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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.
| 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.
| 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.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| 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.
| Long-term monitoring of active stars. X. Photometry collected in 1994
As part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 18 selected stars is presented. UBV(RI)_c observations were collectedat the European Southern Observatory over the intervals 21-28 September1994 and 25 November-05 December 1994. Additional UBV photometryobtained late in 1994 by Catania Astrophysical Observatory AutomaticPhotoelectric Telescope is also presented. Significant evolution of thelight curves, period variations and evidence for long-term variabilityof the global degree of spottedness are found. Some spectralclassifications are revised and photometric parallaxes are compared,whenever possible, with the values measured by the Hipparcos satellite.These observations are finalized to the construction of an extendedphotometric database, which can give important clues on topics such asthe stability of spotted areas, differential stellar rotation,solar-like activity cycles and the correlation between inhomogeneitiesat different atmospheric levels.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables 1, 2 and 4 and the complete data set are only available inelectronic form at the 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/400/659 (Table 3 is alsoavailable at the CDS).
| 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.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/qcat?J/A+A/397/997
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Flare Heating in Stellar Coronae
An open question in the field of solar and stellar astrophysics is thesource of heating that causes stellar coronae to reach temperatures ofmillions of degrees. One possibility is that the coronae are heated by alarge number of small flares. On the Sun, flares with energies as low asthose of microflares are distributed with energy as a power law of theform (dN/dE)~E-α, with α~1.8, and α appearsto increase to values of 2.2-2.7 for flares of lower energy. If theslope exceeds the critical value of 2, then in principle the entirecoronal energy input can be ascribed to flares that are increasinglyless energetic but are more numerous. Previous analyses of flares inlight curves of active stars have shown that this index generally isgreater than 2, although it may be as low as 1.6 when strong flaresalone are considered. Here we investigate the contribution of very weakflares, covering the milliflare energy range, to the coronal luminosityof low-mass active stars. We analyze Extreme Ultraviolet Explorer/DeepSurvey events data from FK Aqr, V1054 Oph, and AD Leo and conclude thatin all these cases, the coronal emission is dominated by flares to suchan extent that in some cases, the entire emission can be ascribed toflare heating. We have developed a new method to directly model for thefirst time stochastically produced flare emission, includingundetectable flares, and their effects on the observed photon arrivaltimes. We find that αFKAqr=2.60+/-0.34,αV1054Oph=2.74+/-0.35, andαADLeo=2.03-2.32, and the flare component accounts fora large fraction (generally greater than 50%) of the total flux.
| The EUVE point of view of AD Leo
All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo,totalling 1.1 Ms of exposure time, have been employed to analyze thecorona of this single M dwarf. The light curves show a well definedquiescent stage, and a distribution of amplitude of variabilityfollowing a power law with a ~ -2.4 index. The flaring behavior exhibitsmuch similarity with other M active stars like FK Aqr or YY Gem, andflares behave differently from late type active giants and subgiants.The Emission Measure Distribution (EMD) of the summed spectrum, as wellas that of quiescent and flaring stages, were obtained using aline-based method. The average EMD is dominated by material at log T(K)~ 6.9, with a second peak around log T(K) ~ 6.3, and a large increase inthe amount of material with log T(K) ga 7.1 during flares, materialalmost absent during quiescence. The results are interpreted as thecombination of three families of loops with maximum temperatures at logT(K) ~ 6.3, ~ 6.9 and somewhere beyond log T(K) ga 7.1. A value of theabundance of [Ne/Fe] = 1.05 +/- 0.08 was measured at log T(K) ~ 5.9. Nosignificative increment of neon abundance was detected betweenquiescence and flaring states.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History
We present high-resolution echelle spectroscopy of 676 nearby M dwarfs.Our measurements include radial velocities, equivalent widths ofimportant chromospheric emission lines, and rotational velocities forrapidly rotating stars. We identify several distinct groups by theirHα properties and investigate variations in chromospheric activityamong early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limitedsample together with a relationship between age and chromosphericactivity, we show that the rate of star formation in the immediate solarneighborhood has been relatively constant over the last 4 Gyr. Inparticular, our results are inconsistent with recent large bursts ofstar formation. We use the correlation between Hα activity and ageas a function of color to set constraints on the properties of L and Tdwarf secondary components in binary systems. We also identify a numberof interesting stars, including rapid rotators, radial velocityvariables, and spectroscopic binaries. Observations were made at the 60inch telescope at Palomar Mountain, which is jointly owned by theCalifornia Institute of Technology and the Carnegie Institution ofWashington.
| The long-period companions of multiple stars tend to have moderate eccentricities
We examined the statistics of an angle gamma between the radius vectorof a visual companion of a multiple star and the vector of its apparentrelative motion in the system. Its distribution f(gamma ) is related tothe orbital eccentricity distribution in the investigated sample. Wefound that for the wide physical subsystems of the 174 objects from theMultiple Star Catalogue f(gamma ) is bell-shaped. The Monte-Carlosimulations have shown that our f(gamma ) corresponds to the populationof the moderate-eccentricity orbits and is not compatible with thelinear distribution f(e)=2e which follows from stellar dynamics andseems to hold for wide binaries. This points to the absence of highlyelongated orbits among the outer subsystems of multiple stars. Theconstraint of dynamical stability of triple systems is not sufficient toexplain the ``rounded-off'' outer orbits; instead, we speculate that itcan result from the angular momentum exchange in multiple systems duringtheir early evolution.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form at the 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/375/670
| The Flux Deficits in Star Spots
The bolometric flux deficits of the photospheres of spotted stars arederived for the first time in the framework of zonal spottedness modelsfor red dwarfs computed at the Crimean Astrophysical Observatory. Theresulting flux deficits are compared to the estimated radiative lossesfrom the chromospheres and coronas measured during quasi-simultaneousobservations. A linear correlation is found between the logarithms ofthese quantities, with the Sun fitting these relations. Radiative lossesfrom the outer stellar atmospheres in quiescence and during individualsporadic flares are significantly lower than the bolometric deficits ofthe spotted photospheres of active stars. This suggests that the fluxdeficit due to spots leads to global reconstruction of the atmospheresof red dwarfs, analogous to the local atmospheric reconstruction thatoccurs during solar and stellar flares. This process may be realized viathe superposition of a large number of weak impulsive flares and otherdynamic events, which develop on these stars and heat their coronas(i.e., in this view, microflaring is favored as the principal coronalheating mechanism for these stars). A brief analysis of the long-termvariations in the chromospheric and photospheric radiation of F-K starsfrom the HK project and of the Sun suggests that such dynamicalreconstruction of the outer atmosphere by energy associated with theflux deficit of the spotted photosphere occurs at times of increasedsurface activity in all F-M stars.
| 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
| X-ray emission from nearby M-dwarfs: the super-saturation phenomenon
A rotation rate and X-ray luminosity analysis is presented for rapidlyrotating single and binary M-dwarf systems. X-ray luminosities for themajority of both single and binary M-dwarf systems with periods below~=5-6d (equatorial velocities, Veq>~6kms-1) areconsistent with the current rotation-activity paradigm, and appear tosaturate at about 10-3 of the stellar bolometric luminosity.The single M-dwarf data show tentative evidence for the super-saturationphenomenon observed in some ultra-fast rotating(>~100kms-1) G- and K-dwarfs in the IC 2391, IC 2602 andAlpha Persei clusters. The IC 2391M star VXR60b is the least X-rayactive and most rapidly rotating of the short period(Prot<~2d) stars considered herein, with a period of0.212d and an X-ray activity level of about 1.5 sigma below the meanX-ray emission level for most of the single M-dwarf sample. For thisstar, and possibly one other, we cautiously believe that we haveidentified the first evidence of super-saturation in M-dwarfs. If we arewrong, we demonstrate that only M-dwarfs rotating close to theirbreak-up velocities are likely to exhibit the super-saturation effect atX-ray wavelengths. The M-dwarf X-ray data also show that there is noevidence for any difference in the X-ray behaviour between the singleand binary systems, because for the single stars, the meanlogLxLbol=-3.21+/-0.04(0.2<~Prot<~10.1d), whereas for the binary stars, themean logLxLbol=-3.19+/-0.10(0.8<~Prot<~10.4d). Furthermore, we show that extremelyX-ray active M-dwarfs exhibit a blue excess of about 0.1magnitudes inU-B compared with less active field M-dwarfs. Such an excess level iscomparable to that observed for extremely chromospherically activeM-dwarfs. Moreover, as is the case for M-dwarf Caii H and K activitylevels, there is an exclusion zone of X-ray activity between theextremely active M-dwarfs and the less active ones.
| Extreme Ultraviolet Astronomy
Astronomical studies in the extreme ultraviolet (EUV) band of thespectrum were dismissed during the early years of space astronomy asimpossible, primarily because of the mistaken view that radiation inthis band would be absorbed by the interstellar medium. Observations inthe 1980s from sounding rockets and limited duration orbital spacecraftbegan to show the potential of this field and led to the deployment oftwo spacecraft devoted to EUV astronomy: the UK Wide Field Camera andthe Extreme Ultraviolet Explorer. The instrumentation in these missions,although quite limited in comparison with instrumentation in otherfields of space astronomy, provided unique and far-reaching results.These included new information on solar system topics, stellarchromospheres and corona, white dwarf astrophysics, cataclysmicvariables, the interstellar medium, galaxies, and clusters of galaxies.We summarize these findings herein.
| 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).
| Catalogue and bibliography of the UV Cet-type flare stars and related objects in the solar vicinity
This new catalogue of flare stars includes 463 objects. It containsastrometric, spectral and photometric data as well as information on theinfrared, radio and X-ray properties and general stellar parameters.From the total reference list of about 3400 articles, partial listsselected by objects, authors, key words and by any pairs of thesecriteria can be obtained Tables 1, 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.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.
| The Wilson-Bappu relation for RS CVn stars
We investigate the extent to which the Wilson-Bappu relationship holdsfor chromospherically active binaries using the Mg ii h&k lines of41 RS CVn stars observed with IUE. The resulting fits are different fromthe relationships obtained for single, less active stars. The parallaxused were those from the hipparcos catalogue, these give a much bettercorrelation than the magnitudes taken from CABS. Within a particularluminosity class the relationship is good, however it tends to breakdown when we incorporate objects ranging in luminosity from class i tov. From model calculations there is very little dependence of the Mg iiline width on effective temperature. The line width does however dependon the column mass at the transition region boundary showing increasedline width at lower column mass. There is also a dependence on thecolumn mass adopted for the temperature minimum, however, the major anddominant parameter is the surface gravity scaling as g(-1/4) . Within aluminosity class more active objects will show larger lines widthsreflecting a higher column mass deeper in the atmosphere, e.g. at thetemperature minimum level.
| Infrared Space Observatory Photometric Search of Main-Sequence Stars for Vega-Type Systems
We obtained 3.6-20 μm photometry of 38 bright [IRAS F_nu(12μm)>0.7 Jy] main-sequence stars with the Infrared SpaceObservatory (ISO). Observations were conducted with the ISOPHOTinstrument, in the single-pointing photometry mode, through filters at3.6, 11.5, and 20.0 mum. We searched for excess (Vega-type) emissionfrom dust at temperatures >~100 K, located at ~1-60 AU from thestars. We thus sampled dust at warm, terrestrial material temperaturesand at cool (~100 K) temperatures of possible Kuiper Belt-type regionsin these systems. We detected 20 μm excesses from ~14% of oursources, but we did not detect 11.5 μm excesses from any of them. Wepresent single-temperature blackbody models of the location and densityof dust emission around 10 stars, two of them (29 Cyg and Gl 816) withexcesses newly reported here. We make a thorough comparison of ISO andIRAS data on our target stars and propose a new calibration procedurefor ISOPHOT staring measurements at 3.6, 11.5, and 20 mum.
| The Second Extreme Ultraviolet Explorer Right Angle Program Catalog
We present the detection of 235 extreme ultraviolet sources, of which169 are new detections, using the Extreme Ultraviolet Explorer's (EUVE)Right Angle Program (RAP) data. This catalog includes observations sincethe first EUVE RAP catalog (1994 January) and covers 17% of the sky. TheEUVE RAP uses the all-sky survey telescopes (also known as``scanners''), mounted at right angles to the Deep Survey andspectrometer instruments, to obtain photometric data in four wavelengthbands centered at ~100 Å (Lexan/B), ~200 Å (Al/Ti/C), ~400Å (Ti/Sb/Al), and ~550 Å (Sn/SiO). This allows the RAP toaccumulate data serendipitously during pointed spectroscopicobservations. The long exposure times possible with RAP observationsprovide much greater sensitivity than the all-sky survey. We presentEUVE source count rates and probable source identifications from theavailable catalogs and literature. The source distribution is similar toprevious extreme ultraviolet (EUV) catalogs with 2% early-type stars,45% late-type stars, 8% white dwarfs, 6% extragalactic, 24% with no firmclassification, and 15% with no optical identification. We also present36 detections of early-type stars that are probably the result ofnon-EUV radiation. We have detected stellar flares from approximately 12sources, including: EUVE J0008+208, M4 star G32-6 (EUVE J0016+198), anew source EUVE J0202+105, EUVE J0213+368, RS CVn V711 Tau (EUVEJ0336+005), BY Draconis type variable V837 Tau (EUVE J0336+259), the newK5 binary EUVE J0725-004, EUVE J1147+050, EUVE J1148-374, EUVE J1334-083(EQ Vir), EUVE J1438-432 (WT 486/487), EUVE J1808+297, and the M5.5estar G208-45 (EUVE J1953+444). We present sample light curves for thebrighter sources.
| 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
| Identification of soft high galactic latitude RASS X-ray sources. I. A complete count-rate limited sample
We present a summary of spectroscopic identifications for a completesample of bright soft high galactic latitude X-ray sources drawn fromthe ROSAT All-Sky Survey which have PSPC count-rates CR > 0.5 {ctss}(-1) and hardness ratios HR1 < 0. Of a total of 397 sources, 270had catalogued counterparts although most of these were not previouslyknown as X-ray sources; of the remaining 127 sources neither X-ray noroptical properties were previously known. Of the whole sample of verysoft X-ray sources 155 were also discovered by the Wide-Field-Camera onboard ROSAT. We present spectroscopic identifications of 108 sources andother identifications for further 18 sources; 1 source remainsunidentified so far. In practically all cases a unique opticalcounterpart exists facilitating identification. The largest sourceclasses are AGN, magnetic cataclysmic variables, and hot white dwarfs.Based in part on observations with the ESO/MPI 2.2m telescope at LaSilla, Chile
| The Galactic Orbits of Nearby UV Ceti Stars
The galactic orbits of 93 UV Ceti stars of the solar neighborhood havebeen numerically integrated. The UV Ceti stars studied are those of theworking list for which Poveda et al. (1996a) determined kinematicproperties and ages. All stars are contained within 25 pc of the Sun(π ≥ 0.04''), and values for their distances, proper motions andradial velocities are available for them (Gliese & Jahreiss 1991).The galactic potential model of Allen & Santillán (1991) wasused, and the orbits were integrated for times similar to the age of theold disk. The galactic orbital parameters are obtained. The orbits areall regular, and the values found for the orbital parameters are similarto those characteristic of the classic young thin disk. The verticalscale height found for the whole sample is 115 pc. However, the orbitalparameters found for the 7 objects with extreme kinematiccharacteristics recognized in Poveda et al. are markedly different fromthose of the rest of the flare stars; in particular, their orbitaleccentricities are larger than 0.3. The vertical scale height of thesample excluding the anomalous objects is just 103 pc. Consequently, wepropose that the anomalous objects belong to the thick disk, which maybe characterized by either e≥ 0.3 or | zmax | > 400 pc.
| Supplementary southern standards for UBV(RI)c photometry
We present UBV(RI)c photometry for 80 southern red and blue stars foruse as additional standards. The data are tied to the Johnson UBV andCousins (RI)c systems and extend the range of the available stars forcolor equation determination, especially in (U-B) for blue stars and(V-R) and (V-I) for red stars. Comparisons with published data are madeand particularly good agreement is found with Bessell for the red(Gliese) stars.
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