629 research outputs found
Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles
We use 3-dimensional numerical simulations of self-gravitating compressible
turbulent gas in combination with Lagrangian tracer particles to investigate
the mixing process of molecular hydrogen (H2) in interstellar clouds. Tracer
particles are used to represent shock-compressed dense gas, which is associated
with H2. We deposit tracer particles in regions of density contrast in excess
of ten times the mean density. Following their trajectories and using
probability distribution functions, we find an upper limit for the mixing
timescale of H2, which is of order 0.3 Myr. This is significantly smaller than
the lifetime of molecular clouds, which demonstrates the importance of the
turbulent mixing of H2 as a preliminary stage to star formation.Comment: 10 pages, 5 figures, conference proceedings "Turbulent Mixing and
Beyond 2007
A Chandra Observation of Supernova Remnant G350.1-0.3 and Its Central Compact Object
We present a new Chandra observation of supernova remnant (SNR) G350.1-0.3.
The high resolution X-ray data reveal previously unresolved filamentary
structures and allow us to perform detailed spectroscopy in the diffuse regions
of this SNR. Spectral analysis demonstrates that the region of brightest
emission is dominated by hot, metal-rich ejecta while the ambient material
along the perimeter of the ejecta region and throughout the remnant's western
half is mostly low-temperature, shocked interstellar/circumstellar medium
(ISM/CSM) with solar-type composition. The data reveal that the emission
extends far to the west of the ejecta region and imply a lower limit of 6.6 pc
on the diameter of the source (at a distance of 4.5 kpc). We show that
G350.1-0.3 is likely in the free expansion (ejecta-dominated) stage and
calculate an age of 600-1200 years. The derived relationship between the shock
velocity and the electron/proton temperature ratio is found to be entirely
consistent with that of other SNRs. We perform spectral fits on the X-ray
source XMMU J172054.5-372652, a candidate central compact object (CCO), and
find that its spectral properties fall within the typical range of other CCOs.
We also present archival 24 um data of G350.1-0.3 taken with the Spitzer Space
Telescope during the MIPSGAL galactic survey and find that the infrared and
X-ray morphologies are well-correlated. These results help to explain this
remnant's peculiar asymmetries and shed new light on its dynamics and
evolution
The Nucleosynthetic Imprint of 15-40 Solar Mass Primordial Supernovae on Metal-Poor Stars
The inclusion of rotationally-induced mixing in stellar evolution can alter
the structure and composition of presupernova stars. We survey the effects of
progenitor rotation on nucleosynthetic yields in Population III and II
supernovae using the new adaptive mesh refinement (AMR) code CASTRO. We examine
spherical explosions in 15, 25 and 40 solar mass stars at Z = 0 and 10^-4 solar
metallicity with three explosion energies and two rotation rates. Rotation in
the Z = 0 models resulted in primary nitrogen production and a stronger
hydrogen burning shell which led all models to die as red supergiants. On the
other hand, the Z=10^-4 solar metallicity models that included rotation ended
their lives as compact blue stars. Because of their extended structure, the
hydrodynamics favors more mixing and less fallback in the metal free stars than
the Z = 10^-4 models. As expected, higher energy explosions produce more
enrichment and less fallback than do lower energy explosions, and less massive
stars produce more enrichment and leave behind smaller remnants than do more
massive stars. We compare our nucleosynthetic yields to the chemical abundances
in the three most iron-poor stars yet found and reproduce the abundance pattern
of one, HE 0557-4840, with a zero metallicity 15 solar mass, 2.4 x 10^51 erg
supernova. A Salpeter IMF averaged integration of our yields for Z=0 models
with explosion energies of 2.4x10^51 ergs or less is in good agreement with the
abundances observed in larger samples of extremely metal-poor stars, provided
15 solar mass stars are included. Since the abundance patterns of extremely
metal-poor stars likely arise from a representative sample of progenitors, our
yields suggest that low-mass supernovae contributed the bulk of the metals to
the early universe.Comment: 16 pages, 11 figures; submitted to Ap
Dosimetry and Gastrointestinal Toxicity Relationships in a Phase II Trial of Pelvic Lymph Node Radiotherapy in Advanced Localised Prostate Cancer.
AIMS:Pelvic lymph node (PLN) radiotherapy for high-risk prostate cancer is limited by late gastrointestinal toxicity. Application of rectal and bowel constraints may reduce risks of side-effects. We evaluated associations between intensity-modulated radiotherapy (IMRT) dose-volume data and long-term gastrointestinal toxicity. MATERIALS AND METHODS:Data from a single-centre dose-escalation trial of PLN-IMRT were analysed, including conventionally fractionated (CFRT) and hypofractionated (HFRT) radiotherapy schedules. Associations between volumes of rectum and bowel receiving specified doses and clinician- and patient-reported toxicity outcomes were investigated independently. A metric, δ median (δM), was defined as the difference in the medians of a volume between groups with and without toxicity at a specified dose and was used to test for statistically significant differences. RESULTS:Constraints were respected in most patients and, when exceeded, led to higher rates of gastrointestinal toxicity. Biologically relevant associations between rectum dose-points and toxicity were more numerous with both mild and moderate toxicity thresholds, but statistical significance was limited after correction for false discovery rate. Rectal V50Gy (CFRT) associated with grade 2+ bleeding; bowel V43Gy and V47 (HFRT/4 days/week schedule) associated with patient-reported loose stools and diarrhoea, respectively. Further investigation showed that CFRT patients with rectal bleeding had a mean rectal V50Gy above the treatment planning constraint. CONCLUSIONS:When dose-volume parameters are kept below tight constraints, toxicity is low. Residual dosimetry loses much of its predictive power for gastrointestinal toxicity in the setting of PLN-IMRT for prostate cancer. We have benchmarked dose-volume constraints for safely delivering PLN-IMRT using CFRT or HFRT
Modeling Collapse and Accretion in Turbulent Gas Clouds: Implementation and Comparison of Sink Particles in AMR and SPH
We implemented sink particles in the adaptive mesh refinement (AMR)
hydrodynamics code FLASH. Sink particles are created in regions of local
gravitational collapse, and their trajectories and accretion can be followed
over many dynamical times. We perform a series of tests including the time
integration of circular and elliptical orbits, the collapse of a Bonnor-Ebert
sphere and a rotating, fragmenting cloud core. We compare the collapse of a
highly unstable singular isothermal sphere to the theory by Shu (1977), and
show that the sink particle accretion rate is in excellent agreement with the
theoretical prediction.
To model eccentric orbits and close encounters of sink particles accurately,
we show that a very small timestep is often required, for which we implemented
subcycling of the N-body system. We emphasize that a sole density threshold for
sink particle creation is insufficient in supersonic flows, if the density
threshold is below the opacity limit. In that case, the density can exceed the
threshold in strong shocks that do not necessarily lead to local collapse.
Additional checks for bound state, gravitational potential minimum, Jeans
instability and converging flows are absolutely necessary for a meaningful
creation of sink particles.
We apply our new sink particle module for FLASH to the formation of a stellar
cluster, and compare to a smoothed particle hydrodynamics (SPH) code with sink
particles. Our comparison shows encouraging agreement of gas properties,
indicated by column density distributions and radial profiles, and of sink
particle formation times and positions. We find excellent agreement in the
number of sink particles formed, and in their accretion and mass distributions.Comment: 30 pages, 17 figures, ApJ accepted, simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
On the interaction of a thin, supersonic shell with a molecular cloud
Molecular clouds (MCs) are stellar nurseries, however, formation of stars
within MCs depends on the ambient physical conditions. MCs, over a free-fall
time are exposed to numerous dynamical phenomena, of which, the interaction
with a thin, dense shell of gas is but one. Below we present results from
self-gravitating, 3-D smoothed particle hydrodynamics ({\small SPH})
simulations of the problem; seven realisations of the problem have been
performed by varying the precollision density within the cloud, the nature of
the post-collision shock, and the spatial resolution in the computational
domain. Irrespective of the type of shock, a complex network of dense
filaments, seeded by numerical noise, readily appears in the shocked cloud.
Segregation of the dense and rarefied gas phases also manifests itself in a
bimodal distribution of gas density. We demonstrate that the power-spectrum for
rarefied gas is Kolomogorov like, while that for the denser gas is considerably
steeper. As a corollary to the main problem, we also look into the possibly
degenerative effect of the {\small SPH} artificial viscosity on the impact of
the incident shell. It is observed that stronger viscosity leads to greater
post-shock dissipation, that strongly decelerates the incident shock-front and
promotes formation of contiguous structure, albeit on a much longer timescale.
We conclude that too much viscosity is likely to enhance the proclivity towards
gravitational boundedness of structure, leading to unphysical fragmentation.On
the other hand, insufficient resolution appears to suppress fragmentation.
Convergence of results is tested at both extremes, first by repeating the test
case with more than a million particles and then with only half the number of
particles in the original test case.Comment: 15 pages, 15 figures, and 1 Table; To appear in Monthly Notices to
the RA
Star Forming Dense Cloud Cores in the TeV {\gamma}-ray SNR RX J1713.7-3946
RX J1713.7-3946 is one of the TeV {\gamma}-ray supernova remnants (SNRs)
emitting synchrotron X rays. The SNR is associated with molecular gas located
at ~1 kpc. We made new molecular observations toward the dense cloud cores,
peaks A, C and D, in the SNR in the 12CO(J=2-1) and 13CO(J=2-1) transitions at
angular resolution of 90". The most intense core in 13CO, peak C, was also
mapped in the 12CO(J=4-3) transition at angular resolution of 38". Peak C shows
strong signs of active star formation including bipolar outflow and a
far-infrared protostellar source and has a steep gradient with a
r^{-2.20.4} variation in the average density within radius r. Peak C and
the other dense cloud cores are rim-brightened in synchrotron X rays,
suggesting that the dense cloud cores are embedded within or on the outer
boundary of the SNR shell. This confirms the earlier suggestion that the X rays
are physically associated with the molecular gas (Fukui et al. 2003). We
present a scenario where the densest molecular core, peak C, survived against
the blast wave and is now embedded within the SNR. Numerical simulations of the
shock-cloud interaction indicate that a dense clump can indeed survive shock
erosion, since shock propagation speed is stalled in the dense clump.
Additionally, the shock-cloud interaction induces turbulence and magnetic field
amplification around the dense clump that may facilitate particle acceleration
in the lower-density inter-clump space leading to the enhanced synchrotron X
rays around dense cores.Comment: 22 pages, 7 figures, to accepted in The Astrophysical Journal. A full
color version with higher resolution figures is available at
http://www.a.phys.nagoya-u.ac.jp/~sano/ApJ10/ms_sano.pd
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
flepiMoP: The evolution of a flexible infectious disease modeling pipeline during the COVID-19 pandemic
The COVID-19 pandemic led to an unprecedented demand for projections of disease burden and healthcare utilization under scenarios ranging from unmitigated spread to strict social distancing policies. In response, members of the Johns Hopkins Infectious Disease Dynamics Group developed flepiMoP (formerly called the COVID Scenario Modeling Pipeline), a comprehensive open-source software pipeline designed for creating and simulating compartmental models of infectious disease transmission and inferring parameters through these models. The framework has been used extensively to produce short-term forecasts and longer-term scenario projections of COVID-19 at the state and county level in the US, for COVID-19 in other countries at various geographic scales, and more recently for seasonal influenza. In this paper, we highlight how the flepiMoP has evolved throughout the COVID-19 pandemic to address changing epidemiological dynamics, new interventions, and shifts in policy-relevant model outputs. As the framework has reached a mature state, we provide a detailed overview of flepiMoP's key features and remaining limitations, thereby distributing flepiMoP and its documentation as a flexible and powerful tool for researchers and public health professionals to rapidly build and deploy large-scale complex infectious disease models for any pathogen and demographic setup
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