Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
An in-depth study of the pre-polar candidate WX Leonis Minoris
Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-rayobservations with full phase coverage are used for an in-depth study ofWX LMi, a system formerly termed a low-accretion rate polar. We find aconstant low-mass accretion rate, dot{M} ˜ 1.5 ×10-13 M_ȯ yr-1, a peculiar accretion geometrywith one spot not accessible via Roche-lobe overflow, a low temperatureof the white dwarf, T_eff< 8000 K, and the secondary very likelyRoche-lobe underfilling. All this lends further support to the changedview on WX LMi and related systems as detached binaries, i.e. magneticpost-common envelope binaries without significant Roche-lobe overflow inthe past. The transfer rate determined here is compatible with accretionfrom a stellar wind. We use cyclotron spectroscopy to determine theaccretion geometry and to constrain the plasma temperatures. Bothcyclotron spectroscopy and X-ray plasma diagnostics reveal low plasmatemperatures below 3 keV on both accretion spots. For the low-dot{m},high-B plasma at the accretion spots in WX LMi, cyclotron coolingdominates thermal plasma radiation in the optical. Optical spectroscopyand X-ray timing reveal atmospheric, chromospheric, and coronal activityat the saturation level on the dM4.5 secondary star.
| Probing long-period companions to planetary hosts. VLT and CFHT near infrared coronographic imaging surveys
Aims.We present the results of a deep imaging survey of stars surroundedby planets detected with the radial velocity technique. The purpose isto search for and to characterize long-period stellar and substellarcompanions. The sample contains a total of 26 stars, among which 6exhibit additional radial velocity drifts. Methods: .We usedNACO, at the ESO Very Large Telescope, and PUEO-KIR, at the CandianFrench Hawaiian Telescope, to conduct a near-infrared coronographicsurvey with adaptive optics of the faint circumstellar environment ofthe planetary hosts. The domain investigated ranges between 0.1'' to15'' (i.e. about 3 to 500 AU, according to the mean distance of thesample). The survey is sensitive to companions within the stellar andthe substellar domains, depending on the distance to the central starsand on the star properties. Results: .The images of 14 stars donot reveal any companions once the field objects are removed. 8 starshave close potential companions that need to be re-observed within 1-2years to check for physical companionship. 4 stars are surrounded byfaint objects which are confirmed or very probable companions. Thecompanion to HD 13445 (Gl 86) is already known. The HD 196885 star is anew close visual binary system with a high probability of being bound.The 2 newly discovered companions, HD 1237 B and HD 27442 B, sharecommon proper motions with the central stars. Orbital motion is detectedfor HD 1237 B. HD 1237 B is likely a low-mass M star, located at 70 AU(projected distance) from the primary. HD 27442 B is most probably awhite dwarf companion located at about 240 AU (projected distance).
| Ca II H and K Chromospheric Emission Lines in Late-K and M Dwarfs
We have measured the profiles of the Ca II H and K chromosphericemission lines in 147 main-sequence stars of spectral type M5-K7 (masses0.30-0.55 Msolar) using multiple high-resolution spectraobtained during 6 years with the HIRES spectrometer on the Keck Itelescope. Remarkably, the average FWHM, equivalent widths, and lineluminosities of Ca II H and K increase by a factor of 3 with increasingstellar mass over this small range of stellar masses. We fit the Ca II Hand K lines with a double-Gaussian model to represent both thechromospheric emission and the non-LTE central absorption. Most of thesample stars display a central absorption that is typically redshiftedby ~0.1 km s-1 relative to the emission. This implies thatthe higher level, lower density chromospheric material has a smalleroutward velocity (or higher inward velocity) by 0.1 km s-1than the lower level material in the chromosphere, but the nature ofthis velocity gradient remains unknown. The FWHM of the Ca II H and Kemission lines increase with stellar luminosity, reminiscent of theWilson-Bappu effect in FGK-type stars. Both the equivalent widths andFWHM exhibit modest temporal variability in individual stars. At a givenvalue of MV, stars exhibit a spread in both the equivalentwidth and FWHM of Ca II H and K, due both to a spread in fundamentalstellar parameters, including rotation rate, age, and possiblymetallicity, and to the spread in stellar mass at a given MV.The K line is consistently wider than the H line, as expected, and itscentral absorption is more redshifted, indicating that the H and K linesform at slightly different heights in the chromosphere where thevelocities are slightly different. The equivalent width of Hαcorrelates with Ca II H and K only for stars having Ca II equivalentwidths above ~2 Å, suggesting the existence of a magneticthreshold above which the lower and upper chromospheres become thermallycoupled.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| LP 349-25: A new tight M8V binary
We present the discovery of a tight M8V binary, with a separation ofonly 1.2 astronomical units, obtained with the PUEO and NACO adaptiveoptics systems, respectively at the CFHT and VLT telescopes. Theestimated period of LP 349-25 is approximately 5 years, and this makesit an excellent candidate for a precise mass measurement.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| New neighbours. III. 21 new companions to nearby dwarfs, discovered with adaptive optics
We present some results of a CFHT adaptive optics search for companionsto nearby dwarfs. We identify 21 new components in solar neighbourhoodsystems, of which 13 were found while surveying a volume-limited sampleof M dwarfs within 12 pc. We are obtaining complete observations forthis subsample, to derive unbiased multiplicity statistics for thevery-low-mass disk population. Additionally, we resolve for the firsttime 6 known spectroscopic or astrometric binaries, for a total of 27newly resolved companions. A significant fraction of the new binarieshas favourable parameters for accurate mass determinations. The newlyresolved companion of Gl 120.1C was thought to have a spectroscopicminimum mass in the brown-dwarf range (Duquennoy & Mayor\cite{duquennoy91}), and it contributed to the statistical evidence thata few percent of solar-type stars might have close-in brown-dwarfcompanions. We find that Gl 120.1C actually is an unrecogniseddouble-lined spectroscopic pair. Its radial-velocity amplitude hadtherefore been strongly underestimated by Duquennoy & Mayor(\cite{duquennoy91}), and it does not truly belong to their sample ofsingle-lined systems with minimum spectroscopic mass below thesubstellar limit. We also present the first direct detection of Gl494B, an astrometric brown-dwarf candidate. Its luminosity straddles thesubstellar limit, and it is a brown dwarf if its age is less than300 Myr. A few more years of observations will ascertain its massand status from first principles.Based on observations made at Canada-France-Hawaii Telescope, operatedby the National Research Council of Canada, the Centre National de laRecherche Scientifique de France and the University of Hawaii. Some ofthe data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| The brown dwarf population in the Chamaeleon I cloud
We present the results of a multiband survey for brown dwarfs in theChamaeleon I dark cloud with the Wide Field Imager(WFI) camera at the ESO/MPG 2.2-m telescope on La Silla (Chile). Thesurvey has revealed a substantial population of brown dwarfs in thissouthern star-forming region. Candidates were selected from R, I andHα imaging observations. We also observed in two medium-bandfilters, M 855 and M 915, for spectral type determination. The formerfilter covers a wavelength range containing spectral featurescharacteristic of M-dwarfs, while the latter lies in a relativelyfeatureless wavelength region for these late-type objects. A correlationwas found between spectral type and (M 855-M 915) colour index for mid-to late M-type objects and early L-type dwarfs. With this method, weidentify most of our object candidates as being of spectral type M 5 orlater. Our results show that there is no strong drop in the number ofobjects for the latest spectral types, hence brown dwarfs may be asabundant as low-mass stars in this region. Also, both kind of objectshave a similar spatial distribution. We derive an index α = 0.6± 0.1 of the mass function in this region of dispersed starformation, in good agreement with the values obtained in other starforming regions and young clusters. Some of the brown dwarfs have strongHα emission, suggesting mass accretion. For objects with publishedinfrared photometry, we find that strong Hα emission is related toa mid-infrared excess, indicative of the existence of a circumstellardisk.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| 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.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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 stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| 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.
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| Variability of the Photospheric Radiation of Active K-M Dwarfs and Their X-ray Luminosities
Variability of the photospheric radiation of 40 (dKe-dMe) dwarfs in thesolar neighborhood due to variations in the spottedness of theirsurfaces is analyzed based on the behavior of their mean annualbrightnesses over long time intervals. The amplitudes and characteristictime scales of the variations of the mean annual brightness are taken tobe indicators of photospheric activity and were used to infer the levelsof photospheric activity in the stars studied. The influence of axialrotation on the development of cyclic activity in young red dwarfs andF-M main-sequence stars is analyzed. The durations and amplitudes of thephotospheric variability of rapidly rotating (dK0e-dK5e) stars testifiesto a higher level of photospheric activity among red dwarfs andsolar-type stars. The X-ray luminosities of these stars grow with theamplitude of the variations of the mean annual brightness. However, thisis not typical of rapidly rotating M dwarfs, for which the X-rayemission varies by more than two orders of magnitude, although theirdegrees of spottedness are all virtually the same. A linear relationshipbetween the X-ray and bolometric luminosities is observed for young(dKe-dMe) stars, with their ratios log(L x/L bol) being about -3. Theseproperties can be used to determine whether a red dwarf is a young staror is already on the main sequence.
| 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.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| 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
| A systematic analysis of X-ray variability of dM stars
We have systematically analyzed X-ray variability of dM stars. Our database is the sample of all dM stars listed in the CNS3 (Gliese &Jahreiss 1991) catalog which have been observed with the ROSAT PSPC. Ourdata sample includes 86 pointed observations of 55 distinct stars ormultiple systems. A large fraction of stars shows significantvariations, regardless of their quiescent flux. Variability is detectedon all observable time scales. The amplitudes of these variations areindependent of both stellar X-ray and visual luminosity. Compared tosolar X-ray variability properties our results suggest that theamplitude distribution of X-ray variability in dM stars is consistentwith the analogous distribution for solar flares. We discuss the effectof variability on the spread observed in the X-ray luminosity functionof M stars. The comparison of our data with those obtained with EinsteinIPC shows that variations on time scales shorter than a few month aremore common than long term variations comparable to, e.g., the 11 yearssolar cycle.
| 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).
| Photometric modelling of starspots - I. A Barnes-Evans-like surface brightness-colour relation using (Ic-K)
In the first part of this work, the empirical correlation of stellarsurface brightness FV with (Ic-K) broad-bandcolour is investigated by using a sample of stars cooler than the Sun. Abilinear correlation is found to represent well the brightness of G, Kand M giant stars. The change in slope occurs at (Ic-K)~2.1or at about the transition from K to M spectral types. The samerelationship is also investigated for dwarf stars and found to bedistinctly different from that of the giants. The dwarf star correlationdiffers by an average of -0.4 in (Ic-K) or by a maximum inFV of ~-0.1, positioning it below that of the giants, withboth trends tending towards convergence for the hotter stars in oursample. The flux distribution derived from theFV-(Ic-K) relationship for the giant stars,together with that derived from an FV-(V-K) relationship andthe blackbody flux distribution, is then utilized to compute syntheticlight V and colour (V-R)c, (V-I)c and (V-K) curvesof cool spotted stars. We investigate the effects on the amplitudes ofthe curves by using these FV-colour relations and by assumingthe effective gravity of the spots to be lower than the gravity of theunspotted photosphere. We find that the amplitudes produced by using theFV-(Ic-K) relationship are larger than thoseproduced by the other two brightness correlations, meaning smallerand/or warmer spots.
| 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.
| Barnes-Evans relations for late-type giants and dwarfs
The visual surface brightness of K/M giants and dwarfs with near-solarmetallicity differ slightly in agreement with the gravity effectspredicted by recent theoretical models. We show that M-dwarfs displayalso a metallicity dependence of the surface brightness in the infraredK-band in agreement with theory. Based on these results, we presentimproved Barnes-Evans type relations and estimate the radii of 60 singleor presumed M and K-dwarfs.
| Observations of radio stars at the Valinhos CCD Meridian Circle
We present the astrometric results from the optical observation of 16stars from the list of \cite[Wendker (1995)]{wend95}, with detected orsuspected radio emission. The results are given on the Hipparcos/Tychoreference frame. The stars are evenly distributed in right ascension andthe declinations range from -15.6(deg) to +28.6(deg) . Thisdistribution, also allowed to obtain positions relative to the TwinAstrographic Catalogue (TAC 1.0). The observations were carried out withthe Valinhos CCD Meridian Circle, in Brazil, operating in drift scanningmode. The average positional precision is at 40 mas and generally below2 mas/yr for proper motions, obtained with the new reductions of theAstrographic Catalogue - AC2000, as first epoch. In the strips takenfrom the observation of the 16 radio stars, 573 Tycho stars, 545 ACTstars and 566 TAC stars were found. The analysis of the independentreductions made relatively to those frames and the large number ofcommon stars enables to focus on the equatorial zone, as represented bythe three catalogues. As a result, the quality of Valinhos observationswith regard to ACT and TAC internal accuracies is verified. Table 1 ofthis paper, plus 16 additional tables with observed positions for theACT reference stars, are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) 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
| The Optical Mass-Luminosity Relation at the End of the Main Sequence (0.08-0.20 M_solar)
The empirical mass-luminosity relation at M_V is presented for starswith masses 0.08-0.20 M_solar based upon new observations made with FineGuidance Sensor 3 on the Hubble Space Telescope. The targets are nearby,red dwarf multiple systems in which the magnitude differences aretypically measured to +/-0.1 mag or better. The M_V values are generatedusing the best available parallaxes and are also accurate to +/-0.1 mag,because the errors in the magnitude differences are the dominant errorsource. In several cases this is the first time the observedsub-arcsecond multiples have been resolved at optical wavelengths. Themass-luminosity relation defined by these data reaches to M_V=18.5 andprovides a powerful empirical test for discriminating the lowest massstars from high-mass brown dwarfs at wavelengths shorter than 1 mum.
| Infrared Colors at the Stellar/Substellar Boundary
We present new infrared JHK photometry for 61 halo and disk stars aroundthe stellar/substellar boundary. We also present new L' photometry for21 of these stars and for 40 low-mass stars taken from the Leggettphotometry compilation. These data are combined with available opticalphotometry and astrometric data to produce color-color and absolutemagnitude-color diagrams; the current sample extends the similar workpresented in the 1992 paper into more metal-poor and lower mass regimes.The disk and halo sequences are compared to the predictions of thelatest model atmospheres and structural models. We find good agreementbetween observation and theory except for known problems in the V and Hpassbands, probably due to incomplete molecular data for TiO, metalhydrides, and H_2O. The metal-poor M subdwarfs are well matched by themodels since oxide opacity sources are less important in this case. Theknown extreme M subdwarfs have metallicities about one-hundredth solar,and the coolest subdwarfs have T_eff ~ 3000 K with masses~0.09M/M_ȯ. The grainless models are not able to reproduce the fluxdistributions of disk objects with T_eff < 2500 K; however, apreliminary version of the NextGen-Dusty models that includeshomogeneous formation and extinction by dust grains is able to match thecolors of these very cool objects. The least luminous objects in thissample are GD 165B, three DENIS objects-DBD 0205, DBD 1058, and DBD1228-and Kelu-1. These have T_eff ~ 2000 K and are at or below thestellar limit with masses <=0.075M/M_ȯ. Photometry alone cannotconstrain these parameters further since the age is unknown, butpublished lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass<= 0.06M/M_ȯ).
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Virgo |
| Ascensió Recta: | 13h00m46.57s |
| Declinació: | +12°22'32.6" |
| Magnitud Aparent: | 9.719 |
| Distancia: | 11.429 parsecs |
| Moviment propi RA: | -640.1 |
| Moviment propi Dec: | -25.1 |
| B-T magnitude: | 11.502 |
| V-T magnitude: | 9.867 |
Catàlegs i designacions:
|
