648 research outputs found
Anisotropic Thermal Conduction in Supernova Remnants: Relevance to Hot Gas Filling Factors in the Magnetized ISM
We explore the importance of anisotropic thermal conduction in the evolution
of supernova remnants via numerical simulations. The mean temperature of the
bubble of hot gas is decreased by a factor of ~3 compared to simulations
without thermal conduction, together with an increase in the mean density of
hot gas by a similar factor. Thus, thermal conduction greatly reduces the
volume of hot gas produced over the life of the remnant. This underscores the
importance of thermal conduction in estimating the hot gas filling fraction and
emissivities in high-stage ions in Galactic and proto-galactic ISMs.Comment: Submitted to Astrophysical Journal Letters. 4 pages, 3 figure
Dust Stratification in Young Circumstellar Disks
We present high-resolution infrared spectra of four YSOs (T Tau N, T Tau S,
RNO 91, and HL Tau). The spectra exhibit narrow absorption lines of 12CO, 13CO,
and C18O as well as broad emission lines of gas phase12CO. The narrow
absorption lines of CO are shown to originate from the colder circumstellar
gas. We find that the line of sight gas column densities resulting from the CO
absorption lines are much higher than expected for the measured extinction for
each source and suggest the gas to dust ratio is measuring the dust settling
and/or grain coagulation in these extended disks. We provide a model of
turbulence, dust settling and grain growth to explain the results. The
techniques presented here allow us to provide some observationally-motivated
bounds on accretion disk alpha in protostellar systems
Direct Evidence for Two-Fluid Effects in Molecular Clouds
We present a combination of theoretical and simulation-based examinations of
the role of two-fluid ambipolar drift on molecular line widths. The dissipation
provided by ion-neutral interactions can produce a significant difference
between the widths of neutral molecules and the widths of ionic species,
comparable to the sound speed. We demonstrate that Alfven waves and certain
families of magnetosonic waves become strongly damped on scales comparable to
the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid
turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show
that the wave damping causes the power spectrum of the ion velocity to drop
below that of the neutral velocity when measured on a relative basis. Following
a set of motivational observations by Li & Houde (2008), we produce synthetic
line width-size relations that shows a difference between the ion and neutral
line widths, illustrating that two-fluid effects can have an observationally
detectable role in modifying the MHD turbulence in the clouds.Comment: 18 pages, 4 figures, submitted to MNRA
Phase vortices from a Young's three-pinhole interferometer
An analysis is presented of the phase vortices generated in the far field, by
an arbitrary arrangement of three monochromatic point sources of complex
spherical waves. In contrast with the case of three interfering plane waves, in
which an infinitely-extended vortex lattice is generated, the spherical sources
generate a finite number of phase vortices. Analytical expressions for the
vortex core locations are developed and shown to have a convenient
representation in a discrete parameter space. Our analysis may be mapped onto
the case of a coherently-illuminated Young's interferometer, in which the
screen is punctured by three rather than two pinholes.Comment: 10 pages, 8 figures, REVTeX4, Submitted to Phys. Rev.
Bogomol'nyi Limit For Magnetic Vortices In Rotating Superconductor
This work is the sequel of a previous investigation of stationary and
cylindrically symmetric vortex configurations for simple models representing an
incompressible non-relativistic superconductor in a rigidly rotating
background. In the present paper, we carry out our analysis with a generalized
Ginzburg-Landau description of the superconductor, which provides a
prescription for the radial profile of the normal density within the vortex.
Within this framework, it is shown that the Bogomol'nyi limit condition marking
the boundary between type I and type II behavior is unaffected by the rotation
of the background.Comment: 7 pages, uses RevTeX, submitted to Phys.Rev.
Gravitational Collapse of Filamentary Magnetized Molecular Clouds
We develop models for the self-similar collapse of magnetized isothermal
cylinders. We find solutions for the case of a fluid with a constant toroidal
flux-to-mass ratio (Gamma_phi=constant) and the case of a fluid with a constant
gas to magnetic pressure ratio (beta=constant). In both cases, we find that a
low magnetization results in density profiles that behave as rho ~ r^{-4} at
large radii, and at high magnetization we find density profiles that behave as
rho ~ r^{-2}. This density behaviour is the same as for hydrostatic filamentary
structures, suggesting that density measurements alone cannot distinguish
between hydrostatic and collapsing filaments--velocity measurements are
required. Our solutions show that the self-similar radial velocity behaves as
v_r ~ r during the collapse phase, and that unlike collapsing self-similar
spheres, there is no subsequent accretion (i.e. expansion-wave) phase. We also
examine the fragmentation properties of these cylinders, and find that in both
cases, the presence of a toroidal field acts to strengthen the cylinder against
fragmentation. Finally, the collapse time scales in our models are shorter than
the fragmentation time scales. Thus, we anticipate that highly collapsed
filaments can form before they are broken into pieces by gravitational
fragmentation.Comment: 20 pages, 4 figures, accepted to Ap
Simulations of Mixed Morphology Supernova Remnants With Anisotropic Thermal Conduction
We explore the role of anisotropic thermal conduction on the evolution of
supernova remnants through interstellar media with a range of densities via
numerical simulations. We find that a remnant expanding in a dense environment
can produce centre-bright hard x-ray emission within 20 kyr, and centre-bright
soft x-ray emission within 60 kyr of the supernova event. In a more tenuous
environment, the appearance of a centre-bright structure in hard x-rays is
delayed until about 60 kyr. The soft x-ray emission from such a remnant may not
become centre bright during its observable lifetime. This can explain the
observations that show that mixed-morphology supernova remnants preferentially
occur close to denser, molecular environments. Remnants expanding into denser
environments tend to be smaller, making it easier for thermal conduction to
make larger changes in the temperatures of their hot gas bubbles. We show that
the lower temperatures make it very favorable to use high-stage ions as
diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of
O VIII transitions from shell-bright at early epochs to centre-bright at later
epochs in the evolution of an SNR expanding in a dense ISM when the physics of
thermal conduction is included.Comment: 8 pages, 5 figures, submitted to Monthly Notice
A Two-Fluid Method for Ambipolar Diffusion
We present a semi-implicit method for isothermal two-fluid ion-neutral
ambipolar drift that is second-order accurate in space and time. The method has
been implemented in the RIEMANN code for astrophysical fluid dynamics. We
present four test problems that show the method works and correctly tracks the
propagation of MHD waves and the structure of two-fluid C-shocks. The accurate
propagation of MHD waves in the two-fluid approximation is shown to be a
stringent test of the algorithm. We demonstrate that highly accurate methods
are required in order to properly capture the MHD wave behaviour in the
presence of ion-neutral friction.Comment: 29 pages, 16 figures, accepted to MNRA
Simulating Anisotropic Thermal Conduction in Supernova Remnants, Implications for the Interstellar Medium
We present a large number of two and a half dimensional simulations of
supernova remnants expanding into interstellar media having a range of
densities, temperatures and magnetic field strengths. The volume of hot gas
produced is strongly dependent on the inclusion of thermal conduction and
magnetic fields. The four-volumes and three-areas of hot gas have been
catalogued and their dependence on interstellar parameters documented.
Simulated line widths of radioactive species ejected by supernovae have also
been catalogued
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