98 research outputs found
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
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
Global mass segregation in hydrodynamical simulations of star formation
Recent analyses of mass segregation diagnostics in star forming regions
invite a comparison with the output of hydrodynamic simulations of star
formation. In this work we investigate the state of mass segregation of 'stars'
(i.e. sink particles in the simulations) in the case of hydrodynamical
simulations which omit feedback. We first discuss methods to quantify mass
segregation in substructured regions, either based on the minimum spanning tree
(Allison's Lambda), or through analysis of correlations between stellar mass
and local stellar surface number densities. We find that the presence of even a
single 'outlier' (i.e. a massive object far from other stars) can cause the
Allison Lambda method to describe the system as inversely mass segregated, even
where in reality the most massive sink particles are overwhelmingly in the
centres of the subclusters. We demonstrate that a variant of the Lambda method
is less susceptible to this tendency but also argue for an alternative
representation of the data in the plane of stellar mass versus local surface
number density.
The hydrodynamical simulations show global mass segregation from very early
times which continues throughout the simulation, being only mildly influenced
during sub-cluster merging.
We find that up to approx. 2-3% of the "massive" sink particles (m > 2.5
Msun) are in relative isolation because they have formed there, although other
sink particles can form later in their vicinity. Ejections of massive sinks
from subclusters do not contribute to the number of isolated massive sink
particles, as the gravitational softening in the calculation suppresses this
process.Comment: 6 pages, 6 figure
Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912
We present bispectrum speckle interferometry of the massive protostellar
object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image
shows the diffuse nebulosity north-east of two point-like sources in
unprecedented detail. The comparison of our near-infrared image with mm
continuum and CO molecular line maps shows that the brighter of the two point
sources lies near the center of the mm peak, indicating that it is a high-mass
protostar. The nebulosity coincides with the blue-shifted molecular outflow
component. The most prominent feature in the nebulosity is a bow-shock-like
arc. We assume that this feature is associated with a precessing jet which has
created an inward-pointed cone in the swept-up material. We present numerical
jet simulations that reproduce this and several other features observed in our
speckle image of the nebulosity. Our data also reveal a linear structure
connecting the central point source to the extended diffuse nebulosity. This
feature may represent the innermost part of a jet that drives the strong
molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative
transfer calculations, we demonstrate that, in general, the observed inner
structures of the circumstellar material surrounding high-mass stars are
strongly influenced by the orientation and symmetry of the bipolar cavity.Comment: accepted by Astronomy & Astrophysics; preprints with high-resolution
images can be obtained from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/iras23151.htm
Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry -- Signatures of flow precession
NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow,
an associated UCHII region, and a linear methanol maser structure, which might
trace a Keplerian-rotating circumstellar disk. The directions of the various
associated axes are misaligned with each other. We investigate the
near-infrared morphology of the source to clarify the relations among the
various axes. K'-band bispectrum speckle interferometry was performed at two
6-meter-class telescopes -- the BTA 6m telescope and the 6.5m MMT.
Complementary IRAC images from the Spitzer Space Telescope Archive were used to
relate the structures detected with the outflow at larger scales. High-dynamic
range images show fan-shaped outflow structure in which we detect 18 stars and
several blobs of diffuse emission. We interpret the misalignment of various
outflow axes in the context of a disk precession model, including numerical
hydrodynamic simulations of the molecular emission. The precession period is
approx. 280 years and its half-opening angle is 40 degrees. A possible
triggering mechanism is non-coplanar tidal interaction of an (undiscovered)
close companion with the circumbinary protostellar disk. Our observations
resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with
separation of 195 mas. We find indications for shock interaction between the
outflow activities in IRS1 and IRS2. Indications of outflow precession have
been discovered to date in a number of massive protostars, all with large
precession angles 20--45 degrees. This might explain the difference between the
outflow widths in low- and high-mass stars and add support to a common
collimation mechanism.Comment: 20 pages; 8 figures; Accepted by A&A on April 10, 2006; Image quality
reduced due to astro-ph file size limitations; Please download a version with
high-quality images from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ngc7538.pd
A VLT/NACO Survey for Triple and Quadruple Systems among Visual Pre-Main Sequence Binaries
This paper describes a systematic search for high-order multiplicity among
wide visual Pre-Main Sequence (PMS) binaries. We conducted an Adaptive Optics
survey of a sample of 58 PMS wide binaries from various star-forming regions,
which include 52 T Tauri systems with mostly K- and M-type primaries, with the
NIR instrument NACO at the VLT. Of these 52 systems, 7 are found to be triple
(2 new) and 7 quadruple (1 new). The new close companions are most likely
physically bound based on their probability of chance projection and, for some
of them, on their position on a color-color diagram. The corresponding degree
of multiplicity among wide binaries (number of triples and quadruples divided
by the number of systems) is 26.9 +/- 7.2% in the projected separation range
0.07-12 arcsec, with the largest contribution from the Taurus-Auriga cloud. We
also found that this degree of multiplicity is twice in Taurus compared to
Ophiuchus and Chamaeleon for which the same number of sources are present in
our sample. Considering a restricted sample composed of systems at distance
140-190pc, the degree of multiplicity is 26.8 +/- 8.1%, in the separation range
10/14 AU - 1700/2300 AU (30 binaries, 5 triples, 6 quadruples). The observed
frequency agrees with results from previous multiplicity surveys within the
uncertainties, although a significant overabundance of quadruple systems
compared to triple systems is apparent. Tentatively including the spectroscopic
pairs in our restricted sample and comparing the multiplicity fractions to
those measured for solar-type main-sequence stars in the solar neighborhood
leads to the conclusion that both the ratio of triples to binaries and the
ratio of quadruples to triples seems to be in excess among young stars. [...]Comment: 24 pages, accepted for publication in Astronomy & Astrophysic
On the stability of radiation-pressure-dominated cavities
Context: When massive stars exert a radiation pressure onto their environment
that is higher than their gravitational attraction, they launch a
radiation-pressure-driven outflow. It has been claimed that a radiative
Rayleigh-Taylor instability should lead to the collapse of the outflow cavity
and foster the growth of massive stars.
Aims: We investigate the stability of idealized radiation-pressure-dominated
cavities, focusing on its dependence on the radiation transport approach for
the stellar radiation feedback.
Methods: We compare two different methods for stellar radiation feedback:
gray flux-limited diffusion (FLD) and ray-tracing (RT). We also derive simple
analytical models to support our findings.
Results: Only the FLD cases lead to prominent instability in the cavity
shell. The RT cases do not show such instability. The gray FLD method
underestimates the opacity at the location of the cavity shell and leads to
extended epochs of marginal Eddington equilibrium in the cavity shell, making
them prone to the radiative Rayleigh-Taylor instability. In the RT cases, the
radiation pressure exceeds gravity by 1-2 orders of magnitude. The radiative
Rayleigh-Taylor instability is then consequently suppressed.
Conclusions: Treating the stellar irradiation in the gray FLD approximation
underestimates the radiative forces acting on the cavity shell. This can lead
artificially to situations that are affected by the radiative Rayleigh-Taylor
instability. The proper treatment of direct stellar irradiation by massive
stars is crucial for the stability of radiation-pressure-dominated cavities.Comment: 14 pages, 8 figures, accepted at A&
Evaluating priority setting success in healthcare: a pilot study
<p>Abstract</p> <p>Background</p> <p>In healthcare today, decisions are made in the face of serious resource constraints. Healthcare managers are struggling to provide high quality care, manage resources effectively, and meet changing patient needs. Healthcare managers who are constantly making difficult resource decisions desire a way to improve their priority setting processes. Despite the wealth of existing priority setting literature (for example, program budgeting and marginal analysis, accountability for reasonableness, the 'describe-evaluate-improve' strategy) there are still no tools to evaluate how healthcare resources are prioritised. This paper describes the development and piloting of a process to evaluate priority setting in health institutions. The evaluation process was designed to examine the procedural and substantive dimensions of priority setting using a multi-methods approach, including a staff survey, decision-maker interviews, and document analysis.</p> <p>Methods</p> <p>The evaluation process was piloted in a mid-size community hospital in Ontario, Canada while its leaders worked through their annual budgeting process. Both qualitative and quantitative methods were used to analyze the data.</p> <p>Results</p> <p>The evaluation process was both applicable to the context and it captured the budgeting process. In general, the pilot test provided support for our evaluation process and our definition of success, (i.e., our conceptual framework).</p> <p>Conclusions</p> <p>The purpose of the evaluation process is to provide a simple, practical way for an organization to better understand what it means to achieve success in its priority setting activities and identify areas for improvement. In order for the process to be used by healthcare managers today, modification and contextualization of the process are anticipated. As the evaluation process is applied in more health care organizations or applied repeatedly in an organization, it may become more streamlined.</p
Hadron Production in Diffractive Deep-Inelastic Scattering
Characteristics of hadron production in diffractive deep-inelastic
positron-proton scattering are studied using data collected in 1994 by the H1
experiment at HERA. The following distributions are measured in the
centre-of-mass frame of the photon dissociation system: the hadronic energy
flow, the Feynman-x (x_F) variable for charged particles, the squared
transverse momentum of charged particles (p_T^{*2}), and the mean p_T^{*2} as a
function of x_F. These distributions are compared with results in the gamma^* p
centre-of-mass frame from inclusive deep-inelastic scattering in the
fixed-target experiment EMC, and also with the predictions of several Monte
Carlo calculations. The data are consistent with a picture in which the
partonic structure of the diffractive exchange is dominated at low Q^2 by hard
gluons.Comment: 16 pages, 6 figures, submitted to Phys. Lett.
Measurement of D* Meson Cross Sections at HERA and Determination of the Gluon Density in the Proton using NLO QCD
With the H1 detector at the ep collider HERA, D* meson production cross
sections have been measured in deep inelastic scattering with four-momentum
transfers Q^2>2 GeV2 and in photoproduction at energies around W(gamma p)~ 88
GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe
the differential cross sections within theoretical and experimental
uncertainties. Using these calculations, the NLO gluon momentum distribution in
the proton, x_g g(x_g), has been extracted in the momentum fraction range
7.5x10^{-4}< x_g <4x10^{-2} at average scales mu^2 =25 to 50 GeV2. The gluon
momentum fraction x_g has been obtained from the measured kinematics of the
scattered electron and the D* meson in the final state. The results compare
well with the gluon distribution obtained from the analysis of scaling
violations of the proton structure function F_2.Comment: 27 pages, 9 figures, 2 tables, submitted to Nucl. Phys.
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