1,229 research outputs found
Jamming Criticality Revealed by Removing Localized Buckling Excitations
Recent theoretical advances offer an exact, first-principle theory of jamming
criticality in infinite dimension as well as universal scaling relations
between critical exponents in all dimensions. For packings of frictionless
spheres near the jamming transition, these advances predict that nontrivial
power-law exponents characterize the critical distribution of (i) small
inter-particle gaps and (ii) weak contact forces, both of which are crucial for
mechanical stability. The scaling of the inter-particle gaps is known to be
constant in all spatial dimensions -- including the physically relevant
and 3, but the value of the weak force exponent remains the object of
debate and confusion. Here, we resolve this ambiguity by numerical simulations.
We construct isostatic jammed packings with extremely high accuracy, and
introduce a simple criterion to separate the contribution of particles that
give rise to localized buckling excitations, i.e., bucklers, from the others.
This analysis reveals the remarkable dimensional robustness of mean-field
marginality and its associated criticality.Comment: 12 pages, 4 figure
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
Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS J13142039+1320011)
We present multi-epoch simultaneous radio, optical, H{\alpha}, UV, and X-ray
observations of the active, young, low-mass binary NLTT 33370 AB (blended
spectral type M7e). This system is remarkable for its extreme levels of
magnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,
and here we show that it is also one of the most X-ray luminous UCDs known. We
detect the system in all bands and find a complex phenomenology of both flaring
and periodic variability. Analysis of the optical light curve reveals the
simultaneous presence of two periodicities, 3.7859 0.0001 and 3.7130
0.0002 hr. While these differ by only ~2%, studies of differential
rotation in the UCD regime suggest that it cannot be responsible for the two
signals. The system's radio emission consists of at least three components:
rapid 100% polarized flares, bright emission modulating periodically in phase
with the optical emission, and an additional periodic component that appears
only in the 2013 observational campaign. We interpret the last of these as a
gyrosynchrotron feature associated with large-scale magnetic fields and a cool,
equatorial plasma torus. However, the persistent rapid flares at all rotational
phases imply that small-scale magnetic loops are also present and reconnect
nearly continuously. We present an SED of the blended system spanning more than
9 orders of magnitude in wavelength. The significant magnetism present in NLTT
33370 AB will affect its fundamental parameters, with the components' radii and
temperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we
suggest spatially resolved observations that could clarify many aspects of this
system's nature.Comment: emulateapj, 22 pages, 15 figures, ApJ in press; v2: fixes low-impact
error in Figure 15; v3: now in-pres
Modeling the Parker instability in a rotating plasma screw pinch
We analytically and numerically study the analogue of the Parker (magnetic
buoyancy) instability in a uniformly rotating plasma screw pinch confined in a
cylinder. Uniform plasma rotation is imposed to create a centrifugal
acceleration, which mimics the gravity required for the classical Parker
instability. The goal of this study is to determine how the Parker instability
could be unambiguously identified in a weakly magnetized, rapidly rotating
screw pinch, in which the rotation provides an effective gravity and a radially
varying azimuthal field is controlled to give conditions for which the plasma
is magnetically buoyant to inward motion. We show that an axial magnetic field
is also required to circumvent conventional current driven magnetohydrodynamic
(MHD) instabilities such as the sausage and kink modes that would obscure the
Parker instability. These conditions can be realized in the Madison Plasma
Couette Experiment (MPCX). Simulations are performed using the extended MHD
code NIMROD for an isothermal compressible plasma model. Both linear and
nonlinear regimes of the instability are studied, and the results obtained for
the linear regime are compared with analytical results from a slab geometry.
Based on this comparison, it is found that in a cylindrical pinch the magnetic
buoyancy mechanism dominates at relatively large Mach numbers (M>5), while at
low Mach numbers (M<1) the instability is due to the curvature of magnetic
field lines. At intermediate values of Mach number (1<M<5) the Coriolis force
has a strong stabilizing effect on the plasma. A possible scenario for
experimental demonstration of the Parker instability in MPCX is discussed
On the compatibility of a flux transport dynamo with a fast tachocline scenario
The compatibility of the fast tachocline scenario with a flux transport
dynamo model is explored. We employ a flux transport dynamo model coupled with
simple feedback formulae relating the thickness of the tachocline to the
amplitude of the magnetic field or to the Maxwell stress. The dynamo model is
found to be robust against the nonlinearity introduced by this simplified fast
tachocline mechanism. Solar-like butterfly diagrams are found to persist and,
even without any parameter fitting, the overall thickness of the tachocline is
well within the range admitted by helioseismic constraints. In the most
realistic case of a time and latitude dependent tachocline thickness linked to
the value of the Maxwell stress, both the thickness and its latitude dependence
are in excellent agreement with seismic results. In the nonparametric models,
cycle related temporal variations in tachocline thickness are somewhat larger
than admitted by helioseismic constraints; we find, however, that introducing a
further parameter into our feedback formula readily allows further fine tuning
of the thickness variations.Comment: Accepted in Solar Physic
Hard sphere crystallization gets rarer with increasing dimension
We recently found that crystallization of monodisperse hard spheres from the
bulk fluid faces a much higher free energy barrier in four than in three
dimensions at equivalent supersaturation, due to the increased geometrical
frustration between the simplex-based fluid order and the crystal [J.A. van
Meel, D. Frenkel, and P. Charbonneau, Phys. Rev. E 79, 030201(R) (2009)]. Here,
we analyze the microscopic contributions to the fluid-crystal interfacial free
energy to understand how the barrier to crystallization changes with dimension.
We find the barrier to grow with dimension and we identify the role of
polydispersity in preventing crystal formation. The increased fluid stability
allows us to study the jamming behavior in four, five, and six dimensions and
compare our observations with two recent theories [C. Song, P. Wang, and H. A.
Makse, Nature 453, 629 (2008); G. Parisi and F. Zamponi, Rev. Mod. Phys, in
press (2009)].Comment: 15 pages, 5 figure
Phase coexistence of cluster crystals: beyond the Gibbs phase rule
We report a study of the phase behavior of multiple-occupancy crystals
through simulation. We argue that in order to reproduce the equilibrium
behavior of such crystals it is essential to treat the number of lattice sites
as a constraining thermodynamic variable. The resulting free-energy
calculations thus differ considerably from schemes used for single-occupancy
lattices. Using our approach, we obtain the phase diagram and the bulk modulus
for a generalized exponential model that forms cluster crystals at high
densities. We compare the simulation results with existing theoretical
predictions. We also identify two types of density fluctuations that can lead
to two sound modes and evaluate the corresponding elastic constants.Comment: 4 pages, 3 figure
Resolving the Surfaces of Extrasolar Planets With Secondary Eclipse Light Curves
We present a method that employs the secondary eclipse light curves of
transiting extrasolar planets to probe the spatial variation of their thermal
emission. This technique permits an observer to resolve the surface of the
planet without the need to spatially resolve its central star. We evaluate the
feasibility of this technique for the HD 209458 system [..]. We consider two
representations of the planetary thermal emission; a simple model parameterized
by a sinusoidal dependence on longitude and latitude, as well as the results of
a three-dimensional dynamical simulation of the planetary atmosphere previously
published by Cooper & Showman. We find that observations of the secondary
eclipse light curve are most sensitive to a longitudinal offset in the
geometric and photometric centroids of the hemisphere of the planet visible
near opposition. To quantify this signal, we define a new parameter, the
``uniform time offset,'' which measures the time lag between the observed
secondary eclipse and that predicted by a planet with a uniform surface flux
distribution. We compare the predicted amplitude of this parameter for HD
209458 with the precision with which it could be measured with IRAC. We find
that IRAC observations at 3.6um a single secondary eclipse should permit
sufficient precision to confirm or reject the Cooper & Showman model of the
surface flux distribution for this planet. We quantify the signal-to-noise
ratio for this offset in the remaining IRAC bands (4.5um, 5.8um, and 8.0um),
and find that a modest improvement in photometric precision (as might be
realized through observations of several eclipse events) should permit a
similarly robust detection.Comment: AASTeX 5.2, 24 pages, 5 figures, accepted for publication in ApJ; v2:
clarifications, updated to version accepted by ApJ; v3: try to reduce spacin
Dimensional dependence of the Stokes--Einstein relation and its violation
We generalize to higher spatial dimensions the Stokes--Einstein relation
(SER) and the leading correction to diffusivity in periodic systems, and
validate them using numerical simulations. Using these results, we investigate
the evolution of the SER violation with dimension in simple hard sphere glass
formers. The analysis suggests that the SER violation disappears around
dimension d=8, above which SER is not violated. The critical exponent
associated to the violation appears to evolve linearly in 8-d below d=8, as
predicted by Biroli and Bouchaud [J. Phys.: Cond. Mat. 19, 205101 (2007)], but
the linear coefficient is not consistent with their prediction. The SER
violation evolution with d establishes a new benchmark for theory, and a
complete description remains an open problem.Comment: 20 pages, 10 figure
Water in HD 209458b's atmosphere from 3.6 - 8 microns IRAC photometric observations in primary transit
The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5,
5.8 and 8.0 microns using the Infrared Array Camera (IRAC) on the Spitzer Space
Telescope. We detail here the procedures we adopted to correct for the
systematic trends present in the IRAC data. The light curves were fitted
including limb darkening effects and fitted using Markov Chain Monte Carlo and
prayer-bead Monte Carlo techniques, finding almost identical results. The final
depth measurements obtained by a combined Markov Chain Monte Carlo fit are at
3.6 microns, 1.469 +- 0.013 % and 1.448 +- 0.013 %; at 4.5 microns, 1.478 +-
0.017 % ; at 5.8 microns, 1.549 +- 0.015 % and at 8.0 microns 1.535 +- 0.011 %.
Our results clearly indicate the presence of water in the planetary atmosphere.
Our broad band photometric measurements with IRAC prevent us from determining
the additional presence of other other molecules such as CO, CO2 and methane
for which spectroscopy is needed. While water vapour with a mixing ratio of
10^-4-10^-3 combined with thermal profiles retrieved from the day-side may
provide a very good fit to our observations, this data set alone is unable to
resolve completely the degeneracy between water abundance and atmospheric
thermal profile.Comment: 14 pages, 6 tables, 10 figures, Accepted for publication in MNRA
- …