3,575 research outputs found
Jet Deflection via Cross winds: Laboratory Astrophysical Studies
We present new data from High Energy Density (HED) laboratory experiments
designed to explore the interaction of a heavy hypersonic radiative jet with a
cross wind. The jets are generated with the MAGPIE pulsed power machine where
converging conical plasma flows are produced from a cylindrically symmetric
array of inclined wires. Radiative hypersonic jets emerge from the convergence
point. The cross wind is generated by ablation of a plastic foil via
soft-X-rays from the plasma convergence region. Our experiments show that the
jets are deflected by the action of the cross wind with the angle of deflection
dependent on the proximity of the foil. Shocks within the jet beam are apparent
in the data. Analysis of the data shows that the interaction of the jet and
cross wind is collisional and therefore in the hydro-dynamic regime. MHD plasma
code simulations of the experiments are able to recover the deflection
behaviour seen in the experiments. We consider the astrophysical relevance of
these experiments applying published models of jet deflection developed for AGN
and YSOs. Fitting the observed jet deflections to quadratic trajectories
predicted by these models allows us to recover a set of plasma parameters
consistent with the data. We also present results of 3-D numerical simulations
of jet deflection using a new astrophysical Adaptive Mesh Refinement code.
These simulations show highly structured shocks occurring within the beam
similar to what was observed in the experimentsComment: Submitted to ApJ. For a version with figures go to
http://web.pas.rochester.edu/~afrank/labastro/CW/Jet-Wind-Frank.pd
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
The effect of a stellar magnetic variation on the jet velocity
Stellar jets are normally constituted by chains of knots with some
periodicity in their spatial distribution, corresponding to a variability of
order of several years in the ejection from the protostar/disk system. A widely
accepted theory for the presence of knots is related to the generation of
internal working surfaces due to variations in the jet ejection velocity. In
this paper we study the effect of variations in the inner disk-wind radius on
the jet ejection velocity. We show that a small variation in the inner
disk-wind radius produce a variation in the jet velocity large enough to
generate the observed knots. We also show that the variation in the inner
radius may be related to a variation of the stellar magnetic field.Comment: 5 pages, 3 figures, accepted for publication in Ap
The variability behavior of CoRoT M-giant Stars
For 6 years the Convection, Rotation, and Planetary Transits (CoRoT) space
mission has acquired photometric data from more than one hundred thousand point
sources towards and directly opposite from the inner and outer regions of the
Galaxy. The high temporal resolution of the CoRoT data combined with the wide
time span of the observations has enabled the study of short and long time
variations in unprecedented detail. From the initial sample of 2534 stars
classified as M-giants in the CoRoT databasis, we selected 1428 targets that
exhibit well defined variability, using visual inspection. The variability
period and amplitude of C1 stars (stars having Teff < 4200 K) were computed
using Lomb-Scargle and harmonic fit methods. The trends found in the V-I vs J-K
color-color diagram are in agreement with standard empirical calibrations for
M-giants. The sources located towards the inner regions of the Galaxy are
distributed throughout the diagram while the majority of the stars towards the
outer regions of the Galaxy are spread between the calibrations of M-giants and
the predicted position for Carbon stars. The stars classified as supergiants
follow a different sequence from the one found for giant stars. We also
performed a KS test of the period and amplitude of stars towards the inner and
outer regions of the Galaxy. We obtained a low probability that the two samples
come from the same parent distribution. The observed behavior of the
period-amplitude and period-Teff diagrams are, in general, in agreement with
those found for Kepler sources and ground based photometry, with pulsation
being the dominant cause responsible for the observed modulation. We also
conclude that short-time variations on M-Giant stars do not exist orare very
rare and the few cases we found are possibly related to biases or background
stars.Comment: 11 pages, 6 figure
An efficient âa prioriâ model reduction for boundary element models
The Boundary Element Method (BEM) is a discretisation technique for solving partial differential equations, which offers, for certain problems, important advantages over domain techniques. Despite the high CPU time reduction that can be achieved, some 3D problems remain today untreatable because the extremely large number of degrees of freedomâdofâinvolved in the boundary description. Model reduction seems to be an appealing choice for both, accurate and efficient numerical simulations. However, in the BEM the reduction in the number of degrees of freedom does not imply a significant reduction in the CPU time, because in this technique the more important part of the computing time is spent in the construction of the discrete system of equations. In this way, a reduction also in the number of weighting functions, seems to be a key point to render efficient boundary element simulations
Effect of ammonia load on efficiency of nitrogen removal in an SBBR with liquid-phase circulation
The removal of biological nitrogen from a synthetic wastewater with different ammonium nitrogen concentrations (50 and 100 mgN-NH4+/L) by a nitrification and denitrification process using a sequencing batch biofilm reactor (SBBR) with liquid-phase circulation was studied. The system with a total working volume of 4.6 L (3.7 L in the reactor and 0.9 L in the reservoir) treated 2.1 L of synthetic wastewater in 12-h cycles. As inoculum two types of biomass were used: an anaerobic/anoxic one from an up-flow anaerobic sludge blanket reactor (UASB) and an aerobic one from a prolonged aeration activated sludge system. The system, maintained at 30 ± 1 ÂșC, operated in batch mode followed by fed-batch mode and was aerated intermittently. During fed-batch operation the reactor was fed with an external carbon source as electron donor in the denitrifying step and with no aeration. When the reactor was fed with 50 mgN-NH4+/L, efficiencies of removal of ammonium nitrogen and total nitrogen from the effluent were 93.8 and 72.2%, respectively, and nitrite, nitrate and organic nitrogen concentrations were 0.07, 6.4 and 0.5 mg/L, respectively. On the other hand, when the influent ammonium nitrogen concentration was 100 mgN-NH4+/L, residual nitrite and nitrate were 0.17 and 20.4, respectively, and no N-Org was found in the effluent. It should be mentioned that residual nitrate remained unaltered at the different C/N ratios used. Consequently, efficiency of total nitrogen removal was reduced to 66.7%, despite efficiency of ammonium nitrogen removal exceeding 90%. These results show the potential of the proposed system in removing ammonium nitrogen from liquid effluents with a moderate ammonium nitrogen concentration.Fundação de Amparo Ă Pesquisa do Estado de SĂŁo Paulo (FAPESP
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
Accretion Disks around Young Objects. I. The Detailed Vertical Structure
We discuss the properties of an accretion disk around a star with parameters
typical of classical T Tauri stars (CTTS), and with the average accretion rate
for these disks. The disk is assumed steady and geometrically thin. The
turbulent viscosity coefficient is expressed using the alpha prescription and
the main heating mechanisms considered are viscous dissipation and irradiation
by the central star. The energy is transported by radiation, turbulent
conduction and convection.
We find that irradiation from the central star is the main heating agent of
the disk, except in the innermost regions, R less than 2 AU. The irradiation
increases the temperature of the outer disk relative to the purely viscous
case. As a consequence, the outer disk (R larger than 5 AU) becomes less dense,
optically thin and almost vertically isothermal, with a temperature
distribution T proportional to R^{-1/2}. The decrease in surface density at the
outer disk, decreases the disk mass by a factor of 4 respect to a purely
viscous case. In addition, irradiation tends to make the outer disk regions
stable against gravitational instabilities.Comment: 41 pages, 14 postscript figures, LaTeX, accepted by Ap
- âŠ