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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 Low-Mass Limit for Total Mass of W UMa-type Binaries
The observations of W UMa type stars show a well-defined short-periodlimit of 0.22 d, which is equivalent to a lower mass limit ofapproximately 1 M_odot for the total binary mass. It is currentlybelieved that cool contact binaries are formed from detached binarieslosing angular momentum (AM) via a magnetized wind. Orbital evolution ofdetached binaries with various component masses was followed until theprimary component reached the critical Roche surface and the Roche lobeoverflow (RLOF) began. It was assumed that the minimum initial, i.e.,ZAMS, orbital period of such binaries is equal to 2 d and that thecomponents lose AM just as single stars. According to the mass-dependentformula for AM loss rate of single stars, derived in this paper, the AMloss time scale increases substantially with decreasing stellar mass.The formula was applied to binaries with the initial primary componentmasses between 1.0 M_odot and 0.6 M_odot and two values of mass ratioq=1 and 0.5.Detailed calculations show that the time needed to reach RLOF by a 1M_odot primary is of the order of 7.5 Gyr, but it increases to more than13 Gyr for a binary with an initial primary mass of 0.7 M_odot. Binarieswith less massive primaries have not yet had time to reach RLOF evenwithin the age of the Universe. This sets a lower mass limit for thepresently existing contact binaries at about 1.0 M_odot--1.2 M_odot, ina good agreement with observations.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| 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.
| Photometric study of the active binary star V1430 Aquilae
New BVR light curves and a photometric analysis of the eclipsing binarystar V1430 Aql are presented. The light curves were obtained at theÇanakkale Onsekiz Mart University Observatory in 2004. The lightcurves are generally those of detached eclipsing binaries, but there arelarge asymmetries between maxima. New BVR light curves were analysedwith an ILOT procedure. Light curve asymmetries of the system wereexplained in terms of large dark starspots on the primary component. Theprimary star shows a long-lived and quasi-poloidal spot distributionwith active longitudes in opposite hemispheres. Absolute parameters ofthe system were derived. We also discuss the evolution of the system:the components are likely to be pre-main sequence stars, but a post-mainsequence stage cannot be ruled out. More observations are needed todecide this point.
| 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.
| 51 Eridani and GJ 3305: A 10-15 Myr old Binary Star System at 30 Parsecs
Following the suggestion of Zuckerman and coworkers, we consider theevidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historicallyclassified as unrelated main-sequence stars in the solar neighborhood,are instead a wide physical binary system and members of the youngβ Pic moving group (BPMG). The BPMG is the nearest (d<~50 pc) ofseveral groups of young stars with ages around 10 Myr that arekinematically convergent with the Oph-Sco-Cen association (OSCA), thenearest OB star association. Combining South African AstronomicalObservatory optical photometry, Hobby-Eberly Telescope high-resolutionspectroscopy, Chandra X-Ray Observatory data, and Second US NavalObservatory CCD Astrograph Catalog kinematics, we confirm with highconfidence that the system is indeed extremely young. GJ 3305 itselfexhibits very strong magnetic activity but has rapidly depleted most ofits lithium. The 51 Eri/GJ 3305 system is the westernmost known memberof the OSCA, lying 110 pc from the main subgroups. The system is similarto the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet,suggesting that dynamically fragile multiple systems can survive theturbulent environments of their natal giant molecular cloud complexes,while still having high dispersion velocities imparted. Nearby youngsystems such as these are excellent targets for evolved circumstellardisk and planetary studies, having stellar ages comparable to that ofthe late phases of planet formation.
| Optical and X-Ray Studies of Chromospherically Active Stars: FR Cancri, HD 95559, and LO Pegasi
We present a multiwavelength study of three chromospherically activestars, namely, FR Cnc (BD +16°1753), HD 95559, and LO Peg (BD+22°4409), including newly obtained optical photometry andlow-resolution optical spectroscopy for FR Cnc, as well as archival IRand X-ray observations. The BVR photometry carried out from 2001 to 2004has found significant photometric variability to be present in all threestars. For FR Cnc, a photometric period of 0.8267+/-0.0004 days has beenestablished. The strong variation in the phase and amplitude of the FRCnc light curves when folded on this period implies the presence ofevolving and migrating spots or spot groups on its surface. Twoindependent spots with migration periods of 0.97 and 0.93 yr,respectively, are inferred. The photometry of HD 95559 suggests theformation of a spot (group) during the interval of our observations. Weinfer the existence of two independent spots or groups in thephotosphere of LO Peg, one of which has a migration period of 1.12 yr.The optical spectroscopy of FR Cnc carried out during 2002-2003 revealsthe presence of strong and variable Ca II H and K, Hβ, and Hαemission features indicative of a high level of chromospheric activity.The value of 5.3 for the ratio of the excess emission in Hα toHβ, EHα/EHβ, suggests that thechromospheric emission may arise from an extended off-limb region. Wehave searched for the presence of color excesses in the near-IR JHKbands of these stars using Two Micron All Sky Survey data, but none ofthem appear to have any significant color excess. We have also analyzedarchival X-ray observations of HD 95559 and LO Peg carried out with theROSAT observatory. The best-fit models to their X-ray spectra imply thepresence of two coronal plasma components of differing temperatures andwith subsolar metal abundances. The inferred emission measures andtemperatures of these systems are similar to those found for otheractive dwarf stars. The kinematics of FR Cnc suggest that it is a veryyoung (35-55 Myr) main-sequence star and a possible member of the IC2391 supercluster. LO Peg also has young disk-type kinematics and hasbeen previously suggested to be a member of the 100 Myr old LocalAssociation (Pleiades moving group). The kinematics of HD 95559 indicateit is a possible member of the 600 Myr old Hyades supercluster.
| A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO
We report on a survey devoted to the search of exoplanets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets versus the angular separationfrom the star is presented. The nondetection of any planetary masscompanion in our survey is used to derive, for the first time, thefrequency of the upper limit of the projected planet-star separation. Inparticular, we find that in 50% of the cases, no 5MJ (or moremassive) planet has been detected at projected separations larger than14 AU, and no 10MJ (or more massive) planet has been detectedat projected separations larger than 8.5 AU. In 100% of the cases, thesevalues increase to 36 and 65 AU, respectively. The excellent sensitivityreached by our study leads to a much lower upper limit of the projectedplanet-star separation compared with previous studies. For example, forthe β Pictoris group (~12 Myr), we did not detect any10MJ planet at distances larger than 15 AU. A previous studycarried out with 4 m class telescopes put an upper limit for10MJ planets at ~60 AU. For our closest target (V2306 Oph;d=4.3 pc), it is shown that it would be possible to detect a10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to mechanisms explainingplanet formation and migration and forthcoming observational strategiesand future planet-finder observations from the ground.Based on observations collected at the European Southern Observatory,Chile. Program 70.C-0777D, 70.C-0777E, and 71.C-0029A.
| HD 34700 is a T Tauri multiple system
We have imaged the young double-lined spectroscopic binary HD 34700 inthe near-infrared and find evidence for two faint, visual, stellarcomponents at 5.2 Arcsec and 9.2 arcsec distance. High-resolutionechelle spectroscopy of both stars shows strong Li I 6708 Åabsorption and Hα emission. The spectral types of the companionsare estimated from the spectra and photometry as M1-M2 and M2-M3. Theirradial velocities are similar to the center-of-mass velocity of thecentral SB2; hence all four stars are most probably physically bound andconstitute a young quadruple stellar system with an inner short-periodbinary. We provide a list of pre-main sequence spectroscopic binarieswith additional components of which HD 34700 is yet another example. Theavailable statistics strengthen the suspicion that dynamical effects inmultiple systems play a key role in the formation of very close binariesearly in their evolution.
| The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin-up
The zero-age main-sequence star AB Dor and the K dwarf component of theV471 Tau close binary have essentially identical rotation rates andspectral types. An analysis of their high-resolution Chandra X-rayspectra reveals remarkably similar coronal characteristics in terms ofboth temperature structure and element abundances. Both stars showdepletions of low first ionization potential (FIP) elements by factorsof ~3, with higher FIP elements showing more mild depletions. Noevidence for enhancements of very low FIP (<7 eV) elements, such asNa, Al, and Ca, as compared to other low-FIP elements, was found. Theabundance anomaly pattern for AB Dor and V471 Tau is similar to,although less extreme than, the abundance anomalies exhibited by activeRS CVn-type binaries. While we find statistically significant structurein the underlying differential emission measure distributions of thesestars over narrow temperature intervals, this structure is stronglydependent on the lines used in the analysis and is probably spurious. Onthe basis of their X-ray similarities, we conclude that the exactevolutionary state of a star has little effect on coronalcharacteristics and that the parameters that dominate coronal structureand composition are simply the rotation rate and spectral type.
| 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.
| Searching for massive extrasolar planets around young and nearby stars: from NACO to CHEOPS
We report on a survey devoted to the search of exo-planets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets vs. the angular separation fromthe star is presented. The non-detection of any planetary mass companionin our survey is used to derive, for the first time, the frequency ofthe upper limit of the projected separation planet/stars. In particular,we find that in 50% of cases, no 5MJ planet (or more massive)has been detected at projected separations larger than 14 AU and no10MJ planet (or more massive) has been detected at projectedseparations larger than 8.5 AU. The excellent sensitivity reached by ourstudy leads to a much lower upper limit of the projected separationplanet-star compared with previous studies. For our closest target(V2306 Oph - d = 4.3 pc) it is shown that it would be possible to detecta 10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to forthcomingobservational strategies (Simultaneous Differential Imaging technique)and future planet finder observations from the ground.
| 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.
| New Aspects of the Formation of the β Pictoris Moving Group
In a previous work, we explored the possibility that the β Pictorismoving group (BPMG), consisting of low-mass post-T Tauri stars, wasformed near the Scorpius-Centaurus OB association. The cause of theformation could be a Type II supernova exploding either in LowerCentaurus Crux (LCC) or the Upper Centaurus Lupus (UCL), the two oldersubgroups of that association. Here we present new results for BPMG. Amore detailed analysis of the orbit confinement in this group leads to astar distribution pattern at birth that can be considered as arepresentation of the density distribution in the natal cloud. We alsopropose a plausible origin for the supernova that could have triggeredthe star formation in BPMG by finding the past position of the runawaystar HIP 46950. We find that this scenario is capable of explaining theorigin of all the members of BPMG proposed by Zuckerman and coworkersand by Song and coworkers, with the exception of HIP 79881, which isprobably an old main-sequence interloper.
| 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
| 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.
| A Multiwavelength Study of CC Eridani
Radio and optical observations from December 2001 and January 2002 ofthe active RS CVn-like binary CC Eri are presented. The star wasmonitored at 4.80 and 8.64GHz over 3×12h allocations with theAustralia Telescope Compact Array on 28 to 30 December 2001. TheAnglo-Australian Telescope was used for simultaneous opticalspectropolarimetry during a 0.5h period on 30 December. Data from fournights of broadband photometry gathered around the same period are alsoincluded in this present multiwavelength study. The low levels of radioemission were circularly polarised at ~20% with slightly positivespectral indices of ~0.26. Two flare-like increases were observed onsuccessive nights with steep positive spectral indices and no detectablepolarisation. Cross-correlation analysis of the 4.80 and 8.64GHzintensities over the stronger flare showed that the higher frequencyemission preceded that at the lower frequency by ~5min, a resultconsistent with the propagation of a hydromagnetic disturbance outwardsthrough the corona. On the same night, a significant cross-correlationin the `quiescent' emission indicates the presence of micro-flaring,although its low intensity does not permit the evaluation of a timedelay. The emission parameters on the three nights are compatible with agyrosyncrotron mechanism, in which the radio source becomes opticallythick during strong flaring. We develop a simple model, which is basedon assuming that the number of radiating electrons is a given functionof the magnetic field in the source region, and derive feasible valuesfor the field, source radius, and number of emitting electrons, whichare not strongly dependent on the field modelling function or the aspectratio of the source. Spectropolarimetry demonstrates the presence of astrong surface magnetic field. Optical photometry, covering a sufficientamount of the orbit, indicates a maculation region of significant size(~14° radius). The results help develop a three-dimensional pictureof a large stellar magnetically active region and encourage moredetailed follow-up multiwavelength studies of this and similar stars.
| From the Solar Corona to Clusters of Galaxies: The Radio Astronomy of Bruce Slee
Owen Bruce Slee is one of the pioneers of Australian radio astronomy.During World War II he independently discovered solar radio emission,and, after joining the CSIRO Division of Radiophysics, used a successionof increasingly more sophisticated radio telescopes to examine anamazing variety of celestial objects and phenomena. These ranged fromthe solar corona and other targets in our solar system, to differenttypes of stars and the ISM in our Galaxy, and beyond to distant galaxiesand clusters of galaxies. Although long retired, Slee continues to carryout research, with emphasis on active stars and clusters of galaxies. Aquiet and unassuming man, Slee has spent more than half a century makingan important, wide-ranging contribution to astronomy, and his workdeserves to be more widely known.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| 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.
| 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.
| Nearby young stars
We present the results of an extensive all-sky survey of nearby stars ofspectral type F8 or later in a systematic search of young (zero-age mainsequence) objects. Our sample has been derived by cross-correlating theROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754candidates distributed more or less randomly over the sky. Follow-upspectroscopy of these candidate objects has been performed on 748 ofthem. We have discovered a tight kinematic group of ten stars withextremely high lithium equivalent widths that are presumably youngerthan the Pleiades, but again distributed rather uniformly over the sky.Furthermore, about 43 per cent of our candidates have detectable levelsof lithium, thus indicating that these are relatively young objects withages not significantly above the Pleiades age.Based on observations collected at the European Southern Observatory,Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D-0236(A)).
| Rotation and differential rotation in field F- and G-type stars
We present a detailed study of rotation and differential rotationanalyzing high resolution high S/N spectra of 142 F-, G- and earlyK-type field stars. Using Least Squares Deconvolution we obtainbroadening profiles for our sample stars and use the Fourier transformmethod to determine projected rotational velocities v sin i.Distributions of rotational velocities and periods are studied in theHR-diagram. For a subsample of 32 stars of spectral type F0-G0 we derivethe amount of differential rotation in terms of alpha = (Omega_Equator- Omega_Pole )/Omega_Equator . We find evidence for differentialrotation in ten of the 32 stars. Differential rotation seems to be morecommon in slower rotators, but deviations from rigid rotation are alsofound in some fast rotators. We search for correlations betweendifferential rotation and parameters relevant for stellar activity andshow indications against strong differential rotation in very activestars. We derive values of Delta P and Delta Omega , which support aperiod dependence of differential rotation. Derived lap times 2pi /DeltaOmega are of the order of 20 d and contradict the assumption thatconstant lap times of the order of the solar one ( ~ 130 d) are therule in stars that are thought to harbour magnetic dynamos.Based on observations collected at the European Southern Observatory, LaSilla.Tables 3 and A1 are only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/647
| 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.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
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