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The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including 11new discoveries made using the 2 Micron All Sky Survey. These objectsare distinguished from warmer (L-type) brown dwarfs by the presence ofmethane absorption bands in the 1-2.5 μm spectral region. A firstattempt at a near-infrared classification scheme for T dwarfs is made,based on the strengths of CH4 and H2O bands andthe shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8V are defined, and spectral indices useful for classification arepresented. The subclasses appear to follow a decreasing Teffscale, based on the evolution of CH4 and H2O bandsand the properties of L and T dwarfs with known distances. However, wespeculate that this scale is not linear with spectral type for cooldwarfs, due to the settling of dust layers below the photosphere andsubsequent rapid evolution of spectral morphology aroundTeff~1300-1500 K. Similarities in near-infrared colors andcontinuity of spectral features suggest that the gap between the latestL dwarfs and earliest T dwarfs has been nearly bridged. This argument isstrengthened by the possible role of CH4 as a minor absorber,shaping the K-band spectra of the latest L dwarfs. Finally, we discussone peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infraredcolors (J-Ks=-0.89+/-0.24) due to suppression of the 2.1μm peak. The feature is likely caused by enhanced collision-inducedH2 absorption in a high-pressure or low-metallicityphotosphere.
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| The position corrections of 1400 stars observed with PA II in San Juan.
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| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| Miscellaneous spectroscopic notes
Results of slit-spectrograph observations are reported for approximately260 stars. The data presented range from recognition of many new Ap, Am,and other unusual stars to H-alpha observations of early-typesupergiants and Be stars. The material discussed was obtained over thepast 40 years at a number of U.S. observatories and at the DominionAstrophysical Observatory in Victoria, B.C.
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