624 research outputs found
The Stability of Magnetized Rotating Plasmas with Superthermal Fields
During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated, rotating fluids and
argue that, for these systems, the curvature of toroidal field lines cannot be
neglected even when short wavelength perturbations are considered. We also
comment on the implications of our results for the validity of shearing box
simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and
5 substantially expanded, added Appendix A and 3 figures with respect to
previous version. Animations are available at
http://www.physics.arizona.edu/~mpessah/research
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
Numerical Models of Viscous Accretion Flows Near Black Holes
We report on a numerical study of viscous fluid accretion onto a black hole.
The flow is axisymmetric and uses a pseudo-Newtonian potential to model
relativistic effects near the event horizon. The numerical method is a variant
of the ZEUS code. As a test of our numerical scheme, we are able to reproduce
results from earlier, similar work by Igumenshchev and Abramowicz and Stone et
al. We consider models in which mass is injected onto the grid as well as
models in which an initial equilibrium torus is accreted. In each model we
measure three ``eigenvalues'' of the flow: the accretion rate of mass, angular
momentum, and energy. We find that the eigenvalues are sensitive to r_{in}, the
location of the inner radial boundary. Only when the flow is always supersonic
on the inner boundary are the eigenvalues insensitive to small changes in
r_{in}. We also report on the sensitivity of the results to other numerical
parameters.Comment: 14 pages, 4 figures, 2 tables, to appear in v573 n2 pt1 ApJ July 10,
200
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling
We consider the nonlinear outcome of gravitational instability in optically
thick disks with a realistic cooling function. We use a numerical model that is
local, razor-thin, and unmagnetized. External illumination is ignored. Cooling
is calculated from a one-zone model using analytic fits to low temperature
Rosseland mean opacities. The model has two parameters: the initial surface
density Sigma_0 and the rotation frequency Omega. We survey the parameter space
and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an
effective cooling time defined as the average internal energy of the model
divided by the average cooling rate. This is consistent with earlier results
that used a simplified cooling function. (2) The initial cooling time t_c0 or a
uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the
nonlinear outcome. The difference is caused by sharp variations in the opacity
with temperature. The condition t_c0 Omega = 1 therefore does not necessarily
indicate where fragmentation will occur. (3) The largest difference between
t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is
largely molecular. (4) In the limit of strong illumination the disk is
isothermal; we find that an isothermal version of our model fragments for Q <
1.4. Finally, we discuss some physical processes not included in our model, and
find that most are likely to make disks more susceptible to fragmentation. We
conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure
Rapidly Spinning Black Holes in Quasars: An Open Question
Wang et al. (2006) estimated an average radiative efficiency of 30% -- 35%
for quasars at moderate redshift. We find that their method is not independent
of quasar lifetimes and thus that quasars do not necessarily have such high
efficiencies. Nonetheless, it is possible to derive interrelated constraints on
quasar lifetimes, Eddington ratios, and radiative efficiencies of supermassive
black holes. We derive such constraints using a statistically complete sample
of quasars with black hole mass estimates from broad Mg II, made both with and
without the radiation pressure correction of Marconi et al. (2008). We conclude
that for quasars with L/LEdd > 0.02, lifetimes can range from 140 to 750 Myr
for Schwarzschild black holes. Coupled with observed black hole masses, quasar
lifetimes of < 140 Myr would imply that radiatively inefficient accretion or BH
mergers must be important in the accretion history of quasars. Given reasonable
assumptions about the quasar population, if the average quasar lifetime is < 1
Gyr, and if radiatively inefficient accretion is unimportant, then not many BHs
with Eddington ratio < 0.2 can be rapidly spinning.Comment: 14 pages, 3 figures, accepted by ApJ on Sep 200
On the evolution of mean motion resonances through stochastic forcing: Fast and slow libration modes and the origin of HD128311
Aims. We clarify the response of extrasolar planetary systems in a 2:1 mean
motion commensurability with masses ranging from the super Jovian range to the
terrestrial range to stochastic forcing that could result from protoplanetary
disk turbulence. The behaviour of the different libration modes for a wide
range of system parameters and stochastic forcing magnitudes is investigated.
The growth of libration amplitudes is parameterized as a function of the
relevant physical parameters. The results are applied to provide an explanation
of the configuration of the HD128311 system.
Methods. We first develop an analytic model from first principles without
making the assumption that both eccentricities are small. We also perform
numerical N-body simulations with additional stochastic forcing terms to
represent the effects of putative disk turbulence.
Results. Systems are quickly destabilized by large magnitudes of stochastic
forcing but some stability is imparted should systems undergo a net orbital
migration. The slow mode, which mostly corresponds to motion of the angle
between the apsidal lines of the two planets, is converted to circulation more
readily than the fast mode which is associated with oscillations of the
semi-major axes. This mode is also vulnerable to the attainment of small
eccentricities which causes oscillations between periods of libration and
circulation.
Conclusions. Stochastic forcing due to disk turbulence may have played a role
in shaping the configurations of observed systems in mean motion resonance. It
naturally provides a mechanism for accounting for the HD128311 system.Comment: 15 pages, 8 figures, added discussion in h and k coordinates,
recommended for publicatio
Altered maternal profiles in corticotropin-releasing factor receptor 1 deficient mice
BACKGROUND: During lactation, the CNS is less responsive to the anxiogenic neuropeptide, corticotropin-releasing factor (CRF). Further, central injections of CRF inhibit maternal aggression and some maternal behaviors, suggesting decreased CRF neurotransmission during lactation supports maternal behaviors. In this study, we examined the maternal profile of mice missing the CRF receptor 1 (CRFR1). Offspring of knockout (CRFR1-/-) mice were heterozygote to offset possible deleterious effects of low maternal glucocorticoids on pup survival and all mice contained a mixed 50:50 inbred/outbred background to improve overall maternal profiles and fecundity. RESULTS: Relative to littermate wild-type (WT) controls, CRFR1-/- mice exhibited significant deficits in total time nursing, including high arched-back, on each test day. Consistent with decreased nursing, pups of CRFR1-deficient dams weighed significantly less than WT offspring. Licking and grooming of pups was significantly higher in WT mice on postpartum Day 2 and when both test days were averaged, but not on Day 3. Time off nest was higher for CRFR1-/- mice on Day 2, but not on Day 3 or when test days were averaged. Licking and grooming of pups did not differ on Day 2 when this measure was examined as a proportion of time on nest. CRFR1-/- mice showed significantly higher nest building on Day 3 and when tests were averaged. Mean pup number was almost identical between groups and no pup mortality occurred. Maternal aggression was consistently lower in CRFR1-/- mice and in some measures these differences approached, but did not reach significance. Because of high variance, general aggression results are viewed as preliminary. In terms of sites of attacks on intruders, CRFR1-/- mice exhibited significantly fewer attacks to the belly of the intruder on Day 5 and when tests were averaged. Performance on the elevated plus maze was similar between genotypes. Egr-1 expression differences in medial preoptic nucleus and c-Fos expression differences in bed nucleus of stria terminalis between genotype suggest possible sites where loss of gene alters behavioral output. CONCLUSION: Taken together, the results suggest that the presence of an intact CRFR1 receptor supports some aspects of nurturing behavior
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