2,787 research outputs found
Review of the kinetics of alkaline degradation of cellulose in view of its relevance for safety assessment of radioactive waste repositories
The degradation of cellulose (a substantial component of low- and intermediate-level radioactive waste) under alkaline conditions occurs via two main processes: a peeling-off reaction and a basecatalyzed cleavage of glycosidic bonds (hydrolysis). Both processes show pseudo-first-order kinetics. At ambient temperature, the peeling-off process is the dominant degradation mechanism, resulting in the formation of mainly isosaccharinic acid. The degradation depends strongly on the degree of polymerization (DP) and on the number of reducing end groups present in cellulose. Beyond pH 12.5, the OH- concentration has only a minor effect on the degradation rate. It was estimated that under repository conditions (alkaline environment, pH 13.3-12.5) about 10% of the cellulosic materials (average DP = 1000-2000) will degrade in the first stage (up to 105 years) by the peeling-off reaction and will cause an ingrowth of isosaccharinic acid in the interstitial cement pore water. In the second stage (105-106 years), alkaline hydrolysis will control the further degradation of the cellulose. The potential role of microorganisms in the degradation of cellulose under alkaline conditions could not be evaluated. Proper assessment of the effect of cellulose degradation on the mobilization of radionuclides basically requires knowing the concentration of isosaccharinic acid in the pore water. This concentration, however, depends on several factors such as the stability of ISA under alkaline conditions, sorption of ISA on cement, formation of sparingly soluble ISA-salts, etc. A discussion of all the relevant processes involved, however, is far beyond the scope of the presented overvie
The circumstellar envelope of AFGL 4106
We present new imaging and spectroscopy of the post-red supergiant binary
AFGL 4106. Coronographic imaging in H-alpha reveals the shape and extent of the
ionized region in the circumstellar envelope (CSE). Echelle spectroscopy with
the slit covering almost the entire extent of the CSE is used to derive the
physical conditions in the ionized region and the optical depth of the dust
contained within the CSE.
The dust shell around AFGL 4106 is clumpy and mixed with ionized gas. H-alpha
and [N II] emission is brightest from a thin bow-shaped layer just outside of
the detached dust shell. On-going mass loss is traced by [Ca II] emission and
blue-shifted absorption in lines of low-ionization species. A simple model is
used to interpret the spatial distribution of the circumstellar extinction and
the dust emission in a consistent way.Comment: 10 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysics Main Journa
Flexible high-voltage supply for experimental electron microscope
Scanning microscope uses a field-emission tip for the electron source, an electron gun that simultaneously accelerates and focuses electrons from the source, and one auxiliary lens to produce a final probe size at the specimen on the order of angstroms
The contribution of massive haloes to the matter power spectrum in the presence of AGN feedback
The clustering of matter, as measured by the matter power spectrum, informs
us about dark matter and cosmology, as well as baryonic effects on the
distribution of matter in the universe. Using cosmological hydrodynamical
simulations from the cosmo-OWLS and BAHAMAS simulation projects, we investigate
the contribution of power in haloes with various masses, defined by particles
within some overdensity region, to the full power spectrum, as well as the
power ratio between baryonic and dark matter only (DMO) simulations for a
matched (between simulations) and an unmatched set of haloes. We find that the
presence of AGN feedback suppresses the power on all scales for haloes of all
masses examined (), by ejecting matter from within
to and potentially beyond in massive
haloes (), and likely
impeding the growth of lower-mass haloes as a consequence. A lower AGN feedback
temperature drastically changes the behaviour of high-mass haloes
(), damping the
effects of AGN feedback at small scales, .
For , group-sized haloes
() dominate the power spectrum, while on
smaller scales the combined contributions of lower-mass haloes to the full
power spectrum rise above that of the group-sized haloes. Finally, we present a
model for the power suppression due to feedback, which combines observed mean
halo baryon fractions with halo mass fractions and halo-matter cross-spectra
extracted from dark matter only simulations to predict the power suppression to
percent-level accuracy down to without
any free parameters.Comment: 20 pages, 11 figures. Submitted to MNRA
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
Discovery of long-period variable stars in the very-metal-poor globular cluster M15
We present a search for long-period variable (LPV) stars among giant branch
stars in M15 which, at [Fe/H] ~ -2.3, is one of the most metal-poor Galactic
globular clusters. We use multi-colour optical photometry from the 0.6-m Keele
Thornton and 2-m Liverpool Telescopes. Variability of delta-V ~ 0.15 mag is
detected in K757 and K825 over unusually-long timescales of nearly a year,
making them the most metal-poor LPVs found in a Galactic globular cluster. K825
is placed on the long secondary period sequence, identified for metal-rich
LPVs, though no primary period is detectable. We discuss this variability in
the context of dust production and stellar evolution at low metallicity, using
additional spectra from the 6.5-m Magellan (Las Campanas) telescope. A lack of
dust production, despite the presence of gaseous mass loss raises questions
about the production of dust and the intra-cluster medium of this cluster.Comment: 13 pages, 9 figures, accepted by MNRA
The M33 Variable Star Population Revealed by Spitzer
We analyze five epochs of Spitzer Space Telescope/Infrared Array Camera
(IRAC) observations of the nearby spiral galaxy M33. Each epoch covered nearly
a square degree at 3.6, 4.5, and 8.0 microns. The point source catalog from the
full dataset contains 37,650 stars. The stars have luminosities characteristic
of the asymptotic giant branch and can be separated into oxygen-rich and
carbon-rich populations by their [3.6] - [4.5] colors. The [3.6] - [8.0] colors
indicate that over 80% of the stars detected at 8.0 microns have dust shells.
Photometric comparison of epochs using conservative criteria yields a catalog
of 2,923 variable stars. These variables are most likely long-period variables
amidst an evolved stellar population. At least one-third of the identified
carbon stars are variable.Comment: Accepted for publication in ApJ. See published article for full
resolution figures and electronic table
A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. II. IC 1613
We present Spitzer Space Telescope IRAC photometry of the Local Group dwarf
irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 micron
photometry with broadband optical photometry and find that the optical data do
not detect 43% and misidentify an additional 11% of the total AGB population,
likely because of extinction caused by circumstellar material. Further, we find
that a narrowband optical carbon star study of IC 1613 detects 50% of the total
AGB population and only considers 18% of this population in calculating the
carbon to M-type AGB ratio. We derive an integrated mass-loss rate from the AGB
stars of 0.2-1.0 x 10^(-3) solar masses per year and find that the distribution
of bolometric luminosities and mass-loss rates are consistent with those for
other nearby metal-poor galaxies. Both the optical completeness fractions and
mass-loss rates in IC 1613 are very similar to those in the Local Group dwarf
irregular, WLM, which is expected given their similar characteristics and
evolutionary histories.Comment: Accepted by ApJ, 26 pages, 10 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
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