336 research outputs found
Calculation of the effect of inertia on the dynamic viscosity of dilute emulsions in a pure straining motion
The dynamic viscosity of a dilute emulsion is calculated for a pure straining motion. The emulsion consists of almost spherical drops of a Newtonian fluid immersed in another Newtonian fluid. The oscillating velocity field of the flow is derived from the Navier-Stokes equation, in which the linear inertia term is included, whereas the non-linear one is neglected. The dynamic viscosity is determined with the aid of a cell model. The results are calculated numerically and typical results are presented graphically. The influence of inertia on the dynamic viscosity appears to be appreciable. Special cases presented in the literature, frequency or density zero and rigid spheres, are confirmed
Simulating Reionization: Character and Observability
In recent years there has been considerable progress in our understanding of
the nature and properties of the reionization process. In particular, the
numerical simulations of this epoch have made a qualitative leap forward,
reaching sufficiently large scales to derive the characteristic scales of the
reionization process and thus allowing for realistic observational predictions.
Our group has recently performed the first such large-scale radiative transfer
simulations of reionization, run on top of state-of-the-art simulations of
early structure formation. This allowed us to make the first realistic
observational predictions about the Epoch of Reionization based on detailed
radiative transfer and structure formation simulations. We discuss the basic
features of reionization derived from our simulations and some recent results
on the observational implications for the high-redshift Ly-alpha sources.Comment: 3 pages, to appear in the Proceedings of First Stars III, Santa Fe,
July 2007, AIP Conference Serie
Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
[Abridged] We use new HST and archived images to clarify the nature of the
knots in the Helix Nebula. We employ published far infrared spectrophotometry
and existing 2.12 micron images to establish that the population distribution
of the lowest ro-vibrational states of H2 is close to the distribution of a gas
in LTE at 988 +- 119 K. We derive a total flux from the nebula in H2 lines and
compare this with the power available from the central star for producing this
radiation. We establish that neither soft X-rays nor FUV radiation has enough
energy to power the H2 radiation, only the stellar EUV radiation shortward of
912 Angstrom does. Advection of material from the cold regions of the knots
produces an extensive zone where both atomic and molecular hydrogen are found,
allowing the H2 to directly be heated by Lyman continuum radiation, thus
providing a mechanism that can explain the excitation temperature and surface
brightness of the cusps and tails. New images of the knot 378-801 reveal that
the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas
zone. This firmly establishes that the "tail" structure is an ionization
bounded radiation shadow behind the optically thick core of the knot. A unique
new image in the HeII 4686 Angstrom line fails to show any emission from knots
that might have been found in the He++ core of the nebula. We also re-examined
high signal-to-noise ratio ground-based telescope images of this same inner
region and found no evidence of structures that could be related to knots.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full resolution version is available at
http://www.ifront.org/wiki/Helix_Nebula_2007_Pape
Diffuse continuum transfer in H II regions
We compare the accuracy of various methods for determining the transfer of
the diffuse Lyman continuum in HII regions, by comparing them with a
high-resolution discrete-ordinate integration. We use these results to suggest
how, in multidimensional dynamical simulations, the diffuse field may be
treated with acceptable accuracy without requiring detailed transport
solutions. The angular distribution of the diffuse field derived from the
numerical integration provides insight into the likely effects of the diffuse
field for various material distributions.Comment: 12 pages, 7 figures, to be published in MNRA
Interaction of Infall and Winds in Young Stellar Objects
The interaction of a stellar or disk wind with a collapsing environment holds
promise for explaining a variety of outflow phenomena observed around young
stars. In this paper we present the first simulations of these interactions.
The focus here is on exploring how ram pressure balance between wind and
ambient gas and post-shock cooling affects the shape of the resulting outflows.
In our models we explore the role of ram pressure and cooling by holding the
wind speed constant and adjusting the ratio of the inflow mass flux to the wind
mass flux (Mdot_a/Mdot_w) Assuming non-spherical cloud collapse, we find that
relatively strong winds can carve out wide, conical outflow cavities and that
relatively weak winds can be strongly collimated into jet-like structures. If
the winds become weak enough, they can be cut off entirely by the infalling
environment. We identify discrepancies between results from standard snowplow
models and those presented here that have important implications for molecular
outflows. We also present mass vs. velocity curves for comparison with
observations.Comment: 35 pages, 11 figures (PNG and EPS
The Kinetic Sunyaev-Zel'dovich Effect from Radiative Transfer Simulations of Patchy Reionization
We present the first calculation of the kinetic Sunyaev-Zel'dovich (kSZ)
effect due to the inhomogeneous reionization of the universe based on detailed
large-scale radiative transfer simulations of reionization. The resulting sky
power spectra peak at l=2000-8000 with maximum values of
l^2C_l~1\times10^{-12}. The peak scale is determined by the typical size of the
ionized regions and roughly corresponds to the ionized bubble sizes observed in
our simulations, ~5-20 Mpc. The kSZ anisotropy signal from reionization
dominates the primary CMB signal above l=3000. This predicted kSZ signal at
arcminute scales is sufficiently strong to be detectable by upcoming
experiments, like the Atacama Cosmology Telescope and South Pole Telescope
which are expected to have ~1' resolution and ~muK sensitivity. The extended
and patchy nature of the reionization process results in a boost of the peak
signal in power by approximately one order of magnitude compared to a uniform
reionization scenario, while roughly tripling the signal compared with that
based upon the assumption of gradual but spatially uniform reionization. At
large scales the patchy kSZ signal depends largely on the ionizing source
efficiencies and the large-scale velocity fields: sources which produce photons
more efficiently yield correspondingly higher signals. The introduction of
sub-grid gas clumping in the radiative transfer simulations produces
significantly more power at small scales, and more non-Gaussian features, but
has little effect at large scales. The patchy nature of the reionization
process roughly doubles the total observed kSZ signal for l~3000-10^4 compared
to non-patchy scenarios with the same total electron-scattering optical depth.Comment: 14 pages, 13 figures (some in color), submitted to Ap
Photoevaporating flows from the cometary knots in the Helix nebula (NGC 7293)
We explain the Ha emission of the cometary knots in the Helix Nebula (NGC
7293) with an analytical model that describes the emission of the head of the
globules as a photoevaporated flow produced by the incident ionizing radiation
of the central star.We compare these models with the Ha emission obtained from
the HST archival images of the Helix Nebula. From a comparison of the Ha
emission with the predictions of the analytical model we obtain a rate of
ionizing photons from the central star of about 5e45 s^-1, which is consistent
with estimates based on the total Hb flux of the nebula. We also model the
tails of the cometary knots as a photoevaporated wind from a neutral shadow
region produced by the diffuse ionizing photon field of the nebula. A
comparison with the HST images allows us to obtain a direct determination of
the value of the diffuse ionizing flux. We compare the ratio of diffuse to
direct stellar flux as a function of radius inside an HII region with those
obtained from the observational data through the analytical tail and head wind
model. The agreement of this model with the values determined from the
observations of the knots is excellent.Comment: 9 pages, 5 figures, accepted for publication in Ap
Jets, knots and tails in planetary nebulae: NGC 3918, K 1-2 and Wray 17-1
We analyze optical images and high-resolution, long-slit spectra of three
planetary nebulae which possess collimated, low-ionization features. NGC 3918
is composed of an inner, spindle-shaped shell mildly inclined with respect to
the plane of the sky. Departing from the polar regions of this shell, we find a
two-sided jet expanding with velocities which increase linearly with distance
from 50 to 100 km/s. The jet is probably coeval with the inner shell (with the
age of approximately 1000 D yr, where D is the distance in kpc), suggesting
that its formation should be ascribed to the same dynamical processes which
also shaped the main nebula, and not to a more recent mass loss episode. We
discuss the formation of the aspherical shell and jet in the light of current
hydrodynamical and magnetohydrodynamical theories. K 1-2 is a planetary nebula
with a close binary nucleus which shows a collimated string of knots embedded
in a diffuse, elliptical shell. The knots expand with a velocity similar to
that of the elliptical nebula (25 km/s), except for an extended tail located
out of the main nebula, which linearly accelerates up to 45 km/s. We estimate
an inclination on the line of the sight of 40 degres for the string of knots;
once the orientation of the orbit is also determined, this information will
allow us to test the prediction of current theories of the occurrence of polar
jets from close binary systems. Wray 17-1 has a complex morphology, showing two
pairs of low-ionization structures located in almost perpendicular directions
from the central star, and embedded in a large, diffuse nebula. The two pairs
show notable similarities and differences, and their origin is very puzzling.Comment: 20 pages plus 10 figures. ApJ recently published (ApJ 523, 721
(1999)
The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion
We present a semi-analytic model for shaping the nebula around eta Carinae
that accounts for the simultaneous production of bipolar lobes and an
equatorial disk through a rotating surface explosion. Material is launched
normal to the surface of an oblate rotating star with an initial kick velocity
that scales approximately with the local escape speed. Thereafter, ejecta
follow ballistic orbital trajectories, feeling only a central force
corresponding to a radiatively reduced gravity. Our model is conceptually
similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we
modify it to an explosion instead of a steady line-driven wind, we include a
rotationally-distorted star, and we treat the dynamics somewhat differently.
Continuum-driving avoids the disk inhibition that normally operates in
line-driven winds. Our model provides a simple method by which rotating hot
stars can simultaneously produce intrinsically bipolar and equatorial mass
ejections, without an aspherical environment or magnetic fields. Although
motivated by eta Carinae, the model may have generic application to other LBVs,
B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less
influential, our model generalizes to produce bipolar morphologies without
disks, as seen in many PNe.Comment: ApJ accepted, 9 page
Jets and the shaping of the giant bipolar envelope of the planetary nebula KjPn 8
A hydrodynamic model involving cooling gas in the stagnation region of a
collimated outflow is proposed for the formation of the giant parsec-scale
bipolar envelope that surrounds the planetary nebula KjPn 8. Analytical
calculations and numerical simulations are presented to evaluate the model. The
envelope is considered to consist mainly of environmental gas swept-up by
shocks driven by an episodic, collimated, bipolar outflow. In this model, which
we call the ``free stagnation knot'' mechanism, the swept-up ambient gas
located in the stagnation region of the bow-shock cools to produce a high
density knot. This knot moves along with the bow-shock. When the central
outflow ceases, pressurization of the interior of the envelope stops and its
expansion slows down. The stagnation knot, however, has sufficient momentum to
propagate freely further along the axis, producing a distinct nose at the end
of the lobe. The model is found to successfully reproduce the peculiar shape
and global kinematics of the giant bipolar envelope of KjPn 8.Comment: 20 pages + 8 figures (in 1 tar-file 0.67 Mb
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