13,428 research outputs found
Line Structure in the Spectrum of FU Orionis
New high-resolution spectra of FU Ori, obtained with the HIRES spectrograph
at the Keck I telescope in 2003-2006, make it possible to compare the optical
line profiles with those predicted by the self-luminous accretion disk model. A
dependence of line width on excitation potential and on wavelength, expected
for a Keplerian disk, is definitely not present in the optical region, nor is
the line duplicity due to velocity splitting. The absorption lines observed in
the optical region of FU Ori must originate in or near the central object, and
here their profiles are shown to be those expected of a rigidly rotating
object. They can be fitted by a rapidly rotating (v sin i = 70 km/s)
high-luminosity G-type star having a large dark polar spot, with axis inclined
toward the line of sight. Over these years, the radial velocity of FU Ori has
remained constant to within +/-0.3 km/s, so there is no indication that the
star is a spectroscopic binary. These results apply to the optical region
( \AA); more distant, cooler regions of the disk contribute in
the infrared.Comment: 14 pages, 11 figures, accepted by A
Periodic radial velocity variations in RU Lupi
Context. RU Lup is a Classical T Tauri star with unusually strong emission
lines, which has been interpreted as manifestations of accretion. Recently,
evidence has accumulated that this star might have a variable radial velocity.
Aims. We intended to investigate in more detail the possible variability in
radial velocity using a set of 68 high-resolution spectra taken at the VLT
(UVES), the AAT (UCLES) and the CTIO (echelle). Methods. Using standard
cross-correlation techniques, we determined the radial velocity of RU Lup. We
analysed these results with Phasedispersion minimization and the Lomb-Scargle
periodogram and searched for possible periodicities in the obtained radial
velocities. We also analysed changes in the absorption line shapes and the
photometric variability of RU Lup. Results. Our analysis indicated that RU Lup
exhibits variations in radial velocity with a periodicity of 3.71 days and an
amplitude of 2.17 km/s. These variations can be explained by the presence of
large spots, or groups of spots, on the surface of RU Lup. We also considered a
low-mass companion and stellar pulsations as alternative sources for these
variations but found these to be unlikely.Comment: 8 pages, 4 figures, Accepted by A&
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
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
Transient currents in a molecular photo-diode
Light-induced charge transmission through a molecular junction (molecular
diode) is studied in the framework of a HOMO-LUMO model and in using a kinetic
description. Expressions are presented for the sequential (hopping) and direct
(tunneling) transient current components together with kinetic equations
governing the time-dependent populations of the neutral and charged molecular
states which participate in the current formation. Resonant and off-resonant
charge transmission processes are analyzed in detail. It is demonstrated that
the transient currents are associated with a molecular charging process which
is initiated by photo excitation of the molecule. If the coupling of the
molecule to the electrodes is strongly asymmetric the transient currents can
significantly exceed the steady state current.Comment: 17 pages, 12 figures, accepted for publication in Chemical Physic
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
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