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Barnes-Evans relations for dwarfs with an application to the determination of distances to cataclysmic variables
Context: . Barnes-Evans type relations provide an empirical relationshipbetween the surface brightness of stars and their color. They are widelyused for measuring the distances to stars of known radii, as theRoche-lobe filling secondaries in cataclysmic variables (CVs).Aims: . The calibration of the surface brightness of field dwarfs ofnear-solar metalicity with spectral types A0 to L8 covers all secondaryspectral types detectable in CVs and related objects and will aid in themeasurement of their distances. Methods: . The calibrations arebased on the radii of field dwarfs measured by the Infrared Flux Methodand by interferometry. Published photometry is used and homogenized tothe Cousins Rc and Ic and the CIT JHK photometricsystems. The narrow band surface brightness at 7500 Å is based onour own and published spectrophotometry. Care is taken to select thedwarfs for near-solar metalicity, appropriate to CVs, and to avoiderrors caused by unrecognized binarity. Results: . Relations areprovided for the surface brightness in V, R_c, I_c, J, H, K and in anarrow band at 7500 Å as functions of V-K and of spectral type.The method is tested with selected CVs for which independent informationon their distances is available. The observed spread in the radii ofearly M-dwarfs of given mass or luminosity and its influence on thedistance measurements of CVs is discussed. Conclusions: . As longas accurate trigonometric parallaxes are not routinely available for alarge number of CVs, the surface brightness method remains a reliablemeans of determining distances to CVs in which a spectral signature ofthe secondary star can be discerned.
| Exploring the Frequency of Close-in Jovian Planets around M Dwarfs
We discuss our high-precision radial velocity results of a sample of 90M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J.Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLTand the Keck I telescopes, within the context of the overall frequencyof Jupiter-mass planetary companions to main-sequence stars. None of thestars in our sample show variability indicative of a giant planet in ashort-period orbit, with a<=1 AU. We estimate an upper limit of thefrequency f of close-in Jovian planets around M dwarfs as <1.27% (atthe 1 σ confidence level). Furthermore, we determine that theefficiency of our survey in noticing planets in circular orbits is 98%for companions with msini>3.8MJ and a<=0.7 AU. Foreccentric orbits (e=0.6) the survey completeness is 95% for all planetswith msini>3.5MJ and a<=0.7 AU. Our results pointtoward a generally lower frequency of close-in Jovian planets for Mdwarfs as compared to FGK-type stars. This is an important piece ofinformation for our understanding of the process of planet formation asa function of stellar mass.Based on data collected with the Hobby-Eberly Telescope, which isoperated by McDonald Observatory on behalf of the University of Texas atAustin, Pennsylvania State University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen. Also based on observationscollected at the European Southern Observatory, Chile (ESO programs65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044,71.C-0498, 072.C-0495, 173.C-0606). Additional data were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration(NASA), and with the McDonald Observatory Harlan J. Smith 2.7 mtelescope.
| Rejecting Astrophysical False Positives from the TrES Transiting Planet Survey: The Example of GSC 03885-00829
Ground-based wide-field surveys for nearby transiting gas giants areyielding far fewer true planets than astrophysical false positives, someof which are difficult to reject. Recent experience has highlighted theneed for careful analysis to eliminate astronomical systems in whichlight from a faint eclipsing binary is blended with that from a brightstar. During the course of the Transatlantic Exoplanet Survey, weidentified a system presenting a transit-like periodic signal. Weobtained the proper motion and infrared color of this target (GSC03885-00829) from publicly available catalogs, which suggested this staris an F dwarf, supporting our transit hypothesis. This spectralclassification was confirmed using spectroscopic observations from whichwe determined the stellar radial velocity. The star did not exhibit anysigns of a stellar mass companion. However, subsequent multicolorphotometry displayed a color-dependent transit depth, indicating that ablend was the likely source of the eclipse. We successfully modeled ourinitial photometric observations of GSC 03885-00829 as the light from aK dwarf binary system superimposed on the light from a late F dwarfstar. High-dispersion spectroscopy confirmed the presence of light froma cool stellar photosphere in the spectrum of this system. With thiscandidate, we demonstrate both the difficulty in identifying certaintypes of false positives in a list of candidate transiting planets andour procedure for rejecting these imposters, which may be useful toother groups performing wide-field transit surveys.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W.M. KeckFoundation.
| 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.
| Metallicity of M dwarfs. I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence
We obtained high resolution ELODIE and CORALIE spectra for bothcomponents of 20 wide visual binaries composed of an F-, G- or K-dwarfprimary and an M-dwarf secondary. We analyse the well-understood spectraof the primaries to determine metallicities ([Fe/H]) for these 20systems, and hence for their M dwarf components. We pool thesemetallicities with determinations from the literature to obtain aprecise (±0.2 dex) photometric calibration of M dwarfmetallicities. This calibration represents a breakthrough in a fieldwhere discussions have had to remain largely qualitative, and it helpsus demonstrate that metallicity explains most of the large dispersion inthe empirical V-band mass-luminosity relation. We examine themetallicity of the two known M-dwarf planet-host stars, Gl876 (+0.02 dex) and Gl 436 (-0.03 dex), inthe context of preferential planet formation around metal-rich stars. Wefinally determine the metallicity of the 47 brightest single M dwarfs ina volume-limited sample, and compare the metallicity distributions ofsolar-type and M-dwarf stars in the solar neighbourhood.
| 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.
| The structure of our stellar system.
Not Available
| RXJ2130.6+4710 - an eclipsing white dwarf-M-dwarf binary star
We report the detection of eclipses in the close white-dwarf-M-dwarfbinary star RXJ2130.6+4710. We present light curves in the B, V and Ibands and fast photometry obtained with the three-channel CCD photometerUltracam of the eclipse in the u', g' and r' bands. The depth of theeclipse varies from 3.0 mag in the u' band to less than 0.1 mag in the Iband. The times of mid-eclipse are given by the ephemerisBJD(mid-eclipse) = 2452785.681876(2) + 0.521035625(3) E, where figuresin parentheses denote uncertainties in the final digit. We presentmedium-resolution spectroscopy from which we have measured thespectroscopic orbits of the M dwarf and white dwarf. We estimate thatthe spectral type of the M dwarf is M3.5Ve or M4Ve, although the data onwhich this is based are not ideal for spectral classification. We havecompared the spectra of the white dwarf with synthetic spectra from purehydrogen model atmospheres to estimate that the effective temperature ofthe white dwarf is Teff= 18000 +/- 1000 K. We have used thewidth of the primary eclipse and duration of totality measured preciselyfrom the Ultracam u' data combined with the amplitude of the ellipsoidaleffect in the I band and the semi-amplitudes of the spectroscopic orbitsto derive masses and radii for the M dwarf and white dwarf. The M dwarfhas a mass of 0.555 +/- 0.023 Msolar and a radius of 0.534+/- 0.053 Rsolar, which is a typical radius for stars of thismass. The mass of the white dwarf is 0.554 +/- 0.017 Msolarand its radius is 0.0137 +/- 0.0014 Rsolar, which is theradius expected for a carbon-oxygen white dwarf of this mass andeffective temperature. The light curves are affected by frequent flaresfrom the M dwarf and the associated dark spots on its surface can bedetected from the distortions to the light curves and radial velocities.RXJ2130.6+4710 is a rare example of a pre-cataclysmic variable star thatwill start mass transfer at a period above the period gap forcataclysmic variables.
| 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.
| Multicolour CCD measurements of nearby visual double stars. II
We present accurate CCD astrometric and photometric data for 31 nearbyvisual double stars in the standard filters BVRI. The observations werecollected with a 1.3-m telescope in 2001-2002 The results consist ofrelative astrometric positions (epoch, angular separation and positionangle) and differential BVRI photometry of the components. Mean errorsare: 0.01 arcsec for the separation; 0.06 ° for the position angle;and 0.015m for the photometric data. Comparing the relative positions atdifferent epochs, we evaluate the physical association of the systems.We additionally derive fractional masses and true separations for themost probable binary systems and, whenever orbits are available, alsototal and component masses.Table \ref{t2} is only available in the electronic form athttp:www.edpsciences.org. Tables \ref{t4} and \ref{t5} are 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/422/1023Based on data obtained at the Skinakas Observatory and by the Hipparcosastrometry satellite. The Skinakas Observatory is a collaborativeproject of the University of Crete, the Foundation for Research andTechnology - Hellas, and the Max-Planck-Institut fürExtraterrestrische Physik.
| Ephemerides of visual binaries via successive approximations method
In this paper, an algorithm based on the successive approximationsmethod was developed for ephemerides computation of visual binaries. Thecomputational algorithm was applied to the binaries ADS 3102 and ADS3672 for elliptic orbits, and to ADS 11632 for hyperbolic orbits. Thecomparison with the observations are in good agreement which proved theefficiency of the developed algorithm.
| 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.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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
| New Hipparcos-based Parallaxes for 424 Faint Stars
We present a catalog of 424 common proper-motion companions to Hipparcosstars with good (>3 σ) parallaxes, thereby effectively providingnew parallaxes for these companions. Compared with typical stars in theHipparcos catalog, these stars are substantially dimmer. The catalogincludes 20 white dwarfs and an additional 29 stars withMV>14, the great majority of the latter being M dwarfs.
| Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars
We present the full target list and prioritization algorithm developedfor use by the microwave search for technological signals at the SETIInstitute. We have included the Catalog of Nearby Habitable StellarSystems (HabCat, described in Paper I), all of the nearest 100 stars and14 old open clusters. This is further augmented by a subset of theTycho-2 catalog based on reduced proper motions, and this larger catalogshould routinely provide at least three target stars within the largeprimary field of view of the Allen Telescope Array. The algorithm forprioritizing objects in the full target list includes scoring based onthe subset category of each target (i.e., HabCat, cluster, Tycho-2, ornearest 100), its distance (if known), and its proximity to the Sun onthe color-magnitude diagram.
| 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.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| Narrow-Angle Astrometry with the Space Interferometry Mission: The Search for Extrasolar Planets. I. Detection and Characterization of Single Planets
A decade after the publication of the Hipparcos Catalogue, the SpaceInterferometry Mission (SIM) will be capable of making selectedhigh-precision astrometric measurements about 3 orders of magnitude moreaccurate than the Hipparcos survey. We present results from a detailedset of end-to-end numerical simulations of SIM narrow-angle astrometricmeasurements and data analysis to illustrate the enormous potential thatSIM has for the discovery and characterization of planets outside thesolar system. Utilizing a template observing scenario, we quantify SIMsensitivity to single planets orbiting single normal nearby stars as afunction of measurement errors and properties of the planet: SIM willdetect over 95% of the planets with periods between a few days and the 5yr nominal mission lifetime that produces astrometric signatures ~2.2times larger than the single-measurement accuracy. We provide accuracyestimates of full-orbit reconstruction and planet mass determination: attwice the discovery limit, orbital elements will be determined with atypical accuracy of 20%-30% the astrometric signature must be ~10 and~15 times the minimum signal required for detection to derive mass andinclination-angle estimates accurate to 10%. We quantify the impact ofdifferent observing strategies on the boundaries for secure detectionand accurate orbit estimation: the results scale with the square root ofboth the number of observations and the number of reference stars. Weinvestigate SIM discovery space, to gauge the instrument ability indetecting very low mass planets: around the nearest stars, SIM will findplanets as small as Earth, if they are present. Some of these might beorbiting inside the parent star's habitable zone. Extrasolar planetsfigure prominently among SIM scientific goals: our results reaffirm theimportance of high-precision astrometric measurements as a uniquecomplement to spectroscopic surveys based on radial velocity. Forexample, establishing the existence of rocky, perhaps habitable planetswould constitute both a fundamental test of theoretical models andprogress toward the understanding of formation and evolution processesof planetary systems. Such discoveries would also provide theTerrestrial Planet Finder with prime targets to investigate with directspectroscopy in terms of the potential for life.
| 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.
| 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.
| A Near-Infrared, Wide-Field, Proper-Motion Search for Brown Dwarfs
A common proper-motion survey of M dwarf stars within 8 pc of the Sunreveals no new stellar or brown dwarf companions at wide separations(~100-1400 AU). This survey tests whether the brown dwarf ``desert''extends to large separations around M dwarf stars and further exploresthe census of the solar neighborhood. The sample includes 66 stars northof -30° and within 8 pc of the Sun. Existing first-epoch images arecompared with new J-band images of the same fields an average of 7 yrlater to reveal proper-motion companions within a ~4' radius of theprimary star. No new companions are detected to a J-band limitingmagnitude of ~16.5, corresponding to a companion mass of ~40 Jupitermasses for an assumed age of 5 Gyr at the mean distance of the objectsin the survey, 5.8 pc.
| The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy
Broadband optical spectra are presented for 34 known and candidatenearby stars in the southern sky. Spectral types are determined using anew method that compares the entire spectrum with spectra of more than100 standard stars. We estimate distances to 13 candidate nearby starsusing our spectra and new or published photometry. Six of these starsare probably within 25 pc, and two are likely to be within the ResearchConsortium on Nearby Stars (RECONS) horizon of 10 pc.
| Toward Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
We present 0.6-2.5 μm, R>~400 spectra of 27 cool, low-luminositystars and substellar objects. Based on these and previously publishedspectra, we develop a preliminary spectral classification system for Land T dwarfs. For late L and T types the classification system is basedentirely on four spectral indices in the 1-2.5 μm interval. Two ofthese indices are derived from water absorption bands at 1.15 and 1.4μm, the latter of which shows a smooth increase in depth through theL and T sequences and can be used to classify both spectral types. Theother two indices make use of methane absorption features in the H and Kbands, with the K-band index also applicable to mid-to-late L dwarfs.Continuum indices shortward of 1 μm used by previous authors toclassify L dwarfs are found to be useful only through mid-L subclasses.We employ the 1.5 μm water index and the 2.2 μm methane index tocomplete the L classification through L9.5 and to link the new systemwith a modified version of the 2MASS ``color-d'' index. By correlatingthe depths of the methane and water absorption features, we establish aT spectral sequence from T0 to T8, based on all four indices, that is asmooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfsas L9 and L9.5 and identify one SDSS object as L9. In the proposedsystem methane absorption appears in the K band approximately at L8, twosubclasses earlier than its appearance in the H band. The L and Tspectral classes are distinguished by the absence and presence,respectively, of H-band methane absorption.
| Statistics of spectroscopic sub-systems in visual multiple stars
A large sample of visual multiples of spectral types F5-M has beensurveyed for the presence of spectroscopic sub-systems. Some 4200 radialvelocities of 574 components were measured in 1994-2000 with thecorrelation radial velocity meter. A total of 46 new spectroscopicorbits were computed for this sample. Physical relations are establishedfor most of the visual systems and several optical components areidentified as well. The period distribution of sub-systems has a maximumat periods from 2 to 7 days, likely explained by a combination of tidaldissipation with triple-star dynamics. The fraction of spectroscopicsub-systems among the dwarf components of close visual binaries withknown orbits is similar to that of field dwarfs, from 11% to 18% percomponent. Sub-systems are more frequent among the components of widevisual binaries and among wide tertiary components to the known visualor spectroscopic binaries - 20% and 30%, respectively. In triple systemswith both outer (visual) and inner (spectroscopic) orbits known, we findan anti-correlation between the periods of inner sub-systems and theeccentricities of outer orbits which must be related to dynamicalstability constraints. Tables 1, 2, and 6 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/382/118
| Kinematics of Hipparcos Visual Binaries. II. Stars with Ground-Based Orbital Solutions
This paper continues kinematical investigations of the Hipparcos visualbinaries with known orbits. A sample, consisting of 804 binary systemswith orbital elements determined from ground-based observations, isselected. The mean relative error of their parallaxes is about 12% andthe mean relative error of proper motions is about 4%. However, even 41%of the sample stars lack radial velocity measurements. The computedGalactic velocity components and other kinematical parameters are usedto divide the stars with known radial velocities into kinematical agegroups. The majority (92%) of binaries from the sample are thin diskstars, 7.6% have thick disk kinematics and only two binaries have halokinematics. Among them, the long-period variable Mira Ceti has a verydiscordant {Hipparcos} and ground-based parallax values. From the wholesample, 60 stars are ascribed to the thick disk and halo population.There is an urgent need to increase the number of the identified halobinaries with known orbits and substantially improve the situation withradial velocity data for stars with known orbits.
| 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.
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