528 research outputs found
Excitation and Disruption of a Giant Molecular Cloud by the Supernova Remnant 3C391
Using the IRAM 30-m telescope, we observed the supernova remnant 3C 391
(G31.9+0.0) and its surroundings in the CO(2-1), HCO+(1-0), CS(2-1), CS(3-2),
and CS(5-4) lines. The ambient molecular gas at the distance (9 kpc) of the
remnant comprises a giant molecular cloud whose edge is closely parallel to a
ridge of bright non-thermal radio continuum, which evidently delineates the
blast-wave into the cloud. We found that in a small (0.6 pc) portion of the
radio shell, the molecular line profiles consist of a narrow (2 km/s)
component, plus a very wide (> 20 km/s) component. Both spectral components
peak within 20" of a previously-detected OH 1720 MHz maser. We name this source
3C 391:BML (broad molecular line); it provides a new laboratory, similar to IC
443 but on a larger scale, to study shock interactions with dense molecular
gas. The wide spectral component is relatively brighter in the
higher-excitation lines. We interpret the wide spectral component as post-shock
gas, either smoothly accelerated or partially dissociated and reformed behind
the shock. The narrow component is either the pre-shock gas or cold gas
reformed behind a fully dissociative shock. Using the 3 observed CS lines, we
measured the temperature, CS column density, and H2 volume density in a dense
clump in the parent molecular cloud as well as the wide-line and narrow-line
portions of the shocked clump. The physical conditions of the narrow-line gas
are comparable to the highest-density clumps in the giant molecular cloud,
while the wide-line gas is both warmer and denser. The mass of compressed gas
in 3C 391:BML is high enough that its self-gravity is significant, and
eventually it could form one or several stars
Cation-selective Mutations in the M2 Domain of the Inhibitory Glycine Receptor Channel Reveal Determinants of Ion-Charge Selectivity
Ligand-gated ion channel receptors mediate neuronal inhibition or excitation depending on their ion charge selectivity. An investigation into the determinants of ion charge selectivity of the anion-selective α1 homomeric glycine receptor (α1 glycine receptor [GlyR]) was undertaken using point mutations to residues lining the extra- and intracellular ends of the ion channel. Five mutant GlyRs were studied. A single substitution at the intracellular mouth of the channel (A-1′E GlyR) was sufficient to convert the channels to select cations over anions with PCl/PNa = 0.34. This result delimits the selectivity filter and provides evidence that electrostatic interactions between permeating ions and pore residues are a critical factor in ion charge selectivity. The P-2′Δ mutant GlyR retained its anion selectivity (PCl/PNa = 3.81), but it was much reduced compared with the wild-type (WT) GlyR (PCl/PNa = 27.9). When the A-1′E and the P-2′Δ mutations were combined (selectivity double mutant [SDM] GlyR), the relative cation permeability was enhanced (PCl/PNa = 0.13). The SDM GlyR was also Ca2+ permeable (PCa/PNa = 0.29). Neutralizing the extracellular mouth of the SDM GlyR ion channel (SDM+R19′A GlyR) produced a more Ca2+-permeable channel (PCa/PNa = 0.73), without drastically altering monovalent charge selectivity (PCl/PNa = 0.23). The SDM+R19′E GlyR, which introduces a negatively charged ring at the extracellular mouth of the channel, further enhanced Ca2+ permeability (PCa/PNa = 0.92), with little effect on monovalent selectivity (PCl/PNa = 0.19). Estimates of the minimum pore diameter of the A-1′E, SDM, SDM+R19′A, and SDM+R19′E GlyRs revealed that these pores are larger than the α1 GlyR, with the SDM-based GlyRs being comparable in diameter to the cation-selective nicotinic acetylcholine receptors. This result provides evidence that the diameter of the ion channel is also an important factor in ion charge selectivity
A Single P-loop Glutamate Point Mutation to either Lysine or Arginine Switches the Cation–Anion Selectivity of the CNGA2 Channel
Cyclic nucleotide-gated (CNG) channels play a critical role in olfactory and visual transduction. Site-directed mutagenesis and inside-out patch-clamp recordings were used to investigate ion permeation and selectivity in two mutant homomeric rat olfactory CNGA2 channels expressed in HEK293 cells. A single point mutation of the negatively charged pore loop (P-loop) glutamate (E342) to either a positively charged lysine or arginine resulted in functional channels, which consistently responded to cGMP, although the currents were generally extremely small. The concentration–response curve of the lysine mutant channel was very similar to that of wild-type (WT) channels, suggesting no major structural alteration to the mutant channels. Reversal potential measurements, during cytoplasmic NaCl dilutions, showed that the lysine and the arginine mutations switched the selectivity of the channel from cations (PCl/PNa = 0.07 [WT]) to anions (PCl/PNa = 14 [Lys] or 10 [Arg]). Relative anion permeability sequences for the two mutant channels, measured with bi-ionic substitutions, were NO3− > I− > Br− > Cl− > F− > acetate−, the same as those obtained for anion-selective GABA and glycine channels. The mutant channels also seem to have an extremely small single-channel conductance, measured using noise analysis of about 1–2 pS, compared to a WT value of about 29 pS. The results showed that it is predominantly the charge of the E342 residue in the P-loop, rather than the pore helix dipoles, which controls the cation–anion selectivity of this channel. However, the outward rectification displayed by both mutant channels in symmetrical NaCl solutions suggests that the negative ends of the pore helix dipoles may play a role in reducing the outward movement of Cl− ions through these anion-selective channels. These results have potential implications for the determinants of anion–cation selectivity in the large family of P-loop–containing channels
Analytic Perturbation Theory: A New Approach to the Analytic Continuation of the Strong Coupling Constant into the Timelike Region
The renormalization group applied to perturbation theory is ordinarily used
to define the running coupling constant in the spacelike region. However, to
describe processes with timelike momenta transfers, it is important to have a
self-consistent determination of the running coupling constant in the timelike
region. The technique called analytic perturbation theory (APT) allows a
consistent determination of this running coupling constant. The results are
found to disagree significantly with those obtained in the standard
perturbative approach. Comparison between the standard approach and APT is
carried out to two loops, and threshold matching in APT is applied in the
timelike region.Comment: 16 pages, REVTeX, 7 postscript figure
Post-Inflationary Reheating
We study a model for reheating that has been much investigated for parametric
resonance, having a quartic interaction of the scalar inflaton with another
scalar field. Attention is particularly on the quantum excitations of the
inflaton field and the metric perturbation with a smooth transition from
quantum to classical stochastic states, followed through from a specific
inflation model to a state including a relativistic fluid. The scalar fields
enter non-perturbatively but the metric enters perturbatively, and the validity
of this latter is assessed. In this model our work seems to point the large
scale curvature parameter changing.Comment: 25 pages, 6 figures. Coding error(misprint) corrected:figures and
some conclusions change
Cosmic microwave background and parametric resonance in reheating
The variation of the perturbative 3-curvature parameter, \zeta, is
investigated in the period of reheating after inflation. The two-field model
used has the inflaton, with an extra scalar field coupled to it, and non-linear
effects of both fields are included as well as a slow decay mechanism into the
hydrodynamic fluid of the radiation era. Changes in \zeta occur and persist
into the succeeding cosmic eras to influence the generation of the cosmic
microwave background fluctuations.Comment: 21 pages, 6 figures.Corrects misprinted formula and 2 number
Effects of Policies Designed to Keep Firearms from High-Risk Individuals
This article summarizes and critiques available evidence from studies published between 1999 and August 2014 on the effects of policies designed to keep firearms from high-risk individuals in the United States. Some prohibitions for high-risk individuals (e.g., those under domestic violence restraining orders, violent misdemeanants) and procedures for checking for more types of prohibiting conditions are associated with lower rates of violence. Certain laws intended to prevent prohibited persons from accessing firearms -- rigorous permit-to-purchase, comprehensive background checks, strong regulation and oversight of gun dealers, and requiring gun owners to promptly report lost or stolen firearms -- are negatively associated with the diversion of guns to criminals. Future research is needed to examine whether these laws curtail nonlethal gun violence and whether the effects of expanding prohibiting conditions for firearm possession are modified by the presence of policies to prevent diversion
Vector Meson Photoproduction with an Effective Lagrangian in the Quark Model
A quark model approach to the photoproduction of vector mesons off nucleons
is proposed. Its starting point is an effective Lagrangian of the interaction
between the vector meson and the quarks inside the baryon, which generates the
non-diffractive s- and u- channel resonance contributions. Additional t-channel
and exchanges are included for the and
production respectively to account for the large diffractive behavior in the
small region as suggested by Friman and Soyeur. The numerical results are
presented for the and productions in four isospin channels with
the same set of parameters, and they are in good agreement with the available
data not only in and productions but also in the charged
productions where the additional t-channel exchange does not
contribute so that it provides an important test to this approach. The
investigation is also extended to the photoproduction, and the initial
results show that the non-diffractive behavior of the productions in the
large region can be described by the s- and u- channel contributions with
significantly smaller coupling constants, which is consistent with the findings
in the similar studies in the QHD framework. The numerical investigation has
also shown that polarization observables are essential for identifying
so-called "missing resonances".Comment: 36 pages, 10 PS figures, extended version of nucl-th/9711061 and
nucl-th/9803021, submitted to PR
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