2,085 research outputs found
Hydrodynamics of photoionized columns in the Eagle Nebula, M 16
We present hydrodynamical simulations of the formation, structure and
evolution of photoionized columns, with parameters based on those observed in
the Eagle Nebula. On the basis of these simulations we argue that there is no
unequivocal evidence that the dense neutral clumps at heads of the columns were
cores in the pre-existing molecular cloud. In our simulations, a variety of
initial conditions leads to the formation and maintenance of near-equilibrium
columns. Therefore, it is likely that narrow columns will often occur in
regions with large-scale inhomogeneities, but that observations of such columns
can tell us little about the processes by which they formed. The manner in
which the columns in our simulations develop suggests that their evolution may
result in extended sequences of radiation-induced star formation.Comment: 12 pages, 9 figures, Latex, MN macros, in press with MNRA
Cardiovascular consequences of cortisol excess
Cushing's syndrome is a consequence of primary or, more commonly, secondary oversecretion of cortisol. Cardiovascular disease is the major cause of morbidity and mortality in Cushing's syndrome, and excess risk remains even in effectively treated patients. The cardiovascular consequences of cortisol excess are protean and include, inter alia, elevation of blood pressure, truncal obesity, hyperinsulinemia, hyperglycemia, insulin resistance, and dyslipidemia. This review analyses the relationship of cortisol excess, both locally and at tissue level, to these cardiovascular risk factors, and to putative mechanisms for hypertension. Previous studies have examined correlations between cortisol, blood pressure, and other parameters in the general population and in Cushing's syndrome. This review also details changes induced by short-term cortisol administration in normotensive healthy men
Submicrosecond comparisons of time standards via the Navigation Technology Satellites (NTS)
An interim demonstration was performed of the time transfer capability of the NAVSTAR GPS system using a single NTS satellite. Measurements of time difference (pseudo-range) are made from the NTS tracking network and at the participating observatories. The NTS network measurements are used to compute the NTS orbit trajectory. The central NTS tracking station has a time link to the Naval Observatory UTC (USNO,MC1) master clock. Measurements are used with the NTS receiver at the remote observatory, the time transfer value UTC (USNO,MC1)-UTC (REMOTE, VIA NTS) is calculated. Intercomparisons were computed using predicted values of satellite clock offset and ephemeus
The fragmentation of expanding shells II: Thickness matters
We study analytically the development of gravitational instability in an
expanding shell having finite thickness. We consider three models for the
radial density profile of the shell: (i) an analytic uniform-density model,
(ii) a semi-analytic model obtained by numerical solution of the hydrostatic
equilibrium equation, and (iii) a 3D hydrodynamic simulation. We show that all
three profiles are in close agreement, and this allows us to use the first
model to describe fragments in the radial direction of the shell. We then use
non-linear equations describing the time-evolution of a uniform oblate spheroid
to derive the growth rates of shell fragments having different sizes. This
yields a dispersion relation which depends on the shell thickness, and hence on
the pressure confining the shell. We compare this dispersion relation with the
dispersion relation obtained using the standard thin-shell analysis, and show
that, if the confining pressure is low, only large fragments are unstable. On
the other hand, if the confining pressure is high, fragments smaller than
predicted by the thin-shell analysis become unstable. Finally, we compare the
new dispersion relation with the results of 3D hydrodynamic simulations, and
show that the two are in good agreement.Comment: 9 pages, 9 figures, accepted by MNRA
SREBF1 links lipogenesis to mitophagy and sporadic Parkinson disease
Mitochondrial quality control has an impact on many diseases, but intense research has focused on the action of 2 genes linked to heritable forms of Parkinson disease (PD), PINK1 and PARK2/parkin, which act in a common pathway to promote mitophagy. However, criticism has been raised that little evidence links this mechanism to sporadic PD. To gain a greater insight into the mechanisms of PINK1-PARK2 mediated mitophagy, we undertook a genome-wide RNAi screen in Drosophila and human cell models. Strikingly, we discovered several components of the lipogenesis pathway, including SREBF1, playing a conserved role in mitophagy. Our results suggest that lipids influence the stabilization of PINK1 during the initiation of mitophagy. Importantly, SREBF1 has previously been identified as a risk locus for sporadic PD, and thus implicates aberrant mitophagy as contributing to sporadic PD. Our findings suggest a role for lipid synthesis in PINK1-PARK2 mediated mitophagy, and propose a mechanistic link between familial and sporadic PD, supporting a common etiology
Stem cell therapies for treating osteoarthritis: prescient or premature?
There has been unprecedented interest in recent years in the use of stem cells as therapy for an array of diseases in companion animals. Stem cells have already been deployed therapeutically in a number of clinical settings, in particular the use of mesenchymal stem cells to treat osteoarthritis in horses and dogs. However, an assessment of the scientific literature highlights a marked disparity between the purported benefits of stem cell therapies and their proven abilities as defined by rigorously controlled scientific studies.Although preliminary data generated from clinical trials in human patients are encouraging, therapies currently available to treat animals are supported by very limited clinical evidence, and the commercialisation of these treatments may be premature. This review introduces the three main types of stem cells relevant to veterinary applications, namely, embryonic stem cells, induced pluripotent stem cells, and mesenchymal stem cells, and draws together research findings from in vitro and in vivo studies to give an overview of current stem cell therapies for the treatment of osteoarthritis in animals. Recent advances in tissue engineering, which is proposed as the future direction of stem cell-based therapy for osteoarthritis, are also discussed
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
Why do starless cores appear more flattened than protostellar cores?
We evaluate the intrinsic three dimensional shapes of molecular cores, by
analysing their projected shapes. We use the recent catalogue of molecular line
observations of Jijina et al. and model the data by the method originally
devised for elliptical galaxies. Our analysis broadly supports the conclusion
of Jones et al. that molecular cores are better represented by triaxial
intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids).
However, we find that the best fit to all of the data is obtained with more
extreme axial ratios () than those derived by Jones et al.
More surprisingly, we find that starless cores have more extreme axial ratios
than protostellar cores -- starless cores appear more `flattened'. This is the
opposite of what would be expected from modeling the freefall collapse of
triaxial ellipsoids. The collapse of starless cores would be expected to
proceed most swiftly along the shortest axis - as has been predicted by every
modeller since Zel'dovich - which should produce more flattened cores around
protostars, the opposite of what is seen.Comment: 7 pages, 3 figure
Gas-cooling by dust during dynamical fragmentation
We suggest that the abrupt switch, from hierarchical clustering on scales
larger than 0.04 pc, to binary (and occasionally higher multiple) systems on
smaller scales, which Larson has deduced from his analysis of the grouping of
pre-Main-Sequence stars in Taurus, arises because pre-protostellar gas becomes
thermally coupled to dust at sufficiently high densities. The resulting change
from gas-cooling by molecular lines at low densities to gas-cooling by dust at
high densities enables the matter to radiate much more efficiently, and hence
to undergo dynamical fragmentation.
We derive the domain where gas-cooling by dust facilitates dynamical
fragmentation. Low-mass (i.e. solar mass) clumps - those supported mainly by
thermal pressure - can probably access this domain spontaneously, albeit rather
quasistatically, provided they exist in a region where external perturbations
are few and far between. More massive clumps probably require an impulsive
external perturbation, for instance a supersonic collision with another clump,
in order for the gas to reach sufficiently high density to couple thermally to
the dust. Impulsive external perturbations should promote fragmentation, by
generating highly non-line ar substructures which can then be amplified by
gravity during the subsequent collapse.Comment: 9 pages, 4 figures, accepted by MNRA
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