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Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| 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.
| Analysis of the Dynamical Instability of Several Multiple Stars with Weak Hierarchy
The dynamical stability of 16 multiple stars is analyzed using MonteCarlo simulations with allowance for the errors in the observationaldata. The analysis was carried out by varying the uncertainties in theinitial observational data. Six different stability criteria wereconsidered, and the dynamical evolution was studied using numericalsimulations. Eleven of the systems are probably stable, whereas fivesystems (HD 40887, HD 136176, HD 150680, HD 217675, and HD 222326) areprobably unstable (the probability that they are unstable is 0.94 ormore accordingt o the results of forward and backward simulations overintervals of 106 yr). The results of the simulations were mostconsistent with the criteria of Mardling-Aarseth (the correlationcoefficient between the probabilities of disruption inferred from thestability criterion and numerical integration was r = 0.998),Valtonen-Karttunen (r = 0.998), and Eggleton-Kiseleva (r = 0.997). Inabout 92 93% of all cases, these criteria yield results that areconsistent with the numerical simulations. These criteria also yieldhigh disruption probabilities for the unstable systems. Scenarios forthe formation of such systems are discussed: temporary capture of afield star by a close binary, perturbation of a stable multiple systemby a massive field object, and disruption of small stellar groups orclusters. The probabilities that these scenarios are realized areanalyzed.
| Analysis of the Dynamic Stability of Selected Multiple Stars with Weak Hierarchy
The stability of multiple systems with known orbital elements and withsubsystems occupying adjacent hierarchy levels is analyzed using sixstability criteria and numerical simulations of their dynamicalevolution. All the stability criteria considered are in qualitativeagreement with the numerical computations. Of the 16 systems studied, 11are confirmed to be stable and five (HD 40887, HD 136176, HD 150680, HD217675, and HD 222326) may be unstable on time scales of 106 yr orless. The small dynamical ages of the unstable systems may indicate thatthey have captured components during encounters between close binariesand field or moving cluster stars. The instability could also resultfrom the perturbation of a stable system when it approaches a massiveobject (star, black hole, or molecular cloud). It is possible that someof the unstable systems are remnants of small clusters or stellargroups.
| Infrared differential photometry of selected orbital binaries
High-angular resolution observations were acquired for a sample of F-Gand K nearby orbital binaries, making use of the ADONIS instrumentmounted at the 3.6m ESO telescope from La Silla observatory. The aim isto derive broad-band near-infrared colours for the individual componentsof those binaries which do not fit well the mean mass-luminosityrelation, but which have accurate parallaxes (from the Hipparcossatellite) and high-quality orbits, thanks to many previous efforts.Such colour determinations allow component temperatures and photometricmass ratios to be derived.
| 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.
| 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.
| Speckle Observations of Binary Stars with the WIYN Telescope. IV. Differential Photometry
Five hundred seventy-six magnitude difference measures are presented for260 binary stars. These measures are derived from CCD-based speckleobservations taken at the WIYN 3.5 m telescope at Kitt Peak NationalObservatory during the period 1997-2000. Separations of the systemsrange from over 1" down to near the diffraction limit of the telescope.A study of multiple measures of the same targets indicates that themeasures have a typical uncertainty of better than 0.13 mag per 2 minuteobservation, and that multiple observations can be averaged to arrive atsmaller uncertainties. Results presented here are also compared, insofaras it is possible, with measures in the Hipparcos Catalogue and toprevious studies using adaptive optics. No major systematic errors wereidentified.The WIYN Observatory is a joint facility of the University ofWisconsin-Madison, Indiana University, Yale University, and the NationalOptical Astronomy Observatory.
| 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
| 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.
| The 7.5 Magnitude Limit Sample of Bright Short-Period Binary Stars. I. How Many Contact Binaries Are There?
A sample of bright contact binary stars (W UMa type or EW, and related:with β Lyr light curves, EB, and ellipsoidal, ELL-in effect, allbut the detached, EA) to the limit of Vmax=7.5 mag is deemedto include all discoverable short-period (P<1 day) binaries withphotometric variation larger than about 0.05 mag. Of the 32 systems inthe final sample, 11 systems have been discovered by the Hipparcossatellite. The combined spatial density is evaluated at(1.02+/-0.24)×10-5 pc-3. The relativefrequency of occurrence (RFO), defined in relation to the main-sequencestars, depends on the luminosity. An assumption of RFO~=1/500 forMV>+1.5 is consistent with the data, although the numberstatistics is poor with the resulting uncertainty in the spatial densityand the RFO by a factor of about 2. The RFO rapidly decreases forbrighter binaries to a level of 1/5000 for MV<+1.5 and to1/30,000 for MV<+0.5. The high RFO of 1/130, previouslydetermined from the deep OGLE-I sample of disk population W UMa typesystems toward Baade's window, is inconsistent with and unconfirmed bythe new results. Possible reasons for the large discrepancy arediscussed. They include several observational effects but also apossibility of a genuine increase in the contact-binary density in thecentral parts of the Galaxy. Based on data from the Hipparcos satellitemission and from the David Dunlap Observatory, University of Toronto.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Speckle Observations of Binary Stars with the WIYN Telescope. II. Relative Astrometry Measures during 1998-2000
Five hundred twelve relative astrometry measures are presented for 253double stars, including 53 double stars discovered by Hipparcos. In 15cases, relative astrometry is reported for the first time for newlyconfirmed pairs. In addition, 20 high-quality nondetections ofcompanions are reported for stars suspected of being nonsingle byHipparcos. Observations were taken using a fast-readout CCD camerasystem at the WIYN 3.5 m telescope at Kitt Peak, Arizona. In comparingthese measures with ephemeris predictions for binary stars with verywell known orbits, we find that the measurement precision is better than3 mas in separation and 1° in position angle per individualobservation. Measurement precision and detection capabilities are fullydiscussed, and confirmed orbital motion is reported in four cases of theHipparcos double star discoveries. The WIYN Observatory is a jointfacility of the University of Wisconsin-Madison, Indiana University,Yale University, and the National Optical Astronomy Observatory.
| 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.
| HD 9770, a southern active-chromosphere system
The visual triple system HD9770 (BB Scl) has been the subject of afour-year programme of UVB(RI)C photometry and Hαéchelle spectroscopy. Analysis of the data obtained over thatperiod shows that star B, and probably also star A, of HD9770 is abinary. The A system comprises a K1V star, which may be in a binarysystem with another K dwarf. The B system is an eclipsing binary of theBY Dra type in which both stars are chromospherically active. An orbitalperiod of 0.476525+/-0.000013d has been derived from the light curve inV. Physical parameters derived from analysis of the light curves inUBV(RI)C are presented.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| 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.
| On the Variability of Early K Stars
We investigate the Hipparcos Satellite photometry of K0-K4 stars tostudy their pattern of activity. Most are not particularly variable. Afew stars for which further study is desirable are identified.
| Dynamical Stability of Triple Stars
The dynamical stability of 38 observed hierarchical triple stars withknown orbital elements of the internal and external binary subsystemsand component masses is considered. Four different criteria of dynamicalstability are used. The observed stability parameters and their criticalvalues are calculated by taking into account errors in the orbitalelements and component masses. Most triple systems are stable. Accordingto some criteria, several triple stars (ADS440, xi Tau, lambda Tau,ADS3358, VV Ori, ADS10157, HZ Her, Gliese 795, ADS15971, and ADS16138)may be dynamically unstable. This result is probably associated withunreliability of the empirical stability criteria and/or with errors inthe observed quantities.
| The active binary star II Pegasi with it BeppoSAX
II Peg is an ideal target to study stellar activityand flares, since intense and long lasting flares have been frequentlydetected from this system at all wavelengths. We report here about aBeppoSAX observation of II Peg. We followed thesystem for ~ 19 hours on December 5 and 6 1997 with BeppoSAX and theX-ray light curve resembles the typical behavior of a decay phase of along-lasting flare. The spectral analysis shows that the IIPeg X-ray spectrum is described by a two-temperaturecomponents, with the two dominant temperatures centered in the range of9-11 and 24-26 MK. The derived coronal metal abundance is low ( Z ~ 0.2Z_sun) compared to recent determinations of the photospheric abundance(Z ~ 0.6 Z_sun). Some possible explanations for this phenomenology arereviewed. As for most other stellar coronal sources observed withBeppoSAX, we find that in order to fit the BeppoSAX spectra aninterstellar column density about a factor ten higher than previouslydetermined is required.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Classification of EUV stellar sources detected by the ROSAT WFC. I. Photometric and radial velocity studies
We present the results of high-precision UBV(RI)_c photometricobservations and of spectroscopic radial velocity measurements obtainedat the European Southern Observatory for a sample of 51 cool starsdetected in the EUV by the ROSAT Wide Field Camera (WFC). Using alsorecent results from HIPPARCOS, we infer spectral types and investigatethe single or binary nature of the sample stars. Optical variability,with periods in the 0.4-13 day range, has been detected for the firsttime in 15 of these stars. based on data collected at the EuropeanSouthern Observatory, La Silla, Chile.}\fnmsep \thanks{Tables 1--5,Figs. 2-27 and the complete data set are available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.html}
| Beppo-SAX observations of AR Lacertae
We present the results of an X-ray observation of the binary system ARLacertae (HD 210334) obtained with the Low and Medium EnergyConcentrator Spectrometers (LECS & MECS) and with the PhoswichDetector System (PDS) on board the Beppo-SAX satellite. Two flareepisodes with comparable peak luminosities were detected, one in each ofthe two runs into which the observations were split. Several lowamplitude flares appear to be superimposed on a rotational modulatedlight curve. A very tight and shallow primary eclipse was observed,which is different from primary eclipses observed at other epochs byother X-ray telescopes. No sign of secondary eclipse can be inferredfrom these data. The PDS data show that the emission in the 15-300 keVrange is generally not detectable, but there is convincing evidence ofhard photons during the onset of a flare on Nov 4 and during the flareon Nov 11. There was no significant emission in the 15-300 keV rangeduring the major flare on Nov 4. The AR Lac spectrum between 0.1 and 9keV is well fit by a two component optical-thin plasma model based onthe MEKAL plasma emission code with subsolar metal abundances. We alsoperformed spectral analyses of the two separate segments of data, andfound that the plasma temperatures, abundances and emission measures didnot significantly change between the two time intervals. Hints ofpossible H I absorption in corona or in the circumstellar enviroment arealso discussed.
| A it BeppoSAX observation of HD 9770: a visual triple system containing a recently discovered short-period eclipsing binary
We have studied the coronal X-ray emission of the recently discoveredshort-period eclipsing binary HD 9770 with the BeppoSAX satellite. Thedata from the Low Energy and Medium Energy Concentrator Spectrometers(LECS & MECS) onboard BeppoSAX allow studying the spectrum of thisstar from 0.1 to 8 keV, confirming that this is a very active coronalsource, with strong flaring activity. The X-ray emission most likelyoriginates from the eclipsing binary itself, rather than from the othervisual component of the system. The X-ray light curves could bemodulated with the orbital period of the eclipsing binary, with a hintfor a different orbital modulation of the cooler and hotter plasma. TheX-ray spectrum is characterized by hot plasma, with the Fe K complex at6.7 keV clearly detected in the MECS spectrum, and it is well fitted bya 2-temperature optically thin plasma model with low metal abundances (~ 0.3 Z_sun). These results are in line with those found for many otheractive stars. As expected, during the flare the X-ray emission isdominated by hotter plasma with a temperature > 4 keV. There is anindication that the metal abundance may be somewhat higher during theflare.
| A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: The Catalog and LOG N-LOG S
We describe the selection of an X-ray flux-limited sample of brightclusters of galaxies in the southern hemisphere, based on the firstanalysis of the ROSAT All-Sky Survey data (RASS1). The sample isconstructed starting from an identification of candidate clusters inRASS1, after which their X-ray fluxes are remeasured using thesteepness-ratio technique. This method is better suited than the RASS1standard algorithm to measuring flux from extended sources. The finalsample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), sothat as a result of the distribution of N_H, its effective flux limitvaries between ~3 and 4x10^-12 ergs cm^-2 s^-1 over the selected area.This covers the delta<2.5d part of the south Galactic cap region(b<-20^deg), excluding patches of low RASS1 exposure time and of theMagellanic Clouds area, for a total of 8235 deg^2. One hundred andthirty candidate sources fulfill our selection criteria for bona fideclusters of galaxies in this area. Of these, 101 are Abell/ACO clusters,while 29 do not have a counterpart in these catalogs. Of these clusters,126 (97%) have a redshift, and for these we compute an X-ray luminosity.20% of the cluster redshifts come from new observations, as part of theESO Key Program survey of RASS clusters that is currently undercompletion. Considering the intrinsic biases and incompletenessesintroduced by the flux selection and source identification processes, weestimate the overall completeness to be better than 90%. The observednumber count distribution, log N-log S, is well fitted by a power lawwith slope alpha=1.34+/-0.15 and normalization A=11.87+/-1.04 sr^-1(10^-11 ergs cm^-2 s^-1)^alpha, in good agreement with othermeasurements.
| Visual binary orbits and masses POST HIPPARCOS
The parallaxes from Hipparcos are an important ingredient to derive moreaccurate masses for known orbital binaries, but in order to exploit theparallaxes fully, the orbital elements have to be known to similarprecision. The present work gives improved orbital elements for some 205systems by combining the Hipparcos astrometry with existing ground-basedobservations. The new solutions avoid the linearity constraints andomissions in the Hipparcos Catalog by using the intermediate TransitData which can be combined with ground-based observations in arbitarilycomplex orbital models. The new orbital elements and parallaxes give newmass-sum values together with realistic total error-estimates. To getindividual masses at least for main-sequence systems, the mass-ratioshave been generally estimated from theoretical isochrones and observedmagnitude-differences. For some 25 short-period systems, however, trueastrometric mass-ratios have been determined through the observedorbital curvature in the 3-year Hipparcos observation interval. Thefinal result is an observed `mass-luminosity relation' which falls closeto theoretical expectation, but with `outliers' due to undetectedmultiplicity or to composition- and age-effects in the nonuniformnear-star sample. Based in part on observations collected with the ESAHipparcos astrometry satellite. Tables~ 1, 3, 4 and 6 are also availablein electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr~(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| HIPPARCOS Parallaxes of Eclipsing Binaries and the Radiative Flux Scale
Eclipsing binaries with Hipparcos parallaxes are found to define aradiative flux relation for main-sequence stars in the B6-F0 range aboutas well as stars with angular diameters and to extend the range to F8.The fluxes of components of binaries showing large intrinsic variationsfall well below the curve for other stars and its extension to lowertemperatures. Angular diameters of main-sequence stars obtained with thenew generation of stellar interferometers should improve and extend theflux calibration. Based on data from the ESA Hipparcos satellite.
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