Harry H. Guetter

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.

The Discovery of an Embedded Cluster of High-Mass Stars near SGR 1900+14

Deep I-band imaging to I approximately 26.5 of the soft gamma-ray repeater SGR 1900+14 region has revealed a compact cluster of massive stars located only a few arcseconds from the fading radio source thought to be the location of the soft gamma-ray repeater (SGR). This cluster was previously hidden in the glare of the pair of M5 supergiant stars (whose light was removed by point-spread function subtraction) proposed by Vrba et al. as likely associated with SGR 1900+14. The cluster has at least 13 members within a cluster radius of approximately 0.6 pc based on an estimated distance of 12-15 kpc. It is remarkably similar to a cluster found associated with SGR 1806-20. That similar clusters have now been found at or near the positions of the two best studied SGRs suggests that young neutron stars, which are thought to be responsible for the SGR phenomenon, have their origins in proximate compact clusters of massive stars.

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.

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.

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

TRIGONOMETRIC PARALLAXES FOR TWO LATE-TYPE SUBDWARFS : LSR 1425+71 (sdM8.0) AND THE BINARY LSR 1610-00 (sd?M6pec)

Trigonometric parallax astrometry and BVI photometry are presented for two late-type subdwarf candidates, LSR 1425+71 (sdM8.0) and LSR 1610?00 (sd?M6pec). For the former we measure an absolute parallax of 13.37 ? 0.51 mas yielding -->MV = 15.25 ? 0.09. The astrometry for LSR 1610?00 shows that this object is an astrometric binary with a period of 1.66 ? 0.01 yr. The photocentric orbit is derived from the data; it has a moderate eccentricity ( -->e ? 0.44 ? 0.02) and a semimajor axis of 0.29 ? 0.01 AU based on our measured absolute parallax of 31.02 ? 0.26 mas. Our radial velocity measure of ?108.1 ? 1.6 km s -->?1 for LSR 1610?00 at epoch 2006.179, when coupled with the observation of ?95 ? 1 km s -->?1 at epoch 2005.167 by Reiners & Basri, indicates a systemic radial velocity of ?101 ? 1 km s -->?1 for the LSR 1610?00AB pair. The Galactic velocity components for LSR 1425+71 and LSR 1610?00AB are (U, V, W) = (84 ? 6, ?202 ? 13, 66 ? 14) km s?1 and (U, V, W) = (36 ? 2, ?232 ? 2, ?61 ? 2) km s?1, respectively. For both stars, the velocities are characteristic of halo population kinematics. However, modeling shows that both stars have orbits around the Galaxy with high eccentricity that pass remarkably close to the Galactic center. LSR 1425+71 has a luminosity and colors consistent with its metal-poor subdwarf spectral classification, while LSR 1610?00 has a luminosity and most colors indicative of being only mildly metal-poor, plus a uniquely red -->B ? V color. The companion to LSR 1610?00 must be a low-mass, substellar brown dwarf. We speculate on the paradoxical nature of LSR 1610?00 and possible sources of its peculiarities.

The Optical Afterglow Light Curve of GRB 980519

We present V-, R-, and I-band observations made at the US Naval Observatory, Flagstaff Station, of the afterglow of GRB 980519 on UT 1998 May 20 and 22. These observations are combined with extensive data from the literature, and all are placed on a uniform magnitude system. The resultant R- and I-band light curves are fit by simple power laws with no breaks and indices of αR = 2.30 ± 0.12 and αI = 2.05 ± 0.07. This makes the afterglow of GRB 980519 one of the two steepest afterglows yet observed. The combined B-, V-, R-, and I-band observations are used to estimate the spectral power-law index, β = 1.4 ± 0.3, after correction for reddening. Unfortunately, GRB 980519 occurred at a relatively low Galactic latitude (b ≈ +15) where the Galactic reddening is poorly known and, hence, the actual value of β is poorly constrained. The observed α and range of likely β-values are, however, found to be consistent with simple relativistic blast-wave models. This afterglow and that of GRB 980326 displayed much steeper declines than the other seven well-observed afterglows, which cluster near α ≈ 1.2. GRB 980519 and GRB 980326 did not display burst characteristics in common that might distinguish them from the gamma-ray bursts with more typical light curves.