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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.
| 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.
| 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.
| 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.
| 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.
| K dwarfs and the chemical evolution of the solar cylinder
K dwarfs have lifetimes older than the present age of the Galactic disc,and are thus ideal stars for investigating the chemical evolution of thedisc. We have developed several photometric metallicity indicators for Kdwarfs, based on a sample of accurate spectroscopic metallicities for 34disc and halo G and K dwarfs. The photometric metallicities lead us todevelop a metallicity index for K dwarfs based only on their position inthe colour-absolute-magnitude diagram. Metallicities have beendetermined for 431 single K dwarfs drawn from the Hipparcos catalogue,selecting the stars by absolute magnitude and removing multiple systems.The sample is essentially a complete reckoning of the metal content innearby K dwarfs. We use stellar isochrones to mark the stars by mass,and select a subset of 220 of the stars, which is complete within anarrow mass interval. We fit the data with a model of the chemicalevolution of the solar cylinder. We find that only a modest cosmicscatter is required to fit our age-metallicity relation. The modelassumes two main infall episodes for the formation of the halo-thickdisc and thin disc, respectively. The new data confirm that the solarneighbourhood formed on a long time-scale of the order of 7 Gyr.
| Starspot mapping with robotic telescopes
Robotic telescopes for photometry and imaging have been around forseveral years. They have been very valuable to monitor light variationsdue to changing spot patterns on active stars. Two new spectroscopicrobotic telescopes will come online in the near future, one in Tenerifeand one in Arizona. Together with the existing photometric robotictelescopes, these new telescopes make several groups of stars a betteraccessible for surface mapping techniques. One of these groups are starswith long rotation periods, since monitoring of several rotations ofthese stars requires too much telecope time. Another group consists ofstars with rotation periods of (almost) exactly one, two, ... days. The100° separation of the two telescopes makes observations of suchtargets possible in less time, which also means that changes on thesurface can be viewed with higher time resolution.
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://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.
| Absolute declinations with the photoelectric astrolabe at Calern Observatory (OCA)
A regular observational programme with a photoelectric astrolabe havebeen performed at ``Observatoire du Calern" (Observatoire de laCôte d'Azur, OCA, phi = +43() o44′55.011″; lambda =-0() h27() m42.44() s, Calern, Caussols, France) for the last twentyyears. It has been almost fully automatized between 1984 and 1987. Since1988 the photoelectric astrolabe was used without any modification. Inaddition to determining the daily orientation of the local vertical, theyearly analysis of the residuals permits to derive corrections to theused star catalogue \cite[(Vigouroux et al. 1992)]{vig92}. A globalreduction method was applied for the ASPHO observations. The new form ofthe equations \cite[(Martin & Leister 1997)]{mar97} give us thepossibility of using the entire set of the observing program using datataken at two zenith distances (30() o and 45() o). The program containsabout 41648 stars' transits of 269 different stars taken at``Observatoire du Calern" (OCA). The reduction was based on theHIPPARCOS system. We discuss the possibility of computing absolutedeclinations through stars belonging simultaneously to the 30() o and45() o zenith distances programmes. The absolute declination correctionswere determined for 185 stars with precision of 0.027arcsec and thevalue of the determined equator correction is -0.018arcsec +/-0.005arcsec . The instrumental effects were also determined. The meanepoch is 1995.29. Catalogue only available at CDS in electronic from viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| UBV(RI)_C and UVBY photometry of HD 81410 and starspot distribution in RS CVn objects
We present UBVRI and uvby photometry of HD 81410 obtained on a total of40 nights during 1987-90, and an analysis of its long-term photometricbehaviour. The star is found to show two well-separated minima in itslight curve most of the time; probably there are two preferred effectivelongitudes about which spots are mainly formed. The migration of thephase of the light minimum usually seen in RS CVn stars is absent in HD81410. The rather small spread in the maximum V amplitudes observed inactive RS CVn objects seen at different inclinations of rotational axisimplies that the longitudinal asymmetry in the distribution of spots,which causes the light modulation, is largely restricted to withinaround +/-40(deg) latitudes. The total ranges of rotational periodswhich are quoted in the literature for several spotted stars based onlong-term photometry also imply a similar latitudinal extent of spotsabout the equator. We find that the light and colour curves produced byan equatorial band of spots limited by latitudes and covering the fullrange of longitudes across the hemisphere visible at light minimum canbe approximated reasonably well by two well-separated circular spots.Further, we find that the net effect in the colours produced bylimb-darkening depends on the exact distribution of spots on the stellarsurface; it could be even negligible for certain spot distributions.From the spot modeling of the light and colour curves of HD 81410 wefind that the maximum temperature difference between the photosphere andspots is around 1400 K. We also find that the light modulation is causedby several small individual spots, and during the epochs of shallowminimum spots are spread out, both latitudinally and longitudinally,over a wider region, as indicated by a higher temperature for theequivalent circular spots at those epochs. It seems that in RS CVn starsthe spots occur largely distributed about the equator as in the case ofthe Sun and the solutions which indicate polar spots result fromlimiting the number of spots in the modeling to a few. Based on theobservations collected at the European Southern Observatory, La Silla
| 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.
| Accurate Positions for Radio Stars as Determined from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1679S&db_key=AST
| Multiwavelength optical observations of chromospherically active binary systems. I. Simultaneous H alpha, NA i D1, D2, and He i D3 observations
This is the first paper of a series aimed at studying the chromosphereof active binary systems using the information provided for severaloptical spectroscopic features. Simultaneous H alpha, Na iD1, D2, and He i D3 spectroscopicobservations are reported here for 18 systems. The chromosphericcontribution in these lines have been determined using the spectralsubtraction technique. Very broad wings have been found in thesubtracted H alpha profile of some of the more active stars. Theseprofiles are well matched using a two-components Gaussian fit (narrowand broad) and the broad component could be interpreted as arising frommicroflaring. Prominence-like extended material have been detected in anear-eclipse H alpha observation of the system AR Lac. The excessemission found in the Na i D1 and D2 lines byapplication of the spectral subtraction technique and the behaviour ofthe H alpha line in the corresponding simultaneous observations indicatethat the filling-in of the core of these lines is a chromosphericactivity indicator. For giant stars of the sample the He i D3line has been detected in absorption in the subtracted spectra. Anoptical flare has been detected in UX Ari and II Peg through thepresence of the He i D3 in emission in coincidence with theenhancement of the H alpha emission. Based on observations made with theIsaac Newton telescope operated on the island of La Palma by the RoyalGreenwich Observatory at the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofisica de Canarias.
| The age-mass relation for chromospherically active binaries. II. Lithium depletion in dwarf components.
We present an extensive study of lithium abundances in dwarf componentsof chromospherically active binary stars (CABS). Since most of thesebinaries have known radii, masses and ages, this kind of data isespecially useful for comparisons with theoretical models which try toexplain the Li depletion phenomenon. We show that a significant part ofthese stars have clear Li overabundances with respect to the typicalvalues for stars of the same mass and evolutionary stage. These excessesare evident when comparing our sample of CABS with binary and singlestars belonging to open clusters of different ages, namely Pleiades,Hyades, NGC 752, M 67 and NGC 188, which have ages ranging from 7x10^7^to 10^10^yr. The Li excesses are more conspicuous for masses in therange 0.75-0.95Msun_, indicating that the rate of Lidepletion has been less pronounced in CABS than in single stars. Thisphenomenon is interpreted in the context of transport of angularmomentum from the orbit to the stellar rotation due to tidal effects.This angular momentum transfer would avoid the radial differentialrotation and the associated turbulent mixing of material in the stellarinterior. Other explanations, however, can not be ruled out. This is thecase of transport of material induced by internal gravity waves, whichcould be inhibited due to the presence of strong magnetic fieldsassociated with the effective dynamo in CABS. The confirmed existence ofa relation between Li abundances and the fluxes in CaII H&K linescan also be accommodated within both scenarios.
| An Optical Atlas of Extreme Ultraviolet Explorer (EUVE) Sources
The Extreme Ultraviolet Explorer (EUVE) has been detecting EUV sourcessince its launch in June 1992. Positions of 540 sources have been madeavailable to the community by the EUVE team. We have extracted 7' X 7'images centered on these 540 EUVE sources from the Space TelescopeScience Institute digitized sky archives. We present these images asmosaic finder charts to aid observers trying to identify EUVE sources,or to characterize known sources. (SECTION: Atlases)
| Rotational Velocities of Late-Type Stars
A calibration based on the results of Gray has been used to determineprojected rotational velocities for 133 bright stars with spectral typesof F, G, or K, most of which appear in {\it The Bright Star Catalogue}.The vast majority have {\it v} sin {\it i} $\leq$ 10 km s$^{-1}$ and,thus, are slow rotators. With the new calibration, projected rotationalvelocities have been determined for a sample of 111 late-type stars,most of which are chromospherically active. Some of the stars have hadtheir rotational velocities measured for the first time. (SECTION:Stars)
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| EUV Emission from RS Canum Venaticorum binaries.
We performed a study of 104 RS CVn systems in the extreme ultraviolet(EUV) using the all-sky survey data obtained by the Extreme UltravioletExplorer (EUVE). The present sample includes several new RS CVndetections; 11 more than in the published EUVE catalogs, and 8 more thanin the ROSAT Wide Field Camera catalog. The ratio of detections tonon-detections remained constant throughout the sky, implying that ourdetections are not limited by the exposure time but are most likelylimited by absorption from the interstellar medium. A general trend ofincreasing Lex/B (50-180Å) flux with decreasing rotational periodis clear. The dwarf systems exhibit a leveling-off for the fasterrotators. In contrast, the evolved systems exhibit no such effect. Forthe RS CVn systems the losses in the EUV represent a smaller fraction ofthe coronal radiative losses, as compared to active late-type dwarfs.
| The X-ray evidence that the 51 Peg companion is a planet
51 Pegasi is a nearby G2--3 V star which is similar to the Sun. Mayor& Queloz (1995) [Natur, 378, 355] have recently found that 51 Peghas sinusoidal radial velocity variations with a period of 4.2 days andamplitude of 59 m/s. The radial velocity, if due to orbital motionaround the system center-of-mass, implies a minimum companion mass of0.47 Jupiter masses, which results from the assumption that theinclination angle of the system is 90 deg from the line of sight.However, because the inclination angle is not directly measured thereremains an uncertainty in the companion mass. In the limit of anear-zero inclination angle the companion could even be a late-typedwarf star. We argue that the low measured X-ray luminosity of the 51Peg system supports the conclusion that the companion is a planet,independent of any assumption about the inclination angle. If 51 Pegwere a binary stellar system with a 4-day orbital period its X-rayemission would be in marked contrast with ALL known binary stellarsystems with similar orbital periods. When compared to the distributionof binary star X-ray luminosities, a system with the 51 Peg X-rayluminosity have an occurrence probability of only 1.7 times 10^-6. Thelow X-ray luminosity also confirms that 51 Peg is a slow rotator basedupon the correlation between X-ray emission and rotational velocity. Wediscuss the lack of synchronization between the 51 Peg orbital androtational periods and calculate model-dependent upper limits on thecompanion mass which also indicate that it is a planet.
| The behaviour of the excess CA II H and K and Hɛ emissions in chromospherically active binaries.
In this work we analyze the behaviour of the excess Ca II H and K andHɛ emissions in a sample of 73 chromospherically active binarysystems (RS CVn and BY Dra classes), of different activity levels andluminosity classes. This sample includes the 53 stars analyzed byFernandez-Figueroa et al. (1994) and the observations of 28 systemsdescribed by Montes et al. (1995c). By using the spectral subtractiontechnique (subtraction of a synthesized stellar spectrum constructedfrom reference stars of spectral type and luminosity class similar tothose of the binary star components) we obtain the active-chromospherecontribution to the Ca II H and K lines in these 73 systems. We havedetermined the excess Ca II H and K emission equivalent widths andconverted them into surface fluxes. The emissions arising from eachcomponent were obtained when it was possible to deblend bothcontributions. We have found that the components of active binaries aregenerally stronger emitters than single active stars for a giveneffective temperature and rotation rate. A slight decline of the excessCa II H and K emissions towards longer rotation periods, P_rot_, andlarger Rossby numbers, R_0_, is found. When we use R_0_ instead ofP_rot_ the scatter is reduced and a saturation at R_0_=~0.3 is observed.A good correlation between the excess Ca II K and Hɛchromospheric emission fluxes has been found. The correlations obtainedbetween the excess Ca II K emission and other activity indicators, (C IVin the transition region, and X-rays in the corona) indicate that theexponents of the power-law relations increase with the formationtemperature of the spectral features.
| A model of zonal spottedness for red dwarf stars
Not Available
| Random Spots on Chromospherically Active Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...462..888E&db_key=AST
| A Search for Optical Flares in Chromospherically Active Stars
We have surveyed four years of archival UBV photometry of 69chromospherically active stars obtained with the Phoenix-10 automaticphotoelectric telescope in search of optical flares on these stars. Wedemonstrate taht, while flares can be detected in these data, the numberof events observed on evolved stars remains very small. Flares werefound only on UX Arietis, II Pegasi, and AR Piscium. (SECTION: Stars)
| Mechanical heating levels in ultra-fast rotating dwarfs.
Using available observational data we have estimated the total radiativeoutput from the chromospheric/coronal plasma as a result of magneticheating for several F, G, K & M dwarfs. When tabulated as a functionof the bolometric flux, a clear 'levelling-off/saturation' exists forthe faster rotators as found previously when using single spectrallines. However, using recent chromospheric radiative transfer modelcalculations it is shown that this 'saturation' can be due to asignificantly larger fraction of the losses being radiated by hydrogenin the ultraviolet/optical/infrared continuum regions. The implicationbeing that these ultra-fast rotators have a high density chromosphere,plus a high temperature minimum, although additional chromospheric modelcalculations are required which include better molecular line opacitiesplus and an extension of the models for stars with higher effectivetemperatures.
| HD 12545, A Study in Spottedness
We have solved two sets of light curves of HD 12545 at its epoch ofextreme spottedness in 1990-91, one of which coincides with anindependent set analyzed recently by Strassmeier and Olah. Even for thehuge amplitudes observed, these light-curve solutions did not givereliable determinations of several important spot properties.Specifically, we find that we could obtain acceptable solutions for awide range of inclination; that spot temperature depends on inclinationassumed, falling in the range Delta-T = T_star - T_spot = 650-1200 K forinclinations of 70-30 deg; that spot latitudes derived from the lightcurves are unreliable; and that our independent spot solutions disagreewith Strassmeier and Olah's. On a more positive note, changes in thelight curve over the past five years seem to have been caused primarilyby rearrangement of persisting spot groups, and we note that the highlevel of activity implied by its H-alpha emission, makes HD 12545 aprime candidate for a white-light flare star. (SECTION: Stars)
| The Second Extreme-Ultraviolet Explorer Source Catalog
We present the second catalog of extreme-ultraviolet objects detected bythe Extreme-Ultraviolet Explorer. The data include (1) all-sky surveydetections from the initial 6 month scanner-survey phase, (2) additionalscanner detections made subsequently during specially programmedobservations designed to fill in low-exposure sky areas of the initialsurvey, (3) sources detected with deep-survey-telescope observationsalong the ecliptic, (4) objects detected by the scanner telescopesduring targeted spectroscopy observations, and ( 3) other observations.We adopt an innovative source detection method that separates the usuallikelihood function into two parts: an intensity diagnostic and aprofile diagnostic. These diagnostics allow each candidate detection tobe tested separately for both signal-to-noise ratio and conformance withthe known instrumental point-spread function. We discuss the dependenceof the false-alarm rate and the survey's completeness on the survey'ssensitivity threshold. We provide three lists of the EUV sourcesdetected: the all-sky survey detections, the deep-survey detections, andsources detected during other phases of the mission. Each list givespositions and intensities in each wave band. The total number of objectslisted is 734. For approximately 65% of these we also provide plausibleoptical, UV, radio, and/or X-ray identifications.
| The emission of the RS CVn binaries in the IRAS passbands.
In the literature, there is an ambiguity pertaining to the existence ofa far-IR excess in RS CVn systems. In the current paper we undertook astudy of the behaviour of 103 such systems in the IRAS passbands. Wefound 72 acceptable detections in the 12μm band, and 40 in the25μm band (50% more than the IRAS Point Source Catalog). Although ourfindings may be interpreted as indicating towards the existence of anexcess beyond 12μm for some systems, the evidence is not conclusivein all but two cases. These are systems GX Lib and HR 7428, with Capellabeing the only system where the IRAS fluxes in all four bands originatefrom the stellar photosphere. Given the accuracy of the data we did notfind 12μm excess for any system. We argue that the IRAS data alonecannot settle the issue, as their uncertainty is, in many cases, higherthan what the Signal-to-Noise ratio of each observation implies.Furthermore, at the higher wavelength bands the IRAS angular resolutiondrops from 0.5' at 12μm to 2' at 100μm and the background becomesvery complex, so one cannot be certain about the origin of the observedflux. The existence of IR excess in the RS CVn stars is important as faras the evolutionary scenarios for these systems are concerned. Futuremissions such as the Infrared Space Observatory will return moresensitive and accurate measurements and the ambiguity can be removed.The fluxes we quote will be helpful when planning these futureobservations, as we provide more accurate photometry and for a largerselection of sources than the IRAS Point Source Catalog.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
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