929 research outputs found
Massive star formation via high accretion rates and early disk-driven outflows
We present an investigation of massive star formation that results from the
gravitational collapse of massive, magnetized molecular cloud cores. We
investigate this by means of highly resolved, numerical simulations of initial
magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing
three different cases - an isothermal collapse, a collapse with radiative
cooling, and a magnetized collapse - we show that massive stars assemble
quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the
mass accretion during the collapsing phase is much more efficient than
predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that
during protostellar assembly the mass accretion reaches 20 - 100 c^3/G.
Furthermore, we determined the self-consistent structure of bipolar outflows
that are produced in our three dimensional magnetized collapse simulations.
These outflows produce cavities out of which radiation pressure can be
released, thereby reducing the limitations on the final mass of massive stars
formed by gravitational collapse. Moreover, we argue that the extraction of
angular momentum by disk-threaded magnetic fields and/or by the appearance of
bars with spiral arms significantly enhance the mass accretion rate, thereby
helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ,
see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf
for high resolution figure
Assessing the association between oral hygiene and preterm birth by quantitative light-induced fluorescence
The aim of this study was to investigate the purported link between oral hygiene and preterm birth by using image analysis tools to quantify dental plaque biofilm. Volunteers (η = 91) attending an antenatal clinic were identified as those considered to be “at high risk” of preterm delivery (i.e., a previous history of idiopathic preterm delivery, case group) or those who were not considered to be at risk (control group). The women had images of their anterior teeth captured using quantitative light-induced fluorescence (QLF). These images were analysed to calculate the amount of red fluorescent plaque (ΔR%) and percentage of plaque coverage. QLF showed little difference in ΔR% between the two groups, 65.00% case versus 68.70% control, whereas there was 19.29% difference with regard to the mean plaque coverage, 25.50% case versus 20.58% control. A logistic regression model showed a significant association between plaque coverage and case/control status (Ρ = 0.031), controlling for other potential predictor variables, namely, smoking status, maternal age, and body mass index (BMI)
Cost efficient operations for Discovery class missions
The Near Earth Asteroid Rendezvous (NEAR) program at The Johns Hopkins University Applied Physics Laboratory is scheduled to launch the first spacecraft in NASA's Discovery program. The Discovery program is to promote low cost spacecraft design, development, and mission operations for planetary space missions. The authors describe the NEAR mission and discuss the design and development of the NEAR Mission Operations System and the NEAR Ground System with an emphasis on those aspects of the design that are conducive to low-cost operations
Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres
Self-similar solutions provide good descriptions for the gravitational
collapse of spherical clouds or stars when the gas obeys a polytropic equation
of state, (with ). We study the behaviors of
nonradial perturbations in the similarity solutions of Larson, Penston and
Yahil, which describe the evolution of the collapsing cloud prior to core
formation. Our global stability analysis reveals the existence of unstable
bar-modes () when . In particular, for the collapse of
isothermal spheres, which applies to the early stages of star formation, the
density perturbation relative to the background, , increases as ,
where denotes the epoch of core formation, and is the cloud
central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal
shape (prolate bar or oblate disk, depending on initial conditions) as the
collapse proceeds. In the context of Type II supernovae, core collapse is
described by the equation of state, and our analysis
indicates that there is no growing mode (with density perturbation) in the
collapsing core before the proto-neutron star forms, although nonradial
perturbations can grow during the subsequent accretion of the outer core and
envelope onto the neutron star. We also carry out a global stability analysis
for the self-similar expansion-wave solution found by Shu, which describes the
post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a
protostar. We show that this solution is unstable to perturbations of all
's, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To
be published in ApJ (V.540, Sept.10, 2000 issue
A PPMAP analysis of the filamentary structures in Ophiuchus L1688 and L1689
We use the PPMAP (Point Process MAPping) algorithm to re-analyse the
\textit{Herschel} and SCUBA-2 observations of the L1688 and L1689 sub-regions
of the Ophiuchus molecular cloud. PPMAP delivers maps with high resolution
(here , corresponding to at ),
by using the observations at their native resolutions. PPMAP also delivers more
accurate dust optical depths, by distinguishing dust of different types and at
different temperatures. The filaments and prestellar cores almost all lie in
regions with (corresponding
to ). The dust temperature, , tends to be correlated
with the dust opacity index, , with low and low tend
concentrated in the interiors of filaments. The one exception to this tendency
is a section of filament in L1688 that falls -- in projection -- between the
two B stars, S1 and HD147889; here and are relatively high, and
there is compelling evidence that feedback from these two stars has heated and
compressed the filament. Filament {\sc fwhm}s are typically in the range
to . Most filaments have line densities in the
range to . If
their only support is thermal gas pressure, and the gas is at the canonical
temperature of , the filaments are highly supercritical. However,
there is some evidence from ammonia observations that the gas is significantly
warmer than this, and we cannot rule out the possibility of additional support
from turbulence and/or magnetic fields. On the basis of their spatial
distribution, we argue that most of the starless cores are likely to disperse
(rather than evolving to become prestellar).Comment: 22 pages, 18 figures, Accepted for publication by MNRAS March 202
Spatial-Distance Cues Influence Economic Decision-Making in a Social Context
Social distance (i.e., the degree of closeness to another person) affects the
way humans perceive and respond to fairness during financial negotiations.
Feeling close to someone enhances the acceptance of monetary offers. Here, we
explored whether this effect also extends to the spatial domain. Specifically,
using an iterated version of the Ultimatum Game in a within-subject design, we
investigated whether different visual spatial distance-cues result in
different rates of acceptance of otherwise identical monetary offers. Study 1
found that participants accepted significantly more offers when they were cued
with spatial closeness than when they were cued with spatial distance. Study 2
replicated this effect using identical procedures but different spatial-
distance cues in an independent sample. Importantly, our results could not be
explained by feelings of social closeness. Our results demonstrate that mere
perceptions of spatial closeness produce analogous–but independent–effects to
those of social closeness
Naturally occurring Ehrlichia chaffeensis infection in coyotes from Oklahoma.
A nested polymerase chain reaction assay was used to determine the presence of Ehrlichia chaffeensis, E. canis, and E. ewingii DNA in blood samples of free-ranging coyotes from central and northcentral Oklahoma. Of the 21 coyotes examined, 15 (71%) were positive for E. chaffeensis DNA; none was positive for E. canis or E. ewingii. Results suggest that E. chaffeensis infections are common in free-ranging coyotes in Oklahoma and that these wild canids could play a role in the epidemiology of human monocytotropic ehrlichiosis
Structure and Evolution of Hot Gas in 30 Dor
We have investigated the structure and evolution of hot gas in the 30 Dor
nebula, based on recent X-ray observations. Our deep ROSAT HRI image shows that
diffuse X-ray emission arises in blister-shaped regions outlined by loops of
HII gas. X-ray spectroscopic data from ASCA confirm the thermal nature of the
emission and indicate that hot gas temperature decreases from the core to the
halo of the nebula. The structure of the nebula can be understood as outflows
of hot and HII gases from the parent giant molecular cloud of the central OB
association. The dynamic mixing between the two gas phases is likely
responsible for the mass loading to the hot gas, as required to explain the
observed thermal structure and X-ray luminosity of the nebula. Such processes
should also be important in the formation of similar giant HII regions and in
their subsequent evolution into supergiant bubbles or galactic chimneys.Comment: 9 page text plus 4 color figures. To appear in ApJ
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