461 research outputs found
The Formation and Role of Vortices in Protoplanetary Disks
We carry out a two-dimensional, compressible, simulation of a disk, including
dust particles, to study the formation and role of vortices in protoplanetary
disks. We find that anticyclonic vortices can form out of an initial random
perturbation of the vorticity field. Vortices have a typical decay time of the
order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk
aspect ratio of H/r = 0.15). If vorticity is continuously generated at a
constant rate in the flow (e.g. by convection), then a large vortex can form
and be sustained (due to the merger of vortices).
We find that dust concentrates in the cores of vortices within a few orbital
periods, when the drag parameter is of the order of the orbital frequency.
Also, the radial drift of the dust induces a significant increase in the
surface density of dust particles in the inner region of the disk. Thus,
vortices may represent the preferred location for planetesimal formation in
protoplanetary disks.
We show that it is very difficult for vortex mergers to sustain a relatively
coherent outward flux of angular momentum.Comment: Sumitted to the Astrophysical Journal, October 20, 199
Far Ultraviolet Spectroscopic Explorer Spectroscopy of the Nova-like BB Doradus
We present an analysis of the Far Ultraviolet Spectroscopic Explorer
({\it{FUSE}}) spectra of the little-known southern nova-like BB Doradus. The
spectrum was obtained as part of our Cycle 8 {\it FUSE} survey of high
declination nova-like stars.
The FUSE spectrum of BB Dor, observed in a high state, is modeled with an
accretion disk with a very low inclination (possibly lower than 10deg).
Assuming an average WD mass of 0.8 solar leads to a mass accretion rate of
1.E-9 Solar mass/year and a distance of the order of 650 pc, consistent with
the extremely low galactic reddening in its direction. The spectrum presents
some broad and deep silicon and sulfur absorption lines, indicating that these
elements are over-abundant by 3 and 20 times solar, respectively
Planets opening dust gaps in gas disks
We investigate the interaction of gas and dust in a protoplanetary disk in
the presence of a massive planet using a new two-fluid hydrodynamics code. In
view of future observations of planet-forming disks we focus on the condition
for gap formation in the dust fluid. While only planets more massive than 1
Jupiter mass (MJ) open up a gap in the gas disk, we find that a planet of 0.1
MJ already creates a gap in the dust disk. This makes it easier to find
lower-mass planets orbiting in their protoplanetary disk if there is a
significant population of mm-sized particles.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Cooling Flows of Self-Gravitating, Rotating, Viscous Systems
We obtain self-similar solutions that describe the dynamics of a
self-gravitating, rotating, viscous system. We use simplifying assumptions; but
explicitly include viscosity and the cooling due to the dissipation of energy.
By assuming that the turbulent dissipation of energy is as power law of the
density and the speed v_{rms} and for a power-law dependence of viscosity on
the density, pressure, and rotational velocity, we investigate turbulent
cooling flows. It has been shown that for the cylindrically and the spherically
cooling flows the similarity indices are the same, and they depend only on the
exponents of the dissipation rate and the viscosity model. Depending on the
values of the exponents, which the mechanisms of the dissipation and viscosity
determine them, we may have solutions with different general physical
properties. The conservation of the total mass and the angular momentum of the
system strongly depends on the mechanisms of energy dissipation and the
viscosity model.Comment: 19 pages, 5 figures, To appear in ApJ (scheduled for the v574, July
20, 2002
Turbulence and the formation of filaments, loops and shock fronts in NGC 1275 in the Perseus Galaxy Cluster
NGC1275, the central galaxy in the Perseus cluster, is the host of gigantic
hot bipolar bubbles inflated by AGN jets observed in the radio as Perseus A. It
presents a spectacular -emitting nebulosity surrounding NGC1275,
with loops and filaments of gas extending to over 50 kpc. The origin of the
filaments is still unknown, but probably correlates with the mechanism
responsible for the giant buoyant bubbles. We present 2.5 and 3-dimensional MHD
simulations of the central region of the cluster in which turbulent energy,
possibly triggered by star formation and supernovae (SNe) explosions is
introduced. The simulations reveal that the turbulence injected by massive
stars could be responsible for the nearly isotropic distribution of filaments
and loops that drag magnetic fields upward as indicated by recent observations.
Weak shell-like shock fronts propagating into the ICM with velocities of
100-500 km/s are found, also resembling the observations. The isotropic outflow
momentum of the turbulence slows the infall of the intracluster medium, thus
limiting further starburst activity in NGC1275. As the turbulence is subsonic
over most of the simulated volume, the turbulent kinetic energy is not
efficiently converted into heat and additional heating is required to suppress
the cooling flow at the core of the cluster. Simulations combining the MHD
turbulence with the AGN outflow can reproduce the temperature radial profile
observed around NGC1275. While the AGN mechanism is the main heating source,
the supernovae are crucial to isotropize the energy distribution.Comment: accepted by ApJ Letter
Dynamics of antibiotic resistance genes and presence of putative pathogens during ambient temperature anaerobic digestion.
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Previous issue date: 2015-03-03201
Far Ultraviolet Observations of the Dwarf Novae SS Aur and RU Peg in Quiescence
We have analyzed the Far Ultraviolet Spectrocopic Explorer (FUSE) spectra of
two U Gem-Type dwarf novae, SS Aur and RU Peg, observed 28 days and 60 days
(respectively) after their last outburst. In both systems the FUSE spectra (905
- 1182 A) reveal evidence of the underlying accreting white dwarf exposed in
the far UV. Our grid of theoretical models yielded a best-fitting photosphere
to the FUSE spectra with Teff=31,000K for SS Aur and Teff=49,000K for RU Peg.
This work provides two more dwarf nova systems with known white dwarf
temperatures above the period gap where few are known. The absence of CIII
(1175 A) absorption in SS Aur and the elevation of N above solar suggests the
possibility that SS Aur represents an additional accreting white dwarf where
the surface C/N ratio derives from CNO processing. For RU Peg, the modeling
uncertainties prevent any reliable conclusions about the surface abundances and
rotational velocity.Comment: AJ, Oct. 200
Impact of infection status and cyclosporine on voriconazole pharmacokinetics in an experimental model of cerebral scedosporiosis
Cerebral Scedosporium infections usually occur in lung transplant recipients as well as in immunocompetent patients in the context of near-drowning. Voriconazole is the first-line treatment. The diffusion of voriconazole through the blood-brain barriers in the context of cerebral infection and cyclosporine administration is crucial and remains a matter of debate. To address this issue, the pharmacokinetics of voriconazole were assessed in the plasma, cerebrospinal fluid (CSF), and brain, in an experimental model of cerebral scedosporiosis in rats receiving or not cyclosporine. A single dose of voriconazole (30 mg/kg, i.v.) was administrated to six groups of rats randomized according to the infection status and the cyclosporine dosing regimen (no cyclosporine, a single dose or three doses 15 mg/kg each). Voriconazole concentrations in plasma, CSF, and brain samples were quantified using UPLC-MS/MS and HPLC-UV methods and documented up to 48 hours after administration. Pharmacokinetic parameters were estimated using a non-compartmental approach. Voriconazole pharmacokinetic profiles were similar for plasma, CSF, and the brain in all groups studied. Voriconazole Cmax and AUC0=>48h were significantly higher in the plasma than in the CSF (CSF/plasma ratio, median [range] = 0.5 [0.39-0.55] for AUC0=>48h and 0.47 [0.35 and 0.75] for Cmax). Cyclosporine administration was significantly associated with an increase in voriconazole exposure in the plasma, CSF, and brain. In the plasma but not in the brain, an interaction between the infection and cyclosporine administration reduced the positive impact of cyclosporine on voriconazole exposure. Together these results emphasize the impact of cyclosporine on the brain voriconazole exposure
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