315 research outputs found
Selfgravitating Gas Spheres in a Box and Relativistic Clusters: Relation between Dynamical and Thermodynamical Stability
We derive a variational principle for the dynamical stability of a cluster as
a gas sphere in a box. Newtonian clusters are always dynamically stable and,
for relativistic clusters, the relation between dynamical and thermodynamical
instabilities is analyzed. The boundaries between dynamically and
thermodynamically stable and unstable models are found numerically for
relativistic stellar systems with different cut off parameters. A criterion
based on binding energy curve is used for determination of the boundary of
dynamical stability.Comment: 10 figure
Multi--Pressure Polytropes as Models for the Structure and Stability of Molecular Clouds. I. Theory
Molecular clouds are supported by thermal pressure, magnetic pressure, and
turbulent pressure. Each of these can be modeled with a polytropic equation of
state, so that overall the total pressure is the sum of the individual
components. We model the turbulent pressure as being due to a superposition of
Alfven waves. The theory of polytropes is generalized to allow for the flow of
entropy in response to a perturbation, as expected for the entropy associated
with wave pressure. The equation of state of molecular clouds is "soft", so
that the properties of the clouds are generally governed by the conditions at
the surface. In general, the polytropes are not isentropic, and this permits
large density and pressure drops to occur between the center and the edge of
the polytropes, as is observed.Comment: Submitted to ApJ with 10 figure
Anion-Catalyzed Dissolution of NO_2 on Aqueous Microdroplets
Fifty-seven years after NOx (NO + NO_2) were identified as essential components of photochemical smog, atmospheric chemical models fail to correctly predict âąOH/HO_2âą concentrations under NO_x-rich conditions. This deficiency is due, in part, to the uncertain rates and mechanism for the reactive dissolution of NO_2(g) (2NO_2 + H_2O = NO_3^â + H^+ + HONO) in fog and aerosol droplets. Thus, state-of-the-art models parametrize the uptake of NO_2 by atmospheric aerosol from data obtained on âdeactivated tunnel wall residueâ. Here, we report experiments in which NO_3^â production on the surface of microdroplets exposed to NO_2(g) for 1 ms is monitored by online thermospray mass spectrometry. NO_2 does not dissolve in deionized water (NO_3^â signals below the detection limit) but readily produces NO_3^â on aqueous NaX (X = Cl, Br, I) microdroplets with NO_2 uptake coefficients Îł that vary nonmonotonically with electrolyte concentration and peak at Îł_(max) ~ 10^(â4) for [NaX] ~ 1 mM, which is >10^3 larger than that in neat water. Since I^â is partially oxidized to I_2âą^â in this process, anions seem to capture NO2(g) into XâNO_2âą^â radical anions for further reaction at the air/water interface. By showing that Îł is strongly enhanced by electrolytes, these results resolve outstanding discrepancies between previous measurements in neat water versus NaCl-seeded clouds. They also provide a general mechanism for the heterogeneous conversion of NO_2(g) to (NO_3^â + HONO) on the surface of aqueous media
Photodesorption of water ice: a molecular dynamics study
Absorption of ultraviolet radiation by water ice coating interstellar grains
can lead to dissociation and desorption of the ice molecules. These processes
are thought to be important in the gas-grain chemistry in molecular clouds and
protoplanetary disks, but very few quantitative studies exist. We compute the
photodesorption efficiencies of amorphous water ice and elucidate the
mechanisms by which desorption occurs. Classical molecular dynamics
calculations were performed for a compact amorphous ice surface at 10 K thought
to be representative of interstellar ice. Dissociation and desorption of H2O
molecules in the top six monolayers are considered following absorption into
the first excited electronic state with photons in the 1300-1500 Angstrom
range. The trajectories of the H and OH photofragments are followed until they
escape or become trapped in the ice. The probability for H2O desorption per
absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to
0.03% in the next two monolayers, and is negligible deeper into the ice. The
main H2O removal mechanism in the top two monolayers is through separate
desorption of H and OH fragments. Removal of H2O molecules from the ice, either
as H2O itself or its products, has a total probability of 2-3% per absorbed UV
photon in the top two monolayers. In the third monolayer the probability is
about 1% and deeper into the ice the probability of photodesorption falling to
insignificant numbers. The probability of any removal of H2O per incident
photon is estimated to be 3.7x10^-4, with the probability for photodesorption
of intact H2O molecules being 1.4x10^-4 per incident photon. When no desorption
occurs, the H and OH products can travel up to 70 and 60 Angstroms inside or on
top of the surface during which they can react with other species.Comment: 12 pages, 10 figures, A&A, in pres
The Lifetimes and Evolution of Molecular Cloud Cores
We discuss the lifetimes and evolution of clumps and cores formed as
turbulent density fluctuations in nearly isothermal molecular clouds. In the
non-magnetic case, clumps are unlikely to reach a hydrostatic state, and
instead are expected to either proceed directly to collapse, or else
``rebound'' towards the mean pressure and density of the parent cloud.
Rebounding clumps are delayed in their re-expansion by their self-gravity. From
a simple virial calculation, we find re-expansion times of a few free-fall
times. In the magnetic case, we present a series of driven-turbulence,
ideal-MHD isothermal numerical simulations in which we follow the evolution of
clumps and cores in relation to the magnetic criticality of their ``parent
clouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps do
not form. A few moderately-gravitationally bound clumps form which however are
dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar
diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only
slightly longer than the dynamical times. In supercritical boxes, some cores
become locally supercritical and collapse in typical times ~ 1 Myr. We also
observe longer-lived supercritical cores that however do not collapse because
they are smaller than the local Jeans length. Fewer clumps and cores form in
these simulations than in their non-magnetic counterpart. Our results suggest
that a) A fraction of the cores may not form stars, and may correspond to some
of the observed starless cores. b) Cores may be out-of-equilibrium structures,
rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the
star formation efficiency by reducing the probability of core formation, rather
than by significantly delaying the collapse of individual cores.Comment: Accepted in ApJ. Originally submitted as astro-ph/0208245. Completely
rewritten, now including numerical simulations. Animations available at
http://www.astrosmo.unam.mx/~e.vazquez/turbulence_HP/movies/VKSB04.htm
Dynamical stability of infinite homogeneous self-gravitating systems: application of the Nyquist method
We complete classical investigations concerning the dynamical stability of an
infinite homogeneous gaseous medium described by the Euler-Poisson system or an
infinite homogeneous stellar system described by the Vlasov-Poisson system
(Jeans problem). To determine the stability of an infinite homogeneous stellar
system with respect to a perturbation of wavenumber k, we apply the Nyquist
method. We first consider the case of single-humped distributions and show
that, for infinite homogeneous systems, the onset of instability is the same in
a stellar system and in the corresponding barotropic gas, contrary to the case
of inhomogeneous systems. We show that this result is true for any symmetric
single-humped velocity distribution, not only for the Maxwellian. If we
specialize on isothermal and polytropic distributions, analytical expressions
for the growth rate, damping rate and pulsation period of the perturbation can
be given. Then, we consider the Vlasov stability of symmetric and asymmetric
double-humped distributions (two-stream stellar systems) and determine the
stability diagrams depending on the degree of asymmetry. We compare these
results with the Euler stability of two self-gravitating gaseous streams.
Finally, we determine the corresponding stability diagrams in the case of
plasmas and compare the results with self-gravitating systems
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
Stable Field Emitters for a Miniature X-ray Tube Using Carbon Nanotube Drop Drying on a Flat Metal Tip
Stable carbon nanotube (CNT) field emitters for a vacuum-sealed miniature X-ray tube have been fabricated. The field emitters with a uniform CNT coating are prepared by a simple drop drying of a CNT mixture solution that is composed of chemically modified multi-walled CNTs, silver nanoparticles, and isopropyl alcohol on flat tungsten tips. A highly thermal- and electrical-conductive silver layer strongly attaches CNTs to the tungsten tips. Consequently, the field emitters exhibit good electron emission stability: continuous electron emission of around 100 ÎŒA at 2.3 V/ÎŒm has stably lasted over 40 h even at non-high vacuum ambient (~10â3 Pa)
Interstudy reproducibility of the second generation, Fourier domain optical coherence tomography in patients with coronary artery disease and comparison with intravascular ultrasound: a study applying automated contour detection
Recently, Fourier domain OCT (FD-OCT) has been introduced for clinical use. This approach allows in vivo, high resolution (15 micron) imaging with very fast data acquisition, however, it requires brief flushing of the lumen during imaging. The reproducibility of such fast data acquisition under intracoronary flush application is poorly understood. To assess the inter-study variability of FD-OCT and to compare lumen morphometry to the established invasive imaging method, IVUS. 18 consecutive patients with coronary artery disease scheduled for PCI were included. In each target vessel a FD-OCT pullback (MGH system, light source 1,310 nm, 105 fps, pullback speed 20 mm/s) was acquired during brief (3 s) injection of X-ray contrast (flow 3 ml/s) through the guiding catheter. A second pullback was repeated under the same conditions after re-introduction of the FD OCT catheter into the coronary artery. IVUS and OCT imaging was performed in random order. FD-OCT and IVUS pullback data were analyzed using a recently developed software employing semi automated lumen contour and stent strut detection algorithms. Corresponding ROI were matched based on anatomical landmarks such as side branches and/or stent edges. Inter-study variability is presented as the absolute difference between the two pullbacks. FD-OCT showed remarkably good reproducibility. Inter-study variability in native vessels (cohort A) was very low for mean and minimal luminal area (0.10 ± 0.38, 0.19 ± 0.57 mm[superscript 2], respectively). Likewise inter-study variability was very low in stented coronary segments (cohort B) for mean lumen, mean stent, minimal luminal and minimal stent area (0.06 ± 0.08, 0.07 ± 0.10, 0.04 ± 0.09, 0.04 ± 0.10 mm[superscript 2], respectively). Comparison to IVUS morphometry revealed no significant differences. The differences between both imaging methods, OCT and IVUS, were very low for mean lumen, mean stent, minimal luminal and minimal stent area (0.10 ± 0.45, 0.10 ± 0.36, 0.26 ± 0.54, 0.05 ± 0.47 mm[superscript 2], respectively). FD-OCT shows excellent reproducibility and very low inter-study variability in both, native and stented coronary segments. No significant differences in quantitative lumen morphometry were observed between FD-OCT and IVUS. Evaluating these results suggest that FD-OCT is a reliable imaging tool to apply in longitudinal coronary artery disease studie
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