77 research outputs found
Delivery of Complex Organic Compounds from Evolved Stars to the Solar System
Stars in the late stages of evolution are able to synthesize complex organic compounds with aromatic and aliphatic structures over very short time scales. These compounds are ejected into the interstellar medium and distributed throughout the Galaxy. The structures of these compounds are similar to the insoluble organic matter found in meteorites. In this paper, we discuss to what extent stellar organics has enriched the primordial Solar System and possibly the early Earth
The circumstellar envelope of the C-rich post-AGB star HD 56126
We present a detailed study of the circumstellar envelope of the
post-asymptotic giant branch ``21 micron object'' HD 56126. We build a detailed
dust radiative transfer model of the circumstellar envelope in order to derive
the dust composition and mass, and the mass-loss history of the star. To model
the emission of the dust we use amorphous carbon, hydrogenated amorphous
carbon, magnesium sulfide and titanium carbide. We present a detailed
parametrisation of the optical properties of hydrogenated amorphous carbon as a
function of H/C content. The mid-infrared imaging and spectroscopy is best
reproduced by a single dust shell from 1.2 to 2.6 arcsec radius around the
central star. This shell originates from a short period during which the
mass-loss rate exceeded 10^(-4) M_sun/yr. We find that the strength of the
``21'' micron feature poses a problem for the TiC identification. The low
abundance of Ti requires very high absorption cross-sections in the ultraviolet
and visible wavelength range to explain the strength of the feature. Other
nano-crystalline metal carbides should be considered as well. We find that
hydrogenated amorphous carbon in radiative equilibrium with the local radiation
field does not reach a high enough temperature to explain the strength of the
3.3-3.4 and 6-9 micron hydrocarbon features relative to the 11-17 micron
hydrocarbon features. We propose that the carriers of these hydrocarbon
features are not in radiative equilibrium but are transiently heated to high
temperature. We find that 2 per cent of the dust mass is required to explain
the strength of the ``30'' micron feature, which fits well within the measured
atmospheric abundance of Mg and S. This further strengthens the MgS
identification of the ``30'' micron feature.Comment: 20 Pages, 10 Figures, accepted for publication in Astronomy and
Astrophysic
The carrier of the "30" micron emission feature in evolved stars. A simple model using magnesium sulfide
We present 2-45 micron spectra of a large sample of carbon-rich evolved stars
in order to study the ``30'' micron feature. We find the ``30'' micron feature
in sources in a wide range of sources: low mass loss carbon stars, extreme
carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles
from the sources by using a simple systematic approach to model the continuum.
We find large variations in the wavelength and width of the extracted profiles
of the ``30'' micron feature. We modelled the whole range of profiles in a
simple way by using magnesium sulfide (MgS) dust grains with a MgS grain
temperature different from the continuum temperature. The systematic change in
peak positions can be explained by cooling of MgS grains as the star evolves
off the AGB. In several sources we find that a residual emission excess at ~26
micron can also be fitted using MgS grains but with a different grains shape
distribution. The profiles of the ``30'' micron feature in planetary nebulae
are narrower than our simple MgS model predicts. We discuss the possible
reasons for this difference. We find a sample of warm carbon-stars with very
cold MgS grains. We discuss possible causes for this phenomenon. We find no
evidence for rapid destruction of MgS during the planetary nebula phase and
conclude that the MgS may survive to be incorporated in the ISM.Comment: 31 pages, accepted for publication in Astronomy and Astrophysics.
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Recent knowledge on hepatitis E virus in Suidae reservoirs and transmission routes to human
Investigation of the influence of oxidation and HF attack on the photoluminescence of silicon nanoparticles
Light-emitting silicon nanocrystals from laser pyrolysis
Crystalline Si nanoparticles with diameters between 2.5 and 20 nm are prepared by CO2-laser-induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone is extracted through a nozzle into a high- acuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles that can be deposited on suitable substrates. The strong visible photoluminescence (PL) of the Si nanocrystals is studied as a function of their size, and as a function of the time for which they are exposed to air. All observations can be explained on the basis of quantum confinement as the only origin of the PL. Chemical methods are exploited to modify the surface of the Si, nanoparticles and to reduce their size, thus shifting their PL to shorter wavelengths. With this technique, the Si nanoparticles, collected in much larger quantities in the filter of the flow reactor, can be made strongly luminescent so that they may be used for various applications
Investigation of the influence of oxidation and HF attack on the photoluminescence of silicon nanoparticles
Investigation of the influence of oxidation and HF attack on the photoluminescence of silicon nanoparticles
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