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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.
| Optical Spectroscopy of a Flare on Barnard's Star
We present optical spectra of a flare on Barnard's star. Severalphotospheric and chromospheric species were enhanced by the flareheating. An analysis of the Balmer lines shows that their shapes arebest explained by Stark broadening rather than chromospheric massmotions. We estimate the temperature of the flaring region in the loweratmosphere to be >=8000 K and the electron density to be~1014 cm-3, similar to values observed in other dMflares. Because Barnard's star is considered to be one of our oldestneighbors, a flare of this magnitude is probably quite rare.
| The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours
The United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Surveyis a set of five surveys of complementary combinations of area, depthand Galactic latitude, which began in 2005 May. The surveys use theUKIRT Wide Field Camera (WFCAM), which has a solid angle of0.21deg2. Here, we introduce and characterize the ZY JHKphotometric system of the camera, which covers the wavelength range0.83-2.37μm. We synthesize response functions for the five passbands,and compute colours in the WFCAM, Sloan Digital Sky Survey (SDSS) andtwo-Micron All Sky Survey (2MASS) bands, for brown dwarfs, stars,galaxies and quasars of different types. We provide a recipe for othersto compute colours from their own spectra. Calculations are presented inthe Vega system, and the computed offsets to the AB system are provided,as well as colour equations between WFCAM filters and the SDSS and 2MASSpassbands. We highlight the opportunities presented by the new Y filterat 0.97-1.07μm for surveys for hypothetical Y dwarfs (brown dwarfscooler than T), and for quasars of very high redshift, z > 6.4.
| First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars
We have measured the angular diameters of six M dwarfs with the CHARAArray, a long-baseline optical interferometer located at Mount WilsonObservatory. Spectral types range from M1.0 V to M3.0 V and linear radiifrom 0.38 to 0.69 Rsolar. These results are consistent withthe seven other M dwarf radii measurements from optical interferometryand with those for 14 stars in eclipsing binary systems. We compare alldirectly measured M dwarf radii to model predictions and find thatcurrent models underestimate the true stellar radii by up to 15%-20%.The differences are small among the metal-poor stars but becomesignificantly larger with increasing metallicity. This suggests thattheoretical models for low-mass stars may be missing some opacity sourcethat alters the computed stellar radii.
| Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
| Probing the LHS Catalog. II. Faint Proper-Motion Stars
We present low-resolution spectroscopic observations of faintproper-motion stars from the LHS Catalogue, concentrating on stars withmr>16.5 and μ>0.5" yr-1. The presentpaper includes observations and spectral classifications for 294 Mdwarfs, M subdwarfs (sdM), and extreme M subdwarfs (esdM). We alsoidentify white dwarfs among the faintest LHS stars. We havecross-referenced this sample against the Two Micron All Sky Survey(2MASS) sources, and list data for the detected objects. We discussstars of individual interest, as well as the characteristics of theoverall sample. As expected, a significant number of the stars in thisproper-motion-selected sample are halo subdwarfs, including an esdMdwarf, LHS 3481, that is likely to lie within 20 pc of the Sun. None ofthe subdwarfs show Hα emission.
| A cannonball star candidate in Canis Minor
We report on the identification of a cannonball star candidate towardthe direction of Canis Minor. The star is called PSS 544-7 and it hasbeen selected as a result of a search in SIMBAD at CDS. With uncorrectedcolors B ‑ V = 1.58 ± 0.15, V ‑ J = 2.66 ±0.14, V ‑ KS = 3.38 ± 0.23, J ‑ H = 0.675± 0.167, H ‑ KS = 0.046 ± 0.244, J‑ KS = 0.721 ± 0.219, the object exhibits asignificant proper motion, 401 ± 13 mas/yr. Based on theobject’s photometry, we conclude that it is likely a M-dwarf withan unusually high velocity perpendicular to the galactic disk. Itsphotometric parallax yields a distance estimate of 210 ± 60 pc, atangential velocity of 399 ± 127 km/s and a W-component >350km/s. If our interpretation is correct, given its location and kinematicsignature, the object is a candidate cannonball star ejected by a starcluster.
| A non-main-sequence secondary in SY Cancri
Simultaneous spectroscopic and photometric observations of the Z Camtype dwarf nova SY Cancri were used to obtain absolute fluxcalibrations. A comparison of the photometric calibration with awide-slit spectrophotometric calibration showed that either method isequally satisfactory. A radial velocity study of the secondary star,made using the far-red NaI doublet, yielded a semi-amplitude ofK2= 127 +/- 23 km s-1. Taking the published valueof 86 +/- 9 km s-1 for K1 gives a mass ratio ofq=M2/M1= 0.68 +/- 0.14; this is very differentfrom the value of 1.13 +/- 0.35 quoted in the literature. Using the newlower mass ratio, and constraining the mass of the white dwarf to bewithin reasonable limits, then leads to a mass for the secondary starthat is substantially less than would be expected for its orbital periodif it satisfied a main-sequence mass-radius relationship. We find aspectral type of M0 that is consistent with that expected for amain-sequence star of the low mass we have found. However, in order tofill its Roche lobe, the secondary must be significantly larger than amain-sequence star of that mass and spectral type. The secondary isdefinitely not a normal main-sequence star.
| Carbon monoxide in low-mass dwarf stars
We compare high-resolution infrared observations of the CO 2-0 bands inthe 2.297-2.310 μm region of M dwarfs and one L dwarf withtheoretical expectations. We find a good match between the observationaland synthetic spectra throughout the 2000-3500 K temperature regimeinvestigated. None the less, for the 2500-3500 K temperature range, thetemperatures that we derive from synthetic spectral fits are higher thanexpected from more empirical methods by several hundred kelvin. In orderto reconcile our findings with the empirical temperature scale, it isnecessary to invoke warming of the model atmosphere used to constructthe synthetic spectra. We consider that the most likely reason for theback-warming is missing high-temperature opacity due to water vapour. Wecompare the water vapour opacity of the Partridge-Schwenke line listused for the model atmosphere with the output from a preliminarycalculation by Barber & Tennyson. While the Partridge-Schwenke linelist is a reasonable spectroscopic match for the new line list at 2000K, by 4000 K it is missing around 25 per cent of the water vapouropacity. We thus consider that the offset between empirical andsynthetic temperature scales is explained by the lack of hot watervapour used for computation of the synthetic spectra. For our coolestobjects with temperatures below 2500 K, we find best fits when usingsynthetic spectra which include dust emission. Our spectra also allow usto constrain the rotational velocities of our sources, and thesevelocities are consistent with the broad trend of rotational velocitiesincreasing from M to L.
| Metallicity measurements using atomic lines in M and K dwarf stars
We report the first survey of chemical abundances in M and K dwarf starsusing atomic absorption lines in high-resolution spectra. We havemeasured Fe and Ti abundances in 35 M and K dwarf stars using equivalentwidths measured from λ/Δλ~ 33000 spectra. Ouranalysis takes advantage of recent improvements in model atmospheres oflow-temperature dwarf stars. The stars have temperatures between 3300and 4700 K, with most cooler than 4100 K. They cover an iron abundancerange of -2.44 < [Fe/H] < +0.16. Our measurements show [Ti/Fe]decreasing with increasing [Fe/H], a trend similar to that measured forwarmer stars, where abundance analysis techniques have been tested morethoroughly. This study is a step towards the observational calibrationof procedures to estimate the metallicity of low-mass dwarf stars usingphotometric and low-resolution spectral indices.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| A Map of the Universe
We have produced a new conformal map of the universe illustrating recentdiscoveries, ranging from Kuiper Belt objects in the solar system to thegalaxies and quasars from the Sloan Digital Sky Survey. This mapprojection, based on the logarithm map of the complex plane, preservesshapes locally and yet is able to display the entire range ofastronomical scales from the Earth's neighborhood to the cosmicmicrowave background. The conformal nature of the projection, preservingshapes locally, may be of particular use for analyzing large-scalestructure. Prominent in the map is a Sloan Great Wall of galaxies 1.37billion light-years long, 80% longer than the Great Wall discovered byGeller and Huchra and therefore the largest observed structure in theuniverse.
| Calibrating models of ultralow-mass stars
Evolutionary and atmospheric models have become available for youngultralow-mass objects. These models are being used to determinefundamental parameters from observational properties. TiO bands are usedto determine effective temperatures in ultralow-mass objects, andtogether with Na- and K-lines to derive gravities at the substellarboundary. Unfortunately, model calibrations in (young) ultralow-massobjects are rare. As a first step towards a calibration of syntheticspectral features, I show molecular bands of TiO, which is a mainopacity source in late M-dwarfs. The TiO \epsilon-band at 8450Å issystematically too weak. This implies that temperatures determined fromthat band are underestimated, and I discuss implications for determiningfundamental parameters from high resolution spectra.
| The Cornell High-Order Adaptive Optics Survey for Brown Dwarfs in Stellar Systems. I. Observations, Data Reduction, and Detection Analyses
In this first of a two-paper sequence, we report techniques and resultsof the Cornell High-Order Adaptive Optics Survey (CHAOS) for brown dwarfcompanions. At the time of this writing, this study represents the mostsensitive published population survey of brown dwarf companions tomain-sequence stars for separations akin to our own outer solar system.The survey, conducted using the Palomar 200 inch (5 m) Hale Telescope,consists of Ks coronagraphic observations of 80 main-sequencestars out to 22 pc. At 1" separation from a typical target system, thesurvey achieves median sensitivities 10 mag fainter than the parentstar. In terms of companion mass, the survey achieves typicalsensitivities of 25MJ (1 Gyr), 50MJ (solar age),and 60MJ (10 Gyr), using the evolutionary models of Baraffeand coworkers. Using common proper motion to distinguish companions fromfield stars, we find that no systems show positive evidence of asubstellar companion (searchable separation ~1"-15" projected separation~10-155 AU at the median target distance). In the second paper of theseries we will present our Monte Carlo population simulations.
| 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).
| A study of Kapteyn's star
We present a review of the current knowledge of Kapteyn's Star (KS) - anearby, low-metallicity M-dwarf, with an eccentric and retrogradeGalactic orbit. A brief survey of its spectroscopic properties isprovided, together with an analysis of its Galactic orbit in a Galaxymodel that incorporates resonances. We propose that KS may have oncebelonged to a dwarf spheroidal galaxy that merged with the Galaxy, andwhose present remnant, if it still exists, is a globular cluster similarto ω Cen.
| A planet-sized transiting star around OGLE-TR-122. Accurate mass and radius near the hydrogen-burning limit
We report the discovery and characterisation of OGLE-TR-122b, thesmallest main-sequence star to date with a direct radius determination.OGLE-TR-122b transits around its solar-type primary every 7.3-days. WithM=0.092±0.009 M_ȯ andR=0.120+0.020-0.013 R_ȯ, it is by far thesmallest known eclipsing M-dwarf. The derived mass and radius forOGLE-TR-122b are in agreement with the theoretical expectations.OGLE-TR-122b is the first observational evidence that stars can indeedhave radii comparable or even smaller than giant planets. In such cases,the photometric signal is exactly that of a transiting planet and thetrue nature of the companion can only be determined with high-resolutionspectroscopy.Based on observations collected with the VLT/UT2 Kueyen telescope(Paranal Observatory, ESO, Chile) using the FLAMES+UVES spectrograph(program ID 072.C-191).
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| The χ Factor: Determining the Strength of Activity in Low-Mass Dwarfs
We describe a new, distance-independent method for calculating themagnetic activity strength in low-mass dwarfs,LHα/Lbol. Using a well-observed sample ofnearby stars and cool standards spanning spectral type M0.5 to L0, wecompute χ, the ratio between the continuum flux near Hα andthe bolometric flux, fλ6560/fbol. Thisratio can be multiplied by the measured equivalent width of the Hαemission line to yield LHα/Lbol. We provideχ values for all objects in our sample, and also fits to χ as afunction of color and average values by spectral type. This method wasused by West et al. to examine trends in magnetic activity strength inlow-mass stars.
| Near-Ultraviolet Spectra of Nine M Dwarf Stars, or a Second Effort to Find Optical Coronal Lines in M Dwarf Stars
We have searched for optical coronal lines in the 3100-3700 Åregion of eight M dwarf stars with rather low levels of activity. Thisbrief survey supplements a similar search in 15 active stars publishedin 1991. No coronal lines could be identified. However, the emissionspectra including lines of H I, He I, Ca II, Ca I, Si I, and Fe I aredescribed and illustrated. Radial velocities of the emission lines showno systematic differences from the stellar absorption lines. Coronaewith temperatures similar to those in the solar corona seem to be rareamong the M dwarfs, although at least one example has been found bySchmitt & Wichmann.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Barnard's Star and the M Dwarf Temperature Scale
The recently measured angular diameter of Barnard's star, together withits large and precise parallax, and a spectral energy distribution thatextends from the near-ultraviolet to almost 12 μm establish some ofthe star's fundamental properties-we find a bolometric luminosityL=(3.46+/-0.17)×10-3Lsolar, radiusR=0.200+/-0.008Rsolar, and effective temperatureTeff=3134+/-102 K. Accurate knowledge of those parametershelps in turn to constrain the star's metallicity and mass. Although itis evidently possible to estimate bolometric fluxes with good accuracyfrom photometry alone, angular diameters present more of a challenge,and we examine alternative methods for determining them, namely, throughthe use of the Barnes-Evans relation and the infrared flux method. Wefind further evidence that even ``state-of-the-art'' M dwarf models,which appear to yield good results for the effective temperatures,nevertheless underestimate the radii of the actual stars.
| The angular sizes of dwarf stars and subgiants. Surface brightness relations calibrated by interferometry
The availability of a number of new interferometric measurements of MainSequence and subgiant stars makes it possible to calibrate the surfacebrightness relations of these stars using exclusively direct angulardiameter measurements. These empirical laws make it possible to predictthe limb darkened angular diameters θLD of dwarfs andsubgiants using their dereddened Johnson magnitudes, or their effectivetemperature. The smallest intrinsic dispersions of σ ≤1% inθLD are obtained for the relations based on the K andL magnitudes, for instance log θLD = 0.0502 (B-L) +0.5133 - 0.2 L or log θLD = 0.0755 (V-K) + 0.5170 -0.2 K. Our calibrations are valid between the spectral types A0 and M2for dwarf stars (with a possible extension to later types when using theeffective temperature), and between A0 and K0 for subgiants. Suchrelations are particularly useful for estimating the angular sizes ofcalibrators for long-baseline interferometry from readily availablebroadband photometry.Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
| NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
We present a final summary of all ROSAT X-ray observations of nearbystars. All available ROSAT observations with the ROSAT PSPC, HRI and WFChave been matched with the CNS4 catalog of nearby stars and the resultsgathered in the Nearby X-ray and XUV-emitting Stars data base, availablevia www from the Home Page of the Hamburger Sternwarte at the URLhttp://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. Newvolume-limited samples of F/G-stars (dlim = 14 pc), K-stars(dlim = 12 pc), and M-stars (dlim = 6 pc) areconstructed within which detection rates of more than 90% are obtained;only one star (GJ 1002) remains undetected in a pointed follow-upobservation. F/G-stars, K-stars and M-stars have indistinguishablesurface X-ray flux distributions, and the lower envelope of the observeddistribution at FX ≈ 104 erg/cm2/sis the X-ray flux level observed in solar coronal holes. Large amplitudevariations in X-ray flux are uncommon for solar-like stars, but maybemore common for stars near the bottom of the main sequence; a largeamplitude flare is reported for the M star LHS 288. Long term X-raylight curves are presented for α Cen A/B and Gl 86, showingvariations on time scales of weeks and demonstrating that α Cen Bis a flare star.Tables 1-3 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/651
| Discovery of very nearby ultracool dwarfs from DENIS
We report new spectroscopic results, obtained with UKIRT/CGS4, of asample of 14 candidate ultracool dwarfs selected from the DENIS (DeepNear-Infrared Survey of the Southern Sky) database. A further object,selected from the 2MASS Second Incremental Release, was observed at alater epoch with the same instrument. Six objects are already known inthe literature; we re-derive their properties. A further four prove tobe very nearby (10 pc) mid-to-late L-dwarfs, three unknownhitherto, two of which are almost certainly substellar. These findingsincrease the number of L-dwarfs known within 10 pc by 25%. Theremainder of the objects discussed here are early L or very late M-typedwarfs lying between 45 and 15 pc and are also new to theliterature. Spectral types have been derived by direct comparison withJ-, H- and K- band spectra of known template ultracool dwarfs given byLeggett et al.thanksftp://ftp . jach . hawaii . edu/pub/ukirt/skl/dL .spectra/ For the known objects, we generally find agreement to within1 subclass with previously derived spectral types. Distances aredetermined from the most recent MJ vs. spectral typecalibrations, and together with our derived proper motions yieldkinematics for most targets consistent with that expected for the diskpopulation; for three probable late M-dwarfs, membership of adynamically older population is postulated. The very nearby L-typeobjects discussed here are of great interest for future studies ofbinarity and parallaxes.
| Library of flux-calibrated echelle spectra of southern late-type dwarfs with different activity levels
We present Echelle spectra of 91 late-type dwarfs, of spectral typesfrom F to M and of different levels of chromospheric activity, obtainedwith the 2.15 m telescope of the CASLEO Observatory located in theArgentinean Andes. Our observations range from 3890 to 6690 Å, ata spectral resolution from 0.141 to 0.249 Å per pixel(R=λ/δ λ ≈ 26 400). The observations were fluxcalibrated with the aid of long slit spectra. A version of thecalibrated spectra is available via the World Wide Web.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/699The spectra are available as FITS and ascii-files at the URL:http://www.iafe.uba.ar/cincunegui/spectra/Table2.html. They are alsoavailable 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/414/699. When convertingthe fits to ascii, the spectra were oversampled to a constant δλ ≈ 0.15 Å.Table 2 is also available in electronic form at the CDS via anonymous
| Roche tomography of cataclysmic variables - II. Images of the secondary stars in AM Her, QQ Vul, IP Peg and HU Aqr
We present a set of Roche tomography reconstructions of the secondarystars in the cataclysmic variables AM Her, QQ Vul, IP Peg and HU Aqr.The image reconstructions show distinct asymmetries in the irradiationpattern for all four systems that can be attributed to shielding of thesecondary star by the accretion stream/column in AM Her, QQ Vul and HUAqr, and increased irradiation by the bright-spot in IP Peg. We use theentropy landscape technique to derive accurate system parameters(M1, M2, i and γ) for the four binaries. Inprinciple, this technique should provide the most reliable massdeterminations available, since the intensity distribution across thesecondary star is known. We also find that the intensity distributioncan systematically affect the value of γ derived from circularorbit fits to radial velocity variations.
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