35 research outputs found
Candidate carriers and synthetic spectra of the 21- and 30-mu protoplanetary nebular bands
Computational chemistry is used here to determine the vibrational line
spectrum of several candidate molecules. It is shown that the thiourea
functional group, associated with various carbonaceous structures (mainly
compact and linear aromatic clusters), is able to mimic the 21-m band
emitted by a number of proto-planetary nebulae. The combination of nitrogen and
sulphur in thiourea is the essential source of emission in this model: the band
disappears if these species are replaced by carbon.
The astronomical 21-m feature extends redward to merge with another
prominent band peaking between 25 and 30 m, also known as the 30-m
band. It is found that the latter can be modelled by the combined spectra of
aliphatic chains, made of CH groups, oxygen bridges and OH groups, which
provide the 30-m emission. The absence of oxygen all but extinguishes the
30-m emission. The emission between the 21- and 30-m bands is
provided mainly by thiourea attached to linear aromatic clusters.
The chemical software reveals that the essential role of the heteroatoms N, S
and O stems from their large electronic charge. It also allows to determine the
type of atomic vibration responsible for the different lines of each structure,
which helps selecting the most relevant structures.
A total of 22 structures have been selected here, but their list is far from
being exhaustive; they are only intended as examples of 3 generic classes. When
background dust emission is added, model spectra are obtained, which are able
to satisfactorily reproduce recent observations of proto-planetary nebulae.
The relative numbers of atomic species used in this model are typically
H:C:O:N:S=53:36:8:2:1.Comment: 9 pages, 14 figure
Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10K
The reactions of cold H atoms with solid O2 molecules were investigated at 10
K. The formation of H2O2 and H2O has been confirmed by in-situ infrared
spectroscopy. We found that the reaction proceeds very efficiently and obtained
the effective reaction rates. This is the first clear experimental evidence of
the formation of water molecules under conditions mimicking those found in cold
interstellar molecular clouds. Based on the experimental results, we discuss
the reaction mechanism and astrophysical implications.Comment: 12 pages, 3 Postscript figures, use package amsmath, amssymb,
graphic
THE MECHANISM OF SURFACE DIFFUSION OF H AND D ATOMS ON AMORPHOUS SOLID WATER: EXISTENCE OF VARIOUS POTENTIAL SITES
To understand elementary processes leading to H{sub 2} formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solid water (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H{sub 2} (J = 0 and 1) and D{sub 2} (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of { =}30 meV, respectively. The present study pictured the outline of H{sub 2} formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with suchmore » trapped H atoms to yield H{sub 2} molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.« les
Formation of Dust in the Ejecta of Type Ia Supernovae
We investigate the formation of dust grains in the ejecta of Type Ia
supernovae (SNe Ia), adopting the carbon-deflagration W7 model. In the
calculations of dust formation, we apply the nucleation and grain growth theory
and consider the two cases with and without formation of CO and SiO molecules.
The results of the calculations show that for the sticking probability of
alpha_j=1, C, silicate, Si, and FeS grains can condense at early times of
~100--300 days after the explosion, whereas Fe and SiC grains cannot form
substantially. Due to the low gas density in SNe Ia with no H-envelope, the
average radii of the newly formed grains are generally below 0.01 micron, being
much smaller than those in Type II-P SNe. This supports our previous conclusion
that the radius of dust formed in the ejecta is smaller in SNe with less
massive envelopes. The total dust mass ranges from 3x10^{-4} M_sun to 0.2 M_sun
for alpha_j=0.1--1, depending on whether or not CO and SiO molecules are
formed. We also estimate the optical depths and thermal emission by the newly
formed dust and compare to the relevant observations of SNe Ia. We find that
the formation of C grains in SNe Ia is suppressed to be consistent with the
observational constraints. This implies that energetic photons and electrons
heavily depress the formation efficiency of C grains or that the outermost C-O
layer of SNe Ia is almost fully burned. Finally, we perform the calculations of
dust destruction in the SN remnants and find that dust grains formed in the
ejecta of SNe Ia are almost completely destroyed in the shocked gas before
being injected into the interstellar medium. This indicates that SNe Ia are
unlikely to be the major sources of interstellar dust.Comment: 13 pages, 7 figures, 2 tables. Accepted for publication in Ap
Silicon carbide absorption features: dust formation in the outflows of extreme carbon stars
Infrared carbon stars without visible counterparts are generally known as
extreme carbon stars. We have selected a subset of these stars with absorption
features in the 10-13 m range, which has been tentatively attributed to
silicon carbide (SiC). We add three new objects meeting these criterion to the
seven previously known, bringing our total sample to ten sources. We also
present the result of radiative transfer modeling for these stars, comparing
these results to those of previous studies. In order to constrain model
parameters, we use published mass-loss rates, expansion velocities and
theoretical dust condensation models to determine the dust condensation
temperature. These show that the inner dust temperatures of the dust shells for
these sources are significantly higher than previously assumed. This also
implies that the dominant dust species should be graphite instead of amorphous
carbon. In combination with the higher condensation temperature we show that
this results in a much higher acceleration of the dust grains than would be
expected from previous work. Our model results suggest that the very optically
thick stage of evolution does not coincide with the timescales for the
superwind, but rather, that this is a very short-lived phase. Additionally, we
compare model and observational parameters in an attempt to find any
correlations. Finally, we show that the spectrum of one source, IRAS
175343030, strongly implies that the 10-13 m feature is due to a solid
state rather than a molecular species.Comment: 13 Figure
The 21 micron and 30 micron circumstellar dust features in evolved C-rich objects
The 21micron and 30micron bands are the strongest dust emission features
detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic
giant branch [AGB] stars, proto-planetary nebulae [PPN], and planetary nebulae
[PN]). While the 21micron feature is rare and exists only in the transient PPN
phase, the 30micron feature is more common and seen in the entire late stage of
stellar evolution, from AGB to PPN and PN phases, as well as in the
low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small
Magellanic Cloud (SMC). The carriers of these features remain unidentified.
Eleven of the twelve well-identified 21micron sources also emit in the 30micron
band, suggesting that their carriers may be somewhat related.Comment: 7 pages, 1 figure, uses eps.cls. Accepted for publication in "Earth,
Planets and Space" (special issue on Cosmic Dust
On the Carriers of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21micron emission feature seen in sixteen C-rich
proto-planetary nebulae (PPNe) remains unidentified since its discovery in
1989. Over a dozen of materials are suggested as the carrier candidates. In
this work we quantitatively investigate eight inorganic and one organic carrier
candidates in terms of elemental abundance constraints, while previous studies
mostly focus on their spectral profiles (which could be largely affected by
grain size, shape and clustering effects). It is found that: (1) five
candidates (TiC nanoclusters, fullerenes coordinated with Ti atoms, SiS,
doped-SiC, and SiO-coated SiC dust) violate the abundance constraints (i.e.
they require too much Ti, S or Si to account for the emission power of the
21micron band, (2) three candidates (carbon and silicon mixtures, FeO,
and FeO),while satisfying the abundance constraints, exhibit secondary
features which are not detected in the 21micron sources, and (3) nano FeO,
neither exceeding the abundance budget nor producing undetected secondary
features, seems to be a viable candidate, supporting the suggestions of Posch
et al. 2004.Comment: 12 pages, 2 figures, accepted for publication in MNRA
Presolar grains from meteorites: Remnants from the early times of the solar system
This review provides an introduction to presolar grains - preserved stardust
from the interstellar molecular cloud from which our solar system formed -
found in primitive meteorites. We describe the search for the presolar
components, the currently known presolar mineral populations, and the chemical
and isotopic characteristics of the grains and dust-forming stars to identify
the grains' most probable stellar sources. Keywords: presolar grains,
interstellar dust, asymptotic giant branch (AGB) stars, novae, supernovae,
nucleosynthesis, isotopic ratios, meteoritesComment: 71 pages, 24 figures, 9 tables. Invited review. to appear in Chemie
der Erd
Origin of Presolar Grains : Formation of TiC Core-Graphitic Spherules in Carbon-rich AGB Stars
Kinetic theory of steady chemical nucleation in the gas phase
We develop a kinetic theory of nucleation involving chemical reactions in the gas phase. For the basis of deriving the chemical nucleation rate, chemical kinetic considerations are presented on the steady current density and the effective rate constants of the overall reaction, which is a sum of a sequential elementary reactions. We formulate the steady rate of chemical nucleation in a multi-component vapor, in which nucleation occurs via the chemical reactions yielding a condensate having a stoichiometric composition. An exact expression of the steady nucleation rate is given together with its approximate formulas for practical applications. The present formulation is not concerned with any particular cluster model. The supersaturation ratio for a many-component vapor is defined so as to be a natural extension of that for a one-component vapor. It is shown that the transition probabilities due to growth and decay of the clusters are of the same form as the growth and evaporation rates in a one-component vapor