1,471 research outputs found
Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores
Stars and more particularly massive stars, have a drastic impact on galaxy
evolution. Yet the conditions in which they form and collapse are still not
fully understood. In particular, the influence of the magnetic field on the
collapse of massive clumps is relatively unexplored, it is thus of great
relevance in the context of the formation of massive stars to investigate its
impact. We perform high resolution, MHD simulations of the collapse of hundred
solar masses, turbulent and magnetized clouds, using the adaptive mesh
refinement code RAMSES. We compute various quantities such as mass
distribution, magnetic field and angular momentum within the collapsing core
and study the episodic outflows and the fragmentation that occurs during the
collapse. The magnetic field has a drastic impact on the cloud evolution. We
find that magnetic braking is able to substantially reduce the angular momentum
in the inner part of the collapsing cloud. Fast and episodic outflows are being
launched with typical velocities of the order of 3-5 km s although the
highest velocities can be as high as 30-40 km s. The fragmentation in
several objects, is reduced in substantially magnetized clouds with respect to
hydrodynamical ones by a factor of the order of 1.5-2. We conclude that
magnetic fields have a significant impact on the evolution of massive clumps.
In combination with radiation, magnetic fields largely determine the outcome of
massive core collapse. We stress that numerical convergence of MHD collapse is
a challenging issue. In particular, numerical diffusion appears to be important
at high density therefore possibly leading to an over-estimation of the number
of fragments.Comment: accepted for publication in A&
An uncertainty principle for star formation -- III. The characteristic emission time-scales of star formation rate tracers
We recently presented a new statistical method to constrain the physics of
star formation and feedback on the cloud scale by reconstructing the underlying
evolutionary timeline. However, by itself this new method only recovers the
relative durations of different evolutionary phases. To enable observational
applications, it therefore requires knowledge of an absolute 'reference
time-scale' to convert relative time-scales into absolute values. The logical
choice for this reference time-scale is the duration over which the star
formation rate (SFR) tracer is visible because it can be characterised using
stellar population synthesis (SPS) models. In this paper, we calibrate this
reference time-scale using synthetic emission maps of several SFR tracers,
generated by combining the output from a hydrodynamical disc galaxy simulation
with the SPS model SLUG2. We apply our statistical method to obtain
self-consistent measurements of each tracer's reference time-scale. These
include H and 12 ultraviolet (UV) filters (from GALEX, Swift, and
HST), which cover a wavelength range 150-350 nm. At solar metallicity, the
measured reference time-scales of H are Myr
with continuum subtraction, and 6-16 Myr without, where the time-scale
increases with filter width. For the UV filters we find 17-33 Myr, nearly
monotonically increasing with wavelength. The characteristic time-scale
decreases towards higher metallicities, as well as to lower star formation rate
surface densities, owing to stellar initial mass function sampling effects. We
provide fitting functions for the reference time-scale as a function of
metallicity, filter width, or wavelength, to enable observational applications
of our statistical method across a wide variety of galaxies.Comment: 24 pages, 18 figures, 7 tables (including Appendices); published in
MNRA
GRB 990123: Reverse and Internal Shock Flashes and Late Afterglow
The prompt (t \siml 0.16 days) light curve and initial 9-th magnitude
optical flash from GRB 990123 can be attributed to a reverse external shock, or
possibly to internal shocks. We discuss the time decay laws and spectral slopes
expected under various dynamical regimes, and discuss the constraints imposed
on the model by the observations, arguing that they provide strongly suggestive
evidence for features beyond those in the simple standard model. The longer
term afterglow behavior is discussed in the context of the forward shock, and
it is argued that, if the steepening after three days is due to a jet geometry,
this is likely to be due to jet-edge effects, rather than sideways expansion.Comment: M.N.R.A.S., subm. 2/26/99; (preprint uses aaspp4.sty), 9 page
Predictors of Venous Thromboembolism in Patients with Advanced Common Solid Cancers
There is uncertainty about risk heterogeneity for venous thromboembolism (VTE) in older
patients with advanced cancer and whether patients can be stratified according to VTE risk. We performed a retrospective cohort study of the linked Medicare-Surveillance, Epidemiology, and End Results cancer registry in older patients with advanced cancer of lung, breast, colon, prostate, or pancreas diagnosed between 1995–1999. We used survival analysis with demographics, comorbidities, and tumor characteristics/treatment as independent variables. Outcome was VTE diagnosed at least one month after cancer diagnosis. VTE rate was highest in the first year (3.4%). Compared to prostate cancer (1.4 VTEs/100 person-years), there was marked variability in VTE risk (hazard ratio (HR) for male-colon cancer 3.73 (95% CI 2.1–6.62), female-colon cancer HR 6.6 (3.83–11.38), up to female-pancreas cancer HR 21.57
(12.21–38.09). Stage IV cancer and chemotherapy resulted in higher risk (HRs 1.75 (1.44–2.12) and 1.31 (1.0–1.57), resp.). Stratifying the cohort by cancer type and stage using recursive partitioning analysis yielded five groups of VTE rates (nonlocalized prostate cancer 1.4 VTEs/100 person-years, to nonlocalized pancreatic cancer 17.4 VTEs/100 patient-years). In a high-risk population with advanced cancer, substantial variability in VTE risk exists, with notable differences according to cancer type and stage
Three-dimensional Continuum Radiative Transfer Images of a Molecular Cloud Core Evolution
We analyze a three-dimensional smoothed particle hydrodynamics simulation of
an evolving and later collapsing pre-stellar core. Using a three-dimensional
continuum radiative transfer program, we generate images at 7 micron, 15
micron, 175 micron, and 1.3 mm for different evolutionary times and viewing
angles. We discuss the observability of the properties of pre-stellar cores for
the different wavelengths. For examples of non-symmetric fragments, it is shown
that, misleadingly, the density profiles derived from a one-dimensional
analysis of the corresponding images are consistent with one-dimensional core
evolution models. We conclude that one-dimensional modeling based on column
density interpretation of images does not produce reliable structural
information and that multidimensional modeling is required.Comment: accepted by ApJL, 4 pages, 4 figure
Simulating Supersonic Turbulence in Magnetized Molecular Clouds
We present results of large-scale three-dimensional simulations of weakly
magnetized supersonic turbulence at grid resolutions up to 1024^3 cells. Our
numerical experiments are carried out with the Piecewise Parabolic Method on a
Local Stencil and assume an isothermal equation of state. The turbulence is
driven by a large-scale isotropic solenoidal force in a periodic computational
domain and fully develops in a few flow crossing times. We then evolve the flow
for a number of flow crossing times and analyze various statistical properties
of the saturated turbulent state. We show that the energy transfer rate in the
inertial range of scales is surprisingly close to a constant, indicating that
Kolmogorov's phenomenology for incompressible turbulence can be extended to
magnetized supersonic flows. We also discuss numerical dissipation effects and
convergence of different turbulence diagnostics as grid resolution refines from
256^3 to 1024^3 cells.Comment: 10 pages, 3 figures, to appear in the proceedings of the DOE/SciDAC
2009 conferenc
Type Ibc supernovae in disturbed galaxies: evidence for a top-heavy IMF
We compare the radial locations of 178 core-collapse supernovae to the R-band
and H alpha light distributions of their host galaxies. When the galaxies are
split into `disturbed' and `undisturbed' categories, a striking difference
emerges. The disturbed galaxies have a central excess of core-collapse
supernovae, and this excess is almost completely dominated by supernovae of
types Ib, Ic and Ib/c, whereas type II supernovae dominate in all other
environments. The difference cannot easily be explained by metallicity or
extinction effects, and thus we propose that this is direct evidence for a
stellar initial mass function that is strongly weighted towards high mass
stars, specifically in the central regions of disturbed galaxies.Comment: 22 pages, 5 figures, accepted for publication in Ap
The Bursty Star Formation Histories of Low-mass Galaxies at Revealed by Star Formation Rates Measured from H and FUV
We investigate the burstiness of star formation histories (SFHs) of galaxies
at by using the ratio of star formation rates (SFRs) measured from
H and FUV (1500 \AA) (H--to--FUV ratio). Our sample contains 164
galaxies down to stellar mass (M*) of in the CANDELS GOODS-N
region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W
images from CANDELS and Hubble Deep UV Legacy Survey are available. When the
{\it ratio} of H- and FUV-derived SFRs is measured, dust extinction
correction is negligible (except for very dusty galaxies) with the Calzetti
attenuation curve. The H--to--FUV ratio of our sample increases with M*
and SFR. The median ratio is 0.7 at M* (or SFR) and increases to 1 at M* (or SFR
). At M*, our median H--to--FUV
ratio is lower than that of local galaxies at the same M*, implying a redshift
evolution. Bursty SFH on a timescale of a few tens of megayears on galactic
scales provides a plausible explanation of our results, and the importance of
the burstiness increases as M* decreases. Due to sample selection effects, our
H--to--FUV ratio may be an upper limit of the true value of a complete
sample, which strengthens our conclusions. Other models, e.g., non-universal
initial mass function or stochastic star formation on star cluster scales, are
unable to plausibly explain our results.Comment: 13 pages, 8 figures. ApJ accepted. The main conclusions are not
changed. Major modifications include: (1) to be consistent with the
literature, now reporting H\beta--to--FUV ratio (rather than FUV--to--H\beta\
in the first version); (2) detailed discussions on dust extinction
correction; (3) new SF bustiness calculation; and (4) enriched discussions in
Introductio
Hospital Performance on Percutaneous Coronary Intervention Process and Outcomes Measures
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139090/1/jah32022_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139090/2/jah32022.pd
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