110,304 research outputs found
Temperature inversion on the surface of externally heated optically thick multigrain dust clouds
It was recently discovered that the temperature in the surface layer of
externally heated optically thick gray dust clouds increases with the optical
depth for some distance from the surface, as opposed to the normal decrease in
temperature with distance in the rest of the cloud. This temperature inversion
is a result of efficient absorption of diffuse flux from the cloud interior by
the surface dust exposed to the external radiation. A micron or bigger size
grains experience this effect when the external flux is of stellar spectrum. We
explore what happens to the effect when dust is a mixture of grain sizes
(multigrain). Two possible boundary conditions are considered: i) a constant
external flux without constrains on the dust temperature, and ii) the maximum
dust temperature set to the sublimation temperature. We find that the first
condition allows small grains to completely suppress the temperature inversion
of big grains if the overall opacity is dominated by small grains. The second
condition enables big grains to maintain the inversion even when they are a
minor contributor to the opacity. In reality, the choice of boundary condition
depends on the dust dynamics. When applied to the physics of protoplanetary
disks, the temperature inversion leads to a previously unrecognized disk
structure where optically thin dust can exist inside the dust destruction
radius of an optically thick disk. We conclude that the transition between the
dusty disk and the gaseous inner clearing is not a sharp edge, but rather a
large optically thin region.Comment: 8 pages, 10 figures, Accepted for publication in the Astrophysical
Journa
On the choice of parameters in solar structure inversion
The observed solar p-mode frequencies provide a powerful diagnostic of the
internal structure of the Sun and permit us to test in considerable detail the
physics used in the theory of stellar structure. Amongst the most commonly used
techniques for inverting such helioseismic data are two implementations of the
optimally localized averages (OLA) method, namely the Subtractive Optimally
Localized Averages (SOLA) and Multiplicative Optimally Localized Averages
(MOLA). Both are controlled by a number of parameters, the proper choice of
which is very important for a reliable inference of the solar internal
structure. Here we make a detailed analysis of the influence of each parameter
on the solution and indicate how to arrive at an optimal set of parameters for
a given data set.Comment: 14 pages, 15 figures. Accepted for publication on MNRA
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Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response
This study deals with shallow sedimentary structure of the Anchorage basin in Alaska. For this purpose, inversion of site response [SR(f)] data in the frequency range 0.5-11.0 Hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness, shear-wave velocity (beta), density, and shear-wave quality factor. The one-dimensional (1D) models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the L-2 norm error between the observed and computed site response attained a global minimum. Next, the spatial distribution of the important parameter beta was obtained by interpolating values yielded by the 1D models. The results indicate the presence of three distinct velocity zones as the source of spatial variation of SR(f) in the Anchorage basin. In the uppermost part of the basin, the beta values of fine-grain Quaternary sediments mainly lie in the range of 180-500 m/sec with thickness varying from 15 to 50 m. This formation overlies relatively thick (80-200 m) coarse-grain Quaternary sediments with beta values in the range of 600-900 m/sec. These two Quaternary units are, in turn, overlain on Tertiary sediments with beta > 1000 m/sec located at depths of 100 and 250 m, respectively, in the central and western side along the Knik Arm parts of the basin. The important implication of the result is that the sources of spatial variation of SR(f) in the Anchorage basin for the frequency band 0.5-11 Hz, besides in the uppermost 30 m, are found to be deeper than this depth. Thus, use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground-motion interpretation will likely yield erroneous results in the Anchorage basin.GIEnvironment and Natural Resources InstituteSchool of Engineering of the University of Alaska, AnchorageGeological Science
On the determination of anti-neutrino spectra from nuclear reactors
In this paper we study the effect of, well-known, higher order corrections to
the allowed beta decay spectrum on the determination of anti-neutrino spectra
resulting from the decays of fission fragments. In particular, we try to
estimate the associated theory errors and find that induced currents like weak
magnetism may ultimately limit our ability to improve the current accuracy and
under certain circumstance could even largely increase the theoretical errors.
We also perform a critical evaluation of the errors associated with our method
to extract the anti-neutrino spectrum using synthetic beta spectra. It turns
out, that a fit using only virtual beta branches with a judicious choice of the
effective nuclear charge provides results with a minimal bias. We apply this
method to actual data for U235, Pu239 and Pu241 and confirm, within errors,
recent results, which indicate a net 3% upward shift in energy averaged
anti-neutrino fluxes. However, we also find significant shape differences which
can in principle be tested by high statistics anti-neutrino data samples.Comment: 20 pages, 5 figures, 9 tables, added references, version accepted for
publication in Phys. Rev. C. Corrected errors in tab. 1 and eqs. 18 and 19.
Results and conclusion unchange
Abundance gradients in spiral disks: is the gradient inversion at high redshift real?
We compute the abundance gradients along the disk of the Milky Way by means
of the two-infall model: in particular, the gradients of oxygen and iron and
their temporal evolution. First, we explore the effects of several physical
processes which influence the formation and evolution of abundance gradients.
They are: i) the inside-out formation of the disk, ii) a threshold in the gas
density for star formation, iii) a variable star formation efficiency along the
disk, iv) radial flows and their speed, and v) different total surface mass
density (gas plus stars) distributions for the halo. We are able to reproduce
at best the present day gradients of oxygen and iron if we assume an inside-out
formation, no threshold gas density, a constant efficiency of star formation
along the disk and radial gas flows. It is particularly important the choice of
the velocity pattern for radial flows and the combination of this velocity
pattern with the surface mass density distribution in the halo. Having selected
the best model, we then explore the evolution of abundance gradients in time
and find that the gradients in general steepen in time and that at redshift z~3
there is a gradient inversion in the inner regions of the disk, in the sense
that at early epochs the oxygen abundance decreases toward the Galactic center.
This effect, which has been observed, is naturally produced by our models if an
inside-out formation of the disk and and a constant star formation efficiency
are assumed. The inversion is due to the fact that in the inside-out formation
a strong infall of primordial gas, contrasting chemical enrichment, is present
in the innermost disk regions at early times. The gradient inversion remains
also in the presence of radial flows, either with constant or variable speed in
time, and this is a new result.Comment: 15 pages, 19 figures, accepted for publication in MNRA
Subsonic structure and optically thick winds from Wolf--Rayet stars
Wolf-Rayet star's winds can be so dense and so optically thick that the
photosphere appears in the highly supersonic part of the outflow, veiling the
underlying subsonic part of the star, and leaving the initial acceleration of
the wind inaccessible to observations. We investigate the conditions and the
structure of the subsonic part of the outflow of Galactic WR stars, in
particular of the WNE subclass; our focus is on the conditions at the sonic
point. We compute 1D hydrodynamic stellar structure models for massive helium
stars adopting outer boundaries at the sonic point. We find that the outflows
of our models are accelerated to supersonic velocities by the radiative force
from opacity bumps either at temperatures of the order of 200kK by the Fe
opacity bump or of the order of 50kK by the HeII opacity bump. For a given
mass-loss rate, the conditions in the subsonic part of the outflow are
independent from the detailed physical conditions in the supersonic part. The
close proximity to the Eddington limit at the sonic point allows us to
construct a Sonic HR diagram, relating the sonic point temperature to the L/M
ratio and the stellar mass-loss rate, thereby constraining the sonic point
conditions, the subsonic structure, and the stellar wind mass-loss rates from
observations. The minimum mass-loss rate necessary to have the flow accelerated
to supersonic velocities by the Fe opacity bump is derived. A comparison of the
observed parameters of Galactic WNE stars to this minimum mass-loss rate
indicates that their winds are launched to supersonic velocities by the
radiation pressure arising from the Fe-bump. Conversely, models which do not
show transonic flows from the Fe opacity bump form inflated envelopes. We
derive an analytic criterion for the appearance of envelope inflation in the
subphotospheric layers.Comment: A&A, Forthcoming article. 13 pages+
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