4,414 research outputs found
Radiative instabilities in simulations of spherically symmetric supernova blast waves
High-resolution simulations of the cooling regions of spherically symmetric
supernova remnants demonstrate a strong radiative instability. This
instability, whose presence is dependent on the shock velocity, causes
large-amplitude fluctuations in the shock velocity. The fluctuations begin
almost immediately after the radiative phase begins (upon shell formation) if
the shock velocity lies in the unstable range; they last until the shock slows
to speeds less than approximately 130 km/s. We find that shock-velocity
fluctuations from the reverberations of waves within the remnant are small
compared to those due to the instability. Further, we find (in plane-parallel
simulations) that advected inhomogeneities from the external medium do not
interfere with the qualitative nature of the instability-driven fluctuations.
Large-amplitude inhomogeneities may alter the phases of shock-velocity
fluctuations, but do not substantially reduce their amplitudes.Comment: 18 pages text, LaTeX/AASTeX (aaspp4); 10 figures; accepted by Ap
Effects of magnetic fields on radiatively overstable shock waves
We discuss high-resolution simulations of one-dimensional, plane-parallel
shock waves with mean speeds between 150 and 240 km/s propagating into gas with
Alfven velocities up to 40 km/s and outline the conditions under which these
radiative shocks experience an oscillatory instability in the cooling length,
shock velocity, and position of the shock front. We investigate two forms of
postshock cooling: a truncated single power law and a more realistic piecewise
power law. The degree of nonlinearity of the instability depends strongly on
the cooling power law and the Alfven Mach number: for power-law indices \alpha
< 0 typical magnetic field strengths may be insufficient either to stabilize
the fundamental oscillatory mode or to prevent the oscillations from reaching
nonlinear amplitudes.Comment: 11 text pages, LaTeX/AASTeX (aaspp4); 5 figures; accepted by Ap
Structuring and support by Alfven waves around prestellar cores
Observations of molecular clouds show the existence of starless, dense cores,
threaded by magnetic fields. Observed line widths indicate these dense
condensates to be embedded in a supersonically turbulent environment. Under
these conditions, the generation of magnetic waves is inevitable. In this
paper, we study the structure and support of a 1D plane-parallel,
self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is
injected in its central plane. Dimensional analysis shows that the solution
must depend on three dimensionless parameters. To study the nonlinear,
turbulent evolution of such a slab, we use 1D high resolution numerical
simulations. For a parameter range inspired by molecular cloud observations, we
find the following. 1) A single source of energy injection is sufficient to
force persistent supersonic turbulence over several hydrostatic scale heights.
2) The time averaged spatial extension of the slab is comparable to the
extension of the stationary, analytical WKB solution. Deviations, as well as
the density substructure of the slab, depend on the wave-length of the injected
wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with
increasing plasma beta. 4) Good spatial resolution is mandatory, making similar
simulations in 3D currently prohibitively expensive.Comment: 13 pages, 8 figures, accepted for publication in A&A. The manuscript
with full color, high-resolution, figures can be downloaded from
http://www.astro.phys.ethz.ch/papers/folini/folini_p_nf.htm
The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution
The stability properties of one-dimensional radiative shocks with a power-law
cooling function of the form are the main
subject of this work. The linear analysis originally presented by Chevalier &
Imamura, is thoroughfully reviewed for several values of the cooling index
and higher overtone modes. Consistently with previous results, it is
shown that the spectrum of the linear operator consists in a series of modes
with increasing oscillation frequency. For each mode a critical value of the
cooling index, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . Particular attention is given to the comparison between
numerical and analytical results (during the early phases of the evolution) and
to the role played by different boundary conditions. It is shown that an
appropriate treatment of the lower boundary yields results that closely follow
the predicted linear behavior. During the nonlinear regime, the shock
oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle.
The modes of oscillations during this phase do not necessarily coincide with
those predicted by linear theory, but may be accounted for by mode-mode
coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical
Journa
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
The Spin Periods and Rotational Profiles of Neutron Stars at Birth
We present results from an extensive set of one- and two-dimensional
radiation-hydrodynamic simulations of the supernova core collapse, bounce, and
postbounce phases, and focus on the protoneutron star (PNS) spin periods and
rotational profiles as a function of initial iron core angular velocity, degree
of differential rotation, and progenitor mass. For the models considered, we
find a roughly linear mapping between initial iron core rotation rate and PNS
spin. The results indicate that the magnitude of the precollapse iron core
angular velocities is the single most important factor in determining the PNS
spin. Differences in progenitor mass and degree of differential rotation lead
only to small variations in the PNS rotational period and profile. Based on our
calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming
angular momentum conservation, we estimate final neutron star rotation periods.
We find periods of one millisecond and shorter for initial central iron core
periods of below ~ 10 s. This is appreciably shorter than what previous studies
have predicted and is in disagreement with current observational data from
pulsar astronomy. After considering possible spindown mechanisms that could
lead to longer periods we conclude that there is no mechanism that can robustly
spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens
to hundreds of milliseconds observed for young pulsars. Our results indicate
that, given current knowledge of the limitations of neutron star spindown
mechanisms, precollapse iron cores must rotate with periods around 50-100
seconds to form neutron stars with periods generically near those inferred for
the radio pulsar population.Comment: 31 pages, including 20 color figures. High-resolution figures
available from the authors upon request. Accepted to Ap
Combined macro- and micro-rheometer for use with Langmuir monolayers
A Langmuir monolayer trough that is equipped for simultaneous microrheology
and standard rheology measurements has been constructed. The central elements
are the trough itself with a full range of optical tools accessing the
air-water interface from below the trough and a portable knife-edge torsion
pendulum that can access the interface from above. The ability to
simultaneously measure the mechanical response of Langmuir monolayers on very
different lengths scales is an important step in for our understanding of the
mechanical response of such systems
- âŠ