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 twelve nebulae have parallax errors less than 20 percent. The parallax for PHL932 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 estimated from spectra of the central stars. Proper motions and tangential velocities are presented. No astrometric perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A

Spin-orbit coupling control of anisotropy, ground state and frustration in 5d2 Sr2MgOsO6

The influence of spin-orbit coupling (SOC) on the physical properties of the 5d2 system Sr2MgOsO6 is probed via a combination of magnetometry, specific heat measurements, elastic and inelastic neutron scattering, and density functional theory calculations. Although a significant degree of frustration is expected, we find that Sr2MgOsO6 orders in a type I antiferromagnetic structure at the remarkably high temperature of 108 K. The measurements presented allow for the first accurate quantification of the size of the magnetic moment in a 5d2 system of 0.60(2) muB - a significantly reduced moment from the expected value for such a system. Furthermore, significant anisotropy is identified via a spin excitation gap, and we confirm by first principles calculations that SOC not only provides the magnetocrystalline anisotropy, but also plays a crucial role in determining both the ground state magnetic order and the size of the local moment in this compound. Through comparison to Sr2ScOsO6, it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5d2 systems relative to their 5d3 counterparts, providing an explanation of the high TN found in Sr2MgOsO6.Comment: Submitted to Scientific Report

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 7,435 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, 0.2 arcsecond astrometric accuracy at J2000, 0.3 magnitude photometric accuracy in up to five colors, and 85% accuracy for distinguishing stars from non-stellar objects. A brief discussion of various issues is given here, but the actual data are available from http://www.nofs.navy.mil and other sites.Comment: Accepted by Astronomical Journa

Trigonometric Parallaxes for Two Late-Type Subdwarfs: LSR1425+71 (sdM8.0) and the Binary LSR1610-00 (sd?M6pec)

Trigonometric parallax astrometry and BVI photometry are presented for two late-type subdwarf candidates, LSR1425+71 (sdM8.0) and LSR1610-00 (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 LSR1610-00 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 semi-major axis of 0.28+/-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 for LSR1610-00 at epoch 2006.179, when coupled with the observation of -95+/-1 km/s at epoch 2005.167 by Reiners & Basri, indicates a systemic radial velocity of -101+/-1 km/s for the LSR1610-00AB pair. The galactic velocity components for LSR1425+71 and LSR1610-00AB -- (U,V,W)=(84+/-6, -202+/-13, 66+/-14) km/s and (U,V,W)=(36+/-2, -232+/-2, -61+/-2) km/s, 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. LSR1425+71 has a luminosity and colors consistent with its metal-poor subdwarf spectral classification, while LSR1610-00 has a luminosity and most colors indicative of being only mildly metal-poor, plus a uniquely red B-V color. The companion to LSR1610-00 must be a low-mass, substellar brown dwarf. We speculate on the paradoxical nature of LSR1610-00 and possible sources of its peculiarities.Comment: Accepted for ApJ. 37 pages, including 8 figure

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) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten 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. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and 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 T_eff ~ 1360 K.Comment: 48 pages, including 7 figures and 6 tables. Accepted for A