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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.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense 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.
| 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.
| 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.
| Spots, activity cycles, and differential rotation on cool stars
The first results are reported from a search for activity cycles instars similar to the sun based on modelling their spotting with analgorithm developed at the Crimean Astrophysical Observatory. Of themore than thirty program stars, 10 manifested a cyclical variation intheir central latitudes and total starspot area. The observed cycleshave durations of 4-15 years, i.e., analogous to the 11 year Schwabesunspot cycle. Most of the stars have a rough analog of the solarbutterfly pattern, with a reduction in the average latitude of the spotsas their area increases. A flip-flop effect during the epoch of themaximum average latitude is noted in a number of these objects (e.g.,the analog LQ Hya of the young sun or the RS CVn-type variable V711Tau), as well as a reduction in the photometric rotation period of astar as the spots drift toward the equator, an analog of thedifferential rotation effect in the sun. Unlike in the sun, the observedspot formation cycles do not correlate uniquely with other indicators ofactivity chromospheric emission in the CaII HK lines (Be Cet, EKDra, Dx Leo), H line emission (LQ Hya, VY Ari, EV Lac), or cyclicalflare activity (EV Lac). In V833 Tau, BY Dra, EK Dra, and VY Ari shortSchwabe cycles coexist with long cycles that are analogous to theGleissberg solar cycle, in which the spotted area can approach half theentire area 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
| 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.
| Statistics of multiple stars
The statistics of stellar systems of multiplicity three and higher isreviewed. They are frequent, 0.15-0.25 of all stellar systems. Some 700multiples are expected among the 3383 stars of spectral type F, G, and Kwithin 50 pc, while only 76 of them are actually known. Many (if notall) close binaries have distant tertiary components, indicating thatangular momentum exchange within multiple systems was probably criticalin forming short-period binaries. The ratio of outer to inner periods inthe best-studied nearby multiples and in low-mass pre-main sequencemultiples does not exceed 104 at the formation epoch; largerratios are produced by subsequent orbital evolution. All multiples withwell-defined orbits are dynamically stable, the eccentricities of outerorbits obey the empirical stability limit P[out](1 - e[out])3/P[in] >5 that is more strict than current theoretical limits. Relativeorientation of orbits in triple stars shows some degree of alignment,especially in weakly-hierarchical systems. The statistics support theidea that most multiple stars originated from dynamical interactions insmall clusters.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| 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.
| Spectral Classification of Stars in A Supplement to the Bright Star Catalogue
MK spectral types are given for about 584 stars in A Supplement to theBright Star Catalogue. These are compared with Hipparcos parallaxes tocheck the reliability of those classifications. The estimated errors are+/-1.2 subtypes, and 10% of the luminosity classes may be wrong.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| 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.
| The brightness variability of active stars on different timescales by robotic observations
The brightness variations shown by late-type stars are caused by thepresence and evolution of photospheric activity centers. In order todisentangle and correctly interpret the causes that contribute to theobserved brightness variability on different time scales, late-typestars need to be observed systematically for several years e.g. byrobotic telescopes.
| 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
| Butterfly Diagram for Starspots on LQ Hya
Information on the latitude distribution of starspots and changes inthis distribution from year to year is very important for ourunderstanding of the nature of stellar activity and for developingdynamo theory. The concept of butterfly diagrams is introduced forhighly spotted stars of late spectral types, by analogy to the Maunderdiagrams for the Sun. Our approach is based on the zonal spottednessmodels constructed by Alekseev and Gershberg. A detailed analysis isgiven for the single active star LQ Hya, and a comparison is made tosimilar analyses for several stars with two well-separated spotbelts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundaryof the butterfly diagram drifts toward the equator during theactivity-rise phase, i.e., during years when the relative spotted areaincreases. This effect is clearly expressed for LQ Hya and other starswhose orientation enables observation of both hemispheres and virtuallyvanishes for V833 Tau, which is viewed nearly pole-on. The upperboundary of the diagram is virtually unchanged for all the consideredspotted stars except V775 Her, for which it moves toward the pole. Thedrift rate of the lower boundary is -1 to -2 deg/yr, a factor of two tothree smaller in magnitude than the corresponding solar value. Ouranalysis provides an independent confirmation of the occurrence ofhigh-latitude spots on stars that are younger than the Sun and whoseactivity is high but less regular than the solar activity; it alsoenables the identification of the starting times of stellar cycles.
| Broad-band Multicolor Photometry and Polarimetry of Spotted Stars
We have confirmed the BY Dra-type variability of the active spottedstars MS Ser, LQ Hya, VY Ari, and EK Dra using simultaneous UBVRIphotometric and polarimetric observations. We have also reliablydetected the intrinsic linear polarization of their radiation and itsrotational modulation in U due to the inhomogeneous distribution ofactive magnetized regions over the surfaces of the stars. Modeling ofthe linear polarization based on the Zeeman effect indicates that allthe stars display strong local magnetic fields (about 2 kG, similar tothose in sunspots), with filling factors of up to 40% of the totalstellar surface. The magnetized regions coincide with cool photosphericspots detected in photoelectric observations.
| The Structure of Stellar Coronae in Active Binary Systems
A survey of 28 stars (22 active binary systems, plus six single stars orwide binaries for comparison) using extreme ultraviolet spectra has beenconducted to establish the structure of stellar coronae in active binarysystems from the emission measure distribution (EMD), electrondensities, and scale sizes. Observations obtained by the ExtremeUltraviolet Explorer satellite (EUVE) during 9 years of operation areincluded for the stars in the sample. EUVE data allow a continuous EMDto be constructed in the range logTe(K)~5.6-7.4, using ironemission lines. These data are complemented with IUE observations tomodel the lower temperature range [logTe(K)~4.0-5.6].Inspection of the EMD shows an outstanding narrow enhancement, or``bump'' peaking around logTe(K)~6.9 in 25 of the stars,defining a fundamental coronal structure. The emission measure per unitstellar area decreases with increasing orbital (or photometric) periodsof the target stars; stars in binaries generally have more material atcoronal temperatures than slowly rotating single stars. High electrondensities (Ne>~1012 cm-3) arederived at ~logTe(K)~7.0 for some targets, implying smallemitting volumes.The observations suggest the magnetic stellar coronae of these stars areconsistent with two basic classes of magnetic loops: solar-like loopswith maximum temperature around logTe(K)~6.3 and lowerelectron densities (Ne>~109-1010.5cm-3), and hotter loops peaking aroundlogTe(K)~6.9 with higher electron densities(Ne>~1012 cm-3). For the most activestars, material exists at much higher temperatures[logTe(K)>=6.9] as well. However, current ab initiostellar loop models cannot reproduce such a configuration. Analysis ofthe light curves of these systems reveals signatures of rotation ofcoronal material, as well as apparent seasonal (i.e., year-to-year)changes in the activity levels.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Spots and Active Regions on Emission Stars. III. LQ Hya
Quasisimultaneous photoelectric, polarimetric, and spectroscopicobservations of the active single red dwarf LQ Hya are presented. Thephotometric variability of LQ Hya is fully described by a zone model.Spotted regions occupy up to 25% of the entire surface of the star witha temperature difference of 800 K between the spots and the calmphotosphere. The spots are localized in the middle and low latitudes. Acyclical variation is observed in the total areas of the spotted regionsand in the average latitude of the spots. The most spotted regions,local magnetic fields, and chromospherically active regions tend toconcentrate in the same distinct active longitudes.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| A study of the Mg II 2796.34 Å emission line in late-type normal and RS CVn stars
We carry out an analysis of the Mg II 2796.34 Å emission line inRS CVn stars and make a comparison with the normal stars studied in aprevious paper (Paper I). The sample of RS CVn stars consists of 34objects with known HIPPARCOS parallaxes and observed at high resolutionwith IUE. We confirm that RS CVn stars tend to possess wider Mg II linesthan normal stars having the same absolute visual magnitude. However, wecould not find any correlation between the logarithmic line width logWdeg and the absolute visual magnitude MV (theWilson-Bappu relationship) for these active stars, contrary to the caseof normal stars addressed in Paper I. On the contrary, we find that astrong correlation exists in the (MV, log LMg II)plane (LMg II is the absolute flux in the line). In thisplane, normal and RS CVn stars are distributed along two nearly parallelstraight lines with RS CVn stars being systematically brighter by ~1dex. Such a diagram provides an interesting tool to discriminate activefrom normal stars. We finally analyse the distribution of RS CVn and ofnormal stars in the (log LMg II, log Wdeg) plane,and find a strong linear correlation for normal stars, which can be usedfor distance determinations.
| Are stellar coronae optically thin in X-rays?. A systematic investigation of opacity effects
The relevance of resonant scattering in the solar corona has always beendiscussed controversially. Ratios of emission lines from identical ionsbut different oscillator strengths have been used in order to estimatedamping of resonance lines due to possible resonant scattering, i.e.,absorption by photo-excitation and re-emission out of the line of sight.The analysis of stellar spectra in analogy to previous works for the Sunis possible now with XMM-Newton and Chandra grating spectra and requiresthis issue to be considered again. In this work we present a sample of45 X-ray spectra obtained for 26 stellar coronae with the RGS on boardXMM-Newton and the LETGS and HETGS on board Chandra. We use ratios ofthe Fe XVII lines at 15.27 Å and 16.78 Å lines to theresonance line at 15.03 Å as well as the He-like f/r ratio of OVII and Ne IX to measure optical depth effects and compare them withratios obtained from optically thin plasma atomic databases such asMEKAL, Chianti, and APEC. From the Fe XVII line ratios we find noconvincing proof for resonance line scattering. Optical depths arebasically identical for all kinds of stellar coronae and we concludethat identical optical depths are more probable when effects fromresonant scattering are generally negligible. The 15.27/15.03Åratio shows a regular trend suggesting blending of the 15.27Åline by a cooler Fe line, possibly Fe XVI. The He-like f/r ratiosfor O and Ne show no indication for significant damping of the resonancelines. We mainly attribute deviations from the atomic databases tostill uncertain emissivities which do not agree well with laboratorymeasurements and which come out with differing results when accountingfor one or the other side effect. We attribute the discrepancies in thesolar data to geometrical effects from observing individual emittingregions in the solar corona but only overall emission for stellarcoronae including photons eventually scattered into the line of sight.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| A study of coronal abundances in RS CVn binaries%
We present XMM-Newton data of several RS CVn binary systems.High-resolution X-ray spectra obtained with the Reflection GratingSpectrometers have been interpreted simultaneously with the EuropeanPhoton Imaging Camera spectra. Highly active stars show a depletion ofelements with a low first ionization potential (FIP) relative tohigh-FIP elements, whereas intermediately active binaries show either noFIP bias or a possible solar-like FIP effect. We find that the low-FIPabundance ratios to oxygen vary with the coronal average temperaturewhereas the ratios for high-FIP elements stay constant. Since we observethat the absolute Fe (low-FIP) abundance increases with decreasingactivity, this suggests that the abundances of elements with low FIPvary with the coronal activity level. Compared with laboratorymeasurements of the intensity ratios of the Fe Xvii lambda lambda 15.01and 15.26 Å lines, the coronal plasmas are in the optically thinregime.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| A possible detection mechanism for pre-low-mass X-ray binaries
This work presents a possible detection mechanism for close, detached,neutron star-red dwarf binaries, which are expected to be theevolutionary precursors of low-mass X-ray binaries (LMXBs). Althoughthis pre-low-mass X-ray binary (pre-LMXB) phase of evolution ispredicted theoretically, as yet no such systems have been identifiedobservationally. The calculations presented here suggest that the X-rayluminosity of neutron star wind accretion in a pre-LMXB system can beexpected to exceed the intrinsic X-ray luminosity of the red dwarfsecondary star. Furthermore, the temperature of the radiation emittedfrom the neutron star wind accretion process is expected, within theconfines of a reasonable set of conditions, to lie within the detectionrange of X-ray satellites. Sources with X-ray luminosities greater thanthat expected for a red dwarf star, but the positions of which coincidewith that of a red dwarf star, are then candidate pre-LMXB systems.These candidate systems should be surveyed for the radial velocityshifts that would occur as a result of the orbital motion of a red dwarfstar within a close binary system containing a high-mass compact object.
| Quiescent and Flaring Structure in RS Canum Venaticorum Stars
Four of the most active RS CVn stars (V711 Tau, II Peg, σ Gem, andUX Ari) have been observed for a total of 3 Ms with the ExtremeUltraviolet Explorer satellite (EUVE) between 1992 and 2000 January.Flaring and quiescent states of extreme ultraviolet spectra(λλ70-740) and light curves (λλ75-175) havebeen analyzed to provide emission measure distributions (EMD) for thesesystems in the range logTe(K)~5.6-7.4, based principally oniron lines. Flux measurements obtained with IUE and the Orbiting andRetrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) completethe EMD in the lower temperature range [logTe(K)~4.0-5.6].Frequent flaring activity has been found in the systems, including anincrease during the rise phase by a factor of ~9 in the flux of σGem, the largest flare enhancement observed with EUVE. Analyses of theEUVE emission in the active single star AB Dor and the low-rotationgiant star β Cet are also included. The EMDs are remarkably similaramong all the stars, showing a narrow enhancement or ``bump'' aroundlogTe(K)~6.9. These narrow bumps are apparently unrelated torotation rate, spectral type, binarity, or evolutionary stage.Significant material is found at logTe(K)>~7.0 for themost active stars. Modulation of the EUV flux outside of flaring occursin four of the stars (σ Gem, V711 Tau, UX Ari, AB Dor). Theelectron density ranges between Ne~1012 and~1013 cm-3, measured at logTe(K)~7.0,and may reach higher values during flares. These densities and EMDvalues imply small scale sizes for emitting regions.
| The X-ray Emission and the Magnetic Fields of Late-Type Stars
We present the results of X-ray luminosities of some active late-typestars, based on data primarily from the ROSAT position sensitiveproportional counter (PSPC). According to the observations, we dividethe stars into four groups: single, wide binary, binary and RS CVn. Weinvestigated the correlation between the X-ray emission and the hardnessratio, coronal temperature, magnetic field strength, magnetic fluxdensity. Our results suggest that the magnetic field plays a veryimportant role in stellar X-ray emission.
| Time series photometric spot modelling V. Phase coherence of spots on UZ Librae
We present spot models for nine years of continuous VI_C photometry ofUZ Lib from 1993-2001. The relatively stable double-humped light curveshape suggests extreme phase coherence. From the spot-modellinganalysis, we found that the major spots or spot groups are alwayslocated on the hemisphere facing the secondary star and exactly in theopposite hemisphere anti-facing the secondary. Several single-humpedlight curves and our suggested binary scenario rule out a pureellipsoidal variability as the cause of the double-humped light curveshape. We try to explain this preferred spot pattern with amagnetic-field structure that connects the two components, as suggestedearlier for RS CVn stars in general. A possible 4.8 years spot cycle isfound from the long-term brightness variations but needs confirmation.We rediscuss the basic astrophysical data of UZ Lib. The Hipparcosparallax is likely wrong, a possible reason could be that UZ Lib ise.g., a triple system.
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