53,727 research outputs found
Titan solar occultation observations reveal transit spectra of a hazy world
High altitude clouds and hazes are integral to understanding exoplanet
observations, and are proposed to explain observed featureless transit spectra.
However, it is difficult to make inferences from these data because of the need
to disentangle effects of gas absorption from haze extinction. Here, we turn to
the quintessential hazy world -- Titan -- to clarify how high altitude hazes
influence transit spectra. We use solar occultation observations of Titan's
atmosphere from the Visual and Infrared Mapping Spectrometer (VIMS) aboard
NASA's Cassini spacecraft to generate transit spectra. Data span 0.88-5 microns
at a resolution of 12-18 nm, with uncertainties typically smaller than 1%. Our
approach exploits symmetry between occultations and transits, producing transit
radius spectra that inherently include the effects of haze multiple scattering,
refraction, and gas absorption. We use a simple model of haze extinction to
explore how Titan's haze affects its transit spectrum. Our spectra show strong
methane absorption features, and weaker features due to other gases. Most
importantly, the data demonstrate that high altitude hazes can severely limit
the atmospheric depths probed by transit spectra, bounding observations to
pressures smaller than 0.1-10 mbar, depending on wavelength. Unlike the usual
assumption made when modeling and interpreting transit observations of
potentially hazy worlds, the slope set by haze in our spectra is not flat, and
creates a variation in transit height whose magnitude is comparable to those
from the strongest gaseous absorption features. These findings have important
consequences for interpreting future exoplanet observations, including those
from NASA's James Webb Space Telescope.Comment: Updated journal reference; data available via
http://sites.google.com/site/tdrobinsonscience/science/tita
Closed-drift thruster investigations
Recent data obtained from a second generation closed-drift thruster design, employing Hall current acceleration is outlined. This type device is emphasized for electric propulsion for geocentric mission applications. Because geocentric mission profiles are best achieved with a specific impulse range of 1000 to 2000 s, closed-drift thrusters are well suited for this application, permitting time payload compromises intermediate of those possible with either electrothermal or electrostatic devices. A discussion is presented of the potential advantages of using a 1000 to 2000 s device for one way orbit raising of nonpower payloads. Because closed-drift thruster operation is not space charge limited, and requires only one power circuit for steady state operation, their application is technically advantageous. Beam, plasma and thrust characteristics are detailed for a range of operating conditions
On the Approximation Performance of Fictitious Play in Finite Games
We study the performance of Fictitious Play, when used as a heuristic for
finding an approximate Nash equilibrium of a 2-player game. We exhibit a class
of 2-player games having payoffs in the range [0,1] that show that Fictitious
Play fails to find a solution having an additive approximation guarantee
significantly better than 1/2. Our construction shows that for n times n games,
in the worst case both players may perpetually have mixed strategies whose
payoffs fall short of the best response by an additive quantity 1/2 -
O(1/n^(1-delta)) for arbitrarily small delta. We also show an essentially
matching upper bound of 1/2 - O(1/n)
The star-formation history of the universe - an infrared perspective
A simple and versatile parameterized approach to the star formation history
allows a quantitative investigation of the constraints from far infrared and
submillimetre counts and background intensity measurements.
The models include four spectral components: infrared cirrus (emission from
interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN
dust torus. The 60 m luminosity function is determined for each chosen
rate of evolution using the PSCz redshift data for 15000 galaxies. The
proportions of each spectral type as a function of 60 m luminosity are
chosen for consistency with IRAS and SCUBA colour-luminosity relations, and
with the fraction of AGN as a function of luminosity found in 12 m
samples. The luminosity function for each component at any wavelength can then
be calculated from the assumed spectral energy distributions. With assumptions
about the optical seds corresponding to each component and, for the AGN
component, the optical and near infrared counts can be accurately modelled.
A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850
m can be found with pure luminosity evolution in all 3 cosmological models
investigated: = 1, = 0.3 ( = 0), and
= 0.3, = 0.7.
All 3 models also give an acceptable fit to the integrated background
spectrum. Selected predictions of the models, for example redshift
distributions for each component at selected wavelengths and fluxes, are shown.
The total mass-density of stars generated is consistent with that observed,
in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details
of models can be found at http://astro.ic.ac.uk/~mrr/countmodel
The Luminous Convolution Model as an alternative to dark matter in spiral galaxies
The Luminous Convolution Model (LCM) demonstrates that it is possible to
predict the rotation curves of spiral galaxies directly from estimates of the
luminous matter. We consider two frame-dependent effects on the light observed
from other galaxies: relative velocity and relative curvature. With one free
parameter, we predict the rotation curves of twenty-three (23) galaxies
represented in forty-two (42) data sets. Relative curvature effects rely upon
knowledge of both the gravitational potential from luminous mass of the
emitting galaxy and the receiving galaxy, and so each emitter galaxy is
compared to four (4) different Milky Way luminous mass models. On average in
this sample, the LCM is more successful than either dark matter or modified
gravity models in fitting the observed rotation curve data.
Implications of LCM constraints on populations synthesis modeling are
discussed in this paper. This paper substantially expands the results in
arXiv:1309.7370.Comment: Implications of LCM constraints on populations synthesis modeling are
discussed in this paper. This paper substantially expands the results in
arxiv:1309.737
Energetics of intrinsic point defects in ZrSiO
Using first principles calculations we have studied the formation energies,
electron and hole affinities, and electronic levels of intrinsic point defects
in zircon. The atomic structures of charged interstitials, vacancies, Frenkel
pairs and anti-site defects are obtained. The limit of high concentration of
point defects, relevant for the use of this material in nuclear waste
immobilization, was studied with a variable lattice relaxation that can
simulate the swelling induced by radiation damage. The limit of low
concentration of defects is simulated with larger cells and fixed lattice
parameters. Using known band offset values at the interface of zircon with
silicon, we analyze the foreseeable effect of the defects on the electronic
properties of zircon used as gate in metal-oxide-semiconductor devices.Comment: preprint 16 pages, 4 figures, and 5 table
Aggregation of Markov flows I : theory
A Markov flow is a stationary measure, with the associated flows and mean first passage times, for a continuous-time regular jump homogeneous semi-Markov process on a discrete state-space. Nodes in the state-space can be eliminated to produce a smaller Markov flow which is a factor of the original one. Some improvements to elimination methods of Wales are given. The main contribution of the paper is to present an alternative, namely a method to aggregate groups of nodes to produce a factor. The method can be iterated to make hierarchical aggregation schemes. The potential benefits are efficient computation, including recomputation to take into account local changes, and insights into the macroscopic behaviour
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