1,794 research outputs found
Preliminary Constraints on 12C(alpha,gamma)16O from White Dwarf Seismology
For many years, astronomers have promised that the study of pulsating white
dwarfs would ultimately lead to useful information about the physics of matter
under extreme conditions of temperature and pressure. In this paper we finally
make good on that promise. Using observational data from the Whole Earth
Telescope and a new analysis method employing a genetic algorithm, we
empirically determine that the central oxygen abundance in the
helium-atmosphere variable white dwarf GD 358 is 84+/-3 percent. We use this
value to place preliminary constraints on the 12C(alpha,gamma)16O nuclear
reaction cross-section. More precise constraints will be possible with
additional detailed simulations. We also show that the pulsation modes of our
best-fit model probe down to the inner few percent of the stellar mass. We
demonstrate the feasibility of reconstructing the internal chemical profiles of
white dwarfs from asteroseismological data, and find an oxygen profile for GD
358 that is qualitatively similar to recent theoretical calculations.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 6
figures, 2 tables, uses emulateapj5.st
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
[N]pT ensemble and finite-size scaling study of the GEM-4 critical isostructural transition
First-order transitions of system where both lattice site occupancy and
lattice spacing fluctuate, such as cluster crystals, cannot be efficiently
studied by traditional simulation methods. These methods necessarily fix one of
these two degrees of freedom, but this difficulty is surmounted by the
generalized [N]pT ensemble [J. Chem. Phys. 136, 214106 (2012)]. Here it is
shown that histogram reweighting and the [N]pT ensemble can be used to study an
isostructural transition between cluster crystals of different occupancy in the
generalized exponential model of index 4 (GEM-4). Extending this scheme to
finite-size scaling studies also allows to accurately determine the critical
point parameters and to verify that it belongs to the Ising universality class.Comment: 5 pages, 4 figure
On the penetration of meridional circulation below the solar convection zone
Meridional flows with velocities of a few meters per second are observed in
the uppermost regions of the solar convection zone. The amplitude and pattern
of the flows deeper in the solar interior, in particular near the top of the
radiative region, are of crucial importance to a wide range of solar
magnetohydrodynamical processes. In this paper, we provide a systematic study
of the penetration of large-scale meridional flows from the convection zone
into the radiative zone. In particular, we study the effects of the assumed
boundary conditions applied at the convective-radiative interface on the deeper
flows. Using simplified analytical models in conjunction with more complete
numerical methods, we show that penetration of the convectively-driven
meridional flows into the deeper interior is not necessarily limited to a
shallow Ekman depth but can penetrate much deeper, depending on how the
convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap
Dynamics of the fast solar tachocline: I. Dipolar field
One possible scenario for the origin of the solar tachocline, known as the
"fast tachocline", assumes that the turbulent diffusivity exceeds eta>10^9
cm^2/s. In this case the dynamics will be governed by the dynamo-generated
oscillatory magnetic field on relatively short timescales. Here, for the first
time, we present detailed numerical models for the fast solar tachocline with
all components of the magnetic field calculated explicitly, assuming axial
symmetry and a constant turbulent diffusivity eta and viscosity nu. We find
that a sufficiently strong oscillatory poloidal field with dipolar latitude
dependence at the tachocline-convective zone boundary is able to confine the
tachocline. Exploring the three-dimensional parameter space defined by the
viscosity in the range log(nu)=9-11, the magnetic Prandtl number in the range
Prm=0.1-10, and the meridional flow amplitude (-3 to +3 cm/s), we also find
that the confining field strength B_conf, necessary to reproduce the observed
thickness of the tachocline, increases with viscosity nu, with magnetic Prandtl
number nu/eta, and with equatorward meridional flow speed. Nevertheless, the
resulting B_conf values remain quite reasonable, in the range 10^3-10^4 G, for
all parameter combinations considered here. The thickness of the tachocline
shows a marked dependence on both time and latitude. A comparison with seismic
constraints suggests that best agreement with our models is achieved for the
highest values of nu and Prm considered here.Comment: 11 page
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
Phase Transformations in Binary Colloidal Monolayers
Phase transformations can be difficult to characterize at the microscopic
level due to the inability to directly observe individual atomic motions. Model
colloidal systems, by contrast, permit the direct observation of individual
particle dynamics and of collective rearrangements, which allows for real-space
characterization of phase transitions. Here, we study a quasi-two-dimensional,
binary colloidal alloy that exhibits liquid-solid and solid-solid phase
transitions, focusing on the kinetics of a diffusionless transformation between
two crystal phases. Experiments are conducted on a monolayer of magnetic and
nonmagnetic spheres suspended in a thin layer of ferrofluid and exposed to a
tunable magnetic field. A theoretical model of hard spheres with point dipoles
at their centers is used to guide the choice of experimental parameters and
characterize the underlying materials physics. When the applied field is normal
to the fluid layer, a checkerboard crystal forms; when the angle between the
field and the normal is sufficiently large, a striped crystal assembles. As the
field is slowly tilted away from the normal, we find that the transformation
pathway between the two phases depends strongly on crystal orientation, field
strength, and degree of confinement of the monolayer. In some cases, the
pathway occurs by smooth magnetostrictive shear, while in others it involves
the sudden formation of martensitic plates.Comment: 13 pages, 7 figures. Soft Matter Latex template was used. Published
online in Soft Matter, 201
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