76 research outputs found
Three-Dimensional Adaptive Mesh Refinement Simulations of Point-Symmetric Nebulae
Previous analytical and numerical work shows that the generalized interacting
stellar winds model can explain the observed bipolar shapes of planetary
nebulae very well. However, many circumstellar nebulae have a multipolar or
point-symmetric shape. With two-dimensional calculations, Icke showed that
these seemingly enigmatic forms can be easily reproduced by a two-wind model in
which the confining disk is warped, as is expected to occur in irradiated
disks. In this contribution we present the extension to fully three-dimensional
adaptive mesh refinement simulations of such an interaction.Comment: 4 pages, 2 figures, to appear in "Asymmetrical Planetary Nebulae III"
editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series).
Movies are available at
http://www.strw.leidenuniv.nl/AstroHydro3D/movies/index.htm
Star Formation with Adaptive Mesh Refinement Radiation Hydrodynamics
I provide a pedagogic review of adaptive mesh refinement (AMR) radiation
hydrodynamics (RHD) methods and codes used in simulations of star formation, at
a level suitable for researchers who are not computational experts. I begin
with a brief overview of the types of RHD processes that are most important to
star formation, and then I formally introduce the equations of RHD and the
approximations one uses to render them computationally tractable. I discuss
strategies for solving these approximate equations on adaptive grids, with
particular emphasis on identifying the main advantages and disadvantages of
various approximations and numerical approaches. Finally, I conclude by
discussing areas ripe for improvement.Comment: 8 pages, to appear in the Proceedings of IAU Symposium 270:
Computational Star Formatio
Numerical nebulae
The late stages of evolution of stars like our Sun are dominated by several episodes of violent mass loss. Space based observations of the resulting objects, known as Planetary Nebulae, show a bewildering array of highly symmetric shapes. The interplay between gasdynamics and radiative processes determines the morphological outcome of these objects, and numerical models for astrophysical gasdynamics have to incorporate these effects. This thesis presents new numerical techniques for carrying out high-resolution three-dimensional radiation hydrodynamical simulations. Such calculations require parallelization of computer codes, and the use of state-of-the-art supercomputer technology. Numerical models in the context of the shaping of Planetary Nebulae are presented, providing insight into their origin and fate.Sterrewacht Leiden - OU
Discovery of a ~7 Hz Quasi-Periodic Oscillation in the low-luminosity low-mass X-ray binary 4U 1820-30
We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the
X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source
4U 1820-30. This QPO was only observable at the highest observed mass accretion
rate, when the source was in the uppermost part of the banana branch, at a 2-25
keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a
FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed
erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms
amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude
increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to
7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately
constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was
consistent with being zero (-1.2+-3.4 ms).
The properties of the QPO (i.e., its frequency and its presence only at the
highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO
observed in the highest luminosity LMXBs (the Z sources) when they are
accreting near the Eddington mass accretion limit. If this is indeed the same
phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which
require the near-Eddington accretion rates, will not hold. Assuming isotropic
emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is
at maximum only 40% (for a companion star with cosmic abundances), but most
likely ~20% (for a helium companion star) of the Eddington accretion limit.Comment: Accepted for publication in ApJ Letters (6 pages, including 3
figures
Interaction of massive stars with their surroundings
Due to their short lifetimes but their enormous energy release in all stages
of their lives massive stars are the major engines for the comic matter
circuit. They affect not only their close environment but are also responsible
to drive mass flows on galactic scales. Recent 2D models of radiation-driven
and wind-blown HII regions are summarized which explore the impact of massive
stars to the interstellar medium but find surprisingly small energy transfer
efficiencies while an observable Carbon self-enrichment in the Wolf-Rayet phase
is detected in the warm ionized gas. Finally, the focus is set on
state-of-the-art modelling of HII regions and its present weaknesses with
respect to uncertainties and simplifications but on a perspective of the
requested art of their modelling in the 21st century.Comment: 7 pages, 3 fig.s, to be published in IAU Symp. No. 252, "The art of
modelling stars in the 21st century", L. Deng & K.L. Chang (eds.), 2008,
invited tal
- …