6,550 research outputs found
Observations of Small Scale ISM Structure in Dense Atomic Gas
We present high resolution (R~170,000) Kitt Peak National Observatory Co'ude
Feed telescope observations of the interstellar KI 7698 angstrom line towards 5
multiple star systems with saturated NaI components. We compare the KI
absorption line profiles in each of the two (or three) lines of sight in these
systems, and find significant differences between the sight-lines in 3 out of
the 5 cases. We infer that the small scale structure traced by previous NaI
observations is also present in at least some of the components with saturated
NaI absorption lines, and thus the small scale structures traced by the neutral
species are occurring at some level in clouds of all column densities. We
discuss the implications of that conclusion and a potential explanation by
density inhomogeneities
Non-linear Preheating with Scalar Metric Perturbations
We have studied preheating of field perturbations in a 3-dimensional lattice
including the effect of scalar metric perturbations, in two generic models of
inflation: chaotic inflation with a quartic potential, and standard hybrid
inflation. We have prepared the initial state for the classical evolution of
the system with vanishing vector and tensor metric perturbations, consistent
with the constraint equations, the energy and momentum constraints. The
non-linear evolution inevitably generates vector and tensor modes, and this
reflects on how well the constraint equations are fulfilled during the
evolution. The induced preheating of the scalar metric perturbations is not
large enough to backreact onto the fields, but it could affect the evolution of
vector and tensor modes. This is the case in hybrid inflation for some values
of the coupling and the height of potential . For example with
GeV, preheating of scalar perturbations is such that
their source term in the evolution equation of tensor and vector becomes
comparable to that of the field anisotropic stress.Comment: 15 pages, 12 eps figure
An extension problem for the CR fractional Laplacian
We show that the conformally invariant fractional powers of the sub-Laplacian
on the Heisenberg group are given in terms of the scattering operator for an
extension problem to the Siegel upper halfspace. Remarkably, this extension
problem is different from the one studied, among others, by Caffarelli and
Silvestre.Comment: 33 pages. arXiv admin note: text overlap with arXiv:0709.1103 by
other author
Low-Metallicity Gas Clouds in a Galaxy Proto-Cluster at Redshift 2.38
We present high resolution spectroscopy of a QSO whose sight-line passes
through the halo of a pair of elliptical galaxies at redshift 2.38. This pair
of galaxies probably lies at the center of a galaxy proto-cluster, and is
embedded in a luminous extended Ly-alpha nebula.
The QSO sight-line intersects two small gas clouds within this halo. These
clouds have properties similar to those of high velocity clouds (HVCs) seen in
the halo of the Milky Way. The gas is in a cool (< 2 x 10^4 K) and at least 20%
neutral phase, with metallicities in the range -3.0 < [Fe/H] < -1.1 and neutral
hydrogen column densities of ~10^19.5 /cm^2.
The origin of these clouds is unclear. The presence of low metallicity gas
within this possible proto-cluster implies either that the intra-cluster medium
has not been enriched with metals at this redshift, or the clouds are embedded
within a hot, ionized, metal-rich gas phase.Comment: Accepted to appear in ApJ Letter
Small-Scale Interstellar Na I Structure Toward M92
We have used integral field echelle spectroscopy with the DensePak
fiber-optic array on the KPNO WIYN telescope to observe the central 27" x 43"
of the globular cluster M92 in the Na I D wavelength region at a spatial
resolution of 4". Two interstellar Na I absorption components are evident in
the spectra at LSR velocities of 0 km/s (Cloud 1) and -19 km/s (Cloud 2).
Substantial strength variations in both components are apparent down to scales
limited by the fiber-to-fiber separations. The derived Na I column densities
differ by a factor of 4 across the Cloud 1 absorption map and by a factor of 7
across the Cloud 2 map. Using distance upper limits of 400 and 800 pc for Cloud
1 and Cloud 2, respectively, the absorption maps indicate structure in the ISM
down to scales of 1600 and 3200 AU. The fiber-to-fiber Na I column density
differences toward M92 are comparable to those found in a similar study of the
ISM toward the globular cluster M15. Overall, the structures in the
interstellar components toward M92 have significantly lower column densities
than those toward M15. We interpret these low column density structures as
small-scale turbulent variations in the gas and compare them to the
larger-scale, higher column density variations toward M15, which may be the
hallmarks of actual H I structures.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter
Voltage-controlled electron tunnelling from a single self-assembled quantum dot embedded in a two-dimensional-electron-gas-based photovoltaic cell
We perform high-resolution photocurrent (PC) spectroscopy to investigate
resonantly the neutral exciton ground-state (X0) in a single InAs/GaAs
self-assembled quantum dot (QD) embedded in the intrinsic region of an
n-i-Schottky photodiode based on a two-dimensional electron gas (2DEG), which
was formed from a Si delta-doped GaAs layer. Using such a device, a single-QD
PC spectrum of X0 is measured by sweeping the bias-dependent X0 transition
energy through that of a fixed narrow-bandwidth laser via the quantum-confined
Stark effect (QCSE). By repeating such a measurement for a series of laser
energies, a precise relationship between the X0 transition energy and bias
voltage is then obtained. Taking into account power broadening of the X0
absorption peak, this allows for high-resolution measurements of the X0
homogeneous linewidth and, hence, the electron tunnelling rate. The electron
tunnelling rate is measured as a function of the vertical electric field and
described accurately by a theoretical model, yielding information about the
electron confinement energy and QD height. We demonstrate that our devices can
operate as 2DEG-based QD photovoltaic cells and conclude by proposing two
optical spintronic devices that are now feasible.Comment: 34 pages, 11 figure
Depolymerization Pathways for Branching Lignin Spirodienone Units Revealed with ab Initio Steered Molecular Dynamics
Lignocellulosic biomass is an abundant, rich source of aromatic compounds, but direct utilization of raw lignin has been hampered by both the high heterogeneity and variability of linking bonds in this biopolymer. Ab initio steered molecular dynamics (AISMD) has emerged both as a fruitful direct computational screening approach to identify products that occur through
mechanical depolymerization (i.e., in sonication or ball-milling) and as a sampling approach. By varying the direction of force and sampling over 750 AISMD trajectories, we identify numerous possible pathways through which lignin depolymerization may occur in pyrolysis or through catalytic depolymerization as well. Here, we present eight unique major depolymerization pathways discovered via AISMD for the recently characterized spirodienone lignin branching linkage that may comprise around 10% weight of all lignin in some softwoods. We extract representative trajectories from AISMD and carry out reaction pathway analysis to identify energetically favorable pathways for lignin depolymerization. Importantly, we identify dynamical effects that could not be observed through more traditional calculations of bond dissociation energies. Such effects include thermodynamically favorable recovery of aromaticity in the dienone ring that leads to near-barrierless subsequent ether cleavage and hydrogen bonding effects that stabilize newly formed radicals. Some of the most stable spirodienone fragments that reside at most 1 eV above the reactant structure are formed with only 2 eV barriers for C-C bond cleavage, suggesting key targets for catalyst design to drive targeted depolymerization of lignin.National Science Foundation (U.S.) (Grant ECCS1449291)Massachusetts Institute of Technology (Research Support Corporation Reed Grant)Burroughs Wellcome Fund (Career Award at the Scientific Interface
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