7,557 research outputs found
Effects of Helium Phase Separation on the Evolution of Extrasolar Giant Planets
We build on recent new evolutionary models of Jupiter and Saturn and here
extend our calculations to investigate the evolution of extrasolar giant
planets of mass 0.15 to 3.0 M_J. Our inhomogeneous thermal history models show
that the possible phase separation of helium from liquid metallic hydrogen in
the deep interiors of these planets can lead to luminosities ~2 times greater
than have been predicted by homogeneous models. For our chosen phase diagram
this phase separation will begin to affect the planets' evolution at ~700 Myr
for a 0.15 M_J object and ~10 Gyr for a 3.0 M_J object. We show how phase
separation affects the luminosity, effective temperature, radii, and
atmospheric helium mass fraction as a function of age for planets of various
masses, with and without heavy element cores, and with and without the effect
of modest stellar irradiation. This phase separation process will likely not
affect giant planets within a few AU of their parent star, as these planets
will cool to their equilibrium temperatures, determined by stellar heating,
before the onset of phase separation. We discuss the detectability of these
objects and the likelihood that the energy provided by helium phase separation
can change the timescales for formation and settling of ammonia clouds by
several Gyr. We discuss how correctly incorporating stellar irradiation into
giant planet atmosphere and albedo modeling may lead to a consistent
evolutionary history for Jupiter and Saturn.Comment: 22 pages, including 14 figures. Accepted to the Astrophysical Journa
Analytic Scattering and Refraction Models for Exoplanet Transit Spectra
Observations of exoplanet transit spectra are essential to understanding the
physics and chemistry of distant worlds. The effects of opacity sources and
many physical processes combine to set the shape of a transit spectrum. Two
such key processes - refraction and cloud and/or haze forward scattering - have
seen substantial recent study. However, models of these processes are typically
complex, which prevents their incorporation into observational analyses and
standard transit spectrum tools. In this work, we develop analytic expressions
that allow for the efficient parameterization of forward scattering and
refraction effects in transit spectra. We derive an effective slant optical
depth that includes a correction for forward scattered light, and present an
analytic form of this correction. We validate our correction against a
full-physics transit spectrum model that includes scattering, and we explore
the extent to which the omission of forward scattering effects may bias models.
Also, we verify a common analytic expression for the location of a refractive
boundary, which we express in terms of the maximum pressure probed in a transit
spectrum. This expression is designed to be easily incorporated into existing
tools, and we discuss how the detection of a refractive boundary could help
indicate the background atmospheric composition by constraining the bulk
refractivity of the atmosphere. Finally, we show that opacity from Rayleigh
scattering and collision induced absorption will outweigh the effects of
refraction for Jupiter-like atmospheres whose equilibrium temperatures are
above 400-500 K.Comment: ApJ accepted; submitted Feb. 7, 201
A Theory for the Radius of the Transiting Giant Planet HD 209458b
Using a full frequency-dependent atmosphere code that can incorporate
irradiation by a central primary star, we calculate self-consistent boundary
conditions for the evolution of the radius of the transiting planet HD 209458b.
Using a well-tested extrasolar giant planet evolutionary code, we then
calculate the behavior of this planet's radius with age. The measured radius is
in fact a transit radius that resides high in HD 209458b's inflated atmosphere.
Using our derived atmospheric and interior structures, we find that irradiation
plus the proper interpretation of the transit radius can yield a theoretical
radius that is within the measured error bars. We conclude that if HD 209458b's
true transit radius is at the lower end of the measured range, an extra source
of core heating power is not necessary to explain the transit observations.Comment: 6 pages in emulateapj format, plus 2 figures (one color), accepted to
the Astrophysical Journa
A noise study of the A-6 airplane and techniques for reducing its aural detection distance
A study was undertaken to determine the noise reduction potential of the A-6 airplane in order to reduce its aural detection distance. Static and flyby noise measurements were taken to document the basic airplane signature. The low-frequency noise which is generally most critical for aural detection was found to be broad-band in nature from this airplane, and its source is the turbojet engine exhaust. High-frequency compressor noise, which is characteristic of turbojet powerplants, and which is prominent at close range for this airplane, has no measurable effect on aural detection distance. The use of fluted-engine exhaust nozzles to change the far-field noise spectra is suggested as a possible means for reducing the aural detection distances. Detection distances associated with eight-lobe and four-lobe nozzles are estimated for a 1,000-foot altitude and grassy terrain to decrease from 4 miles to about 3 miles, and from 3 miles to about 2 miles for a 300-foot altitude and grassy terrain
Investigations of meltwater refreezing and density variations in the snowpack and firn within the percolation zone of the Greenland Ice Sheet
The mass balance of polythermal ice masses is critically dependent on the proportion of surface-generated meltwater that subsequently refreezes in the snowpack and firn. In order to quantify this effect and to characterize its spatial variability, we measured near-surface (26%, resulting in a 32% increase in net accumulation. This 'seasonal densification' increased at lower elevations, rising to 47% 10 km closer to the ice-sheet margin at 1860 m a. s. l. Density/depth profiles from nine sites within 1 km2 at ∼1945 m a.s.l. reveal complex stratigraphies that change over short spatial scales and seasonally. We conclude that estimates of mass-balance change cannot be calculated solely from observed changes in surface elevation, but that near-surface densification must also be considered. However, predicting spatial and temporal variations in densification may not be straightforward. Further, the development of complex firn-density profiles both masks discernible annual layers in the near-surface firn and ice stratigraphy and is likely to introduce error into radar-derived estimates of surface elevation
Effect of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition
The Mott-Hubbard metal-insulator transition is one of the most important
problems in correlated electron systems. In the past decade, much progress has
been made on examining a particle-hole symmetric form of the transition in the
Hubbard model with dynamical mean field theory where it was found that the
electronic self energy develops a pole at the transition. We examine the
particle-hole asymmetric metal-insulator transition in the Falicov-Kimball
model, and find that a number of features change when the noninteracting
density of states has a finite bandwidth. Since, generically particle-hole
symmetry is broken in real materials, our results have an impact on
understanding the metal-insulator transition in real materials.Comment: 5 pages, 3 figure
Fractional Aharonov-Bohm effect in mesoscopic rings
We study the effects of correlations on a one dimensional ring threaded by a
uniform magnetic flux. In order to describe the interaction between particles,
we work in the framework of the U Hubbard and - models. We focus
on the dilute limit. Our results suggest the posibility that the persistent
current has an anomalous periodicity , where is an integer in
the range ( is the number of particles in the ring
and is the flux quantum). We found that this result depends neither
on disorder nor on the detailed form of the interaction, while remains the on
site infinite repulsion.Comment: 14 pages (Revtex), 5 postscript figures. Send e-mail to:
[email protected]
Noise reduction studies for the U-10 airplane
A study was undertaken by the NASA Langley Research Center to determine the noise reduction potential of the U-10 airplane in order to reduce its aural detection distance. Static and flyover noise measurements were made to document the basic airplane noise signature. Two modifications to the airplane configuration are suggested as having the best potential for substantially reducing aural detection distance with small penalty to airplane performance or stability and control. These modifications include changing the present 3-blade propeller to a 5-blade propeller, changing the propeller diameter, and changing the propeller gear ratio, along with the use of an engine exhaust muffler. The aural detection distance corresponding to normal cruising flight at an altitude of 1,000 ft over grassy terrain is reduced from 28,000 ft (5.3 miles) to about 50 percent of that value for modification 1, and to about 25 percent for modification 2. For the aircraft operating at an altitude of 300 ft, the analysis indicates that relatively straightforward modifications could reduce the aural detection distance to approximately 0.9 mile. Operation of the aircraft at greatly reduced engine speed (1650 rpm) with a 1.3-cu-ft muffler provides aural detection distances slightly lower than modification 1
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