealed.Significant variability in the hydrogen Hα line profile wasobserved on II Peg and EI Eri, and the evidence of a rotationalmodulation was found in KT Peg and II Peg.
| 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.
| 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.
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