Richard L. Walker

The USNO-B Catalog

USNO-B is an all-sky catalog that presents positions, proper motions, magnitudes in various optical passbands, and star/galaxy estimators for 1,042,618,261 objects derived from 3,643,201,733 separate observations. The data were obtained from scans of 7435 Schmidt plates taken for the various sky surveys during the last 50 years. USNO-B1.0 is believed to provide all-sky coverage, completeness down to V = 21, 02 astrometric accuracy at J2000, 0.3 mag photometric accuracy in up to five colors, and 85% accuracy for distinguishing stars from nonstellar objects. A brief discussion of various issues is given here, but the actual data are available from the US Naval Observatory Web site and others.

Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs

Trigonometric parallax determinations are presented for 28 late-type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz*) and/or near-IR wavelengths (JHK) is presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including 10 L and two T dwarfs with parallaxes established by association with bright, usually Hipparcos primaries, this material forms the basis for studying various color-color and color?absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late M and L dwarfs. MJ becomes monotonically fainter with I-J color and with spectral type through late L dwarfs, then brightens for early T dwarfs. The combination of z*JK colors alone can be used to classify late M, early L, and T dwarfs accurately, as well as to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have Teff ~ 1360 K.

Trigonometric Parallaxes of Central Stars of Planetary Nebulae

Trigonometric parallaxes of 16 nearby planetary nebulae are presented, including reduced errors for seven objects with previous initial results and results for six new objects. The median error in the parallax is 0.42 mas, and 12 nebulae have parallax errors of less than 20%. The parallax for PHL 932 is found here to be smaller than was measured by Hipparcos, and this peculiar object is discussed. Comparisons are made with other distance estimates. The distances determined from these parallaxes tend to be intermediate between some short distance estimates and other long estimates; they are somewhat smaller than those estimated from spectra of the central stars. Proper motions and tangential velocities are presented. No astrometric perturbations from unresolved close companions are detected.

U. S. Naval Observatory photographic parallaxes. List IX

Trigonometric parallaxes, relative proper motions, and photometry are presented for 122 stars in 111 systems. Of these stars, 70 are brighter than V=10.0

Upgrades to the Flagstaff Astrometric Scanning Transit Telescope: A Fully Automated Telescope for Astrometry

The Flagstaff Astrometric Scanning Transit Telescope (FASTT) is a fully automated telescope that takes about 41,000 CCD frames of data a year for various research projects. All aspects of the telescopes operation have been automated (e.g., target selection, observing, reduction of data, and collation of results), and manpower needs are now under one person per year, mostly involved with routine maintenance and the dissemination of data. This paper describes the FASTT instrumental system, methods used with its automated operation, and the various FASTT research projects. Among the projects, astrometry is provided in support of various spacecraft missions, to predict occultation events, calculate dynamical masses for selective asteroids, and improve the ephemerides for thousands of asteroids, the planets Jupiter to Pluto, and 17 satellites of Jupiter through Neptune. Although most of the FASTT observing program involves the solar system, FASTT stellar astrometry was used to set up a number of astrometric calibration regions along the celestial equator, verify the Hipparcos link to the International Celestial Reference Frame, determine accurate positions for a large sample of radio stars, and investigate systematic errors in the FK5 star catalog. Furthermore, the FASTT produces accurate magnitudes that are being used to investigate the shapes of thousands of asteroids. By the end of year 2003, the FASTT will have produced over 190,000 positions of solar system objects in a program to provide a very large and homogeneous database for each object that will extend over many years and include positions accurate to ±47 to ±300 mas, depending on the magnitude of each observed object (3.5 < V < 17.5). Moreover, extensive efforts have been undertaken to improve the systematic accuracy of FASTT equatorial positions by applying corrections in the reductions for differential color refraction, distortions in the focal plane, and correcting for a positional error that is dependent on magnitude. The systematic accuracy of FASTT observations is now about ±20 mas in both right ascension and declination. FASTT data have contributed very significantly to recent successful spacecraft missions and to a dramatic improvement in the predictions made for occultation events.

Astrometry and Photometry for Two Dwarf Carbon Stars

Preliminary trigonometric parallaxes and BVI photometry are presented for two dwarf carbon stars, LP 765-18 (=LHS 1075) and LP 328-57 (=CLS 96). The data are combined with the literature values for a third dwarf carbon star, G77-61 (=LHS 1555). All three stars have very similar luminosities (9.6 < MV < 10.0) and very similar broadband colors across the entire visual-to-near-IR (BVIJHK) wavelength range. Their visual (BVI) colors differ from all known red dwarfs, subdwarfs, and white dwarfs. In the MV versus V-I color-magnitude diagram, they are approximately 2 mag subluminous compared to normal disk dwarfs with solar-like metallicities, occupying a region also populated by O-rich subdwarfs with -1.5 < [M/H] < -1.0. The kinematics indicate that they are members of the Galactic spheroid population. The subluminosity of all three stars is due to an as yet unknown combination of (undoubtedly low) metallicity, possibly enhanced helium abundance, and unusual line blanketing in the bandpasses considered. The properties of the stars are compared with models for the production of dwarf carbon stars.

Astrometry and Photometry for Brown Dwarf Candidates in the Hyades

Updated VIJHK photometry is presented for 12 stars from Leggett & Hawkins, along with trigonometric parallaxes and proper motions for three of the stars, that are candidate brown dwarfs in the Hyades. The three stars with parallaxes are found to lie well beyond the Hyades cluster and to fall 3–4 mag above the main sequence, indicating that they are pre–main-sequence objects. These results illustrate the problem that contaminating field stars pose for attempts to identify rare objects such as brown dwarfs as physical members of clusters.

PARALLAXES AND PROPER MOTIONS OF QSOs: A TEST OF ASTROMETRIC PRECISION AND ACCURACY

Optical astrometry of 12 fields containing quasi-stellar objects (QSOs) is presented. The targets are radio sources in the International Celestial Reference Frame with accurate radio positions that also have optical counterparts. The data are used to test several quantities: the internal precision of the relative optical astrometry, the relative parallaxes and proper motions, the procedures to correct from relative to absolute parallax and proper motion, the accuracy of the absolute parallaxes and proper motions, and the stability of the optical photocenters for these optically variable QSOs. For these 12 fields, the mean error in absolute parallax is 0.38 mas and the mean error in each coordinate of absolute proper motion is 1.1 mas yr−1. The results yield a mean absolute parallax of −0.03 ± 0.11 mas. For 11 targets, we find no significant systematic motions of the photocenters at the level of 1–2 mas over the 10 years of this study; for one BL Lac object, we find a possible motion of 4 mas correlated with its brightness.

CCD Parallaxes for 309 Late-type Dwarfs and Subdwarfs

New, updated, and/or revised CCD parallaxes determined with the Strand Astrometric Reflector at the Naval Observatory Flagstaff Station (NOFS) are presented. Included are results for 309 late-type dwarf and subdwarf stars observed over the 30+ years that the program operated. For 124 of the stars, parallax determinations from other investigators have already appeared in the literature and we compare the different results. Also included here is new or updated

Parallaxes of Brown Dwarfs at USNO

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