14,595 research outputs found
Simulation of the Gravitational Collapse and Fragmentation of Rotating Molecular Clouds
In this paper we study the process of the subsequent (runaway) fragmentation
of the rotating isothermal Giant Molecular Cloud (GMC) complex. Our own
developed Smoothed Particle Hydrodynamics (SPH) gas-dynamical model
successfully reproduce the observed Cloud Mass-distribution Function (CMF) in
our Galaxy (even the differences between the inner and outer parts of our
Galaxy). The steady state CMF is established during the collapse within a
free-fall timescale of the GMC. We show that one of the key parameters, which
defines the observed slope of the present day CMF, is the initial ratio of the
rotational (turbulent) and gravitational energy inside the fragmented GMC.Comment: 8 pages, 9 EPS figures, special forma.cls class file is use
Bremsstrahlung radiation from the interaction of short laser pulses with dielectrics
An intense, short laser pulse incident on a transparent dielectric can excite
electrons from valence to the conduction band. As these electrons undergo
scattering, both from phonons and ions, they emit bremsstrahlung radiation.
Here we present a theory of bremsstrahlung emission appropriate for laser
pulse-dielectric interactions. Simulations of the interaction, incorporating
this theory, illustrate characteristics of the radiation (power, energy and
spectra) for arbitrary ratios of electron collision frequency to radiation
frequency. The conversion efficiency of laser pulse energy into bremsstrahlung
radiation depends strongly on both the intensity and duration of the pulse,
saturating at values of about 10e-5. Depending on whether the intensity is
above or below the damage threshold of the material, the emission can originate
either from the surface or the bulk of the dielectric respectively. The
bremsstrahlung emission may provide a broadband light source for diagnostics
Cyclic phase in F=2 spinor condensate: Long-range order, kinks, and roughening transition
We study the effect of thermal fluctuations on homogeneous infinite
Bose-Einstein condensate with spin F=2 in the cyclic state, when atoms occupy
three hyperfine states with . We use both the approach of
small-amplitude oscillations and mapping of our model on the sine-Gordon model.
We show that thermal fluctuations lead to the existence of the rough phase in
one- and two-dimensional systems, when presence of kinks is favorable. The
structure and energy of a single kink are found. We also discuss the effect of
thermal fluctuations on spin degrees of freedom in F=1 condensate.Comment: 6 pages, 1 figure; final version, accepted for publication in Phys.
Rev.
Unveiling extremely veiled T Tauri stars
Photospheric absorption lines in classical T Tauri stars (CTTS) are weak
compared to normal stars. This so-called veiling is normally identified with an
excess continuous emission formed in shock-heated gas at the stellar surface
below the accretion streams. We have selected four stars (RW Aur A, RU Lup, S
CrA NW and S CrA SE) with unusually strong veiling to make a detailed
investigation of veiling versus stellar brightness and emission line strengths
for comparisons to standard accretion models. We have monitored the stars
photometrically and spectroscopically at several epochs. In standard accretion
models a variable accretion rate will lead to a variable excess emission.
Consequently, the stellar brightness should vary accordingly. We find that the
veiling of absorption lines in these stars is strongly variable and usually so
large that it would require the release of several stellar luminosities of
potential energy. At states of very large line dilution, the correspondingly
large veiling factors derived correlate only weakly with brightness. Moreover,
the emission line strengths violate the expected trend of veiling versus line
strength. The veiling can change dramatically in one night, and is not
correlated with the phase of the rotation periods found for two stars. We show
that in at least three of the stars, when the veiling becomes high, the
photospheric lines become filled-in by line emission, which produces large
veiling factors unrelated to changes in any continuous emission from shocked
regions. We also consider to what extent extinction by dust and electron
scattering in the accretion stream may affect veiling measures in CTTS. We
conclude that the degree of veiling cannot be used as a measure of accretion
rates in CTTS with rich emission line spectra.Comment: Accepted for publication in A&A Letters. New language-edited version.
(4 pages, 3 figures
Facing the wind of the pre-FUor V1331 Cyg
The mass outflows in T Tauri stars (TTS) are thought to be an effective
mechanism to remove angular momentum during the pre-main-sequence contraction
of a low-mass star. The most powerful winds are observed at the FUor stage of
stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor
stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007
and 20-d series of spectra taken in 2012. For the first time the photospheric
spectrum of the star is detected and stellar parameters are derived: spectral
type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric
spectrum is highly veiled, but the amount of veiling is not the same in
different spectral lines, being lower in weak transitions and much higher in
strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of
metals are accompanied by a `shell' absorption at radial velocity of about -240
km/s. We show that these absorptions form in the post-shock gas in the jet,
i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta
indicate the terminal wind velocity of about 500 km/s, which vary on
time-scales from several days to years. A model of the stellar wind is
developed to interpret the observations. The model is based on calculation of
hydrogen spectral lines using the radiative transfer code TORUS. The observed
H-alpha and H-beta line profiles and their variability can be well reproduced
with a stellar wind model, where the mass-loss rate and collimation (opening
angle) of the wind are variable. The changes of the opening angle may be
induced by small variability in magetization of the inner disc wind. The
mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the
accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS.
Typographical errors have been corrected after the proof stag
Mathematical Models of Video-Sequences of Digital Half-Tone Images
This chapter is devoted to Mathematical Models (MM) of Digital Half-Tone Images (DHTI) and their
video-sequences presented as causal multi-dimensional Markov Processes (MP) on discrete meshes.
The difficulties of MM development for DHTI video-sequences of Markov type are shown. These difficulties are related to the enormous volume of computational operations required for their realization.
The method of MM-DHTI construction and their statistically correlated video-sequences on the basis
of the causal multi-dimensional multi-value MM is described in detail. Realization of such operations
is not computationally intensive; Markov models from the second to fourth order demonstrate this. The
proposed method is especially effective when DHTI is represented by low-bit (4-8 bits) binary numbers
Electroweak gauge boson production at hadron colliders through O(alpha_s^2)
We describe a calculation of the O(alpha_s^2) QCD corrections to the fully
differential cross section for W and Z boson production in hadronic collisions.
The result is fully realistic in that it includes spin correlations, finite
width effects, gamma-Z interference and allows for the application of arbitrary
cuts on the leptonic decay products of the W and Z. We have implemented this
calculation into a numerical program. We demonstrate the use of this code by
presenting phenomenological results for several future LHC analyses and recent
Tevatron measurements, including the W cross section in the forward rapidity
region and the central over forward cross section ratio.Comment: 11 pages, 4 figure
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