64,760 research outputs found
Dynamics of Circumstellar Disks III: The case of GG Tau A
(abridged) We present 2-dimensional hydrodynamic simulations using the
Smoothed Particle Hydrodynamic (SPH) code, VINE, to model a self-gravitating
binary system similar to the GG Tau A system. We simulate systems configured
with semi-major axes of either ~AU (`wide') or ~AU (`close'), and
with eccentricity of either or . Strong spiral structures are
generated with large material streams extending inwards. A small fraction
accretes onto the circumstellar disks, with most returning to the torus.
Structures also propagate outwards, generating net outwards mass flow and
eventually losing coherence at large distances. The torus becomes significantly
eccentric in shape. Accretion onto the stars occurs at a rate of a few
\msun/yr implying disk lifetimes shorter than ~yr,
without replenishment. Only wide configurations retain disks by virtue of
robust accretion. In eccentric configurations, accretion is episodic, occurs
preferentially onto the secondary at wrates peaked near binary periapse. We
conclude that the \ggtaua\ torus is strongly self gravitating and that a major
contribution to its thermal energy is shock dissipation. We interpret its
observed features as manifestations of spiral structures and the low density
material surrounding it as an excretion disk created by outward mass flux. We
interpret GG Tau A as a coplanar system with an eccentric torus, and account
for its supposed mutual inclination as due to degeneracy between the
interpretation of inclination and eccentricity. Although the disks persist for
long enough to permit planet formation, the environment remains unfavorable due
to high temperatures. We conclude that the GG Tau A system is in an eccentric,
~AU orbit.Comment: Accepted for publication in the Astrophysical Journa
Quasinormal modes and dispersion relations for quarkonium in a plasma
Recent investigations show that the thermal spectral function of heavy and vector mesons can be described using holography.
These studies consider a bottom up model that captures the heavy flavour
spectroscopy of masses and decay constants in the vacuum and is consistently
extended to finite temperature. The corresponding spectral functions provide a
picture of the dissociation process in terms of the decrease of the quasi-state
peaks with temperature.
Another related tool that provides important information about the thermal
behaviour is the analysis of the quasinormal modes. They are field solutions in
a curved background assumed to represent, in gauge/gravity duality,
quasi-particle states in a thermal medium. The associated complex frequencies
are related to the thermal mass and width. We present here the calculation of
quasinormal modes for charmonium and bottomonium using the holographic
approach. The temperature dependence of mass and thermal width are
investigated. Solutions corresponding to heavy mesons moving into the plasma
are also studied. They provide the dependence of the real and imaginary parts
of the frequency with the quasi-particle momenta, the so called dispersion
relations.Comment: V2: enlarged version with clarifications, more comparison with
previous articles and additional references included. 11 figures, 2 tables,
62 references. Version accepted for publication in JHE
Bottomonium dissociation in a finite density plasma
We present a holographic description of the thermal behavior of
heavy vector mesons inside a plasma at finite temperature and density. The
meson dissociation in the medium is represented by the decrease in the height
of the spectral function peaks. In order to find a description for the
evolution of the quasi-states with temperature and chemical potential it is
crucial to use a model that is consistent with the decay constant behavior. The
reason is that the height of a spectral function peak is related to the value
of the zero temperature decay constant of the corresponding particle. AdS/QCD
holographic models are in general not consistent with the observation that
decay constants of heavy vector mesons decrease with radial excitation level.
However, it was recently shown that using a soft wall background and
calculating the correlation functions at a finite position of anti-de Sitter
space, associated with an ultraviolet energy scale, it is possible to describe
the observed behavior. Here we extend this proposal to the case of finite
temperature and chemical potential . A clear picture of the
dissociation of bottomonium states as a function of and emerges
from the spectral function. The energy scales where the change in chemical
potential leads to changes in the thermal properties of the mesons is
consistent with QCD expectations.Comment: In V3: errors in reference citations corrected. Version published in
Physics Letters B. 15 pages, 3 figure
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