147 research outputs found
Stellar capture by an accretion disc
Long-term evolution of a stellar orbit captured by a massive galactic center
via successive interactions with an accretion disc has been examined. An
analytical solution describing evolution of the stellar orbital parameters
during the initial stage of the capture was found. Our results are applicable
to thin Keplerian discs with an arbitrary radial distribution of density and
rather general prescription for the star-disc interaction. Temporal evolution
is given in the form of quadrature which can be carried out numerically.Comment: Letter to MNRAS, 5 pages and 3 figures; also available at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
Symplectic integration of space debris motion considering several Earth's shadowing models
In this work, we present a symplectic integration scheme to numerically
compute space debris motion. Such an integrator is particularly suitable to
obtain reliable trajectories of objects lying on high orbits, especially
geostationary ones. Indeed, it has already been demonstrated that such objects
could stay there for hundreds of years. Our model takes into account the
Earth's gravitational potential, luni-solar and planetary gravitational
perturbations and direct solar radiation pressure. Based on the analysis of the
energy conservation and on a comparison with a high order non-symplectic
integrator, we show that our algorithm allows us to use large time steps and
keep accurate results. We also propose an innovative method to model Earth's
shadow crossings by means of a smooth shadow function. In the particular
framework of symplectic integration, such a function needs to be included
analytically in the equations of motion in order to prevent numerical drifts of
the energy. For the sake of completeness, both cylindrical shadows and penumbra
transitions models are considered. We show that both models are not equivalent
and that big discrepancies actually appear between associated orbits,
especially for high area-to-mass ratios
Emission Line Profiles from Self-Gravitating Thin Disks
We have constructed general relativistic models of a stationary, axially
symmnetric, Keplerian thin disk around a rotating black hole. We computed
profiles of a spectral line, emitted in the inner region of the disk. In our
models we have taken into account also the self-gravity of the disk. The aim of
this work is to study gravitational effects on the line profiles in connection
with the X-ray features observed in spectra of active galactic nuclei. In some
cases, the calculated profiles are clearly affected by the disk gravity but
relativistic dragging effects are found to be negligible.Comment: 26 pages, 8 figures, uuencoded postscript file, to appear in The
Astrophysical Journal, Part I. Printed version available upon request from
the author
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