1,567 research outputs found
Can Nonlinear Hydromagnetic Waves Support a Self-Gravitating Cloud?
Using self-consistent magnetohydrodynamic (MHD) simulations, we explore the
hypothesis that nonlinear MHD waves dominate the internal dynamics of galactic
molecular clouds. We employ an isothermal equation of state and allow for
self-gravity. We adopt ``slab-symmetry,'' which permits motions
and fields perpendicular to the mean field, but permits gradients
only parallel to the mean field. The Alfv\'en speed exceeds the sound
speed by a factor . We simulate the free decay of a spectrum of
Alfv\'en waves, with and without self-gravity. We also perform simulations with
and without self-gravity that include small-scale stochastic forcing.
Our major results are as follows: (1) We confirm that fluctuating transverse
fields inhibit the mean-field collapse of clouds when the energy in Alfv\'en-
like disturbances remains comparable to the cloud's gravitational binding
energy. (2) We characterize the turbulent energy spectrum and density structure
in magnetically-dominated clouds. The spectra evolve to approximately
with ,
i.e. approximately consistent with a ``linewidth-size'' relation . The simulations show large density contrasts, with high
density regions confined in part by the fluctuating magnetic fields. (3) We
evaluate the input power required to offset dissipation through shocks, as a
function of , the velocity dispersion , and the scale
of the forcing. In equilibrium, the volume dissipation rate is
, for a cloud of
linear size and density . (4) Somewhat speculatively, we apply our
results to a ``typical'' molecular cloud. The mechanical power input requiredComment: Accepted for publication in Ap.J. 47 pages, 13 postscript figures.
Report also available at http://cfa-www.harvard.edu/~gammie/MHD.p
Dissipation in Compressible MHD Turbulence
We report results of a three dimensional, high resolution (up to 512^3)
numerical investigation of supersonic compressible magnetohydrodynamic
turbulence. We consider both forced and decaying turbulence. The model
parameters are appropriate to conditions found in Galactic molecular clouds. We
find that the dissipation time of turbulence is of order the flow crossing time
or smaller, even in the presence of strong magnetic fields. About half the
dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by
the turbulence, while strong fields remain well ordered.Comment: 5 pages, 3 Postscript figures, LaTeX, accepted by Ap.J.Let
On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds
Measurements of the star formation efficiency (SFE) of giant molecular clouds
(GMCs) in the Milky Way generally show a large scatter, which could be
intrinsic or observational. We use magnetohydrodynamic simulations of GMCs
(including feedback) to forward-model the relationship between the true GMC SFE
and observational proxies. We show that individual GMCs trace broad ranges of
observed SFE throughout collapse, star formation, and disruption. Low measured
SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall"
SFE where most stars actually form can be much larger. Very high (>>10%) values
are often artificially enhanced by rapid gas dispersal. Simulations including
stellar feedback reproduce observed GMC-scale SFEs, but simulations without
feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms
simulated. An anticorrelation of SFE with cloud mass is shown to be an
observational artifact. We also explore individual dense "clumps" within GMCs
and show that (with feedback) their bulk properties agree well with
observations. Predicted SFEs within the dense clumps are ~2x larger than
observed, possibly indicating physics other than feedback from massive (main
sequence) stars is needed to regulate their collapse.Comment: Fixed typo in the arXiv abstrac
Reactive vaccination as an effective tool for measles outbreak control in measles mortality reduction settings, Democratic Republic of Congo, 2005–2006
Neuropsychological and neurobehavioral outcome following childhood arterial ischemic stroke: Attention deficits, emotional dysregulation, and executive dysfunction.
Objectives. To investigate neuropsychological and neurobehavioral outcome in children with arterial ischemic stroke (AIS). Background. Childhood stroke can have consequences on motor, cognitive, and behavioral development. We present a cross-sectional study of neuropsychological and neurobehavioral outcome at least one year poststroke in a uniquely homogeneous sample of children who had experienced AIS. Method. Forty-nine children with AIS aged 6 to 18 years were recruited from a specialist clinic. Neuropsychological measures of intelligence, reading comprehension, attention, and executive function were administered. A triangulation of data collection included questionnaires completed by the children, their parents, and teachers, rating behavior, executive functions, and emotions. Key Findings. Focal neuropsychological vulnerabilities in attention (response inhibition and dual attention) and executive function were found, beyond general intellectual functioning, irrespective of hemispheric side of stroke. Difficulties with emotional and behavioral regulation were also found. Consistent with an "early plasticity" hypothesis, earlier age of stroke was associated with better performance on measures of executive function. Conclusions. A significant proportion of children poststroke are at long-term risk of difficulties with emotional regulation, executive function, and attention. Data also suggest that executive functions are represented in widespread networks in the developing brain and are vulnerable to unilateral injury
Chemistry and radiative shielding in star forming galactic disks
To understand the conditions under which dense, molecular gas is able to form
within a galaxy, we post-process a series of three-dimensional
galactic-disk-scale simulations with ray-tracing based radiative transfer and
chemical network integration to compute the equilibrium chemical and thermal
state of the gas. In performing these simulations we vary a number of
parameters, such as the ISRF strength, vertical scale height of stellar
sources, cosmic ray flux, to gauge the sensitivity of our results to these
variations. Self-shielding permits significant molecular hydrogen (H2)
abundances in dense filaments around the disk midplane, accounting for
approximately ~10-15% of the total gas mass. Significant CO fractions only form
in the densest, n>~10^3 cm^-3, gas where a combination of dust, H2, and
self-shielding attenuate the FUV background. We additionally compare these
ray-tracing based solutions to photochemistry with complementary models where
photo-shielding is accounted for with locally computed prescriptions. With some
exceptions, these local models for the radiative shielding length perform
reasonably well at reproducing the distribution and amount of molecular gas as
compared with a detailed, global ray tracing calculation. Specifically, an
approach based on the Jeans Length with a T=40K temperature cap performs the
best in regards to a number of different quantitative measures based on the H2
and CO abundances.Comment: 21 Pages, 15 figures. Submitted to MNRAS. Comments welcom
Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models
We use 3D numerical MHD simulations to follow the evolution of cold,
turbulent, gaseous systems with parameters representing GMC conditions. We
study three cloud simulations with varying mean magnetic fields, but identical
initial velocity fields. We show that turbulent energy is reduced by a factor
two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically
supercritical cloud models collapse after ~6 Myr, while the subcritical cloud
does not collapse. We compare density, velocity, and magnetic field structure
in three sets of snapshots with matched Mach numbers. The volume and column
densities are both log-normally distributed, with mean volume density a factor
3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4
times the unperturbed value. We use a binning algorithm to investigate the
dependence of kinetic quantities on spatial scale for regions of column density
contrast (ROCs). The average velocity dispersion for the ROCs is only weakly
correlated with scale, similar to the mean size-linewidth relation for clumps
within GMCs. ROCs are often superpositions of spatially unconnected regions
that cannot easily be separated using velocity information; the same difficulty
may affect observed GMC clumps. We analyze magnetic field structure, and show
that in the high density regime, total magnetic field strengths increase with
density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field
strengths vary widely across a projected cloud, and do not correlate with
column density. We compute simulated interstellar polarization maps at varying
orientations, and determine that the Chandrasekhar-Fermi formula multiplied by
a factor ~0.5 yields a good estimate of the plane-of sky magnetic field
strength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte
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