2,425 research outputs found
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
Inversion of synrift normal faults in the High Atlas Mountains, Morocco
Copyright 1997, Society for Exploration Geophysics.
See also:
http://segdl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=LEEDFF000016000008001171000001&idtype=cvips&gifs=Yes;
http://atlas.geo.cornell.edu/morocco/publications/beauchamp1997.htmStructural inversion related to intracontinental rifting occurs when extensional rift faults reverse their sense of motion during subsequent episodes of compressional tectonics. Features generated by extension, such as half grabens, are uplifted to form positive anticlinal structures
Coherent Error Suppression in Multi-Qubit Entangling Gates
We demonstrate a simple pulse shaping technique designed to improve the
fidelity of spin-dependent force operations commonly used to implement
entangling gates in trapped-ion systems. This extension of the
M{\o}lmer-S{\o}rensen gate can theoretically suppress the effects of certain
frequency and timing errors to any desired order and is demonstrated through
Walsh modulation of a two-qubit entangling gate on trapped atomic ions. The
technique is applicable to any system of qubits coupled through collective
harmonic oscillator modes
Vortex Lock-In Deep in the Bose Glass
We use a Bi gaussmeter of micron dimensions to explore the magnetic field dependence of the magnetization relaxation rate and the critical current down to millikelvin temperatures in untwinned single crystals of YBa_2Cu_3O_(7-δ) with columnar defects. The response separates into three regimes as a function of the ratio of vortex density to columnar defect density B/B_φ: enhancements in both critical current and quantum creep in the dilute limit, vanishing magnetization relaxation at the matching density (the proposed "Mott insulator'' phase line), and the emergence of temperature-dependent vortex motion for B≫B_φ
Saturation in phosphene size with increasing current levels delivered to human visual cortex
Electrically stimulating early visual cortex results in a visual percept known as a phosphene. Although phosphenes can be evoked by a wide range of electrode sizes and current amplitudes, they are invariably described as small. To better understand this observation, we electrically stimulated 93 electrodes implanted in the visual cortex of 13 human subjects who reported phosphene size while stimulation current was varied. Phosphene size increased as the stimulation current was initially raised above threshold, but then rapidly reached saturation. Phosphene size also depended on the location of the stimulated site, with size increasing with distance from the foveal representation. We developed a model relating phosphene size to the amount of activated cortex and its location within the retinotopic map. First, a sigmoidal curve was used to predict the amount of activated cortex at a given current. Second, the amount of active cortex was converted to degrees of visual angle by multiplying by the inverse cortical magnification factor for that retinotopic location. This simple model accurately predicted phosphene size for a broad range of stimulation currents and cortical locations. The unexpected saturation in phosphene sizes suggests that the functional architecture of cerebral cortex may impose fundamental restrictions on the spread of artificially evoked activity and this may be an important consideration in the design of cortical prosthetic devices
Vortex Plastic Flow, , Deep in the Bose Glass and Mott-Insulator Regimes
We present simulations of flux-gradient-driven superconducting vortices
interacting with strong columnar pinning defects as an external field is
quasi-statically swept from zero through a matching field . We
analyze several measurable quantities, including the local flux density , magnetization , critical current , and the
individual vortex flow paths. We find a significant change in the behavior of
these quantities as the local flux density crosses , and quantify it
for many microscopic pinning parameters. Further, we find that for a given pin
density can be enhanced by maximizing the distance between the pins
for .Comment: 4 pages, 4 PostScript Figure
Local probe of vortex pinning energies in the Bose glass
Columnar defects provide strong pinning centers for vortices in high-T_c superconductors, increasing global critical currents. Using a magnetometer array of micron dimensions, we characterize the local held profiles in untwinned single crystals of YBa_2Cu_3O_(7-δ) with equivalent columnar defect densities B_φ. We find that the critical current is large only where the internal magnetic field BB _ φ, the critical current is sharply reduced. We model both local and global critical current measurements by generalizing the Bean picture to the case of irradiated high-T_c superconductors
Intracontinental rifting and inversion: Missour Basin and Atlas Mountains, Morocco
An edited version of this paper was published by the American Association of Petroleum Geologists (AAPG). Copyright 1996, AAPG.
See also:
http://www.aapg.org/bulletin/index.cfm;
http://atlas.geo.cornell.edu/morocco/publications/beauchamp1996.htmThe intracontinental High and Middle Atlas mountain belts in Morocco intersect to form the southern and western margins of the Missour Basin, an intermontane basin formed as a result of the uplift and inversion of the Mesozoic Atlas paleorifts. These rifts were areas where the crust was greatly attenuated and more subject to deformation in response to nearby plate boundary tectonics. Data from observations based on seismic reflection profiles and wells over the Missour basin for hydrocarbon exploration and field mapping were used to understand the basin evolution, structural styles, and inversion timing of the nearby Atlas Mountains. Hercynian and Mesozoic normal faults were reactivated into high-angle reverse and thrust faults in the Mesozoic during the Jurassic, early Cretaceous (early Alpine phase), and the Paleogene (late Alpine phase). The reactivation of synrift normal faults of the paleo-Atlas rifts inverted previous half grabens into anticlinal structures, with the axis of the half graben centered below the axis of the inverted anticline. The resulting inverted fold geometries are controlled by the geometries of the extensional planar or listric faults
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