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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| The coronal structure of Speedy Mic - II. Prominence masses and off-disc emission
Observations of stellar prominences on young rapidly rotating starsprovide unique probes of their magnetic fields out to many stellarradii. We compare two independently obtained data sets of the K3 dwarfSpeedy Mic (BO Mic, HD 197890) using the Anglo-Australian Telescope(AAT) and the European Southern Observatory (ESO) Very Large Telescope(VLT). Taken more than a fortnight apart, they provide the first insightinto the evolution of the prominence system on such a young rapidlyrotating star. The largest prominences observed transiting the stellardisc are found at very similar rotational phases between the epochs.This suggests that the magnetic structures supporting the prominencesretain their identity on a two to three week time-scale. By takingadvantage of the high signal-to-noise ratio and large wavelength rangeof the VLT observations, we identify prominences as transient absorptionfeatures in all lines of the hydrogen Balmer series down toH10. We use the ratios of the prominence equivalent widths(EWs) in these lines to determine their column densities in the firstexcited state of hydrogen. We determine the optical depths, findingprominences to be rather optically thick (τ ~ 20) in the Hαline. The total hydrogen column density and thus the prominence massesare determined via observations of the CaII H&K lines. We findtypical masses for four of the largest prominences to be in the range0.5-2.3 × 1014 kg, slightly larger than giant solarprominence masses. Rotationally modulated emission is seen outside ofthe Hα line. These loops of emission are shown to be caused byprominences seen off the stellar disc. We find that all of the largeemission loops can be associated with prominences we see transiting thestellar disc. This, combined with the fact that many prominences appearto eclipse the off-disc emission of others, strongly suggests that theprominence system is highly flattened and likely confined to low stellarlatitudes.Based on observations made with the ESO telescopes obtained from the ESOScience Archive Facility.E-mail: njd2@st-andrews.ac.uk
| Photoelectric radial velocities, Paper XVI 625 ninth-magnitude K0 stars in the six southern Clube Selected Areas
Paper XIII of this series presented radial velocities for 406 stars incertain of the Clube Selected Areas, a set of areas systematicallyarranged in Galactic coordinates. We now complete the survey byproviding the radial velocities, mostly obtained at the EuropeanSouthern Observatory (ESO), of 625 stars in the six southernmost Areas.Each star has been measured at least twice; the mean velocities havestandard errors typically of 0.2-0.3 km s-1. Additionalobservations made from Haute-Provence of many of the stars that areobservable from there have helped to identify, and in some instances tocharacterize, the ~70 spectroscopic binaries discovered in thisprogramme. The final results of the programme, complementing those givenin table 3 of Paper XIII, are summarized in Table 12, which presents themean velocities and velocity dispersions in the six southern Areas. Itis noted that the difference between Paper XIII and this one as regardsthe provenance of the radial velocities has led to a small difference inzero-points, which is discussed in Section 3 and needs to be taken intoaccount in any analysis of the combined data.
| A radio-emitting outflow in the quiescent state of A0620-00: implications for modelling low-luminosity black hole binaries
Deep observations with the Very Large Array of A0620-00, performed in2005 August, resulted in the first detection of radio emission from ablack hole binary at X-ray luminosities as low as 10-8.5times the Eddington limit. The measured radio flux density, of 51 +/- 7μJy at 8.5 GHz, is the lowest reported for an X-ray binary system sofar, and is interpreted in terms of partially self-absorbed synchrotronemission from outflowing plasma. Making use of the estimated outeraccretion rate of A0620-00 in quiescence, we demonstrate that theoutflow kinetic power must be energetically comparable to the totalaccretion power associated with such rate, if it was to reach the blackhole with the standard radiative efficiency of 10 per cent. This favoursa model for quiescence in which a radiatively inefficient outflowaccounts for a sizable fraction of the missing energy, and, in turn,substantially affects the overall dynamics of the accretion flow.Simultaneous observations in the X-ray band, with Chandra, confirm thevalidity of a non-linear radio/X-ray correlation for hard state blackhole binaries down to low quiescent luminosities, thereby contradictingsome theoretical expectations. Taking the mass term into account, theA0620-00 data lie on the extrapolation of the so-called FundamentalPlane of black hole activity, which has thus been extended by more thantwo orders of magnitude in radio and X-ray luminosity. With the additionof the A0620-00 point, the plane relation provides an empirical prooffor the scale invariance of the jet-accretion coupling in accretingblack holes over the entire parameter space observable with currentinstrumentation.
| 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.
| The coronal structure of Speedy Mic - I. A densely packed prominence system beyond corotation
We present new observations of the prominence system on the K3 dwarfSpeedy Mic (BO Mic, HD 197890). Using an improved technique to track theabsorption features in Hα we find a very active prominence systemwith approximately 10 prominences on the observable hemisphere perrotation. From a total of 25 prominences, we find an average axialdistance of (2.85 +/- 0.54) R* which is twice the corotationradius above the stellar surface. We discuss the consequences of theseobservations on the nature of the supporting magnetic structures. Twoconsecutive nights, with complete phase coverage, combined with afurther night after a three-night gap allow us to study the evolution ofthe prominence system on two different time-scales. Several of theprominences have counterparts at similar phases on consecutive nights.During this interval, many prominences show evidence for evolution intheir heights and phases of observation. Five nights (13 rotationcycles) later, we recover many prominences at approximately the samephases. Whilst individual prominences change axial distances orappear/reappear from night-to-night, the underlying prominencesupporting structures appear to be stable over as many as 13 stellarrotations.
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| The highly spotted photosphere of the young rapid rotator Speedy Mic
We present high-resolution images of the young rapidly rotating K3 dwarfSpeedy Mic (BO Mic, HD 197890). The photospheric spot maps reveal aheavily and uniformly spotted surface from equatorial to high-latituderegions. Contrary to many images of similar objects, Speedy Mic does notpossess a uniform filling at high latitudes, but exhibits structure inthe polar regions showing greatest concentration in a particularlongitude range. The asymmetric rotation profile of Speedy Mic indicatesthe presence of a companion or nearby star which shows radial velocityshifts over a time-scale of several years. Using a simple dynamicalargument, we show that Speedy Mic is unlikely to be a binary system, andconclude that the feature must be the result of a chance alignment witha background binary. Complete phase coverage on two consecutive nightsin addition to 60 per cent phase coverage after a three-night gap hasenabled us to track the evolution of spots with time. By incorporating asolar-like differential rotation model into the image reconstructionprocess, we find that the equator laps the polar regions once every 191+/- 17 d. This finding is in close agreement with measurements for otherlate-type rapid rotators.
| An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 μm) and the coronagraph on theNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) on theHubble Space Telescope to survey 45 single stars with a median age of0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7mag. For the median age we were capable of detecting a 30MJcompanion at separations between 15 and 200 AU. A 5MJ objectcould have been detected at 30 AU around 36% of our primaries. Forseveral of our targets that were less than 30 Myr old, the lower masslimit was as low as 1MJ, well into the high mass planetregion. Results of the entire survey include the proper-motionverification of five low-mass stellar companions, two brown dwarfs(HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Doppler imaging and surface differential rotation of young open cluster stars - I. HD 307938 (R58) in IC 2602
In this paper we present Doppler images of a young active G dwarf (HD307938) in the southern open cluster IC 2602. Spectroscopic data wereobtained over a four-night period in 2000 January at the 3.9-mAnglo-Australian Telescope using the University College London EchelleSpectrograph. Simultaneous photometric observations (in the V and Rbands) were obtained at the 1.0-m Australian National Universitytelescope. By applying least-squares deconvolution (LSD) to the 2500+photospheric lines in each echelle spectrum a single highsignal-to-noise ratio LSD profile was produced for each phase of thespectroscopic observations. Maximum-entropy image reconstruction,incorporating both the LSD profiles and the photometric data, was usedto produce maps of the surface features of the star, with the inclusionof the photometric data producing an increase (compared with the use ofspectroscopic data alone) in the spot occupancy in both low- andmid-latitude regions of the star. The maps show that HD 307938 possessesa large, broken polar spot extending down to ~60° latitude, as wellas lower-latitude spots similar to other rapidly rotating G dwarfs. Byincorporating a solar-like differential rotation law into the imagingprocess the surface differential rotation of HD 307938 was determined.This gave a surface shear of dΩ= 0.025 +/- 0.015 radd-1 (for an inclination angle of 60°). Thus the equatorof HD 307938 laps the poles every ~250 d and has a photospheric sheararound half that of the Sun.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Localizing plages on BO Mic. Rapid variability and rotational modulation of stellar Ca H&K core emission
We have obtained a densely sampled time series of Ca ii H&K lineprofiles of the ultrafast rotating K-dwarf star BOMic. Taken at high resolution, the spectra reveal pronouncedvariations of the emission core profiles. We interpret these variationsas signs of concentrated chromospherically active regions, in analogy tosolar plages. We further interpret the variations as partly due to therapid growth and decay of plages, while other variations appear to becaused by plages moved over the visible stellar disk by rotation. Theequivalent width of the Ca K core emission changes approximately inanti-phase to the photospheric brightness, suggesting an association ofthe chromospheric plage regions with pronounced dark photospheric spots.We believe that further analysis of the presented spectral time serieswill lead to a chromospheric Doppler image of BO mic.
| Doppler imaging of Speedy Mic using the VLT. Fast spot evolution on a young K-dwarf star
We study the short-term evolution of starspots on the ultrafast-rotatingstar HD 197890 (“Speedy Mic” = BO Mic, K 0-2 V,Prot=0.380 d) based on two Doppler images taken about 13stellar rotations apart. Each image is based on spectra densely samplinga single stellar rotation. The images were reconstructed by our Dopplerimaging code CLDI (Clean-like Doppler imaging) from line profilesextracted by spectrum deconvolution. Our Doppler images constructed fromtwo independent wavelength ranges agree well on scales down to 10°on the stellar surface. In conjunction with nearly parallel V-bandphotometry our observations reveal a significant evolution of the spotpattern during as little as two stellar rotations. We suggest that sucha fast spot evolution demands care when constructing Doppler images ofhighly active stars based on spectral time series extending over severalstellar rotations. The fast intrinsic spot evolution on BO Mic impedesthe determination of a surface differential rotation; in agreement withearlier results by other authors we determine an upper limit of |α | < 0.004 ± 0.002.
| The dependence of differential rotation on temperature and rotation
We use Doppler imaging techniques to determine the dependence ofstarspot rotation rates on latitude in an homogeneous sample of young,rapidly rotating solar analogues. A solar-like differential rotation lawis used, where the rotation depends on sin2θ, whereθ is the stellar latitude. By including this term in the imagereconstruction process, using starspots as tracers, we are able todetermine the magnitude of the shear over more than one rotation cycle.We also consider results from matched filter starspot trackingtechniques, where individual starspot rotation rates are determined. Inaddition we have re-analysed published results and present a newmeasurement for the K3 dwarf Speedy Mic.A total of 10 stars of spectral type G2-M2 are considered. We find atrend towards decreasing surface differential rotation with decreasingeffective temperature. The implied approach to solid body rotation withincreasing relative convection zone depth implies that the dynamomechanism operating in low-mass stars may be substantially differentfrom that in the Sun.
| 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
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The Brown Dwarf Desert at 75-1200 AU
We present results of a comprehensive infrared coronagraphic search forsubstellar companions to nearby stars. The research consisted of (1) a178-star survey at Steward and Lick observatories, with opticalfollow-up from Keck Observatory, capable of detecting companions withmasses greater than 30 MJ, and semimajor axes between about140 to 1200 AU; (2) a 102-star survey using the Keck Telescope, capableof detecting extrasolar brown dwarfs and planets typically more massivethan 10 MJ, with semimajor axes between about 75 and 300 AU.Only one brown dwarf companion was detected, and no planets. Thefrequency of brown dwarf companions to G, K, and M stars orbitingbetween 75 and 300 AU is measured to be 1%+/-1%, the most precisemeasurement of this quantity to date. The frequency of massive (greaterthan 30 MJ) brown dwarf companions at 120-1200 AU is found tobe f=0.7%+/-0.7%. The frequency of giant planet companions with massesbetween 5 and 10 MJ orbiting between 75 and 300 AU ismeasured here for the first time to be no more than ~3%. Together withother surveys that encompass a wide range of orbital separations, theseresults imply that substellar objects with masses between 12 and 75MJ form only rarely as companions to stars. Theories of starformation that could explain these data are only now beginning toemerge.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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 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
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| Solar spots as prototypes for stellar spots
What is the nature of what we commonly refer to as starspots? The answerto that question determines what we can learn from the study ofstarspots about stars, their internal dynamics, and their magneticactivity. Observations of the Sun, the only cool star that we canobserve in detail, naturally led to the hypothesis that starspots inother cool stars are magneto-convective phenomena in which a magneticfield substantially affects the structure of a stellar atmosphere.Spectral line features, eclipse light curves, and intensity modulationsare consistent with that hypothesis. Yet for almost all stars for whichstarspots are reported, the surface coverage is very much higher thanfor the Sun, while many stars manifest spots at high latitudes where theSun has never been seen to do so. In this review, I address how wellstarspots compare to sunspots, discuss some differences expected whenmoving away from the Sun in the HR diagram or in level of activity, andidentify resources to further deepen our understanding.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| Dusty Debris around Solar-Type Stars: Temporal Disk Evolution
Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to searchfor evidence of emission from dust orbiting young main-sequence stars,both in clusters and isolated systems. Over half of the detections arenew examples of dusty stellar systems and demonstrate that such dust canbe detected around numerous stars older than a few times 106yr. Fluxes at 60 and either 90 or 100 μm for the new excess sourcestogether with improved fluxes for a number of IRAS-identified sourcesare presented. Analysis of the excess luminosity relative to the stellarphotosphere shows a systematic decline of this excess with stellar ageconsistent with a power-law index of -2.
| Doppler images from dual-site observations of southern rapidly rotating stars - II. Starspot patterns and differential rotation on Speedy Mic
We have secured high spatial and temporal resolution spectra of therapidly rotating K dwarf Speedy Mic (HD 197890) at two sites and acommon epoch of observations. The 0.38-d axial rotation period and theV-band magnitude of 9.33 make it a difficult target for Doppler imaging.In order to obtain high signal-to-noise ratio profiles from 300-sexposures, we apply the technique of least-squares deconvolution to thelarge number of photospheric absorption lines available in each of ourspectra. This allows us to derive high-resolutionmaximum-entropy-regularized Doppler images of the stellar surface. Usingthese techniques, we also derive radial velocities and accurateprojected equatorial rotation velocities which are consistent to within~1kms-1. Our surface maps reveal one of the most heavilyspotted photospheres seen on a rapid rotator, with starspots occurringat all latitudes. At the time of observations, Speedy Mic had no strongpolar spot, but it shows spots concentrated in low- andintermediate-latitude bands. We attempt a differential rotationmeasurement, but lack of sufficient phase coverage allows determinationof only a lower limit of 59d for the time it takes the equatorialregions to lap the polar regions. We also find variations in the heavilyfilled-in Hα line which can be attributed to prominences passingin front of the stellar disc. Despite the rapid rotation, the appearanceof the same features on consecutive nights of observations shows theclouds to be stable on time-scales of at least a day.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| The Coronal Metallicity of the Intermediate Activity Dwarf ξ Bootis A
Extreme Ultraviolet Explorer (EUVE) spectra from a 270 ks observation ofthe intermediate activity star ξ Boo A (G8 V) have been analyzed inorder to determine the metallicity of its coronal plasma. This analysiswas based on a new method that used both the EUVE spectroscopic andphotometric (Deep Survey) data, together with theoretical model spectra.By comparison of observed and synthetic spectra computed using theCHIANTI database, we estimated that about 20% of the observed line fluxis not accounted for in the theoretical model. Allowing for the``missing lines'' in our analysis, we obtain a coronal metallicity forξ Boo A of [M/H]=0.1+0.2-0.15 relative to thesolar photosphere. This is slightly higher than the photosphericmetallicity of [M/H]=-0.15+/-0.05 and lends support to the results ofour earlier analysis of the same spectral data that indicated thatcoronal abundances of elements with low first ionization potentials(FIPs) are systematically higher than those of elements with high FIPs.These results, coupled with other evidence, imply that stars ofintermediate activity level-stars more active than the Sun butsignificantly less active than the most active single and binarystars-do not suffer from any depletion of metals in their coronae assome coronal metallicities estimated for more active stars have appearedto suggest. The observation of a solar-like FIP effect on ξ Boo Aadds support to the notion that stars of intermediate activity levelhave coronae characterized by a high surface coverage of brightsolar-like active regions.
| The Mg II and Lyα Lines of Nearby K Dwarfs: Interstellar Medium Components and Flux Measurements
We analyze local interstellar medium (LISM) absorption observed in theLyα and Mg II h and k lines of six nearby K dwarf stars, using UVspectra of these stars obtained with the Goddard High ResolutionSpectrograph on the Hubble Space Telescope. For four of the six stars,we detect an absorption component with a velocity and column densityconsistent with the Local Interstellar Cloud (LIC). For HD 197890, thereis no observed component at the expected LIC velocity or at theprojected velocity of the G cloud, which is a nearby cloud in thegeneral direction of the Galactic center. It also seems doubtful thateither of the two components seen toward HD 82558 are LIC or G cloudabsorption. The total H I column density toward HD 82558 (d=18.3 pc) isextremely high (logNH=19.05+/-0.15), representing the largestaverage H I density detected for any line of sight through the nearbyLISM (nH~0.2 cm-3). This is particularlyremarkable considering that this star is only 39° from the``interstellar tunnel'' toward ɛ CMa, where column densities are anorder of magnitude lower than this toward stars that are an order ofmagnitude farther away. Based on observations with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| X-rays from quiescent low-mass X-ray binary transients
I argue that it is very unlikely that X- rays from quiescent black-holelow-mass X-ray binary transients are emitted by coronae of companionstars. I show that in a simple model in which these X-rays are emittedby an ADAF filling the inner part of an unsteady, dwarf-nova type disc,the X-ray luminosity is correlated with the orbital period. I predictwhat values of X-ray luminosities from black-hole transient systemsshould be observed by Chandra and XMM-Newton.
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