Blaise Canzian

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.

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs, and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices from Geballe et al. that measure the strengths of the near infrared water and methane bands. We identify nine new L8?9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954-0935066 as T9, the latest and coolest dwarf found to date. We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L?T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the Geballe et al. spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions, and thicknesses of the clouds. We have identified a small group of late-L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H2O, and K I absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late-T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages greater than108 yr and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15?75MJupiter. One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ~ 4.2?4.5 and mass ~ 10?15MJupiter.

Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the US Naval Observatory (USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based on observations obtained between 2000 September and 2002 November; about half of the objects have an observational time baseline of Δt = 1.3 yr and half Δt = 2.0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the European Southern Observatory 3.5 m New Technology Telescope parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing Two Micron All Sky Survey and/or Sloan Digital Sky Survey photometry for reference-frame stars. We combine USNO infrared and optical parallaxes with the best available California Institute of Technology (CIT) system photometry to determine MJ, MH, and MK values for 37 L dwarfs between spectral types L0 and L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects that are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early to mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity-excess region indicate that it cannot be due entirely to selection effects. The T dwarf sequence is extended to MJ ≈ 16.9 by 2MASS J041519-0935, which, at d = 5.74 pc, is found to be the least luminous [log(L/L⊙) = -5.58] and coldest (Teff ≈ 760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of low-velocity (Vtan < 20 km s-1) T dwarfs. This is consistent with the T dwarfs in this study being generally older than the L dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.

An Improved Proper-Motion Catalog Combining USNO-B and the Sloan Digital Sky Survey

An improved proper-motion catalog is presented, combining the USNO-B and Sloan Digital Sky Survey (SDSS) catalogs in the area of sky covered by SDSS Data Release 1 (DR1; 2099 deg2). USNO-B positions are recalibrated using SDSS galaxies, and proper motions are recomputed including both the USNO-B and SDSS positions. Statistical errors in the USNO-B proper motions are decreased by roughly 20%?30%, systematic errors are greatly reduced, and the proper motions are placed on an absolute reference frame. Requiring a match to an SDSS object removes the large number of false high proper motion objects in USNO-B. The resultant catalog is 90% complete to g < 19.7, with statistical errors in the component proper motions of roughly 3?3.5 mas yr-1, substantially smaller systematic errors, and a contamination rate of less than 0.5%. A number of studies are currently underway using proper motions from this catalog. The catalog is available via ftp.

A new way to locate corotation resonances in spiral galaxies

Kinematical information over the entire disk of a spiral galaxy can be used in a new way to locate the position of the corotation resonance. The spiral residual velocity field has a global appearance that is distinctly different inside and outside the corotation resonance radius. Inside the corotation radius, the spiral velocity field shows a single spiral feature (that is, a single approaching-receding spiral arm pair). Outside the corotation radius, there are three spiral features (approaching-receding arm pairs). The corotation radius is located where this morphological change occurs. The effect is a consequence of geometric phase. It should apply generally, regardless of the functional form of the velocity perturbation due to a spinal arm, if the spiral structure is wave-based

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.

ERRATUM: “AN IMPROVED PROPER-MOTION CATALOG COMBINING USNO-B AND THE SLOAN DIGITAL SKY SURVEY” (2004, AJ, 127, 3034)

The authors have discovered an error in the calculation of the proper motions in right ascension (the proper motions in declination were not affected by the error, and are thus correct). The error involves the use of an incorrect sign when interpreting the residuals in right ascension in the USNO-B catalog. The results of the error are as follows. For users working with samples of stars chosen to have significantly detected proper motions, the impact of the error should be small. However, for users working closer to the statistical error limit, or averaging over many stars to determine small mean motions, the additional errors are clearly significant. A corrected version of the catalog has been generated. The corrected motions will be available in Data Release 7 of the Sloan Digital Sky Survey Catalog Archive Server, due out in October 2008. Until then, corrected motions may be obtained by contacting the first author.

Spiral Density Wave Theory, Corotation Resonance, and the Velocity Field of NGC 4321

We extend the exposition of a new kinematic method (exploiting geometric phase) for determining the corotation resonance radius of a grand-design spiral. We present an explicit formulation, in terms of first-order linear spiral density wave theory, for fitting to the velocity field of a grand-design spiral galaxy. We test the new formulation on the velocity field of the ionized gas in the grand-design spiral NGC 4321 in the Virgo Cluster using Fabry-Perot Hα data. The kinematic method as currently implemented is useful specifically for determining an upper bound for the corotation resonance radius. We illustrate the kinematic evidence for the corotation resonance in the disk of NGC 4321.

Meeting the Cool Neighbors. IV. 2MASS 1835+32, a Newly Discovered M8.5 Dwarf within 6 Parsecs of the Sun

We present observations of 2MASSI J1835379+325954, a previously unrecognized late-type M dwarf within 6 pc of the Sun. Identified based analysis of the Two Micron All Sky Survey optical spectroscopy and photometry indicate a spectral type of M8.5. The star has a proper motion of 0759 yr-1 and is clearly visible on both POSS I and POSS II photographic plate material, but it may have escaped detection in previous surveys through its proximity to the Galactic plane. We discuss potential implications for the completeness of the local stellar census.