5,592 research outputs found
Stability and asymptotic behavior of periodic traveling wave solutions of viscous conservation laws in several dimensions
Under natural spectral stability assumptions motivated by previous
investigations of the associated spectral stability problem, we determine sharp
estimates on the linearized solution operator about a multidimensional
planar periodic wave of a system of conservation laws with viscosity, yielding
linearized stability for all and dimensions and nonlinear stability and
-asymptotic behavior for and . The behavior can in
general be rather complicated, involving both convective (i.e., wave-like) and
diffusive effects
Turing patterns in parabolic systems of conservation laws and numerically observed stability of periodic waves
Turing patterns on unbounded domains have been widely studied in systems of
reaction-diffusion equations. However, up to now, they have not been studied
for systems of conservation laws. Here, we (i) derive conditions for Turing
instability in conservation laws and (ii) use these conditions to find families
of periodic solutions bifurcating from uniform states, numerically continuing
these families into the large-amplitude regime. For the examples studied,
numerical stability analysis suggests that stable periodic waves can emerge
either from supercritical Turing bifurcations or, via secondary bifurcation as
amplitude is increased, from sub-critical Turing bifurcations. This answers in
the affirmative a question of Oh-Zumbrun whether stable periodic solutions of
conservation laws can occur. Determination of a full small-amplitude stability
diagram-- specifically, determination of rigorous Eckhaus-type stability
conditions-- remains an interesting open problem.Comment: 12 pages, 20 figure
Nonlinear stability of spatially-periodic traveling-wave solutions of systems of reaction diffusion equations
Using spatial domain techniques developed by the authors and Myunghyun Oh in
the context of parabolic conservation laws, we establish under a natural set of
spectral stability conditions nonlinear asymptotic stability with decay at
Gaussian rate of spatially periodic traveling-waves of systems of reaction
diffusion equations. In the case that wave-speed is identically zero for all
periodic solutions, we recover and slightly sharpen a well-known result of
Schneider obtained by renormalization/Bloch transform techniques; by the same
arguments, we are able to treat the open case of nonzero wave-speeds to which
Schneider's renormalization techniques do not appear to appl
Fluorescence-Controlled Er:YAG Laser For Caries Removal In Permanent Mandibular Molars
Poster presentation of research proposal addressing the question: Do fluorescence-controlled Er:YAG lasers more effectively remove dentinal carious hard tissue on permanent mandibular molars than conventional rotary burs?https://dune.une.edu/cdm_studpost/1000/thumbnail.jp
Dust Formation in Very Massive Primordial Supernovae
At redshift z>5 Type II supernovae (SNII) are the only known dust sources
with evolutionary timescales shorter than the Hubble time. We extend the model
of dust formation in the ejecta of SNII by Todini & Ferrara (2001) to
investigate the same process in pair-instability supernovae (PISN), which are
though to arise from the explosion of the first, metal free, very massive
(140-260 Msun) cosmic stars. We find that 15%-30% of the PISN progenitor mass
is converted into dust, a value >10 times higher than for SNII; PISN dust
depletion factors (fraction of produced metals locked into dust grains) range
between 0.3 and 0.7. These conclusions depend very weakly on the mass of the
PISN stellar progenitor, which instead affects considerably the composition and
size distribution. For the assumed temperature evolution, grain condensation
starts 150-200 days after the explosion; the dominant compounds for all
progenitor masses are SiO2 and Mg2SiO4 while the contribution of amorphous
carbon and magnetite grains grows with progenitor mass; typical grain sizes
range between 0.001 and a few 0.1 micron and are always smaller than 1 micron.
We give a brief discussion of the implications of dust formation for the IMF
evolution of the first stars, cosmic reionization and the intergalactic medium.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Rapidly Accreting Supergiant Protostars: Embryos of Supermassive Black Holes?
Direct collapse of supermassive stars (SMSs) is a possible pathway for
generating supermassive black holes in the early universe. It is expected that
an SMS could form via very rapid mass accretion with Mdot ~ 0.1 - 1 Msun/yr
during the gravitational collapse of an atomic-cooling primordial gas cloud. In
this paper we study how stars would evolve under such extreme rapid mass
accretion, focusing on the early evolution until the stellar mass reaches 1000
Msun. To this end we numerically calculate the detailed interior structure of
accreting stars with primordial element abundances. Our results show that for
accretion rates higher than 0.01 Msun/yr, stellar evolution is qualitatively
different from that expected at lower rates. While accreting at these high
rates the star always has a radius exceeding 100 Rsun, which increases
monotonically with the stellar mass. The mass-radius relation for stellar
masses exceeding ~ 100 Msun follows the same track with R_* \propto M_*^0.5 in
all cases with accretion rates > 0.01 Msun/yr; at a stellar mass of 1000 Msun
the radius is about 7000 Rsun (~= 30 AU). With higher accretion rates the onset
of hydrogen burning is shifted towards higher stellar masses. In particular,
for accretion rates exceeding Mdot > 0.1 Msun/yr, there is no significant
hydrogen burning even after 1000 Msun have accreted onto the protostar. Such
"supergiant" protostars have effective temperatures as low as Teff ~= 5000 K
throughout their evolution and because they hardly emit ionizing photons, they
do not create an HII region or significantly heat their immediate surroundings.
Thus, radiative feedback is unable to hinder the growth of rapidly accreting
stars to masses in excess of 1000 Msun, as long as material is accreted at
rates Mdot > 0.01 Msun/yr.Comment: 11 pages, 10 figure
Lyalpha heating and its impact on early structure formation
In this paper we have calculated the effect of Lyalpha photons emitted by the
first stars on the evolution of the IGM temperature. We have considered both a
standard Salpeter IMF and a delta-function IMF for very massive stars with mass
300 M_sun. We find that the Lyalpha photons produced by the stellar populations
considered here are able to heat the IGM at z<25, although never above ~100 K.
Stars with a Salpeter IMF are more effective as, due to the contribution from
small-mass long-living stars, they produce a higher Lyalpha background. Lyalpha
heating can affect the subsequent formation of small mass objects by producing
an entropy floor that may limit the amount of gas able to collapse and reduce
the gas clumping.We find that the gas fraction in halos of mass below ~ 5 x
10^6 M_sun is less than 50% (for the smallest masses this fraction drops to 1%
or less) compared to a case without Lyalpha heating. Finally, Lyalpha photons
heat the IGM temperature above the CMB temperature and render the 21cm line
from neutral hydrogen visible in emission at z<15.Comment: 7 pages, 5 figures, to be printed in MNRA
Nonlinear modulational stability of periodic traveling-wave solutions of the generalized Kuramoto-Sivashinsky equation
In this paper we consider the spectral and nonlinear stability of periodic
traveling wave solutions of a generalized Kuramoto-Sivashinsky equation. In
particular, we resolve the long-standing question of nonlinear modulational
stability by demonstrating that spectrally stable waves are nonlinearly stable
when subject to small localized (integrable) perturbations. Our analysis is
based upon detailed estimates of the linearized solution operator, which are
complicated by the fact that the (necessarily essential) spectrum of the
associated linearization intersects the imaginary axis at the origin. We carry
out a numerical Evans function study of the spectral problem and find bands of
spectrally stable periodic traveling waves, in close agreement with previous
numerical studies of Frisch-She-Thual, Bar-Nepomnyashchy,
Chang-Demekhin-Kopelevich, and others carried out by other techniques. We also
compare predictions of the associated Whitham modulation equations, which
formally describe the dynamics of weak large scale perturbations of a periodic
wave train, with numerical time evolution studies, demonstrating their
effectiveness at a practical level. For the reader's convenience, we include in
an appendix the corresponding treatment of the Swift-Hohenberg equation, a
nonconservative counterpart of the generalized Kuramoto-Sivashinsky equation
for which the nonlinear stability analysis is considerably simpler, together
with numerical Evans function analyses extending spectral stability analyses of
Mielke and Schneider.Comment: 78 pages, 11 figure
Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption
We discuss expectations for the absorption of high-energy gamma-rays by
gamma-gamma pair production with intergalactic radiation fields (IRFs) at very
high redshifts (z~5-20), and the prospects thereof for probing the cosmic
reionization era. For the evolving IRF, a semi-analytical model incorporating
both Population II and Population III stars is employed, which is consistent
with a wide variety of existing high-z observations including QSO spectral
measurements, WMAP Thomson depth constraints, near-IR source count limits, etc.
We find that the UV IRF below the Lyman edge energy with intensities in the
range of a few times 10^{-19} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1} can cause
appreciable attenuation above ~12 GeV at z~5, down to ~6-8 GeV at z>~8-10. This
may be observable in the spectra of blazars or gamma-ray bursts by the Fermi
Gamma-ray Space Telescope or next generation facilities such as the Cherenkov
Telescope Array, Advanced Gamma-ray Imaging System or 5@5, providing invaluable
insight into early star formation and cosmic reionization.Comment: MNRAS in press with minor revisions, 5 pages, 5 figures. Numerical
data of the model results will be available at
http://www-tap.scphys.kyoto-u.ac.jp/~inoue/hizabs
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