13,311 research outputs found
Line-dependent veiling in very active T Tauri Stars
The T Tauri stars with active accretion disks show veiled photospheric
spectra. This is supposedly due to non-photospheric continuum radiated by hot
spots beneath the accretion shocks at stellar surface and/or chromospheric
emission lines radiated by the post-shocked gas. The amount of veiling is often
considered as a measure of the mass-accretion rate. We analysed high-resolution
photospheric spectra of accreting T Tauri stars LkHa 321, V1331 Cyg, and AS 353
A with the aim of clarifying the nature of the line-dependent veiling. Each of
these objects shows a highly veiled, strong emission line spectrum and powerful
wind features indicating high rates of accretion and mass loss. Equivalent
widths of hundreds of weak photospheric lines were measured in the observed
spectra and compared with those in synthetic spectra with the same spectral
type. We found that the veiling is strongly line-dependent: larger in stronger
photospheric lines and weak or absent in the weakest ones. No dependence of
veiling on excitation potential within 0 to 5 eV was found. Different physical
processes responsible for these unusual veiling effects are discussed in the
framework of the magnetospheric accretion model. The observed veiling has two
origins: 1) an abnormal structure of stellar atmosphere heated up by the
accreting matter, and 2) a non-photospheric continuum radiated by a hot spot
with temperature lower than 10000 K. The true level of the veiling continuum
can be derived by measuring the weakest photospheric lines with equivalent
widths down to 10 m\AA. A limited spectral resolution and/or low
signal-to-noise ratio results in overestimation of the veiling continuum. In
the three very active stars, the veiling continuum is a minor contributor to
the observed veiling, while the major contribution comes from the
line-dependent veiling.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Equilibrium and dynamics of a trapped superfluid Fermi gas with unequal masses
Interacting Fermi gases with equal populations but unequal masses are
investigated at zero temperature using local density approximation and the
hydrodynamic theory of superfluids in the presence of harmonic trapping.
We derive the conditions of energetic stability of the superfluid
configuration with respect to phase separation and the frequencies of the
collective oscillations in terms of the mass ratio and the trapping frequencies
of the two components. We discuss the behavior of the gas after the trapping
potential of a single component is switched off and show that, near a Feshbach
resonance, the released component can still remain trapped due to many-body
interaction effects. Explicit predictions are presented for a mixture of Li
and K with resonant interaction.Comment: 4 pages, 2 figure
Nonlinearity of vacuum reggeons and exclusive diffractive production of vector mesons at HERA
The processes of exclusive photo- and electroproduction of vector mesons
(770), (1020) and (3096) at collision energies and transferred momenta squared are considered in
the framework of a phenomenological Regge-eikonal scheme with nonlinear Regge
trajectories in which their QCD asymptotic behavior is taken into account
explicitly. By comparison of available experimental data from ZEUS and H1
Collaborations with the model predictions it is demonstrated that corresponding
angular distributions and integrated cross-sections in the above-mentioned
kinematical range can be quantitatively described with use of two -even
vacuum Regge trajectories. These are the "soft" pomeron dominating the high
energy reactions without a hard scale and the "hard" pomeron giving an
essential contribution to photo- and electroproduction of heavy vector mesons
and deeply virtual electroproduction of light vector mesons.Comment: 25 pages, 12 figure
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
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