11 research outputs found
A hypothetical effect of the Maxwell-Proca electromagnetic stresses on galaxy rotation curves
The Maxwell-Proca electrodynamics corresponding to a finite photon mass
causes a substantial change of the Maxwell stress tensor and, under certain
circumstances, may cause the electromagnetic stresses to act effectively as
"negative pressure." The paper describes a model where this negative pressure
imitates gravitational pull and may produce forces comparable to gravity and
even become dominant. The effect is associated with the random magnetic fields
in the galactic disk with a scale exceeding the photon Compton wavelength. The
presence of a weaker regular field does not affect the forces under
consideration. The stresses act predominantly on the interstellar gas and cause
an additional force pulling the gas towards the center and towards the galactic
plane. The stars do not experience any significant direct force but get
involved in this process via a "recycling loop" where rapidly evolving massive
stars are formed from the gas undergoing galactic rotation and then lose their
masses back to the gas within a time shorter than roughly 1/6 of the rotation
period. This makes their dynamics inseparable from that of the rotating gas.
The lighter, slowly evolving stars, as soon as they are formed, lose connection
to the gas and are confined within the galaxy only gravitationally. Numerical
examples based on the parameters of our galaxy reveal both opportunities and
challenges of this model and motivate further analysis. The critical issue is
the plausibility of formation of the irregular magnetic field that would be
force free. Another challenge is developing a predictive model of the evolution
of the gaseous and stellar population of the galaxy under the aforementioned
scenario. It may be interesting to also explore possible broader cosmological
implications of the negative-pressure model.Comment: 29 pages, 1 figur
Formation of Pillars at the Boundaries between H II Regions and Molecular Clouds
We investigate numerically the hydrodynamic instability of an ionization
front (IF) accelerating into a molecular cloud, with imposed initial
perturbations of different amplitudes. When the initial amplitude is small, the
imposed perturbation is completely stabilized and does not grow. When the
initial perturbation amplitude is large enough, roughly the ratio of the
initial amplitude to wavelength is greater than 0.02, portions of the IF
temporarily separate from the molecular cloud surface, locally decreasing the
ablation pressure. This causes the appearance of a large, warm HI region and
triggers nonlinear dynamics of the IF. The local difference of the ablation
pressure and acceleration enhances the appearance and growth of a multimode
perturbation. The stabilization usually seen at the IF in the linear regimes
does not work due to the mismatch of the modes of the perturbations at the
cloud surface and in density in HII region above the cloud surface. Molecular
pillars are observed in the late stages of the large amplitude perturbation
case. The velocity gradient in the pillars is in reasonably good agreement with
that observed in the Eagle Nebula. The initial perturbation is imposed in three
different ways: in density, in incident photon number flux, and in the surface
shape. All cases show both stabilization for a small initial perturbation and
large growth of the second harmonic by increasing amplitude of the initial
perturbation above a critical value.Comment: 21 pages, 8 figures, accepted for publication in ApJ. high resolution
figures available upon reques
Modelling of Field-Reversed Configuration Experiment with Large Safety Factor Physics of Plasmas Modelling of Field-reversed configuration experiment with large safety factor
Abstract The Translation-Confinement-Sustainment facility has been operated in the "translationformation" mode in which a plasma is ejected at high-speed from a θ-pinch-like source into a confinement chamber where it settles into a field-reversed-configuration state. Measurements of the poloidal and toroidal field have been the basis of modeling to infer the safety factor. It is found that the edge safety factor exceeds two, and that there is strong forward magnetic shear. The high-q arises because the large elongation compensates for the modest ratio of toroidal-topoloidal field in the plasma. This is the first known instance of a very high-β plasma with a safety factor greater than unity. Two-fluid modeling of the measurements also indicate several other significant features: a broad "transition layer" at the plasma boundary with probable linetying effects, complex high-speed flows, and the appearance of a two-fluid minimum-energy state in the plasma core. All these features may contribute to both the stability and good confinement of the plasma
The effect of artificial diffusivity on the flute instability
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Nonlinear Dynamics of Ionization Fronts in HII Regions Nonlinear Dynamics of Ionization Fronts in HII Regions
Abstract Hydrodynamic instability of an accelerating ionization front (IF) is investigated with 2D hydrodynamic simulations, including absorption of incident photoionizing photons, recombination in the HII region, and radiative molecular cooling. When the amplitude of the perturbation is large enough, nonlinear dynamics of the IF triggered by the separation of the IF from the cloud surface is observed. This causes the second harmonic of the imposed perturbation to appear on the cloud surfaces, whereas the perturbation in density of ablated gas in the HII region remains largely single mode. This mismatch of modes between the IF and the density perturbation in the HII region prevents the strong stabilization effect seen in the linear regime. Large growth of the perturbation caused by Rayleigh-Taylor-like instability is observed late in time