2,586 research outputs found
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
Transits of Earth-Like Planets
Transmission spectroscopy of Earth-like exoplanets is a potential tool for
habitability screening. Transiting planets are present-day "Rosetta Stones" for
understanding extrasolar planets because they offer the possibility to
characterize giant planet atmospheres and should provide an access to
biomarkers in the atmospheres of Earth-like exoplanets, once they are detected.
Using the Earth itself as a proxy we show the potential and limits of the
transiting technique to detect biomarkers on an Earth-analog exoplanet in
transit. We quantify the Earths cross section as a function of wavelength, and
show the effect of each atmospheric species, aerosol, and Rayleigh scattering.
Clouds do not significantly affect this picture because the opacity of the
lower atmosphere from aerosol and Rayleigh losses dominates over cloud losses.
We calculate the optimum signal-to-noise ratio for spectral features in the
primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and
also M stars, for a 6.5-m telescope in space. We find that the signal to noise
values for all important spectral features are on the order of unity or less
per transit - except for the closest stars - making it difficult to detect such
features in one single transit, and implying that co-adding of many transits
will be essential.Comment: 17 pages, 3 figures, 6 tables, to appear in ApJ (accepted) V2:
corrected transit times, corrected values for M4 star radiu
A posteriori detection of the planetary transit of HD189733b in the Hipparcos photometry
Thanks to observations performed at the Haute-Provence Observatory, Bouchy et
al. recently announced the detection of a 2.2-day orbital period extra-solar
planet that transits the disk of its parent star, HD189733. With high level of
confidence, we find that Hipparcos likely observed one transit of HD189733b in
October 1991, and possibly two others in February 1991 and February 1993. Using
the range of possible periods for HD189733b, we find that the probability that
none of those events are due to planetary transits but are instead all due to
artifacts is lower than 0.15%. Thanks to the 15-year temporal baseline
available, we can measure the orbital period of the planet HD189733b with a
particularly high accuracy. We obtain a period of 2.218574
(+0.000006/-0.000010) days, corresponding to an accuracy of ~1 second. Such
accurate measurements might provide clues for companions presence.Comment: 7 pages, to be published in Astronomy & Astrophysic
Phase behavior and far-from-equilibrium gelation of charged attractive colloids
In this Rapid Communication we demonstrate the applicability of an augmented
Gibbs ensemble Monte Carlo approach for the phase behavior determination of
model colloidal systems with short-ranged depletion attraction and long-ranged
repulsion. This technique allows for a quantitative determination of the phase
boundaries and ground states in such systems. We demonstrate that gelation may
occur in systems of this type as the result of arrested microphase separation,
even when the equilibrium state of the system is characterized by compact
microphase structures.Comment: 5 pages, 3 figures, final versio
Ground-based NIR emission spectroscopy of HD189733b
We investigate the K and L band dayside emission of the hot-Jupiter HD
189733b with three nights of secondary eclipse data obtained with the SpeX
instrument on the NASA IRTF. The observations for each of these three nights
use equivalent instrument settings and the data from one of the nights has
previously reported by Swain et al (2010). We describe an improved data
analysis method that, in conjunction with the multi-night data set, allows
increased spectral resolution (R~175) leading to high-confidence identification
of spectral features. We confirm the previously reported strong emission at
~3.3 microns and, by assuming a 5% vibrational temperature excess for methane,
we show that non-LTE emission from the methane nu3 branch is a physically
plausible source of this emission. We consider two possible energy sources that
could power non-LTE emission and additional modelling is needed to obtain a
detailed understanding of the physics of the emission mechanism. The validity
of the data analysis method and the presence of strong 3.3 microns emission is
independently confirmed by simultaneous, long-slit, L band spectroscopy of HD
189733b and a comparison star.Comment: ApJ accepte
A Ground-Based Search for Thermal Emission from the Exoplanet TrES-1
Eclipsing planetary systems give us an important window on extrasolar planet
atmospheres. By measuring the depth of the secondary eclipse, when the planet
moves behind the star, we can estimate the strength of the thermal emission
from the day side of the planet. Attaining a ground-based detection of one of
these eclipses has proven to be a significant challenge, as time-dependent
variations in instrument throughput and atmospheric seeing and absorption
overwhelm the small signal of the eclipse at infrared wavelengths. We gathered
a series of simultaneous L grism spectra of the transiting planet system TrES-1
and a nearby comparison star of comparable brightness, allowing us to correct
for these effects in principle. Combining the data from two eclipses, we
demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to
the stellar flux. This approaches the sensitivity required to detect the
planetary emission, which theoretical models predict should lie between
0.05-0.1% of the stellar flux in our 2.9-4.3 micron bandpass. We explore the
factors that ultimately limit the precision of this technique, and discuss
potential avenues for future improvements.Comment: 10 pages, 1 table, four figures, accepted for publication in PAS
Universal microstructure and mechanical stability of jammed packings
Jammed packings' mechanical properties depend sensitively on their detailed
local structure. Here we provide a complete characterization of the pair
correlation close to contact and of the force distribution of jammed
frictionless spheres. In particular we discover a set of new scaling relations
that connect the behavior of particles bearing small forces and those bearing
no force but that are almost in contact. By performing systematic
investigations for spatial dimensions d=3-10, in a wide density range and using
different preparation protocols, we show that these scalings are indeed
universal. We therefore establish clear milestones for the emergence of a
complete microscopic theory of jamming. This description is also crucial for
high-precision force experiments in granular systems.Comment: 11 pages, 7 figure
Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the dayside atmosphere of HD 209458b have a thermal
inversion and a stratosphere. This inversion is caused by the capture of
optical stellar flux by an absorber of uncertain origin that resides at
altitude. One consequence of stratospheric heating and temperature inversion is
the flipping of water absorption features into emission features from the near-
to the mid-infrared and we see evidence of such a water emission feature in the
recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na D feature
seen in transit and the fact that the transit radius at 24 m is smaller
than the corresponding radius in the optical. Moreover, it may be a factor in
why HD 209458b's optical transit radius is as large as it is. We speculate on
the nature of this absorber and the planets whose atmospheres may, or may not,
be affected by its presence.Comment: Accepted to the Astrophysical Journal Letters on August 28, 2007, six
pages in emulateapj forma
Geometrical Frustration and Static Correlations in Hard-Sphere Glass Formers
We analytically and numerically characterize the structure of hard-sphere
fluids in order to review various geometrical frustration scenarios of the
glass transition. We find generalized polytetrahedral order to be correlated
with increasing fluid packing fraction, but to become increasingly irrelevant
with increasing dimension. We also find the growth in structural correlations
to be modest in the dynamical regime accessible to computer simulations.Comment: 21 pages; part of the "Special Topic Issue on the Glass Transition
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