Stephen E. Levine

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.

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.

THE WHITE DWARF LUMINOSITY FUNCTION FROM SLOAN DIGITAL SKY SURVEY IMAGING DATA

A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify completeness and contamination of the sample; kinematic models are used to understand and correct for velocity-dependent selection biases. A luminosity function is constructed covering the range 7 < Mbol < 16, and its sensitivity to various assumptions and selection limits is discussed. The white dwarf luminosity function based on 6000 stars is remarkably smooth, and rises nearly monotonically to Mbol = 15.3. It then drops abruptly, although the small number of low-luminosity stars in the sample and their unknown atmospheric composition prevent quantitative conclusions about this decline. Stars are identified that may have high tangential velocities, and a preliminary luminosity function is constructed for them.

The recent expansion of Pluto's atmosphere.

Stellar occultations—the passing of a relatively nearby body in front of a background star—can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Plutos atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of ∼1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Plutos atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Plutos atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

Cool White Dwarfs in the Sloan Digital Sky Survey

A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity, main-sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7 m telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the reduced proper motion diagram and that the contamination by subdwarfs is small near the cool white dwarf locus. This enables large, statistically complete samples of white dwarfs, particularly the poorly understood cool white dwarfs, to be created from the SDSS imaging survey, with important implications for white dwarf luminosity function studies. SDSS photometry for our sample of cool white dwarfs is compared to current white dwarf models.

Size and albedo of Kuiper belt object 55636 from a stellar occultation

The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 TX300) is a member of the water-ice-rich Haumea KBO collisional family. The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago. Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 ut. We find that it has a mean radius of 143 ± 5 km (assuming a circular solution). Allowing for possible elliptical shapes, we find a geometric albedo of in the V photometric band, which establishes that KBO 55636 is smaller than previously thought and that, like its parent body, it is highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales.

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.

Structure of the Uranian rings. II - Ring orbits and widths

We have used the square-well model of J. L. Elliot, R. G. French, K. J. Meech, and J. H. Elias (1984, Astron. J. 89, 1587–1603) to determine the midtimes, widths, and optical depths of all available Uranus ring occultation observations from 1977 to 1983. We have fitted an improved kinematical model for the nine known rings, from which we find: (i) J2 = (3.3461 ± 0.0030) × 10−3, J4 = (−3.21 ± 0.37) × 10−5, pole of the equatorial plane (1950.0) α = 5h06m25.6s ± 4.6s, δ =+15°01′56″ ± 2′09″; typical uncertainties in ring orbital elements are as follows: σ (semimajor axis) ⋍ 5 km, σ (eccenticity) ⋍2×10−5, σ (inclination) ⋍0.005°; (ii) typical post-fit residuals of 0.2–0.06 km in the ring plane radius for rings 6, 5, 4, α, β, η, and e are comparable to the uncertainties in the midtimes for many data sets; (iii) upper limits to shepherd satellite masses of m < 1018 g (Δ/100km)2 for shepherds near these seven rings, from observed radial perturbations, and m < 4 × 1017 g (Δ/100 km)2 from anomalous ring precession rates (where Δ is the distance between the ring and the shepherd satellite); (iv) the γ and δ rings have rms radial perturbations ⋍ 3 km, well above the uncertainties in the occultation profile midtimes; (v) these perturbations appear to vary slowly with true anomaly, raising the possibility that they may be associated with Lindblad resonances. From the ring width determinations, we find: (vi) all nine rings have significant width perturbations, well above the uncertainties in the width measurements; (vii) typical width perturbations are 0.5–2.0 km, much larger than the orbit radius perturbations for all but the γ and δ rings; (viii) newly determined width-longitude relations, including apsidal twists, and dynamical models yield ring masses of (4.2 ± 0.9) × 1016, (3.8 ± 0.6) × 1016, and (6.1 ± 0.1) × 1018 g, surface densities of 2.0 ± 0.4, 1.5 ± 0.2, and 32.9 ± 0.6 cm−2, and dispersion velocities of 0.15 ± 0.03, 0.17 ± 0.02, and (0.17 ± 0.25) cm sec−1 for the α, β, and e rings, respectively, and a typical ring thickness of ⋍8 m, consistent with the results of P.D. Nicholson and K. Matthews (1983, Bull. Amer. Astron. Soc. 15, 816); (ix) there are significant deviations (>1 km) from the best-fitting sinusoidal width-longitude relations for the α, β, and e rings; (x) the narrow eccentric rings 6, 5, and 4 do not follow simple width-longitude relations; (xi) the quasi-circular rings η, γ, and δ show large width variations (greater than a factor of 2), even for ring profiles at the same true anomaly obtained at different times. From optical depth measurements of the rings, we find: (xii) accurate width-optical depth products (“equivalent depths”), based on a square-well model, for profiles of all nine rings; (xiii) the rings are not optical monolayers-there is significant particle shadowing for all nine rings; (xiv) the “equivalent depths” of the γ and δ rings are not conserved, probably indicative of unresolved radial structure within the rings.

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.