2,786 research outputs found
BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution
We present a radiative-transfer calculation which reproduces the infrared
spectrum of the planetary nebula BD~+303639. We calculate the
transfer process through absorption and scattering in a spherical-symmetric
multi-grain dust shell. The emission of transiently heated particles is taken
into account, as well as polycyclic aromatic hydrocarbons. We obtain an
acceptable fit to most of the spectrum, including the PAH infrared bands. At
submillimetre wavelengths the observed emission is larger than the model
predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be
needed as an additional constituent. The fit favours a distance of kpc, which implies that BD~+303639 has evolved from a massive
progenitor of several solar masses. A low dust-to-gas mass ratio is found in
the ionised region. The calculations yield an original mass-loss rate of
2\times10^{-4} \msolar \peryr on the Asymptotic Giant Branch. Using this
mass-loss rate, we calculate how the infrared spectrum has evolved during the
post-AGB evolution. We show in particular the evolution of the IRAS colours
during the preceding post-AGB evolution.Comment: accepted for publication in MNRAS. LaTeX, 15 pages, hardcopy and 8
figures available from [email protected] or [email protected]
Non-spiky density of states of an icosahedral quasicrystal
The density of states of the ideal three-dimensional Penrose tiling, a
quasicrystalline model, is calculated with a resolution of 10 meV. It is not
spiky. This falsifies theoretical predictions so far, that spikes of width
10-20 meV are generic for the density of states of quasicrystals, and it
confirms recent experimental findings. The qualitative difference between our
results and previous calculations is partly explained by the small number of k
points that has usually been included in the evaluation of the density of
states of periodic approximants of quasicrystals. It is also shown that both
the density of states of a small approximant of the three-dimensional Penrose
tiling and the density of states of the ideal two-dimensional Penrose tiling do
have spiky features, which also partly explains earlier predictions.Comment: 8 pages, 4 figures. Changes in this version: longer introduction,
details of figures shown in inset
Structure and shaping processes within the extended atmospheres of AGB stars
We present recent studies using the near-infrared instrument AMBER of the VLT
Interferometer (VLTI) to investigate the structure and shaping processes within
the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER
observations of the Mira variable S Ori have revealed wavelength-dependent
apparent angular sizes. These data were successfully compared to dynamic model
atmospheres, which predict wavelength-dependent radii because of geometrically
extended molecular layers. Most recently, AMBER closure phase measurements of
several AGB stars have also revealed wavelength-dependent deviations from 0/180
deg., indicating deviations from point symmetry. The variation of closure phase
with wavelength indicates a complex non-spherical stratification of the
extended atmosphere, and may reveal whether observed asymmetries are located
near the photosphere or in the outer molecular layers. Concurrent observations
of SiO masers located within the extended molecular layers provide us with
additional information on the morphology, conditions, and kinematics of this
shell. These observations promise to provide us with new important insights
into the shaping processes at work during the AGB phase. With improved imaging
capabilities at the VLTI, we expect to extend the successful story of imaging
studies of planetary nebulae to the photosphere and extended outer atmosphere
of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F.
Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for
Astrophysics, Manchester, UK, 201
All-electron theory of the coupling between laser-induced coherent phonons in bismuth
Using first principles, all-electron calculations and dynamical simulations
we study the behavior of the A_1g and E_g coherent phonons induced in Bi by
intense laser pulses. We determine the potential landscapes in the laser heated
material and show that they exhibit phonon-softening, phonon-phonon coupling,
and anharmonicities. As a consequence the E_g mode modulates the A_1g
oscillations and higher harmonics of both modes appear, which explains recent
isotropic reflectivity measurements. Our results offer a unified description of
the different experimental observations performed so far on bismuth.Comment: 3 figure
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
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