72 research outputs found
The spin rate of pre-collapse stellar cores: wave-driven angular momentum transport in massive stars
The core rotation rates of massive stars have a substantial impact on the
nature of core-collapse supernovae and their compact remnants. We demonstrate
that internal gravity waves (IGW), excited via envelope convection during a red
supergiant phase or during vigorous late time burning phases, can have a
significant impact on the rotation rate of the pre-SN core. In typical () supernova progenitors, IGW may
substantially spin down the core, leading to iron core rotation periods . Angular momentum (AM) conservation during the
supernova would entail minimum NS rotation periods of . In most cases, the combined effects of magnetic torques and IGW
AM transport likely lead to substantially longer rotation periods. However, the
stochastic influx of AM delivered by IGW during shell burning phases inevitably
spin up a slowly rotating stellar core, leading to a maximum possible core
rotation period. We estimate maximum iron core rotation periods of in typical core-collapse supernova
progenitors, and a corresponding spin period of for newborn neutron stars. This is comparable to the typical birth
spin periods of most radio pulsars. Stochastic spin-up via IGW during shell
O/Si burning may thus determine the initial rotation rate of most neutron
stars. For a given progenitor, this theory predicts a Maxwellian distribution
in pre-collapse core rotation frequency that is uncorrelated with the spin of
the overlying envelope.Comment: Published in Ap
Order Out of Chaos: Slowly Reversing Mean Flows Emerge from Turbulently Generated Internal Waves
We demonstrate via direct numerical simulations that a periodic, oscillating
mean flow spontaneously develops from turbulently generated internal waves. We
consider a minimal physical model where the fluid self-organizes in a
convective layer adjacent to a stably stratified one. Internal waves are
excited by turbulent convective motions, then nonlinearly interact to produce a
mean flow reversing on timescales much longer than the waves' period. Our
results demonstrate for the first time that the three-scale dynamics due to
convection, waves, and mean flow is generic and hence can occur in many
astrophysical and geophysical fluids. We discuss efforts to reproduce the mean
flow in reduced models, where the turbulence is bypassed. We demonstrate that
wave intermittency, resulting from the chaotic nature of convection, plays a
key role in the mean-flow dynamics, which thus cannot be captured using only
second-order statistics of the turbulent motions
Connection between nonlinear energy optimization and instantons.
How systems transit between different stable states under external perturbation is an important practical issue. We discuss here how a recently developed energy optimization method for identifying the minimal disturbance necessary to reach the basin boundary of a stable state is connected to the instanton trajectory from large deviation theory of noisy systems. In the context of the one-dimensional Swift-Hohenberg equation, which has multiple stable equilibria, we first show how the energy optimization method can be straightforwardly used to identify minimal disturbances-minimal seeds-for transition to specific attractors from the ground state. Then, after generalizing the technique to consider multiple, equally spaced-in-time perturbations, it is shown that the instanton trajectory is indeed the solution of the energy optimization method in the limit of infinitely many perturbations provided a specific norm is used to measure the set of discrete perturbations. Importantly, we find that the key features of the instanton can be captured by a low number of discrete perturbations (typically one perturbation per basin of attraction crossed). This suggests a promising new diagnostic for systems for which it may be impractical to calculate the instanton
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
- …