536 research outputs found
Astrophysical gyrokinetics: Turbulence in pressure-anisotropic plasmas at ion scales and beyond
We present a theoretical framework for describing electromagnetic kinetic
turbulence in a multi-species, magnetized, pressure-anisotropic plasma.
Turbulent fluctuations are assumed to be small compared to the mean field, to
be spatially anisotropic with respect to it, and to have frequencies small
compared to the ion cyclotron frequency. At scales above the ion Larmor radius,
the theory reduces to the pressure-anisotropic generalization of kinetic
reduced magnetohydrodynamics (KRMHD) formulated by Kunz et al. (2015). At
scales at and below the ion Larmor radius, three main objectives are achieved.
First, we analyse the linear response of the pressure-anisotropic gyrokinetic
system, and show it to be a generalisation of previously explored limits. The
effects of pressure anisotropy on the stability and collisionless damping of
Alfvenic and compressive fluctuations are highlighted, with attention paid to
the spectral location and width of the frequency jump that occurs as Alfven
waves transition into kinetic Alfven waves. Secondly, we derive and discuss a
general free-energy conservation law, which captures both the KRMHD free-energy
conservation at long wavelengths and dual cascades of kinetic Alfven waves and
ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in
the distribution function change the amount of phase mixing and the efficiency
of magnetic stresses, and thus influence the partitioning of free energy
amongst the cascade channels. Thirdly, a simple model is used to show that
pressure anisotropy can cause large variations in the ion-to-electron heating
ratio due to the dissipation of Alfvenic turbulence. Our theory provides a
foundation for determining how pressure anisotropy affects the turbulent
fluctuation spectra, the differential heating of particle species, and the
ratio of parallel and perpendicular phase mixing in space and astrophysical
plasmas.Comment: 59 pages, 6 figures, accepted for publication in Journal of Plasma
Physics (original 28 Nov 2017); abstract abridge
Flame Enhancement and Quenching in Fluid Flows
We perform direct numerical simulations (DNS) of an advected scalar field
which diffuses and reacts according to a nonlinear reaction law. The objective
is to study how the bulk burning rate of the reaction is affected by an imposed
flow. In particular, we are interested in comparing the numerical results with
recently predicted analytical upper and lower bounds. We focus on reaction
enhancement and quenching phenomena for two classes of imposed model flows with
different geometries: periodic shear flow and cellular flow. We are primarily
interested in the fast advection regime. We find that the bulk burning rate v
in a shear flow satisfies v ~ a*U+b where U is the typical flow velocity and a
is a constant depending on the relationship between the oscillation length
scale of the flow and laminar front thickness. For cellular flow, we obtain v ~
U^{1/4}. We also study flame extinction (quenching) for an ignition-type
reaction law and compactly supported initial data for the scalar field. We find
that in a shear flow the flame of the size W can be typically quenched by a
flow with amplitude U ~ alpha*W. The constant alpha depends on the geometry of
the flow and tends to infinity if the flow profile has a plateau larger than a
critical size. In a cellular flow, we find that the advection strength required
for quenching is U ~ W^4 if the cell size is smaller than a critical value.Comment: 14 pages, 20 figures, revtex4, submitted to Combustion Theory and
Modellin
A phenomenological theory of nonphotochemical laser induced nucleation
Our analysis of the experimental data related to nonphotochemical laser
induced nucleation in solutions leads to the inevitable conclusion that the
phase transformation is initiated by particles that are metallic in nature.
This conclusion appears paradoxical because the final products are dielectric
crystals. We show that the experimental results are well accounted for by the
theory of electric field induced nucleation of metallic particles that are
elongated in the direction of the field. However, new physical and chemical
insights are required to understand the structure of the metallic precursor
particles and the kinetics of subsequent dielectric crystallization.Comment: 5 pages 4 figure
Brane Gases in the Early Universe
Over the past decade it has become clear that fundamental strings are not the
only fundamental degrees of freedom in string theory. D-branes are also part of
the spectrum of fundamental states. In this paper we explore some possible
effects of D-branes on early Universe string cosmology, starting with two key
assumptions: firstly that the initial state of the Universe corresponded to a
dense, hot gas in which all degrees of freedom were in thermal equilibrium, and
secondly that the topology of the background space admits one-cycles. We argue
by t-duality that in this context the cosmological singularities are not
present. We derive the equation of state of the brane gases and apply the
results to suggest that, in an expanding background, the winding modes of
fundamental strings will play the most important role at late times. In
particular, we argue that the string winding modes will only allow four
space-time dimensions to become large. The presence of brane winding modes with
may lead to a hierarchy in the sizes of the extra dimensions.Comment: 8 pages, 1 figure; typos corrected; published in PR
Cosmological Creation of D-branes and anti-D-branes
We argue that the early universe may be described by an initial state of
space-filling branes and anti-branes. At high temperature this system is
stable. At low temperature tachyons appear and lead to a phase transition,
dynamics, and the creation of D-branes. These branes are cosmologically
produced in a generic fashion by the Kibble mechanism. From an entropic point
of view, the formation of lower dimensional branes is preferred and
brane-worlds are exponentially more likely to form than higher dimensional
branes. Virtually any brane configuration can be created from such phase
transitions by adjusting the tachyon profile. A lower bound on the number
defects produced is: one D-brane per Hubble volume.Comment: 30 pages, 5 eps figures; v2 more references added; v3 section 4
slightly improve
Functional characterization of generalized Langevin equations
We present an exact functional formalism to deal with linear Langevin
equations with arbitrary memory kernels and driven by any noise structure
characterized through its characteristic functional. No others hypothesis are
assumed over the noise, neither the fluctuation dissipation theorem. We found
that the characteristic functional of the linear process can be expressed in
terms of noise's functional and the Green function of the deterministic
(memory-like) dissipative dynamics. This object allow us to get a procedure to
calculate all the Kolmogorov hierarchy of the non-Markov process. As examples
we have characterized through the 1-time probability a noise-induced interplay
between the dissipative dynamics and the structure of different noises.
Conditions that lead to non-Gaussian statistics and distributions with long
tails are analyzed. The introduction of arbitrary fluctuations in fractional
Langevin equations have also been pointed out
[CII] 158 micron Luminosities and Star Formation Rate in Dusty Starbursts and AGN
Results are presented for [CII] 158 micron line fluxes observed with the
Herschel PACS instrument in 112 sources with both starburst and AGN
classifications, of which 102 sources have confident detections. Results are
compared with mid-infrared spectra from the Spitzer Infrared Spectrometer and
with L(IR) from IRAS fluxes; AGN/starburst classifications are determined from
equivalent width of the 6.2 micron PAH feature. It is found that the [CII] line
flux correlates closely with the flux of the 11.3 micron PAH feature
independent of AGN/starburst classification, log [f([CII] 158 micron)/f(11.3
micron PAH)] = -0.22 +- 0.25. It is concluded that [CII] line flux measures the
photodissociation region associated with starbursts in the same fashion as the
PAH feature. A calibration of star formation rate for the starburst component
in any source having [CII] is derived comparing [CII] luminosity L([CII]) to
L(IR) with the result that log SFR = log L([CII)]) - 7.08 +- 0.3, for SFR in
solar masses per year and L([CII]) in solar luminosities. The decreasing ratio
of L([CII]) to L(IR) in more luminous sources (the "[CII] deficit") is shown to
be a consequence of the dominant contribution to L(IR) arising from a luminous
AGN component because the sources with largest L(IR) and smallest
L([CII])/L(IR) are AGN.Comment: Accepted for publication in The Astrophysical Journa
Aspects of String-Gas Cosmology at Finite Temperature
We study string-gas cosmology in dilaton gravity, inspired by the fact that
it naturally arises in a string theory context. Our main interest is the
thermodynamical treatment of the string-gas and the resulting implications for
the cosmology. Within an adiabatic approximation, thermodynamical equilibrium
and a small, toroidal universe as initial conditions, we numerically solve the
corresponding equations of motions in two different regimes describing the
string-gas thermodynamics: (i) the Hagedorn regime, with a single scale factor,
and (ii) an almost-radiation dominated regime, which includes the leading
corrections due to the lightest Kaluza Klein and winding modes, with two scale
factors. The scale factor in the Hagedorn regime exhibits very slow time
evolution with nearly constant energy and negligible pressure. By contrast, in
case (ii) we find interesting cosmological solutions where the large dimensions
continue to expand and the small ones are kept undetectably small.Comment: 21 pages, 5 eps figure
New Physics in CP Asymmetries and Rare B Decays
We review and update the effects of physics beyond the standard model on CP
asymmetries in B decays. These asymmetries can be significantly altered if
there are important new-physics contributions to \bqbqbar mixing. This same new
physics will therefore also contribute to rare, flavor-changing B decays.
Through a study of such decays, we show that it is possible to partially
distinguish the different models of new physics.Comment: 42 pages, plain TeX (macros included), 1 figure (included). A few
sentences added, references updated. Present manuscript is now identical to
the version accepted for publication in Phys. Rev.
The Formation and Evolution of the First Massive Black Holes
The first massive astrophysical black holes likely formed at high redshifts
(z>10) at the centers of low mass (~10^6 Msun) dark matter concentrations.
These black holes grow by mergers and gas accretion, evolve into the population
of bright quasars observed at lower redshifts, and eventually leave the
supermassive black hole remnants that are ubiquitous at the centers of galaxies
in the nearby universe. The astrophysical processes responsible for the
formation of the earliest seed black holes are poorly understood. The purpose
of this review is threefold: (1) to describe theoretical expectations for the
formation and growth of the earliest black holes within the general paradigm of
hierarchical cold dark matter cosmologies, (2) to summarize several relevant
recent observations that have implications for the formation of the earliest
black holes, and (3) to look into the future and assess the power of
forthcoming observations to probe the physics of the first active galactic
nuclei.Comment: 39 pages, review for "Supermassive Black Holes in the Distant
Universe", Ed. A. J. Barger, Kluwer Academic Publisher
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