12 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
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
Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests
Radiative transfer simulations are now at the forefront of numerical
astrophysics. They are becoming crucial for an increasing number of
astrophysical and cosmological problems; at the same time their computational
cost has come to the reach of currently available computational power. Further
progress is retarded by the considerable number of different algorithms
(including various flavours of ray-tracing and moment schemes) developed, which
makes the selection of the most suitable technique for a given problem a
non-trivial task. Assessing the validity ranges, accuracy and performances of
these schemes is the main aim of this paper, for which we have compared 11
independent RT codes on 5 test problems: (0) basic physics, (1) isothermal H II
region expansion and (2) H II region expansion with evolving temperature, (3)
I-front trapping and shadowing by a dense clump, (4) multiple sources in a
cosmological density field. The outputs of these tests have been compared and
differences analyzed. The agreement between the various codes is satisfactory
although not perfect. The main source of discrepancy appears to reside in the
multi-frequency treatment approach, resulting in different thicknesses of the
ionized-neutral transition regions and different temperature structure. The
present results and tests represent the most complete benchmark available for
the development of new codes and improvement of existing ones. To this aim all
test inputs and outputs are made publicly available in digital form.Comment: 32 pages, 39 figures (all color), comments welcom
Strategy for online correction of rotational organ motion for intensity-modulated radiotherapy of prostate cancer
A novel Bayesian respiratory motion model to estimate and resolve uncertainty in image-guided cardiac interventions
Strategies for online organ motion correction for intensity-modulated radiotherapy of prostate cancer: prostate, rectum, and bladder dose effects.
PURPOSE: To quantify and evaluate the accumulated prostate, rectum, and bladder dose for several strategies including rotational organ motion correction for intensity-modulated radiotherapy (IMRT) of prostate cancer using realistic organ motion data. METHODS AND MATERIALS: Repeat computed tomography (CT) scans of 19 prostate patients were used. Per patient, two IMRT plans with different uniform margins were created. To quantify prostate and seminal vesicle motion, repeat CT clinical target volumes (CTVs) were matched onto the planning CTV using deformable registration. Four different strategies, from online setup to full motion correction, were simulated. Rotations were corrected for using gantry and collimator angle adjustments. Prostate, rectum, and bladder doses were accumulated for each patient, plan, and strategy. Minimum CTV dose (D(min)), rectum equivalent uniform dose (EUD, n = 0.13), and bladder surface receiving >/=78 Gy (S78), were calculated. RESULTS: With online CTV translation correction, a 7-mm margin was sufficient (i.e., D(min) >/= 95% of the prescribed dose for all patients). A 4-mm margin required additional rotational correction. Margin reduction lowered the rectum EUD(n = 0.13) by approximately 2.6 Gy, and the bladder S78 by approximately 1.9%. CONCLUSIONS: With online correction of both translations and rotations, a 4-mm margin was sufficient for 15 of 19 patients, whereas the remaining four patients had an underdosed CTV volume <1%. Margin reduction combined with online corrections resulted in a similar or lower dose to the rectum and bladder. The more advanced the correction strategy, the better the planned and accumulated dose agree