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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.
| Discovery of a nearby M9 dwarf
We report the discovery of a new M9.0 dwarf at only 8.2 pc, which weidentified in our search for nearby ultracool dwarf (I-J>= 3.0, laterthan M8.0) in the DENIS data base. We measure a very high proper motionof 2.5arcsecyr-1. The PC3 index measured from itslow-resolution spectrum gives a spectrophotometric distance of 8.2 pc.This makes it the third closest M9.0 dwarf.
| New Debris Disks around Nearby Main-Sequence Stars: Impact on the Direct Detection of Planets
Using the MIPS instrument on Spitzer, we have searched for infraredexcesses around a sample of 82 stars, mostly F, G, and K main-sequencefield stars, along with a small number of nearby M stars. These starswere selected for their suitability for future observations by a varietyof planet-finding techniques. These observations provide information onthe asteroidal and cometary material orbiting these stars, data that canbe correlated with any planets that may eventually be found. We havefound significant excess 70 μm emission toward 12 stars. Combinedwith an earlier study, we find an overall 70 μm excess detection rateof 13%+/-3% for mature cool stars. Unlike the trend for planets to befound preferentially toward stars with high metallicity, the incidenceof debris disks is uncorrelated with metallicity. By newly identifyingfour of these stars as having weak 24 μm excesses (fluxes ~10% abovethe stellar photosphere), we confirm a trend found in earlier studieswherein a weak 24 μm excess is associated with a strong 70 μmexcess. Interestingly, we find no evidence for debris disks around 23stars cooler than K1, a result that is bolstered by a lack of excessaround any of the 38 K1-M6 stars in two companion surveys. Onemotivation for this study is the fact that strong zodiacal emission canmake it hard or impossible to detect planets directly with futureobservatories such as the Terrestrial Planet Finder (TPF). Theobservations reported here exclude a few stars with very high levels ofemission, >1000 times the emission of our zodiacal cloud, from directplanet searches. For the remainder of the sample, we set relatively highlimits on dust emission from asteroid belt counterparts.
| A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs
We present a low-resolution (R≡λ/Δλ~90),5.5-38 μm spectral sequence of a sample of M, L, and T dwarfsobtained with the Infrared Spectrograph (IRS) on board the Spitzer SpaceTelescope. The spectra exhibit prominent absorption bands ofH2O at 6.27 μm, CH4 at 7.65 μm, andNH3 at 10.5 μm and are relatively featureless atλ>~15 μm. Three spectral indices that measure the strengthsof these bands are presented; H2O absorption features arepresent throughout the MLT sequence, while the CH4 andNH3 bands first appear at roughly the L/T transition.Although the spectra are, in general, qualitatively well matched bysynthetic spectra that include the formation of spatially homogeneoussilicate and iron condensate clouds, the spectra of the mid-type Ldwarfs show an unexpected flattening from roughly 9 to 11 μm. Wehypothesize that this may be a result of a population of small silicategrains that are not predicted in the cloud models. The spectrum of thepeculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5 to 7.5 μmrelative to typical T6 dwarfs and may be a consequence of its mildlymetal-poor/high surface gravity atmosphere. Finally, we computebolometric luminosities of a subsample of the M, L, and T dwarfs bycombining the IRS spectra with previously published 0.6-4.1 μmspectra and find good agreement with the values of Golimowski et al.,who use L'- and M'-band photometry to account forthe flux emitted at λ>2.5 μm.
| 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.
| 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 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.
| The HARPS search for southern extra-solar planets. VI. A Neptune-mass planet around the nearby M dwarf Gl 581
We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M =0.31 Mȯ), based on precise Doppler measurements with the HARPSspectrograph at La Silla Observatory. The radial velocities reveal acircular orbit of period P = 5.366 days and semi-amplitude K1= 13.2 m s-1. The resulting minimum mass of the planet(M2 sin i) is only 0.052 MJup = 0.97MNep = 16.6 MEarth making Gl 581b one of thelightest extra-solar planet known to date. The Gl 581 planetary systemis only the third centered on an M dwarf, joining the Gl 876three-planet system and the lone planet around Gl 436. Its discoveryreinforces the emerging tendency of such planets to be of low mass, andfound at short orbital periods. The statistical properties of theplanets orbiting M dwarfs do not seem to match a simple mass scaling oftheir counterparts around solar-type stars.
| 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.
| 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.
| 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.
| 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).
| L' and M' Photometry of Ultracool Dwarfs
We have compiled L' (3.4-4.1 μm) and M' (4.6-4.8 μm) photometry of63 single and binary M, L, and T dwarfs obtained at the United KingdomInfrared Telescope using the Mauna Kea Observatory filter set. Thiscompilation includes new L' measurements of eight L dwarfs and 13 Tdwarfs and new M' measurements of seven L dwarfs, five T dwarfs, and theM1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6,respectively, the numbers of ultracool dwarfs (Teff<~2400K) for which L' and M' measurements have been reported. We computeLbol, BCK, and Teff for 42 dwarfs whoseflux-calibrated JHK spectra, L' photometry, and trigonometric parallaxesare available, and we estimate these quantities for nine other dwarfswhose parallaxes and flux-calibrated spectra have been obtained.BCK is a well-behaved function of near-infrared spectral typewith a dispersion of ~0.1 mag for types M6-T5 it is significantly morescattered for types T5-T9. Teff declines steeply andmonotonically for types M6-L7 and T4-T9, but it is nearly constant at~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constantTeff is evidenced by nearly unchanging values of L'-M'between types L6 and T3. It also supports recent models that attributethe changing near-infrared luminosities and spectral features across theL-T transition to the rapid migration, disruption, and/or thinning ofcondensate clouds over a narrow range of Teff. The L' and M'luminosities of early-T dwarfs do not exhibit the pronounced humps orinflections previously noted in the I through K bands, but insufficientdata exist for types L6-T5 to assert that ML' andMM' are strictly monotonic within this range of types. Wecompare the observed K, L', and M' luminosities of L and T dwarfs in oursample with those predicted by precipitating-cloud and cloud-free modelsfor varying surface gravities and sedimentation efficiencies. The modelsindicate that the L3-T4.5 dwarfs generally have higher gravities(logg=5.0-5.5) than the T6-T9 dwarfs (logg=4.5-5.0). The predicted M'luminosities of late-T dwarfs are 1.5-2.5 times larger than thosederived empirically for the late-T dwarfs in our sample. Thisdiscrepancy is attributed to absorption at 4.5-4.9 μm by CO, which isnot expected under the condition of thermochemical equilibrium assumedin the models. Our photometry and bolometric calculations indicate thatthe L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57-041403.5 areprobable binary systems. We computelog(Lbol/Lsolar)=-5.73+/-0.05 andTeff=600-750 K for the T9 dwarf 2MASSI J0415195-093506, whichsupplants Gl 570D as the least luminous and coolest brown dwarfpresently known.
| 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
| 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
| 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.
| Astrophysics of Young Star Binaries
This paper describes our study of the astrophysics of individualcomponents in close pre-main-sequence binaries. We observed both starsin 17 systems, located in four nearby star-forming regions, usinglow-resolution (R=760) infrared spectroscopy and photometry. For 29components we detected photospheric absorption lines and were able todetermine spectral type, extinction, K-band excess, and luminosity. Theother five objects displayed featureless or pure emission line spectra.In ~50% of the systems, the extinction and K-band excess of the primarystars dominate those of the secondaries. Masses and ages were determinedfor these 29 objects by placing them on the H-R diagram, overlaid withtheoretical pre-main-sequence tracks. Most of the binaries appear to becoeval. The ages span 5×105 to 1×107yr. The derived masses range from the substellar, 0.06Msolar, to 2.5 Msolar, and the mass ratios fromM2/M1=0.04 to 1.0. Fourteen stars show evidence ofcircumstellar disks. The K-band excess is well correlated with the K-Lcolor for stars with circumstellar material.
| 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.
| Kinematics and Luminosity Function of Dwarf Populations in Three Areas of the Calán-ESO Proper-Motion Catalog
We have completed the analysis of a sample of 112 stars in the solarneighborhood taken from the statistically complete Calán-ESOcatalog. From medium-resolution spectroscopy we classified every star,both by direct comparison with spectroscopic standards and by usingspectral indices. The latter also allowed discrimination betweenmain-sequence (MS) dwarfs and subdwarfs. Several useful spectral typeversus color relations were obtained from CCD photometry of the sample(observed magnitudes were dereddened). Distances and absolute magnitudeswere determined. From measured radial velocities and proper motions, wedetermined the kinematics [Galactocentric velocity components (U,V,W)],which allowed the classification of each star as belonging to the diskor halo population. Luminosity functions (LFs) were then obtained usingthe 1/Vmax method for the different populations. The maximumin the LF for MS dwarfs was found to be near MV=12.5+/-0.5,in accord with previous determinations. On the other hand, we found anincrease in the LF of the subdwarf at its faint end, which is in strongdisagreement with determinations by other authors. A mass density of MSdwarfs of ~0.047+/-0.021 Msolar pc-3 was derived,while the contribution of subdwarfs was found to be negligible.Based on observations obtained with the VLT (ESO), project 67.D-0224A.
| 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.
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
| 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.
| Component Masses of the Young Spectroscopic Binary UZ Tau E
We report estimates of the masses of the component stars in thepre-main-sequence spectroscopic binary UZ Tau E. These results come fromthe combination of our measurements of the mass ratio,M2/M1=0.28+/-0.01, obtained using high-resolutionH-band spectroscopy, with the total mass of the system,(1.31+/-0.08)(D/140 pc) Msolar, derived from millimeterobservations of the circumbinary disk (Simon, Dutrey, & Guilloteau).The masses of the primary and secondary are (1.016+/-0.065)(D/140 pc)Msolar and (0.294+/-0.027)(D/140 pc) Msolar,respectively. Using the orbital parameters determined from our sixepochs of observation, we find that the inclination of the binary orbit,59.8d+/-4.4d, is consistent with that determined for the circumbinarydisk from the millimeter observations, indicating that the disk andbinary orbits are probably coplanar.
| The Smallest Mass Ratio Young Star Spectroscopic Binaries
Using high-resolution near-infrared spectroscopy with the KeckTelescope, we have detected the radial velocity signatures of the coolsecondary components in four optically identified pre-main-sequence,single-lined spectroscopic binaries. All are weak-lined T Tauri starswith well-defined center-of-mass velocities. The mass ratio for oneyoung binary, NTTS 160905-1859, isM2/M1=0.18+/-0.01, the smallest yet measureddynamically for a pre-main-sequence spectroscopic binary. These newresults demonstrate the power of infrared spectroscopy for the dynamicalidentification of cool secondaries. Visible-light spectroscopy, to date,has not revealed any pre-main-sequence secondary stars with masses<0.5 Msolar, while two of the young systems reported hereare in that range. We compare our targets with a compilation of thepublished young, double-lined spectroscopic binaries and discuss ourunique contribution to this sample.
| 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.
| Meeting the Cool Neighbors. III. Spectroscopy of Northern NLTT Stars
We present initial results of an all-sky search for late-type dwarfswithin 20 pc of the Sun using the New Luyten Two-Tenths (NLTT) catalogcross-referenced with the Two Micron All Sky Survey (2MASS) database.The results were obtained with low-resolution optical spectroscopicfollow-up of candidate nearby stars as a preliminary test of ourmethodology. MJ, derived using spectral indices, and 2MASS Jare used to estimate distances. Out of the 70 objects observed, 28 areidentified as previously unrecognized objects within 25 pc of the Sun,and up to 19 of these are within 20 pc. One, LP 647-13, is an M9-typedwarf at 10.5 pc, making it one of the four closest M9 dwarfs currentlyknown. We also discuss the chromospheric activity of the observeddwarfs.
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