1,721 research outputs found
Characterizing the transition from diffuse atomic to dense molecular clouds in the Magellanic clouds with [CII], [CI], and CO
We present and analyze deep Herschel/HIFI observations of the [CII] 158um,
[CI] 609um, and [CI] 370um lines towards 54 lines-of-sight (LOS) in the Large
and Small Magellanic clouds. These observations are used to determine the
physical conditions of the line--emitting gas, which we use to study the
transition from atomic to molecular gas and from C^+ to C^0 to CO in their low
metallicity environments. We trace gas with molecular fractions in the range
0.1<f(H2)<1, between those in the diffuse H2 gas detected by UV absorption
(f(H2)<0.2) and well shielded regions in which hydrogen is essentially
completely molecular. The C^0 and CO column densities are only measurable in
regions with molecular fractions f(H2)>0.45 in both the LMC and SMC. Ionized
carbon is the dominant gas-phase form of this element that is associated with
molecular gas, with C^0 and CO representing a small fraction, implying that
most (89% in the LMC and 77% in the SMC) of the molecular gas in our sample is
CO-dark H2. The mean X_CO conversion factors in our LMC and SMC sample are
larger than the value typically found in the Milky Way. When applying a
correction based on the filling factor of the CO emission, we find that the
values of X_CO in the LMC and SMC are closer to that in the Milky Way. The
observed [CII] intensity in our sample represents about 1% of the total
far-infrared intensity from the LOSs observed in both Magellanic Clouds.Comment: 32 pages, 21 figures, Accepted to Ap
A boron-oxygen transborylation strategy for a catalytic Midland reduction
The enantioselective hydroboration of ketones is a textbook reaction requiring stoichiometric amounts of an enantioenriched borane, with the Midland reduction being a seminal example. Here, a turnover strategy for asymmetric catalysis, boron.oxygen transborylation, has been developed and used to transform the stoichiometric borane reagents of the Midland reduction into catalysts. This turnover strategy was demonstrated by the enantioselective reduction of ketones, including derivatives of biologically active molecules and those containing reducible groups. The enantioenriched borane catalyst was generated in situ from commercially available reagents, 9-borabicyclo[3.3.1]nonane (H-B-9-BBN) and β-pinene, and B.O transborylation with pinacolborane (HBpin) was used for catalytic turnover. Mechanistic studies indicated that B.O transborylation proceeded by B.O/B.H boron exchange through a stereoretentive, concerted transition state, resembling σ-bond metathesis
Stable Carbon Isotope Signature of Methane Released From Phytoplankton
Unidad de excelencia María de Maeztu CEX2019-000940-MAquatic ecosystems play an important role in global methane cycling and many field studies have reported methane supersaturation in the oxic surface mixed layer (SML) of the ocean and in the epilimnion of lakes. The origin of methane formed under oxic condition is hotly debated and several pathways have recently been offered to explain the "methane paradox." In this context, stable isotope measurements have been applied to constrain methane sources in supersaturated oxygenated waters. Here we present stable carbon isotope signatures for six widespread marine phytoplankton species, three haptophyte algae and three cyanobacteria, incubated under laboratory conditions. The observed isotopic patterns implicate that methane formed by phytoplankton might be clearly distinguished from methane produced by methanogenic archaea. Comparing results from phytoplankton experiments with isotopic data from field measurements, suggests that algal and cyanobacterial populations may contribute substantially to methane formationobserved in the SML of oceans and lakes
Chaotic behavior and damage spreading in the Glauber Ising model - a master equation approach
We investigate the sensitivity of the time evolution of a kinetic Ising model
with Glauber dynamics against the initial conditions. To do so we apply the
"damage spreading" method, i.e., we study the simultaneous evolution of two
identical systems subjected to the same thermal noise. We derive a master
equation for the joint probability distribution of the two systems. We then
solve this master equation within an effective-field approximation which goes
beyond the usual mean-field approximation by retaining the fluctuations though
in a quite simplistic manner. The resulting effective-field theory is applied
to different physical situations. It is used to analyze the fixed points of the
master equation and their stability and to identify regular and chaotic phases
of the Glauber Ising model. We also discuss the relation of our results to
directed percolation.Comment: 9 pages RevTeX, 4 EPS figure
Conductance and density of states as the Kramers-Kronig dispersion relation
By applying the Kramers-Kronig dispersion relation to the transmission
amplitude a direct connection of the conductance with the density of states is
given in quantum scattering systems connected to two one-channel leads.
Using this method we show that in the Fano resonance the peak position of the
density of states is generally different from the position of the corresponding
conductance peak, whereas in the Breit-Wigner resonance those peak positions
coincide.
The lineshapes of the density of states are well described by a Lorentz type
in the both resonances.
These results are verified by another approach using a specific form of the
scattering matrix to describe scattering resonances.Comment: 9 pages, 4 figure
Synthesis and SAR evaluation of coumarin derivatives as potent cannabinoid receptor agonists
We report the development and extensive structure-activity relationship evaluation of a series of modified coumarins as cannabinoid receptor ligands. In radioligand, and [S-35]GTP gamma S binding assays the CB receptor binding affinities and efficacies of the new ligands were determined. Furthermore, we used a ligand-based docking approach to validate the empirical observed results. In conclusion, several crucial structural requirements were identified. The most potent coumarins like 3-butyl-7-(1-butylcyclopentyl)-5-hydroxy-2H-chromen-2-one (36b, K-i CB2 13.7 nM, EC50 18 nM), 7-(1-butylcyclohexyl)-5-hydroxy-3-propyl-2H-chromen-2-one (39b, K-i CB2 6.5 nM, EC50 4.51 nM) showed a CB2 selective agonistic profile with low nanomolar affinities. (C) 2021 Published by Elsevier Masson SAS.Peer reviewe
Rigid Chiral Membranes
Statistical ensembles of flexible two-dimensional fluid membranes arise
naturally in the description of many physical systems. Typically one encounters
such systems in a regime of low tension but high stiffness against bending,
which is just the opposite of the regime described by the Polyakov string. We
study a class of couplings between membrane shape and in-plane order which
break 3-space parity invariance. Remarkably there is only {\it one} such
allowed coupling (up to boundary terms); this term will be present for any
lipid bilayer composed of tilted chiral molecules. We calculate the
renormalization-group behavior of this relevant coupling in a simplified model
and show how thermal fluctuations effectively reduce it in the infrared.Comment: 11 pages, UPR-518T (This replaced version has fonts not used
removed.
Magnetic and Dynamic Properties of the Hubbard Model in Infinite Dimensions
An essentially exact solution of the infinite dimensional Hubbard model is
made possible by using a self-consistent mapping of the Hubbard model in this
limit to an effective single impurity Anderson model. Solving the latter with
quantum Monte Carlo procedures enables us to obtain exact results for the one
and two-particle properties of the infinite dimensional Hubbard model. In
particular we find antiferromagnetism and a pseudogap in the single-particle
density of states for sufficiently large values of the intrasite Coulomb
interaction at half filling. Both the antiferromagnetic phase and the
insulating phase above the N\'eel temperature are found to be quickly
suppressed on doping. The latter is replaced by a heavy electron metal with a
quasiparticle mass strongly dependent on doping as soon as . At half
filling the antiferromagnetic phase boundary agrees surprisingly well in shape
and order of magnitude with results for the three dimensional Hubbard model.Comment: 32 page
Stress Propagation through Frictionless Granular Material
We examine the network of forces to be expected in a static assembly of hard,
frictionless spherical beads of random sizes, such as a colloidal glass. Such
an assembly is minimally connected: the ratio of constraint equations to
contact forces approaches unity for a large assembly. However, the bead
positions in a finite subregion of the assembly are underdetermined. Thus to
maintain equilibrium, half of the exterior contact forces are determined by the
other half. We argue that the transmission of force may be regarded as
unidirectional, in contrast to the transmission of force in an elastic
material. Specializing to sequentially deposited beads, we show that forces on
a given buried bead can be uniquely specified in terms of forces involving more
recently added beads. We derive equations for the transmission of stress
averaged over scales much larger than a single bead. This derivation requires
the Ansatz that statistical fluctuations of the forces are independent of
fluctuations of the contact geometry. Under this Ansatz, the
-component stress field can be expressed in terms of a d-component
vector field. The procedure may be generalized to non-sequential packings. In
two dimensions, the stress propagates according to a wave equation, as
postulated in recent work elsewhere. We demonstrate similar wave-like
propagation in higher dimensions, assuming that the packing geometry has
uniaxial symmetry. In macroscopic granular materials we argue that our approach
may be useful even though grains have friction and are not packed
sequentially.=17Comment: 15 pages, 4 figures, revised vertion for Phys. Rev.
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