686 research outputs found
Wave scattering on a domain wall in a chain of PT-symmetric couplers
We study wave propagation in linear arrays composed of pairs of conjugate
waveguides with balanced gain and loss, i.e. arrays of the PT-symmetric
couplers, where the linear spectrum is known to feature high-frequency and
low-frequency branches. We introduce a domain wall by switching the gain and
loss in a half of the array, and analyze the scattering of linear waves on this
defect. The analysis reveals two major effects: amplification of both reflected
and transmitted waves, and excitation of the reflected and transmitted
low-frequency and high-frequency waves by the incident high-frequency and
low-frequency waves, respectively.Comment: 7 pages, 9 figures, Physical Review A, in pres
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
Solitons in a chain of PT-invariant dimers
Dynamics of a chain of interacting parity-time invariant nonlinear dimers is
investigated. A dimer is built as a pair of coupled elements with equal gain
and loss. A relation between stationary soliton solutions of the model and
solitons of the discrete nonlinear Schrodinger (DNLS) equation is demonstrated.
Approximate solutions for solitons whose width is large in comparison to the
lattice spacing are derived, using a continuum counterpart of the discrete
equations. These solitons are mobile, featuring nearly elastic collisions.
Stationary solutions for narrow solitons, which are immobile due to the pinning
by the effective Peierls-Nabarro potential, are constructed numerically,
starting from the anti-continuum limit. The solitons with the amplitude
exceeding a certain critical value suffer an instability leading to blowup,
which is a specific feature of the nonlinear PT-symmetric chain, making it
dynamically different from DNLS lattices. A qualitative explanation of this
feature is proposed. The instability threshold drops with the increase of the
gain-loss coefficient, but it does not depend on the lattice coupling constant,
nor on the soliton's velocity.Comment: 9 pages, 9 figure
Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data
We propose a method that uses near-infrared plus uvby photometry to identify
potentially extensive circumstellar dusty environment about F and A stars. The
method has been applied to a sample of ~900 metal rich reddened F stars with
2MASS and uvby data, suggesting the presence of circumstellar dust emitting in
the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most,
if not all, of them are likely pre-mainsequence (PMS). They seem to be
consistent with being a continuation of the class of Herbig Ae/Be PMS stars
into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84
(spectral types later than ~F5), which may provide new clues to the PMS
evolution of stars with 1 to 2 solar mass. We present a list of 21 most
conspicuous candidate stars with circumstellar dust. About half of them are
associated with the extended star-forming region around rho Oph. The brightest
of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the
presence of heated dust emitting in the far infrared. Also in this list, HD
81270 is reported as a very unusual star moving away from the Galactic plane at
a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002
May
Three-Dimensional Simulations of a Starburst-Driven Galactic Wind
We have performed a series of three-dimensional simulations of a
starburst-driven wind in an inhomogeneous interstellar medium. The introduction
of an inhomogeneous disk leads to differences in the formation of a wind, most
noticeably the absence of the ``blow-out'' effect seen in homogeneous models. A
wind forms from a series of small bubbles that propagate into the tenuous gas
between dense clouds in the disk. These bubbles merge and follow the path of
least resistance out of the disk, before flowing freely into the halo.
Filaments are formed from disk gas that is broken up and accelerated into the
outflow. These filaments are distributed throughout a biconical structure
within a more spherically distributed hot wind. The distribution of the
inhomogeneous interstellar medium in the disk is important in determining the
morphology of this wind, as well as the distribution of the filaments. While
higher resolution simulations are required in order to ascertain the importance
of mixing processes, we find that soft X-ray emission arises from gas that has
been mass-loaded from clouds in the disk, as well as from bow shocks upstream
of clouds, driven into the flow by the ram pressure of the wind, and the
interaction between these shocks.Comment: 37 pages, 16 figures, mpg movie can be obtained at
http://www.mso.anu.edu.au/~jcooper/movie/video16.mpg, accepted for
publication in Ap
Impact of climate changes on indicator specie and predicted socio-economical effects
Impact of the above-mentioned factors can be seen on particular representatives of flora and fauna, communities, landscape elements and elementary associations, and is demonstrated on the example of model species and communities
Definition of Time Induction of Self-Ignition of the Substance on the Prognostic Extrapolation Depending on the Basis of Indicators Fire and Explosion Hazard
In this article the research directed on development of a technique of definition of time of induction of the self-ignition of substances and materials which is an indicator of the beginning of development of an emergency is conducted. The experiment consisting in supervision over process of self-ignition of coal and oil deposits was the basis for research. On the basis of experimental data the curve expressing analytic - expected dependence of size of temperature of ignition on induction time was constructed. Proceeding from graphical representation of process, functional dependence of time of induction on a temperature indicator was received: y=16920•x{0.537}. By means of known indicators of such substances as bitumen oil oxidized (the combustible solid substance received by oxidation of residual product of oil refining) and tar oil (the combustible solid substance which is residual product of oil refining) and the received algorithm, verification of reliability of the received dependence and a technique of definition of time of induction of spontaneous ignition of deposits of oil in general was carried out. The practical importance of the conducted research is that having data on time of induction of process of self-ignition, by means of preventive measures becomes possible to avoid and prevent accidents in oil and oil processing branches, at the same time loss of property and loss of human life
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