41 research outputs found
On the formation of density filaments in the turbulent interstellar medium
This study is motivated by recent observations on ubiquitous interstellar
density filaments and guided by modern theories of compressible
magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the
observed density structures. As the fundamental dynamics of compressible MHD
turbulence, perpendicular turbulent mixing of density fluctuations entails
elongated density structures aligned with the local magnetic field, accounting
for low-density parallel filaments seen in diffuse atomic and molecular gas.
The elongation of low-density parallel filaments depends on the turbulence
anisotropy. When taking into account the partial ionization, we find that the
minimum width of parallel filaments in the cold neutral medium and molecular
clouds is determined by the neutral-ion decoupling scale perpendicular to
magnetic field. In highly supersonic MHD turbulence in molecular clouds, both
low-density parallel filaments due to anisotropic turbulent mixing and
high-density filaments due to shock compression exist.Comment: 13 pages, 6 figures, accepted for publication in ApJ. arXiv admin
note: text overlap with arXiv:1802.0098
The Efficiency of Magnetic Field Amplification at Shocks by Turbulence
Turbulent dynamo field amplification has often been invoked to explain the
strong field strengths in thin rims in supernova shocks (G)
and in radio relics in galaxy clusters (G). We present high
resolution MHD simulations of the interaction between pre-shock turbulence,
clumping and shocks, to quantify the conditions under which turbulent dynamo
amplification can be significant. We demonstrate numerically converged field
amplification which scales with Alfv\'en Mach number, , up to . This implies that the
post-shock field strength is relatively independent of the seed field.
Amplification is dominated by compression at low , and
stretching (turbulent amplification) at high . For high
, the -field grows exponentially and saturates at
equipartition with turbulence, while the vorticity jumps sharply at the shock
and subsequently decays; the resulting field is orientated predominately along
the shock normal (an effect only apparent in 3D and not 2D). This agrees with
the radial field bias seen in supernova remnants. By contrast, for low
, field amplification is mostly compressional, relatively
modest, and results in a predominantly perpendicular field. The latter is
consistent with the polarization seen in radio relics. Our results are
relatively robust to the assumed level of gas clumping. Our results imply that
the turbulent dynamo may be important for supernovae, but is only consistent
with the field strength, and not geometry, for cluster radio relics. For the
latter, this implies strong pre-existing -fields in the ambient cluster
outskirts.Comment: 15 pages, 11 figures, published version on MNRA
Magnetohydrodynamic Simulations of the Tayler Instability in Rotating Stellar Interiors
The Tayler instability is an important but poorly studied magnetohydrodynamic
instability that likely operates in stellar interiors. The nonlinear saturation
of the Tayler instability is poorly understood and has crucial consequences for
dynamo action and angular momentum transport in radiative regions of stars. We
perform three-dimensional MHD simulations of the Tayler instability in a
cylindrical geometry, including strong buoyancy and Coriolis forces as
appropriate for its operation in realistic rotating stars. The linear growth of
the instability is characterized by a predominantly oscillation with
growth rates roughly following analytical expectations. The non-linear
saturation of the instability appears to be caused by secondary shear
instabilities and is also accompanied by a morphological change of the flow. We
argue, however, that non-linear saturation likely occurs via other mechanisms
in real stars where the separation of scales is larger than those reached by
our simulations. We also observe dynamo action via the amplification of the
axisymmetric poloidal magnetic field, suggesting that Tayler instability could
be important for magnetic field generation and angular momentum transport in
the radiative regions of evolving stars.Comment: 11 pages, 10 figures, submitted to MNRA
Spiral Disk Instability Can Drive Thermonuclear Explosions in Binary White Dwarf Mergers
Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion
of carbon--oxygen white dwarfs, and serve as standardizable cosmological
candles. However, despite their importance, the nature of the progenitor
systems that give rise to SNe Ia has not been hitherto elucidated.
Observational evidence favors the double-degenerate channel in which merging
white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies
exist between observations and theory, and to date, there has been no
self-consistent merger model that yields a SNe Ia. Here we show that a spiral
mode instability in the accretion disk formed during a binary white dwarf
merger leads to a detonation on a dynamical timescale. This mechanism sheds
light on how white dwarf mergers may frequently yield SNe Ia.Comment: Final version (as in ApJL) with minor edit
A Simple Sub-Grid Model For Cosmic Ray Effects on Galactic Scales
Many recent numerical studies have argued that cosmic rays (CRs) from
supernovae (SNe) or active galactic nuclei (AGN) could play a crucial role in
galaxy formation, in particular by establishing a CR-pressure dominated
circum-galactic medium (CGM). But explicit CR-magneto-hydrodynamics (CR-MHD)
remains computationally expensive, and it is not clear whether it even makes
physical sense in simulations that do not explicitly treat magnetic fields or
resolved ISM phase structure. We therefore present an intentionally
extremely-simplified 'sub-grid' model for CRs, which attempts to capture the
key qualitative behaviors of greatest interest for those interested in
simulations or semi-analytic models including some approximate CR effects on
galactic (>kpc) scales, while imposing negligible computational overhead. The
model is numerically akin to some recently-developed sub-grid models for
radiative feedback, and allows for a simple constant parameterization of the CR
diffusivity and/or streaming speed; it allows for an arbitrary distribution of
sources (proportional to black hole accretion rates or star-particle SNe rates
or gas/galaxy star formation rates), and interpolates between the limits where
CRs escape the galaxies with negligible losses and those where CRs lose most of
their energy catastrophically before escape (relevant in e.g. starburst
galaxies). The numerical equations are solved trivially alongside gravity in
most codes. We compare this to explicit CR-MHD simulations and discuss where
the (many) sub-grid approximations break down, and what drives the major
sources of uncertainty.Comment: 12 pages, 4 figures. Submitted to MNRAS. Comments welcom
Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations
Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR
scattering from magnetic fluctuations. In the ISM, for CRs with energies
~MeV-TeV, these fluctuations are usually attributed either to 'extrinsic
turbulence' (ET) - a cascade from larger scales - or 'self-confinement' (SC) -
self-generated fluctuations from CR streaming. Using simple analytic arguments
and detailed live numerical CR transport calculations in galaxy simulations, we
show that both of these, in standard form, cannot explain even basic
qualitative features of observed CR spectra. For ET, any spectrum that obeys
critical balance or features realistic anisotropy, or any spectrum that
accounts for finite damping below the dissipation scale, predicts qualitatively
incorrect spectral shapes and scalings of B/C and other species. Even if
somehow one ignored both anisotropy and damping, observationally-required
scattering rates disagree with ET predictions by orders-of-magnitude. For SC,
the dependence of driving on CR energy density means that it is nearly
impossible to recover observed CR spectral shapes and scalings, and again there
is an orders-of-magnitude normalization problem. But more severely, SC
solutions with super-Alfvenic streaming are unstable. In live simulations, they
revert to either arbitrarily-rapid CR escape with zero secondary production, or
to bottleneck solutions with far-too-strong CR confinement and secondary
production. Resolving these fundamental issues without discarding basic plasma
processes requires invoking different drivers for scattering fluctuations.
These must act on a broad range of scales with a power spectrum obeying several
specific (but plausible) constraints.Comment: 36 pages, 7 figures. Updated to match published version, added
section discussing 'meso-scale' phenomenolog
On the Formation of Density Filaments in the Turbulent Interstellar Medium
This study is motivated by recent observations of ubiquitous interstellar density filaments and guided by modern theories of compressible magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structures. As the fundamental dynamics of compressible MHD turbulence, perpendicular turbulent mixing of density fluctuations entails elongated density structures aligned with the local magnetic field, accounting for low-density parallel filaments seen in diffuse atomic and molecular gas. The elongation of low-density parallel filaments depends on the turbulence anisotropy. When taking into account the partial ionization, we find that the minimum width of parallel filaments in the cold neutral medium and molecular clouds is determined by the neutral–ion decoupling scale perpendicular to magnetic field. In highly supersonic MHD turbulence in molecular clouds, both low-density parallel filaments due to anisotropic turbulent mixing and high-density filaments due to shock compression exist