15,702 research outputs found
Heterogeneous distribution of Al-26 at the birth of the Solar System
It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct
radionuclide, was uniformly distributed in the nascent Solar System with the
initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar
origin. However, the stellar source of Al-26 and the manner in which it was
injected into the solar system remain controversial: the Al-26 could have been
produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star
and injected either into the protosolar molecular cloud or protoplanetary disk.
Corundum (Al2O3) is thermodynamically predicted to be the first condensate from
a cooling gas of solar composition. Here we show that micron-sized corundum
condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition
recorded heterogeneous distribution of Al-26 at the birth of the solar system:
the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50%
of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich
refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich
calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite
crystals in CM chondrites, and FUN (fractionation and unidentified nuclear
isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the
apparently early and short duration (<0.3 Ma) of condensation of refractory
O-16-rich solids in the solar system, we infer that Al-26 was injected into the
collapsing protosolar molecular cloud and later homogenized in the
protoplanetary disk. The apparent lack of correlation between Al-26 abundance
and O-isotope compositions of corundum grains put important constraints on the
stellar source of Al-26 in the solar system.Comment: Astrophysical Journal Letters 733, L3
Optical constants of refractory oxides at high temperatures
Many cosmic dust species, among them refractory oxides, form at temperatures
higher than 300 K. Nevertheless, most astrophysical studies are based on the
room-temperature optical constants of solids, such as corundum and spinel. A
more realistic approach is needed for these materials, especially in the
context of modeling late-type stars. We aimed at deriving sets of optical
constants of selected, astrophysically relevant oxide dust species with high
melting points. A high-temperature-high-pressure-cell and a Fourier-transform
spectrometer were used to measure reflectance spectra of polished samples. For
corundum (alpha-AlO), spinel (MgAlO), and alpha-quartz
(SiO), temperature-dependent optical constants were measured from 300 K up
to more than 900 K. Small particle spectra were also calculated from these
data. All three examined oxides show a significant temperature dependence of
their mid-IR bands. For the case of corundum, we find that the 13m
emission feature - seen in the IR spectra of many AGB stars - can very well be
assigned to this mineral species. The best fit of the feature is achieved with
oblate corundum grains at mean temperatures around 550 K. Spinel remains a
viable carrier of the 13m feature as well, but only for T < 300 K and
nearly spherical grain shapes. Under such circumstances, spinel grains may also
account for the 31.8m band that is frequently seen in sources of the
13m feature and which has not yet been identified with certainty.Comment: Astronomy & Astrophysics, accepted, 26 February 2013. Article with 18
pages and 15 figure
Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge
After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools
Dust in the wind: Crystalline silicates, corundum and periclase in PG 2112+059
We have determined the mineralogical composition of dust in the Broad
Absorption Line (BAL) quasar PG 2112+059 using mid-infrared spectroscopy
obtained with the Spitzer Space Telescope. From spectral fitting of the solid
state features, we find evidence for Mg-rich amorphous silicates with olivine
stoichiometry, as well as the first detection of corundum (Al_2O_3) and
periclase (MgO) in quasars. This mixed composition provides the first direct
evidence for a clumpy density structure of the grain forming region. The
silicates in total encompass 56.5% of the identified dust mass, while corundum
takes up 38 wt.%. Depending on the choice of continuum, a range of mass
fractions is observed for periclase ranging from 2.7% in the most conservative
case to 9% in a less constrained continuum. In addition, we identify a feature
at 11.2 micron as the crystalline silicate forsterite, with only a minor
contribution from polycyclic aromatic hydrocarbons. The 5% crystalline silicate
fraction requires high temperatures such as those found in the immediate quasar
environment in order to counteract rapid destruction from cosmic rays.Comment: 2 figure
Fe-substituted mullite powders for the in situ synthesis of carbon nanotubes by catalytic chemical vapor deposition
Powders of iron-substituted mullite were prepared by combustion and further calcination in air at different temperatures. A detailed study involving notably Mo¨ssbauer spectroscopy showed that the Fe3+ ions are distributed between the mullite phase and a corundum phase that progressively dissolves into mullite upon the increase in calcination temperature. Carbon nanotube-Fe-mullite nanocomposites were prepared for the first time by a direct method involving a reduction of these powders in H2-CH4 and without any mechanical mixing step. The carbon nanotubes formed by the catalytic decomposition of CH4 on the smallest metal particles are mostly double-walled and multiwalled, although some carbon nanofibers are also observed
Uncovering the mechanism of the impurity-selective Mott transition in paramagnetic VO
While the phase diagrams of the one- and multi-orbital Hubbard model have
been well studied, the physics of real Mott insulators is often much richer,
material dependent, and poorly understood. In the prototype Mott insulator
VO, chemical pressure was initially believed to explain why the
paramagnetic-metal to antiferromagnetic-insulator transition temperature is
lowered by Ti doping while Cr doping strengthens correlations, eventually
rendering the high-temperature phase paramagnetic insulating. However, this
scenario has been recently shown both experimentally and theoretically to be
untenable. Based on full structural optimization, we demonstrate via the charge
self-consistent combination of density functional theory and dynamical
mean-field theory that changes in the VO phase diagram are driven
by defect-induced local symmetry breakings resulting from dramatically
different couplings of Cr and Ti dopants to the host system. This finding
emphasizes the high sensitivity of the Mott metal-insulator transition to the
local environment and the importance of accurately accounting for the
one-electron Hamiltonian, since correlations crucially respond to it.Comment: 5 pages, 5 figures, supplementary informatio
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