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Gravity Indicators in the Near-Infrared Spectra of Brown Dwarfs
We investigate the sensitivity to temperature and gravity of the strongabsorption features in the J- and K-band spectra of substellar objects.We compare the spectra of giants and young M dwarfs (of low gravity) tofield M and L dwarfs (of high gravity) and to model spectra from theLyon group. We find that low-resolution spectra of M4-M9 stars and youngbrown dwarfs at R~350 and signal-to-noise ratios greater than 70 candetermine the spectral type to a precision of +/-1 subtype using theH2O and CO bands and can measure the surface gravity to+/-0.5 dex using the atomic lines of K I and Na I. This result pointstoward the development of photometric spectral indices to separatelow-mass members from foreground and background objects in youngclusters and associations. We also emphasize the complexity of theinterpretation of the empirical quantities (e.g., spectral types) interms of the physical variables (e.g., temperature, opacities) in thecool atmospheres of young brown dwarfs.Based on observations obtained with the Multiple Mirror TelescopeObservatory, a joint facility of the University of Arizona and theSmithsonian Institution.
| 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.
| Photoelectric Vilnius Photometry of Hipparcos Turn-Off Region Stars
Seven-color photometry in the Vilnius system and photometricclassification in terms of spectral type, absolute magnitude andmetallicity are presented for 145 Hipparcos stars of the turn-offregion, most of which have parallaxes determined to an accuracy of atleast 15%. The stars selected for the observing program included thoseidentified kinematically as intermediate between the thin disk and halo,plus a number of weak-lined stars discovered previously from objectiveprism surveys. The metallicity distribution we find for a kinematicallydefined sample of possible members of the thick disk has a meanabundance [Fe/H]= --0.3 dex and a dispersion of 0.3 dex. Our data seemto suggest a large age for this intermediate population.
| Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
| Radial-velocity measurements. V - Ground support of the HIPPARCOS satellite observation program
The paper presents data on 1070 radial velocity measurements of starsdistributed in 39 fields measuring 4 deg x 4 deg. The PPO series ofFehrenbach et al. (1987) and Duflot et al. (1990) is continued using theFehrenbach objective prism method.
| Photometry of 50 suspected variable stars
Fifty stars have been chosen as suspected variable stars and analyzedfor variability. A large portion of this sample are stars that areeither proved active chromosphere stars or are candidates for suchactivity. The photometric data base consists of differential Vmeasurements of the Vanderbilt 16 inch (41 cm) automatic photoelectrictelescope and 25 observers at 26 observatories worldwide. Publishedphotometric data have also been utilized, with proper adjustments madeto ensure that all magnitudes are differential. Searches for photometricperiod, amplitudes, and times of minimum light showed 68 percent of thesample to be photometrically variable with periods found for 34. Twostars were deemed norvariable for the period of observation. Conclusivestatements could not be made concerning the photometric variability ofthe 14 remaining stars.
| ICCD speckle observations of binary stars. III - A survey for duplicity among high-velocity stars
An analysis of speckle interferometry data for 182 stars has resulted inthe detection of 10 binaries, four of which are newly resolved systems.After correcting for selection effects, the data are found to becompatible with a total frequency for high-velocity long-period doubleswhich is as large as that for low-velocity stars. Based on spectroscopicparallaxes and visual magnitudes, these binaries are found to be within100 pc of the sun, with eight having linear separations of less than 20AU. Four of the binaries are determined to have periods of less than 20yr.
| Three-dimensional calssification of F-M type halo stars in the Vilnius photometric system
Not Available
| Colors, luminosities, and motions of the nearer G-type stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1964AJ.....69..570E&db_key=AST
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