1,477 research outputs found
Toward realistic simulations of magneto-thermal winds from weakly-ionized protoplanetary disks
Protoplanetary disks (PPDs) accrete onto their central T Tauri star via
magnetic stresses. When the effect of ambipolar diffusion (AD) is included, and
in the presence of a vertical magnetic field, the disk remains laminar between
1-5 au, and a magnetocentrifugal disk wind forms that provides an important
mechanism for removing angular momentum. We present global MHD simulations of
PPDs that include Ohmic resistivity and AD, where the time-dependent gas-phase
electron and ion fractions are computed under FUV and X-ray ionization with a
simplified recombination chemistry. To investigate whether the mass loading of
the wind is potentially affected by the limited vertical extent of our existing
simulations, we attempt to develop a model of a realistic disk atmosphere. To
this end, by accounting for stellar irradiation and diffuse reprocessing of
radiation, we aim at improving our models towards more realistic thermodynamic
properties.Comment: 8 pages, 3 figures, ASTRONUM-2016 proceeding
fNL - gNL mixing in the matter density field at higher orders
In this paper we examine how primordial non-Gaussianity contributes to
nonlinear perturbative orders in the expansion of the density field at large
scales in the matter dominated era. General Relativity is an intrinsically
nonlinear theory, establishing a nonlinear relation between the metric and the
density field. Representing the metric perturbations with the curvature
perturbation zeta, it is known that nonlinearity produces effective
non-Gaussian terms in the nonlinear perturbations of the matter density field,
even if the primordial zeta is Gaussian. Here we generalise these results to
the case of a non-Gaussian primordial zeta. Using a standard parametrization of
primordial non-Gaussianity in zeta in terms of fNL, gNL, hNL..., we show how at
higher order (from third and higher) nonlinearity also produces a mixing of
these contributions to the density field at large scales, e.g. both fNL and gNL
contribute to the third order in the density contrast. This is the main result
of this paper. Our analysis is based on the synergy between a gradient
expansion (aka long-wavelength approximation) and standard perturbation theory
at higher order. In essence, mathematically the equations for the gradient
expansion are equivalent to those of first order perturbation theory, thus
first-order results convert into gradient expansion results and, vice versa,
the gradient expansion can be used to derive results in perturbation theory at
higher order and large scales
Toward a hybrid dynamo model for the Milky Way
(Abridged) Based on the rapidly increasing all-sky data of Faraday rotation
measures and polarised synchrotron radiation, the Milky Way's magnetic field is
now modelled with an unprecedented level of detail and complexity. We aim to
complement this heuristic approach with a physically motivated, quantitative
Galactic dynamo model -- a model that moreover allows for the evolution of the
system as a whole, instead of just solving the induction equation for a fixed
static disc. Building on the framework of mean-field magnetohydrodynamics and
extending it to the realm of a hybrid evolution, we perform three-dimensional
global simulations of the Galactic disc. Closure coefficients embodying the
mean-field dynamo are calibrated against resolved box simulations of
supernova-driven interstellar turbulence. The emerging dynamo solutions
comprise a mixture of the dominant axisymmetric S0 mode, with even parity, and
a subdominant A0 mode, with odd parity. Notably, such a superposition of modes
creates a strong localised vertical field on one side of the Galactic disc. We
moreover find significant radial pitch angles, which decay with radius --
explained by flaring of the disc. In accordance with previous work, magnetic
instabilities appear to be restricted to the less-stirred outer Galactic disc.
Their main effect is to create strong fields at large radii such that the
radial scale length of the magnetic field increases from 4 kpc (for the case of
a mean-field dynamo alone) to about 10 kpc in the hybrid models. There remain
aspects (e.g., spiral arms, X-shaped halo fields, fluctuating fields) that are
not captured by the current model and that will require further development
towards a fully dynamical evolution. Nevertheless, the work presented
demonstrates that a hybrid modelling of the Galactic dynamo is feasible and can
serve as a foundation for future efforts.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in A&
Do magnetic fields influence gas rotation in galaxies?
We aim to estimate the contribution of the radial component of the Lorentz
force to the gas rotation in several types of galaxies. Using typical
parameters for the exponential scale of synchrotron emission and the scale
length of HI gas, under the assumption of equipartition between the energies of
cosmic rays and total magnetic fields, we derive the Lorentz force and compare
it to the gravitational force in the radial component of the momentum equation.
We distinguish the different contributions between the large-scale and the
small-scale turbulent fields by Reynolds averaging. We compare these findings
with a dynamical dynamo model. We find a possible reduction of circular gas
velocity in the very outer parts and an increase inside a radius of four times
the synchrotron scale length. Sufficiently localized radial reversals of the
magnetic field may cause characteristic modulations in the gas rotation curve
with typical amplitudes of 10-20 km/s. It is unlikely that the magnetic field
contributes to the flat rotation in the outer parts of galaxies. If anything,
it will \emph{impede} the gravitationally supported rotation, demanding for an
even higher halo mass to explain the observed rotation profile. We speculate
that this may have consequences for ram pressure stripping and the truncation
of the stellar disc
The pause-initiation limit restricts transcription activation in human cells.
Eukaryotic gene transcription is often controlled at the level of RNA polymerase II (Pol II) pausing in the promoter-proximal region. Pausing Pol II limits the frequency of transcription initiation ('pause-initiation limit'), predicting that the pause duration must be decreased for transcriptional activation. To test this prediction, we conduct a genome-wide kinetic analysis of the heat shock response in human cells. We show that the pause-initiation limit restricts transcriptional activation at most genes. Gene activation generally requires the activity of the P-TEFb kinase CDK9, which decreases the duration of Pol II pausing and thereby enables an increase in the productive initiation frequency. The transcription of enhancer elements is generally not pause limited and can be activated without CDK9 activity. Our results define the kinetics of Pol II transcriptional regulation in human cells at all gene classes during a natural transcription response
New scaling for the alpha effect in slowly rotating turbulence
Using simulations of slowly rotating stratified turbulence, we show that the
alpha effect responsible for the generation of astrophysical magnetic fields is
proportional to the logarithmic gradient of kinetic energy density rather than
that of momentum, as was previously thought. This result is in agreement with a
new analytic theory developed in this paper for large Reynolds numbers. Thus,
the contribution of density stratification is less important than that of
turbulent velocity. The alpha effect and other turbulent transport coefficients
are determined by means of the test-field method. In addition to forced
turbulence, we also investigate supernova-driven turbulence and stellar
convection. In some cases (intermediate rotation rate for forced turbulence,
convection with intermediate temperature stratification, and supernova-driven
turbulence) we find that the contribution of density stratification might be
even less important than suggested by the analytic theory.Comment: 10 pages, 9 figures, revised version, Astrophys. J., in pres
Compulsory Sterilization: Equal Protection and the Quality of Life
North Carolina Ass\u27n for Retarded Children v. North Carolina, 420 F. Supp. 451 (M.D.N.C. 1975).
Compulsory sterilization statutes grew out of the eugenics movement of the early twentieth century. Eugenic theory maintains that human defects are the result of heredity and that the race can be improved by encouraging procreation of superior persons and preventing the procreation of inferior persons. This theory was once widely accepted, and at the height of its popularity the Supreme Court upheld the constitutionality of a compulsory sterilization statute. Although the scientific community now largely rejects compulsory eugenic sterilization, the Supreme Court has not reconsidered the matter since 1927. In light of scientific rejection of eugenics as a basis for involuntary sterilization and recent decisions regarding familial and personal autonomy and privacy, the Court could be expected to find compulsory sterilization statutes unconstitutional. Yet a lower federal court, recently presented with a challenge to North Carolina\u27s modern compulsory eugenic sterilization statute, upheld it against equal protection and procedural and substantive due process attacks, although it held that the right to procreate is fundamental. The statute challenged in North Carolina Ass\u27n for Retarded Children v. North Carolina permits sterilization not only of persons likely to produce a retarded child, but also of persons who would, because retarded, be unable to care for normal children born to them. This was considered an expression of two state interests: a eugenic interest; that is, the prevention of the birth of retarded children, and an interest in prevention of poor parenting. Using an equal protection analysis, the court found that these state interests were compelling enough to permit interference with a fundamental right.
This note proposes to examine these state interests and to suggest that they, in fact, are not compelling. Additionally, this note suggests that, although the court\u27s construction of the statute has emasculated it with respect to the state\u27s eugenic interest, acceptance of a state interest in preventing poor parenting allows sterilization of citizens on the basis of a theory of questionable constitutionality
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