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The atmospheric parameters, abundances and magnetic field of the AR piscium primary
The high resolution, high S/N spectra for the chromospherically activebinary AR Psc (=HD 8357) has been obtained by using the Coudéechelle spectrograph of the 2.16 m telescope of Beijing AstronomicalObservatory. The Hα lines present emission above thecontinuum and the profiles of CaII IRT2 (λ 854.2 nm) & IRT3(λ 866.2 nm) show core emission. The net chromospheric flux ofthe had been calculated by using spectral subtraction. A detailedspectroscopic analysis and model calculation have yielded theatmospheric parameters of the primary of AR Psc. The abundances of 9elements has been obtained. And, the measurement of Magnetic field havebeen made using the Stenflo Lindegren statistical analysis and theprofile-addition technique. A magnetic field strength B = 2400 ±300 G and a filling factor α = 0.45 ± 0.05 are obtained.
| 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.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| Zonal Model of Starspots. Application to RS CVn Systems
Model results on starspots in 15 chromospherically active type RS CVnbinary systems are presented. The dependences of the parameters of thespots on the principal characteristics of the stars (spectral class,luminosity class, rotation, Rossby number) are examined. Latitudinaldrift of the spots, cycles in the spot activity, and differentialrotation are found in 9 of the stars.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| The 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 Structure of Stellar Coronae in Active Binary Systems
A survey of 28 stars (22 active binary systems, plus six single stars orwide binaries for comparison) using extreme ultraviolet spectra has beenconducted to establish the structure of stellar coronae in active binarysystems from the emission measure distribution (EMD), electrondensities, and scale sizes. Observations obtained by the ExtremeUltraviolet Explorer satellite (EUVE) during 9 years of operation areincluded for the stars in the sample. EUVE data allow a continuous EMDto be constructed in the range logTe(K)~5.6-7.4, using ironemission lines. These data are complemented with IUE observations tomodel the lower temperature range [logTe(K)~4.0-5.6].Inspection of the EMD shows an outstanding narrow enhancement, or``bump'' peaking around logTe(K)~6.9 in 25 of the stars,defining a fundamental coronal structure. The emission measure per unitstellar area decreases with increasing orbital (or photometric) periodsof the target stars; stars in binaries generally have more material atcoronal temperatures than slowly rotating single stars. High electrondensities (Ne>~1012 cm-3) arederived at ~logTe(K)~7.0 for some targets, implying smallemitting volumes.The observations suggest the magnetic stellar coronae of these stars areconsistent with two basic classes of magnetic loops: solar-like loopswith maximum temperature around logTe(K)~6.3 and lowerelectron densities (Ne>~109-1010.5cm-3), and hotter loops peaking aroundlogTe(K)~6.9 with higher electron densities(Ne>~1012 cm-3). For the most activestars, material exists at much higher temperatures[logTe(K)>=6.9] as well. However, current ab initiostellar loop models cannot reproduce such a configuration. Analysis ofthe light curves of these systems reveals signatures of rotation ofcoronal material, as well as apparent seasonal (i.e., year-to-year)changes in the activity levels.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| VLA multifrequency observations of RS CVn binaries
We present multiepoch Very Large Array (VLA) observations at 1.4 GHz,4.9 GHz, 8.5 GHz and 14.9 GHz for a sample of eight RS CVn binarysystems. Circular polarization measurements of these systems are alsoreported. Most of the fluxes observed are consistent with incoherentemission from mildly relativistic electrons. Several systems show anincrease of the degree of circular polarization with increasingfrequency in the optically thin regime, in conflict with predictions bygyrosynchrotron models. We observed a reversal in the sense of circularpolarization with increasing frequency in three non-eclipsing systems:EI Eri, DM UMa and HD8358. We find clear evidence for coherent plasma emission at1.4 GHz in the quiescent spectrum of HD 8358 duringthe helicity reversal. The degrees of polarization of the other twosystems could also be accounted for by a coherent emission process. Theobservations of ER Vul revealed two U-shaped fluxspectra at the highest frequencies. The U-shape of the spectra may beaccounted for by an optically thin gyrosynchrotron source for the lowfrequency part whereas the high frequency part is dominated by a thermalemission component.
| Hα variations of the RS CVn type binary ER Vulpeculae
We present the results of spectroscopic observations of the activeeclipsing binary ER Vul in the 5860-6700 Å spectral range carriedout at the Catania Astrophysical Observatory. Accurate measurements ofradial velocities by using the cross-correlation technique have beenmade and a new orbital solution is given. All spectra displaychromospheric emission which fills in the Hα absorption lines ofboth components. The equivalent width (EW) of the Hα emission hasbeen measured by subtraction of a synthetic spectrum built up withspectra of inactive standard stars. The EW of total Hα emission,arisen from both components, shows a phase-dependent variation. Itreaches its maximum value just before the primary eclipse. We were ableto separate the contributions to the Hα emission from theindividual components at phases far from the eclipses. We have foundthat the secondary, cooler component is the most active and its Hαresidual emission shows the same trend as the total Hα emission,along orbital phase. This Hα equivalent width variation can bedue to plage-like structures on the chromosphere of secondary star. Inaddition, a decrease of the Hα EW around the ingress phase of theprimary eclipse is apparent.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Hα spectroscopy and BV photometry of RT Lacertae
Contemporaneous spectroscopic and photometric B V observations of the RSCVn type eclipsing binary RT Lacertae were performedin summer 2000. The photometric observations were obtained at the EgeUniversity Observatory, while the spectroscopic ones were carried out atCatania Astrophysical Observatory in the spectral range 5860-6700Å. We obtained a high quality radial velocity curve of the systemthat allowed us to give more accurate values of the orbital parameters.A steady decrease of the barycentric velocity from 1920 to 2000 has beenpointed out and has been discussed in the context of a third bodyhypothesis. Through the subtraction of a ``synthetic'' spectrum, builtup with spectra of inactive standard stars, we detected Hα excessemission which fills in the photospheric absorption profiles of bothcomponents. With the exception of a few spectra, taken close to theeclipses, in which some extra absorption or a faint double-peaked broademission appears, there is no further evidence of circumstellar matterin this system, as suggested in previous works. The hotter and moremassive star appears also as the more active at a chromospheric level,since it has a Hα flux about ten times greater than the companion,on average. Rotational modulation of the Hα emission has beendetected in both stars. The hemisphere of the more massive star facingthe observer at phase 0fp75 appears brighter (in Hα ) than thatseen at phase 0fp25 , while for the less massive G9 IV star the maximumHα emission is seen around phase 0fp0-0fp1 . From the analysis ofthe contemporaneous light curve (Lanza et al. \cite{Lanza2001}), themore massive G5 IV star results to be more active than the companion ata photospheric level, in agreement with the chromospheric behaviourobserved in Hα . In addition, the starspots of the G5 IV star aremainly located in the Hα brighter hemisphere, suggesting a closespatial association of spots and plages in this star. The G9 IV stardisplays instead the maximum Hα emission at the phase of maximumvisibility of the smaller spotted area found from the light-curveanalysis. The minimum Hα emission occurs when the more heavilyspotted region is visible. Based on observations collected at CataniaAstrophysical Observatory, Italy, and at Ege University Observatory,Turkey. Table 1 only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/298
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| BVR photometry and Hα spectroscopy of RS CVn type binary MM Herculis
The RS CVn type eclipsing binary MM Herculis wasobserved photo-electrically using B, V and R filters in 1998 and 1999and the light and colour curves were obtained. Spectroscopicobservations were carried out in 1999. The new light and colour curvesare anti-correlated with the observations of 1997, i.e. the system isbluer when it is faintest. The variations of the brightnesses at eachspecial phase (0.0, 0.25, 0.5, and 0.75) show an almost cyclic changewith a period of about 6 years. This value is in good agreement with themigration periods of the spots suggested by us previously. Theoutside-of-eclipse wave in the light curve shows a minimum at phase~0.50, and the mean colour of the system is the bluest at the samephase. When the spots located on the cooler component are seen aroundphase 0.50, the amplitude of the light variation outside eclipse islarger than the others. Such variation may be caused by the effect ofthe hotter component. Spectroscopic observations of the system werecarried out in the spectral range 5860-6700 Å. The subtraction ofa ``synthetic'' spectrum, built up with spectra of inactive standardstars, allows us to detect an Hα emission excess only from thecool component. New radial velocity measurements of the system wereobtained and analyzed for the orbital parameters. Based on observationscollected at the Ege University Observatory, Turkey, and at the CataniaAstrophysical Observatory, Italy.
| Long-term monitoring of active stars. IX. Photometry collected in 1993
As a part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 31 selected stars is presented. The UBV(RI)_c observations werecollected at the European Southern Observatory over the 31 December1992-18 January 1993 and the 20 November-3 December 1993 intervals.Additional UBV photometry obtained by the ``Phoenix" and by the CataniaAstrophysical Observatory Automatic Photoelectric Telescopes from 1990to 1993 is also presented for some of the program stars. Significantevolution of the light curves, period variations and evidence forlong-term variability of the global degree of spottedness are found.Some spectral classifications are revised and the inferred photometricparallaxes are compared, whenever possible, with the values measured bythe Hipparcos satellite. These observations are finalized to theconstruction of an extended photometric database, which can giveimportant clues on topics such as the stability of spotted areas,differential rotation, solar-like cycles and the correlation betweeninhomogeneities at different atmospheric levels. Based on data collectedat the European Southern Observatory, La Silla, Chile. Tables and thecomplete data set are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/910
| 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).
| Time series analysis of V815 Herculis photometry between 1984 and 1998
As a case study of the solar-stellar connection, we have analysed aprolonged time series of BV photometry of the chromospherically activebinary V815 Her . The surface differential rotation in the rapidlyrotating G5{v} primary caused changes of 4.6% in the seasonalphotometric rotation periods. This would imply a differential rotationcoefficient of k = 0.184, if the rotation of the starspots follows thesolar law of differential rotation and the activity is confined withinthe same latitudinal range as in the Sun, having k = 0.189 and thespectral-type of G2{v}. Our analysis of the primary and secondary minimaof the seasonal light curves indicated that the regions of strongeractivity have concentrated on one active longitude, which has maintaineda constant rotation period of 1.d79244 for about 14 years. Noregular activity cycle was detected in the mean brightness changes ofV815 Her. Table~1 is available only in electronic form at CDS viaanonymous ftp to edarc.u-strasbg.fr (130.79.1285) or via http://cdsweb.u-strasbg.fr/Abstract.html. These data are also available athttp: //schwab.tsuniv.edu/t3/v815her/v815her.html
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| Extreme Ultraviolet Explorer Photometry of RS Canum Venaticorum Systems: Four Flaring Megaseconds
We present an analysis of 12.2 Ms of Extreme Ultraviolet Explorer (EUVE)photometry of 16 RS CVn systems (including four flaring megaseconds).Our study attempts a systematic categorization of stellar coronalemission in RS CVn binary systems. The temporal resolution of the EUVEsatellite and the opportunity for long observations enables us to studyseparately the quiescent and flaring states of RS CVn systems. Thirty of31 observations are statistically variable. We examine light curves,characterizing both phase-dependent variations and large- andsmall-scale flaring. There is evidence for small-scale stochasticvariability on the one short-period (P_orb<1 day) system in oursample (ER Vul), which we interpret as small-scale flaring. CF Tuc isthe only system for which phase-dependent variations over multipleorbital periods are seen. We analyze 30 flares on nine systems, fittingrise and decay times. Several flares have durations >0.5P_orb andthus cannot originate from compact emission regions close to the stellarsurfaces. Many flares have unusual flare morphologies, with rise timescomparable to or greater than the decay time, and emission plateaus. Inaddition, eight flares have decay phases that are fit better by a brokenpower law than a single power law. The decay times for the second powerlaw are statistically different than the single decay times and arecorrelated with the flare rise times. This observed connection points tothe potential importance of the change in decay rate as a means ofexamining the physical processes operating during the flare. Thedistribution of orbital phases of flare onsets on V711 Tau (HR 1099),the best-studied RS CVn in our sample, is consistent with a Poissondistribution that is random in time. In two systems that are partiallyeclipsing (CF Tuc and ER Vul), we find no evidence for an eclipse in thelight curve, indicating that the emission region is large compared tothe stellar radius. We parameterize the distribution of flare energiesas a power law, finding a cut-off energy of ~10^33 ergs and a slope of-0.6. Flare energy increases with flare duration as E~Deltat^1.42,confirming the long-duration nature of high-energy flares on RS CVnsystems. The integrated flare luminosity depends on the quiescentluminosity as L_flare~L^1.05_quiescent. Flare frequencies range from 0.1day^-1 to 1.5 day^-1. For observations not affected by the dead spot onthe Deep Survey detector, 40% of the observed time was spent in aflaring state, indicating that flaring is a ``normal'' state for thecoronae of these systems. We note the presence of a quasi-periodicity inEUVE photometric data on the order of 1 day, which is not corrected forin the standard reduction and analysis software.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| 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}
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Time series analysis of V 1794 Cygni long-term photometry
Standard Johnson UBVRI photometry of {V 1794 Cyg} (HD199178) between1975 and 1995 is analysed. Instead of the traditional constant periodephemeris, we determine the seasonal periodicities (Pphot)and the primary and secondary minima epochs (t_{min,1}, t_{min,2}) ofthe normalized UBVRI magnitudes using the three stage period analysis(TSPA) and complementary methods. Our t_{min,1} and t_{min,2} estimateswith variable Pphot can adapt easily to both differentialrotation and longitudinal activity migration. The seasonalPphot are utilized in modelling the mean (M) and totalamplitude (A) of contemporary light curves in UBVRI. TSPA reveals thatthe long-term M and A changes of {V 1794 Cyg} are unpredictable. Wesearch for active longitudes from the t_{min,1} and t_{min,2} series oftime points with nonparametric methods. The critical level of 0.0029 forthe best 3.d3175 period detected with the Kuiper method ishigh, but exceeds the 0.001 significance for rejecting the hypothesisthat the phases of t_{min,1} and t_{min,2} are randomly distributed. Theactivity centres in {V 1794 Cyg} are rarely disrupted, and most probablyundergo continuous longitudinal migration, because only one abruptdisruption is observed during 20 years. As for differential rotation,the irregular changes of seasonal Pphot are 7.5%. Thesurprisingly regular 3.3% changes of yearly Pphot may providea stellar analogy of the solar ``butterfly'' diagram.
| Further discussion of binary star radio survey data
Statistical information on 8280 individual radio observations of binarystars, predominantly at 8.4 GHz using the Parkes 64 m antenna, ispresented. Three main groups are distinguished: (i) RS CVn stars, (ii)classical Algol binaries (EA2s) and (iii) detached pairs of generallyearly type (ETBs). The RS CVn stars more frequently gave rise todetectable fluxes, while the ETBs, in this data, are a small and ratherheterogeneous class. The Algols' emission appears to increase nearconjunction phases, though we cannot clearly distinguish any specialproperty of the Algols' phase-dependent behaviour which is not alsoshared by the RS CVn binaries. Both these categories' data show abimodal, phase-dependent pattern to the distribution of detections,suggesting both binary types share similar underlying physicalproperties, though there could also be other factors at play. The samplesizes of the Algols and particularly the ETB detections are too smallfor effective, discriminatory statistics, however.
| On the rotation-activity correlation for active binary stars
We present an investigation of rotation-activity correlations usingInternational Ultraviolet Explorer (IUE) SWP measurements of the Civemission line at 1550Angstroms for 72 active binary systems. We use astandard stellar evolution code to derive non-empirical Rossby numbers,R_0, for each star in our sample and compare the resulting Civrotation-activity correlation to that found for empirically derivedvalues of the Rossby number and that based on rotation alone. For dwarfstars our values of R_0 do not differ greatly from empirical ones and wefind a corresponding lack of improvement in correlation. Only a marginalimprovement in correlation is found for evolved components in oursample. We discuss possible additional factors, other than rotation orconvection, that may influence the activity levels in active binaries.Our observational data imply, in contrast to the theoretical predictionsof convective motions, that activity is only weakly related to mass inevolved stars. We conclude that current dynamo theory is limited in itsapplication to the study of active stars because of the uncertainty inthe angular velocity-depth profile in stellar interiors and the unknowneffects of binarity and surface gravity.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | דגים |
| התרוממות ימנית: | 01h22m56.76s |
| סירוב: | +07°25'09.3" |
| גודל גלוי: | 7.337 |
| מרחק: | 45.188 פארסק |
| תנועה נכונה: | 94.9 |
| תנועה נכונה: | 229.7 |
| B-T magnitude: | 8.385 |
| V-T magnitude: | 7.424 |
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INHABITED SKY
