A double-frequency dwarf nova oscillation

We have detected coherent oscillations (``dwarf nova oscillations'') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared to the many other examples in the literature for two reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to 20% of the total flux over the range 1100 to 2500A. However, most remarkably we find that there are two components present simultaneously. Both have periods close to 18sec (equivalent to 4800 cycles/day) but they are separated by 57.7+/-0.5 cycles/day. The lower frequency component of the pair has a strong second harmonic while its companion, which has about twice its amplitude, does not. The oscillation spectra appear hotter than the mean spectrum and approximately follow the continuum distribution of a black-body with a temperature in the range 30,000 to 50,000K. We tentatively suggest that the weaker non-sinusoidal component could represent the rotation of the white dwarf, although we have been unable to recover any such signal in quiescent data.Comment: 9 pages, 9 figure

A new method for measuring slipperiness of airport runways and other paved surfaces

Aircraft stopping distances on wet runways are accurately predicted by measurements taken with a conventional automobile equipped with diagonal braking system and simple instrumentation for recording stopping distances

How fast do Jupiters grow? Signatures of the snowline and growth rate in the distribution of gas giant planets

We present here observational evidence that the snowline plays a significant role in the formation and evolution of gas giant planets. When considering the population of observed exoplanets, we find a boundary in mass-semimajor axis space that suggests planets are preferentially found beyond the snowline prior to undergoing gap-opening inward migration and associated gas accretion. This is consistent with theoretical models suggesting that sudden changes in opacity -- as would occur at the snowline -- can influence core migration. Furthermore, population synthesis modelling suggests that this boundary implies that gas giant planets accrete ~ 70 % of the inward flowing gas, allowing ~ 30$ % through to the inner disc. This is qualitatively consistent with observations of transition discs suggesting the presence of inner holes, despite there being ongoing gas accretion.Comment: 7 pages, 6 figures, accepted for publication in Monthly Notices of the Royal Astronomical Societ

Extending emission line Doppler tomography ; mapping modulated line flux

Emission line Doppler tomography is a powerful tool that resolves the accretion flow in binaries on micro-arcsecond scales using time-resolved spectroscopy. I present an extension to Doppler tomography that relaxes one of its fundamental axioms and permits the mapping of time-dependent emission sources. Significant variability on the orbital period is a common characteristic of the emission sources that are observed in the accretion flows of cataclysmic variables and X-ray binaries. Modulation Doppler tomography maps sources varying harmonically as a function of the orbital period through the simultaneous reconstruction of three Doppler tomograms. One image describes the average flux distribution like in standard tomography, while the two additional images describe the variable component in terms of its sine and cosine amplitudes. I describe the implementation of such an extension in the form of the maximum entropy based fitting code MODMAP. Test reconstructions of synthetic data illustrate that the technique is robust and well constrained. Artifact free reconstructions of complex emission distributions can be achieved under a wide range of signal to noise levels. An application of the technique is illustrated by mapping the orbital modulations of the asymmetric accretion disc emission in the dwarf nova IP Pegasi.Comment: 8 pages, 4 figures; accepted for publication in MNRA

Friction evaluation of unpaved, gypsum-surface runways at Northrup Strip, White Sands Missile Range, in support of Space Shuttle Orbiter landing and retrieval operations

Friction measurement results obtained on the gypsum surface runways at Northrup Strip, White Sands Missile Range, N. M., using an instrumented tire test vehicle and a diagonal braked vehicle, are presented. These runways were prepared to serve as backup landing and retrieval sites to the primary sites located at Dryden Flight Research Center for shuttle orbiter during initial test flights. Similar friction data obtained on paved and other unpaved surfaces was shown for comparison and to indicate that the friction capability measured on the dry gypsum surface runways is sufficient for operations with the shuttle orbiter and the Boeing 747 aircraft. Based on these ground vehicle friction measurements, estimates of shuttle orbiter and aircraft tire friction performance are presented and discussed. General observations concerning the gypsum surface characteristics are also included and several recommendations are made for improving and maintaining adequate surface friction capabilities prior to the first shuttle orbiter landing

Acoustic and aerodynamic study of a pusher-propeller aircraft model

An aerodynamic and acoustic study was made of a pusher-propeller aircraft model in the NASA-Ames 7 x 10 ft Wind Tunnel. The test section was changed to operate as an open jet. The 591 mm diameter unswept propeller was operated alone and in the wake of three empennages: an I tail, Y tail, and a V tail. The radiated noise and detailed wake properties were measured. Results indicate that the unsteady blade loading caused by the blade interactions with the wake mean velocity distribution had a strong effect on the harmonics of blade passage noise. The blade passage harmonics above the first were substantially increased in all horizontal directions by the empennage/propeller interaction. Directivity in the plane of the propeller was maximum perpendicular to the blade surface. Increasing the tail loading caused the propeller harmonics to increase 3 to 5 dB for an empennage/propeller spacing of 0.38 mean empennage chords. The interaction noise became weak as empennage propeller spacing was increased beyond 1.0 mean empennage chord lengths. Unlike the mean wake deficit, the wake turbulence had only a small effect on the propeller noise, that effect being a small increase in the broadband noise

Flow-field Survey of an Empennage Wake Interacting with a Pusher Propeller

The flow field between a model empennage and a 591-mm-diameter pusher propeller was studied in the Ames 7- by 10-Foot Wind Tunnel with directional pressure probes and hot-wire anemometers. The region probed was bounded by the empennage trailing edge and downstream propeller. The wake properties, including effects of propeller operation on the empennage wake, were investigated for two empennage geometries: one, a vertical tail fin, the other, a Y-tail with a 34 deg dihedral. Results showed that the effect of the propeller on the empennage wake upstream of the propeller was not strong. The flow upstream of the propeller was accelerated in the streamwise direction by the propeller, but the empennage wake width and velocity defect were relatively unaffected by the presence of the propeller. The peak turbulence in the wake near the propeller tip station, 0.66 diameter behind the vertical tail fin, was approximately 3 percent of the free-stream velocity. The velocity field data can be used in predictions of the acoustic field due to propeller-wake interaction

Quasar Tomography: Unification of Echo Mapping and Photoionisation Models

Reverberation mapping uses time-delayed variations in photoionised emission lines to map the geometry and kinematics of emission-line gas in active galactic nuclei. In previous work, the light travel time delay tau=R(1+cos(theta))/c and Doppler shift v give a 2-d map Psi(tau,v) for each emission line. Here we combine the velocity-delay information with photoionisation physics in a maximum entropy fit to the full reverberating spectrum F_lam(lam,t) to recover a 5-d map of the differential covering fraction f(R,theta,n,N,v), with n and N the density and column density of the gas clouds. We test the method for a variety of geometries (shells, rings, disks, clouds, jets) by recovering a 3-d map f(R,theta,n) from reverberations in 7 uv emission lines. The best test recovers a hollow shell geometry, defining R to 0.15 dex, n to 0.3 dex, and ionisation parameter U ~ 1/(n R^2) to 0.1 dex. The results are sensitive to the adopted distance and luminosity, suggesting that these parameters may be measurable as well.Comment: Accepted 4 Sep 2002 for publication in MNRA

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interface