3,067 research outputs found
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
The Evolution of Large-Amplitude Internal Gravity Wavepackets
The interaction between internal gravity wavepackets and their associated wave-induced mean flow is shown
to dominate their weakly nonlinear evolution at early times. Sufficiently large amplitude waves either grow in
amplitude as the wavepacket narrows or decrease in amplitude as the wavepacket broadens due to nonlinearly
enhanced dispersion depending on whether the frequency is respectively greater or less than the frequency of
internal waves having the fastest vertical group velocity. This is true whether the waves are horizontally periodic
or horizontally compact. In non-uniformly stratified fluid, the weakly nonlinear effect acts to enhance or retard
transmission of the waves across a localized evanescent region depending upon the wave amplitude and relative
frequency
Spitzer Space Telescope Spectroscopy of Ices toward Low-Mass Embedded Protostars
Sensitive 5-38 μm Spitzer Space Telescope and ground-based 3-5 μm spectra of the embedded low-mass protostars B5 IRS1 and HH 46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO_2 bending mode at 15 μm is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN^- bands, however. The observed CO2 profile indicates an intimate mixture with H_(2)O, but not necessarily with CH_(3)OH, in contrast to some high-mass protostars. This is consistent with the low CH_(3)OH abundance derived from the ground-based L-band spectra. The CO_2 : H_(2)O column density ratios are high in both B5 IRS1 and HH 46 IRS (~35%). Clearly, the Spitzer spectra are essential for studying ice evolution in low-mass protostellar environments and for eventually determining the relation between interstellar and solar system ices
Adaptively-refined overlapping grids for the numerical solution of systems of hyperbolic conservation laws
Adaptive mesh refinement (AMR) in conjunction with higher-order upwind finite-difference methods have been used effectively on a variety of problems in two and three dimensions. In this paper we introduce an approach for resolving problems that involve complex geometries in which resolution of boundary geometry is important. The complex geometry is represented by using the method of overlapping grids, while local resolution is obtained by refining each component grid with the AMR algorithm, appropriately generalized for this situation. The CMPGRD algorithm introduced by Chesshire and Henshaw is used to automatically generate the overlapping grid structure for the underlying mesh
A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
We determine the parallax and proper motion of the flaring, non-thermal radio
star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline
Array observations. Based on the parallax, we measure a distance of 389 +24/-21
parsecs to the source. Our measurement places the Orion Nebula Cluster
considerably closer than the canonical distance of 480 +/- 80 parsecs
determined by Genzel et al. (1981). A change of this magnitude in distance
lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We
briefly discuss two effects of this change--an increase in the age spread of
the pre-main sequence stars and better agreement between the zero-age
main-sequence and the temperatures and luminosities of massive stars.Comment: 10 pages, 4 figures, emulateapj, accepted to Ap
The atmospheres of the hot-Jupiters Kepler-5b and Kepler-6b observed during occultations with Warm-Spitzer and Kepler
This paper reports the detection and the measurements of occultations of the
two transiting hot giant exoplanets Kepler-5b and Kepler-6b by their parent
stars. The observations are obtained in the near infrared with Spitzer Space
Telescope and at optical wavelengths by combining more than a year of Kepler
photometry. The investigation consists of constraining the eccentricities of
these systems and of obtaining broad band emergent spectra for individual
planets. For both targets, the occultations are detected at 3 sigma level at
each wavelength with mid-occultation times consistent with circular orbits. The
brightness temperatures of these planets are deduced from the infrared
observations and reach T=1930+/-100K and T=1660+/-120K for Kepler-5b and
Kepler-6b respectively. We measure optical geometric albedos A_g in the Kepler
bandpass and find A_g=0.12+/-0.04 for Kepler-5b and A_g=0.11+/-0.04 for
Kepler-6b leading to an upper limit for the Bond albedo of A_B < 0.17 in both
cases. The observations for both planets are best described by models for which
most of the incident energy is redistributed on the dayside, with only less
than 10% of the absorbed stellar flux redistributed to the night side of these
planets. The data for Kepler-5b favor a model without a temperature inversion,
whereas for Kepler-6b they do not allow distinguishing between models with and
without inversion.Comment: 26 pages, 18 figures, 3 tables, submitted to Ap
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