7,446 research outputs found
Mixing and reaction studies of hydrazine and nitrogen tetroxide using photographic and spectral techniques
Mixing and reaction studies of hydrazine and nitrogen tetroxide using photographic and spectral technique
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data
We present theoretical atmosphere, spectral, and light-curve models for
extrasolar giant planets (EGPs) undergoing strong irradiation for which {\it
Spitzer} planet/star contrast ratios or light curves have been published (circa
June 2007). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD
179949b, and And b. By comparing models with data, we find that a
number of EGP atmospheres experience thermal inversions and have stratospheres.
This is particularly true for HD 209458b, HD 149026b, and And b.
This finding translates into qualitative changes in the planet/star contrast
ratios at secondary eclipse and in close-in EGP orbital light curves. Moreover,
the presence of atmospheric water in abundance is fully consistent with all the
{\it Spitzer} data for the measured planets. For planets with stratospheres,
water absorption features invert into emission features and mid-infrared fluxes
can be enhanced by a factor of two. In addition, the character of near-infrared
planetary spectra can be radically altered. We derive a correlation between the
importance of such stratospheres and the stellar flux on the planet, suggesting
that close-in EGPs bifurcate into two groups: those with and without
stratospheres. From the finding that TrES-1 shows no signs of a stratosphere,
while HD 209458b does, we estimate the magnitude of this stellar flux
breakpoint. We find that the heat redistribution parameter, P, for the
family of close-in EGPs assumes values from 0.1 to 0.4. This paper
provides a broad theoretical context for the future direct characterization of
EGPs in tight orbits around their illuminating stars.Comment: Accepted to Ap. J., provided here in emulateapj format: 28 pages, 8
figures, many with multiple panel
Fornax: a Flexible Code for Multiphysics Astrophysical Simulations
This paper describes the design and implementation of our new multi-group,
multi-dimensional radiation hydrodynamics (RHD) code Fornax and provides a
suite of code tests to validate its application in a wide range of physical
regimes. Instead of focusing exclusively on tests of neutrino radiation
hydrodynamics relevant to the core-collapse supernova problem for which Fornax
is primarily intended, we present here classical and rigorous demonstrations of
code performance relevant to a broad range of multi-dimensional hydrodynamic
and multi-group radiation hydrodynamic problems. Our code solves the
comoving-frame radiation moment equations using the M1 closure, utilizes
conservative high-order reconstruction, employs semi-explicit matter and
radiation transport via a high-order time stepping scheme, and is suitable for
application to a wide range of astrophysical problems. To this end, we first
describe the philosophy, algorithms, and methodologies of Fornax and then
perform numerous stringent code tests, that collectively and vigorously
exercise the code, demonstrate the excellent numerical fidelity with which it
captures the many physical effects of radiation hydrodynamics, and show
excellent strong scaling well above 100k MPI tasks.Comment: Accepted to the Astrophysical Journal Supplement Series; A few more
textual and reference updates; As before, one additional code test include
Electron-Capture and Low-Mass Iron-Core-Collapse Supernovae: New Neutrino-Radiation-Hydrodynamics Simulations
We present new 1D (spherical) and 2D (axisymmetric) simulations of
electron-capture (EC) and low-mass iron-core-collapse supernovae (SN). We
consider six progenitor models: the ECSN progenitor from Nomoto (1984, 1987);
two ECSN-like low-mass low-metallicity iron core progenitors from Heger
(private communication); and the 9-, 10-, and 11- (zero-age main
sequence) progenitors from Sukhbold et al. (2016). We confirm that the ECSN and
ESCN-like progenitors explode easily even in 1D with explosion energies of up
to a 0.15 Bethes (), and are a viable
mechanism for the production of very low-mass neutron stars. However, the 9-,
10-, and 11- progenitors do not explode in 1D and are not even
necessarily easier to explode than higher-mass progenitor stars in 2D. We study
the effect of perturbations and of changes to the microphysics and we find that
relatively small changes can result in qualitatively different outcomes, even
in 1D, for models sufficiently close to the explosion threshold. Finally, we
revisit the impact of convection below the protoneutron star (PNS) surface. We
analyze, 1D and 2D evolutions of PNSs subject to the same boundary conditions.
We find that the impact of PNS convection has been underestimated in previous
studies and could result in an increase of the neutrino luminosity by up to
factors of two.Comment: 18 pages, 17 figures, 3 tables. Major revisions following a fix in
the code input physics. Accepted on Ap
A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and Planets
We show that under certain circumstances the differences between the
absorption mean and Planck mean opacities can lead to multiple solutions for an
LTE atmospheric structure. Since the absorption and Planck mean opacities are
not expected to differ significantly in the usual case of radiative
equilibrium, non-irradiated atmospheres, the most interesting situations where
the effect may play a role are strongly irradiated stars and planets, and also
possibly structures where there is a significant deposition of mechanical
energy, such as stellar chromospheres and accretion disks. We have presented an
illustrative example of a strongly irradiated giant planet where the
bifurcation effect is predicted to occur for a certain range of distances from
the star.Comment: 22 pages, 6 figures, submitted to Ap
Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres
We calculate detailed chemical abundance profiles for a variety of brown
dwarf and extrasolar giant planet atmosphere models, focusing in particular on
Gliese 229B, and derive the systematics of the changes in the dominant
reservoirs of the major elements with altitude and temperature. We assume an
Anders and Grevesse (1989) solar composition of 27 chemical elements and track
330 gas--phase species, including the monatomic forms of the elements, as well
as about 120 condensates. We address the issue of the formation and composition
of clouds in the cool atmospheres of substellar objects and explore the rain
out and depletion of refractories. We conclude that the opacity of clouds of
low--temperature (900 K), small--radius condensibles (specific chlorides
and sulfides), may be responsible for the steep spectrum of Gliese 229B
observed in the near infrared below 1 \mic. Furthermore, we assemble a
temperature sequence of chemical transitions in substellar atmospheres that may
be used to anchor and define a sequence of spectral types for substellar
objects with Ts from 2200 K to 100 K.Comment: 57 pages total, LaTeX, 14 figures, 5 tables, also available in
uuencoded, gzipped, and tarred form via anonymous ftp at
www.astrophysics.arizona.edu (cd to pub/burrows/chem), submitted to Ap.
Spectroscopic Constants, Abundances, and Opacities of the TiH Molecule
Using previous measurements and quantum chemical calculations to derive the
molecular properties of the TiH molecule, we obtain new values for its
ro-vibrational constants, thermochemical data, spectral line lists, line
strengths, and absorption opacities. Furthermore, we calculate the abundance of
TiH in M and L dwarf atmospheres and conclude that it is much higher than
previously thought. We find that the TiH/TiO ratio increases strongly with
decreasing metallicity, and at high temperatures can exceed unity. We suggest
that, particularly for subdwarf L and M dwarfs, spectral features of TiH near
0.52 \mic, 0.94 \mic, and in the band may be more easily measureable
than heretofore thought. The recent possible identification in the L subdwarf
2MASS J0532 of the 0.94 \mic feature of TiH is in keeping with this
expectation. We speculate that looking for TiH in other dwarfs and subdwarfs
will shed light on the distinctive titanium chemistry of the atmospheres of
substellar-mass objects and the dimmest stars.Comment: 37 pages, including 4 figures and 13 tables, accepted to the
Astrophysical Journa
Relative entropy via non-sequential recursive pair substitutions
The entropy of an ergodic source is the limit of properly rescaled 1-block
entropies of sources obtained applying successive non-sequential recursive
pairs substitutions (see P. Grassberger 2002 ArXiv:physics/0207023 and D.
Benedetto, E. Caglioti and D. Gabrielli 2006 Jour. Stat. Mech. Theo. Exp. 09
doi:10.1088/1742.-5468/2006/09/P09011). In this paper we prove that the cross
entropy and the Kullback-Leibler divergence can be obtained in a similar way.Comment: 13 pages , 2 figure
Line Intensities and Molecular Opacities of the FeH Transition
We calculate new line lists and opacities for the
transition of FeH. The 0-0 band of this transition is responsible for the
Wing-Ford band seen in M-type stars, sunspots and brown dwarfs. The new
Einstein A values for each line are based on a high level ab initio calculation
of the electronic transition dipole moment. The necessary rotational line
strength factors (H\"onl-London factors) are derived for both the Hund's case
(a) and (b) coupling limits. A new set of spectroscopic constants were derived
from the existing FeH term values for v=0, 1 and 2 levels of the and
states. Using these constants extrapolated term values were generated for v=3
and 4 and for values up to 50.5. The line lists (including Einstein A
values) for the 25 vibrational bands with v4 were generated using a
merged list of experimental and extrapolated term values. The FeH line lists
were use to compute the molecular opacities for a range of temperatures and
pressures encountered in L and M dwarf atmospheres. Good agreement was found
between the computed and observed spectral energy distribution of the L5 dwarf
2MASS-1507.Comment: 52 pages, 3 figures, many tables, accepted for publication in the
Astrophysical Journal Supplement
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