905 research outputs found
Dissipation-Scale Turbulence in the Solar Wind
We present a cascade model for turbulence in weakly collisional plasmas that
follows the nonlinear cascade of energy from the large scales of driving in the
MHD regime to the small scales of the kinetic Alfven wave regime where the
turbulence is dissipated by kinetic processes. Steady-state solutions of the
model for the slow solar wind yield three conclusions: (1) beyond the observed
break in the magnetic energy spectrum, one expects an exponential cut-off; (2)
the widely held interpretation that this dissipation range obeys power-law
behavior is an artifact of instrumental sensitivity limitations; and, (3) over
the range of parameters relevant to the solar wind, the observed variation of
dissipation range spectral indices from -2 to -4 is naturally explained by the
varying effectiveness of Landau damping, from an undamped prediction of -7/3 to
a strongly damped index around -4.Comment: 6 pages, 2 figures, accepted for publication in AIP Conference
Proceedings on "Turbulence and Nonlinear Processes in Astrophysical Plasmas
Shearing Box Simulations of the MRI in a Collisionless Plasma
We describe local shearing box simulations of turbulence driven by the
magnetorotational instability (MRI) in a collisionless plasma. Collisionless
effects may be important in radiatively inefficient accretion flows, such as
near the black hole in the Galactic Center. The MHD version of ZEUS is modified
to evolve an anisotropic pressure tensor. A fluid closure approximation is used
to calculate heat conduction along magnetic field lines. The anisotropic
pressure tensor provides a qualitatively new mechanism for transporting angular
momentum in accretion flows (in addition to the Maxwell and Reynolds stresses).
We estimate limits on the pressure anisotropy due to pitch angle scattering by
kinetic instabilities. Such instabilities provide an effective ``collision''
rate in a collisionless plasma and lead to more MHD-like dynamics. We find that
the MRI leads to efficient growth of the magnetic field in a collisionless
plasma, with saturation amplitudes comparable to those in MHD. In the saturated
state, the anisotropic stress is comparable to the Maxwell stress, implying
that the rate of angular momentum transport may be moderately enhanced in a
collisionless plasma.Comment: 20 pages, 9 figures, submitted to Ap
The Cooling Flow to Accretion Flow Transition
Cooling flows in galaxy clusters and isolated elliptical galaxies are a
source of mass for fueling accretion onto a central supermassive black hole. We
calculate the dynamics of accreting matter in the combined gravitational
potential of a host galaxy and a central black hole assuming a steady state,
spherically symmetric flow (i.e., no angular momentum). The global dynamics
depends primarily on the accretion rate. For large accretion rates, no simple,
smooth transition between a cooling flow and an accretion flow is possible; the
gas cools towards zero temperature just inside its sonic radius, which lies
well outside the region where the gravitational influence of the central black
hole is important. For accretion rates below a critical value, however, the
accreting gas evolves smoothly from a radiatively driven cooling flow at large
radii to a nearly adiabatic (Bondi) flow at small radii. We argue that this is
the relevant parameter regime for most observed cooling flows. The transition
from the cooling flow to the accretion flow should be observable in M87 with
the {\it Chandra X-ray Observatory}.Comment: emulateapj.sty, 10 pages incl. 5 figures, to appear in Ap
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
On the Conditions for Neutron-Rich Gamma-Ray Burst Outflows
We calculate the structure and neutron content of neutrino-heated MHD winds
driven from the surface of newly-formed magnetars (``proto-magnetars'') and
from the midplane of hyper-accreting disks, two of the possible central engines
for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the
surface of proto-magnetars and the midplane of neutrino-cooled accretion flows
(NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p
>> 1). If this substantial free neutron excess is preserved to large radii in
ultra-relativistic outflows, several important observational consequences may
result. Weak interaction processes, however, can drive n/p to ~1 in the
nondegenerate regions that obtain just above the surfaces of NDAFs and
proto-magnetars. Our calculations show that mildly relativistic neutron-rich
outflows from NDAFs are possible in the presence of a strong poloidal magnetic
field. However, we find that neutron-rich winds possess a minimum mass-loss
rate that likely precludes simultaneously neutron-rich and ultra-relativistic
(Lorentz factor > 100) NDAF winds accompanying a substantial accretion power.
In contrast, proto-magnetars are capable of producing neutron-rich
long-duration GRB outflows ~10-30 seconds following core bounce for
sub-millisecond rotation periods; such outflows would, however, accompany only
extremely energetic events, in which the GRB + SN energy budget exceeds ~ 4e52
ergs. Neutron-rich highly relativistic outflows may also be produced during
some short-duration GRBs by geometrically thick accretion disks formed from
compact object mergers. The implications for r-process nucleosynthesis, optical
transients due to non-relativistic neutron-rich winds, and Nickel production in
proto-magnetar and NDAF winds are also briefly discussed.Comment: 24 pages, 7 figures, submitted to Ap
Analysis of Climate Policy Targets under Uncertainty
Abstract and PDF report are also available on the MIT Joint Program on the Science and Policy of Global Change website (http://globalchange.mit.edu/).Although policymaking in response to the climate change is essentially a challenge of risk management, most studies of the relation of emissions targets to desired climate outcomes are either deterministic or subject to a limited representation of the underlying uncertainties. Monte Carlo simulation, applied to the MIT Integrated Global System Model (an integrated economic and earth system model of intermediate complexity), is used to analyze the uncertain outcomes that flow from a set of century-scale emissions targets developed originally for a study by the U.S. Climate Change Science Program. Results are shown for atmospheric concentrations, radiative forcing, sea ice cover and temperature change, along with estimates of the odds of achieving particular target levels, and for the global costs of the associated mitigation policy. Comparison with other studies of climate targets are presented as evidence of the value, in understanding the climate challenge, of more complete analysis of uncertainties in human emissions and climate system response.This study received support from the MIT Joint Program on the Science and Policy of Global Change, which is funded by a consortium of government, industry and foundation sponsors
Molecular Cloning and Heterologous Expression of the Dehydrophos Biosynthetic Gene Cluster
SummaryDehydrophos is a vinyl phosphonate tripeptide produced by Streptomyces luridus with demonstrated broad-spectrum antibiotic activity. To identify genes necessary for biosynthesis of this unusual compound we screened a fosmid library of S. luridus for the presence of the phosphoenolpyruvate mutase gene, which is required for biosynthesis of most phosphonates. Integration of one such fosmid clone into the chromosome of S. lividans led to heterologous production of dehydrophos. Deletion analysis of this clone allowed identification of the minimal contiguous dehydrophos cluster, which contained 17 open reading frames (ORFs). Bioinformatic analyses of these ORFs are consistent with a proposed biosynthetic pathway that generates dehydrophos from phosphoenolpyruvate. The early steps of this pathway are supported by analysis of intermediates accumulated by blocked mutants and in vitro biochemical experiments
Tree-Based Methods for Discovery of Association between Flow Cytometry Data and Clinical Endpoints
We demonstrate the application and comparative interpretations of
three tree-based algorithms for the analysis of data arising from
flow cytometry: classification and regression trees (CARTs), random
forests (RFs), and logic regression (LR). Specifically, we consider
the question of what best predicts CD4 T-cell recovery in HIV-1
infected persons starting antiretroviral therapy with CD4 count
between 200 and 350 cell/μL. A comparison to a more standard
contingency table analysis is provided. While contingency table
analysis and RFs provide information on the importance of each
potential predictor variable, CART and LR offer additional insight
into the combinations of variables that together are predictive of
the outcome. In all cases considered, baseline CD3-DR-CD56+CD16+
emerges as an important predictor variable, while the tree-based
approaches identify additional variables as potentially informative.
Application of tree-based methods to our data suggests that a
combination of baseline immune activation states, with emphasis on
CD8 T-cell activation, may be a better predictor than any single
T-cell/innate cell subset analyzed. Taken together, we show that
tree-based methods can be successfully applied to flow cytometry data
to better inform and discover associations that may not emerge in
the context of a univariate analysis
Electron Heating in Hot Accretion Flows
Local (shearing box) simulations of the nonlinear evolution of the
magnetorotational instability in a collisionless plasma show that angular
momentum transport by pressure anisotropy (, where the
directions are defined with respect to the local magnetic field) is comparable
to that due to the Maxwell and Reynolds stresses. Pressure anisotropy, which is
effectively a large-scale viscosity, arises because of adiabatic invariants
related to and in a fluctuating magnetic field. In a
collisionless plasma, the magnitude of the pressure anisotropy, and thus the
viscosity, is determined by kinetic instabilities at the cyclotron frequency.
Our simulations show that % of the gravitational potential energy is
directly converted into heat at large scales by the viscous stress (the
remaining energy is lost to grid-scale numerical dissipation of kinetic and
magnetic energy). We show that electrons receive a significant fraction () of this dissipated energy. Employing this heating by an
anisotropic viscous stress in one dimensional models of radiatively inefficient
accretion flows, we find that the radiative efficiency of the flow is greater
than 0.5% for . Thus a low accretion
rate, rather than just a low radiative efficiency, is necessary to explain the
low luminosity of many accreting black holes. For Sgr A* in the Galactic
Center, our predicted radiative efficiencies imply an accretion rate of
and an electron temperature of
K at Schwarzschild radii; the latter is
consistent with the brightness temperature inferred from VLBI observations.Comment: to appear in Oct 1 2007 issue of ApJ; corrected grant information and
the firehose threshold typ
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