6,467 research outputs found
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
Sectoral r modes and periodic RV variations of Sun-like stars
Radial velocity (RV) measurements are used to search for planets orbiting
late-type main-sequence stars and confirm the transiting planets. The most
advanced spectrometers are approaching a precision of cm/s that
implies the need to identify and correct for all possible sources of RV
oscillations intrinsic to the star down to this level and possibly beyond. The
recent discovery of global-scale equatorial Rossby waves in the Sun, also
called r modes, prompted us to investigate their possible signature in stellar
RV measurements. R modes are toroidal modes of oscillation whose restoring
force is the Coriolis force and propagate in the retrograde direction in a
frame that corotates with the star. The solar r modes with azimuthal orders were identified unambiguously because of their dispersion
relation and their long e-folding lifetimes of hundreds of days. Here we
simulate the RV oscillations produced by sectoral r modes with assuming a stellar rotation period of 25.54 days and a maximum amplitude of
the surface velocity of each mode of 2 m/s. This amplitude is representative of
the solar measurements, except for the mode which has not yet been
observed. Sectoral r modes with azimuthal orders and would produce RV
oscillations with amplitudes of 76.4 and 19.6 cm/s and periods of 19.16 and
10.22 days, respectively, for a star with an inclination of the rotation axis
. Therefore, they may produce rather sharp peaks in the Fourier
spectrum of the radial velocity time series that could lead to spurious
planetary detections. Sectoral r~modes may represent a source of confusion in
the case of slowly rotating inactive stars that are preferential targets for RV
planet search. The main limitation of the present investigation is the lack of
observational constraint on the amplitude of the mode on the Sun.Comment: 7 pages; 4 figures; 1 table; accepted to Astronomy & Astrophysic
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
Irreversible magnetization under rotating fields and lock-in effect on ErBa_2Cu_3O_7 single crystal with columnar defects
We have measured the irreversible magnetization M_i of an ErBa_2Cu_3O_7
single crystal with columnar defects (CD), using a technique based on sample
rotation under a fixed magnetic field H. This method is valid for samples whose
magnetization vector remains perpendicular to the sample surface over a wide
angle range - which is the case for platelets and thin films - and presents
several advantages over measurements of M_L(H) loops at fixed angles. The
resulting M_i(\Theta) curves for several temperatures show a peak in the CD
direction at high fields. At lower fields, a very well defined plateau
indicative of the vortex lock-in to the CD develops. The H dependence of the
lock-in angle \phi_L follows the H^{-1} theoretical prediction, while the
temperature dependence is in agreement with entropic smearing effects
corresponding to short range vortex-defects interactions.Comment: 7 pages, 6 figures, to be published in Phys. Rev.
Searching for star-planet magnetic interaction in CoRoT observations
Close-in massive planets interact with their host stars through tidal and
magnetic mechanisms. In this paper, we review circumstantial evidence for
star-planet interaction as revealed by the photospheric magnetic activity in
some of the CoRoT planet-hosting stars, notably CoRoT-2, CoRoT-4, and CoRoT-6.
The phenomena are discussed in the general framework of activity-induced
features in stars accompanied by hot Jupiters. The theoretical mechanisms
proposed to explain the activity enhancements possibly related with hot Jupiter
are also briefly reviewed with an emphasis on the possible effects at
photospheric level. The unique advantages of CoRoT and Kepler observations to
test these models are pointed out.Comment: Invited review paper accepted by Astrophysics and Space Science, 13
pages, 5 figure
Microscopic calculations of double and triple Giant Resonance excitation in heavy ion collisions
We perform microscopic calculations of the inelastic cross sections for the
double and triple excitation of giant resonances induced by heavy ion probes
within a semicalssical coupled channels formalism. The channels are defined as
eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective
RPA phonons. Therefore, they are superpositions of several multiphonon states,
also with different numbers of phonons and the spectrum is anharmonic. The
inclusion of (n+1) phonon configurations affects the states whose main
component is a n-phonon one and leads to an appreacible lowering of their
energies. We check the effects of such further anharmonicities on the previous
published results for the cross section for the double excitation of Giant
Resonances. We find that the only effect is a shift of the peaks towards lower
energies, the double GR cross section being not modified by the explicity
inclusion of the three-phonon channels in the dynamical calculations. The
latters give an important contribution to the cross section in the triple GR
energy region which however is still smaller than the experimental available
data. The inclusion of four phonon configurations in the structure calculations
does not modify the results.Comment: Revtex4, to be published in PR
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