1,224 research outputs found
Dissociative recombination of NH: A revisited study
Dissociative recombination of NH is explored in a two-step
theoretical study. In a first step, a diatomic (1D) rough model with frozen NN
bond and frozen angles is adopted, in the framework of the multichannel quantum
defect theory (MQDT). The importance of the indirect mechanism and of the
bending mode is revealed, in spite of the disagreement between our cross
section and the experimental one. In a second step, we use our recently
elaborated 3D approach based on the normal mode approximation combined with
R-matrix theory and MQDT. This approach results in satisfactory agreement with
storage-ring measurements, significantly better at very low energy than the
former calculations.Comment: 9 pages, 5 figures, 1 tabl
Electronic structure of the candidate 2D Dirac semimetal SrMnSb2: a combined experimental and theoretical study
SrMnSb is suggested to be a magnetic topological semimetal. It contains
square, 2D Sb planes with non-symmorphic crystal symmetries that could protect
band crossings, offering the possibility of a quasi-2D, robust Dirac semi-metal
in the form of a stable, bulk (3D) crystal. Here, we report a combined and
comprehensive experimental and theoretical investigation of the electronic
structure of SrMnSb, including the first ARPES data on this compound.
SrMnSb possesses a small Fermi surface originating from highly 2D, sharp
and linearly dispersing bands (the Y-states) around the (0,/a)-point in
-space. The ARPES Fermi surface agrees perfectly with that from
bulk-sensitive Shubnikov de Haas data from the same crystals, proving the
Ystates to be responsible for electrical conductivity in SrMnSb. DFT and
tight binding (TB) methods are used to model the electronic states, and both
show good agreement with the ARPES data. Despite the great promise of the
latter, both theory approaches show the Y-states to be gapped above E,
suggesting trivial topology. Subsequent analysis within both theory approaches
shows the Berry phase to be zero, indicating the non-topological character of
the transport in SrMnSb, a conclusion backed up by the analysis of the
quantum oscillation data from our crystals.Comment: 26 pages, 10 figures, revised submission to SciPost after including
changes requested by referees. All referee reports are open and can be viewed
here: https://scipost.org/submissions/1711.07165v2
The extremely sharp transition between molecular and ionized gas in the Horsehead nebula
(Abridged) Massive stars can determine the evolution of molecular clouds with
their strong ultraviolet (UV) radiation fields. Moreover, UV radiation is
relevant in setting the thermal gas pressure in star-forming clouds, whose
influence can extend from the rims of molecular clouds to entire star-forming
galaxies. Probing the fundamental structure of nearby molecular clouds is
therefore crucial to understand how massive stars shape their surrounding
medium and how fast molecular clouds are destroyed, specifically at their
UV-illuminated edges, where models predict an intermediate zone of neutral
atomic gas between the molecular cloud and the surrounding ionized gas whose
size is directly related to the exposed physical conditions. We present the
highest angular resolution (~", corresponding to au) and
velocity-resolved images of the molecular gas emission in the Horsehead nebula,
using CO J=3-2 and HCO J=4-3 observations with ALMA. We find that CO and
HCO are present at the edge of the cloud, very close to the ionization
(H/H) and dissociation fronts (H/H), suggesting a very thin layer of
neutral atomic gas (< au) and a small amount of CO-dark gas
( mag) for stellar UV illumination conditions typical of
molecular clouds in the Milky Way. The new ALMA observations reveal a web of
molecular gas filaments with an estimated thermal gas pressure of
K cm, and the presence of a
steep density gradient at the cloud edge that can be well explained by
stationary isobaric PDR models with pressures consistent with our estimations.
However, in the HII region and PDR interface, we find , suggesting the gas is slightly compressed. Therefore,
dynamical effects cannot be completely ruled out and even higher angular
observations will be needed to unveil their role.Comment: 15 pages, 1 table, 9 figures; Accepted for publication in A&
Fibromuscular Arterial Disease
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75604/1/j.1440-1673.1970.tb01633.x.pd
Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)
HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
We present 5 deg^2 (~250 pc^2) HCN, HNC, HCO+, and CO J=1-0 maps of the Orion
B GMC, complemented with existing wide-field [CI] 492 GHz maps, as well as new
pointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines.
We detect anomalous HCN J=1-0 hyperfine structure line emission almost
everywhere in the cloud. About 70% of the total HCN J=1-0 luminosity arises
from gas at A_V < 8 mag. The HCN/CO J=1-0 line intensity ratio shows a bimodal
behavior with an inflection point at A_V < 3 mag typical of translucent gas and
UV-illuminated cloud edges. We find that most of the HCN J=1-0 emission arises
from extended gas with n(H2) < 10^4 cm^-3, even lower density gas if the
ionization fraction is > 10^-5 and electron excitation dominates. This result
explains the low-A_V branch of the HCN/CO J=1-0 intensity ratio distribution.
Indeed, the highest HCN/CO ratios (~0.1) at A_V < 3 mag correspond to regions
of high [CI] 492 GHz/CO J=1-0 intensity ratios (>1) characteristic of
low-density PDRs. Enhanced FUV radiation favors the formation and excitation of
HCN on large scales, not only in dense star-forming clumps. The low surface
brightness HCN and HCO+ J=1-0 emission scale with I_FIR (a proxy of the stellar
FUV radiation field) in a similar way. Together with CO J=1-0, these lines
respond to increasing I_FIR up to G0~20. On the other hand, the bright HCN
J=1-0 emission from dense gas in star-forming clumps weakly responds to I_FIR
once the FUV radiation field becomes too intense (G0>1500). The different power
law scalings (produced by different chemistries, densities, and line excitation
regimes) in a single but spatially resolved GMC resemble the variety of
Kennicutt-Schmidt law indexes found in galaxy averages. As a corollary for
extragalactic studies, we conclude that high HCN/CO J=1-0 line intensity ratios
do not always imply the presence of dense gas.Comment: accepted for publication in A&A. 24 pages, 18 figures, plus Appendix.
Abridged Abstract. English language not edite
Pt-Sn/C as a possible methanol-tolerant cathode catalyst for DMFC
An effective method was developed for preparing highly dispersed nano-sized PtâSn/C electrocatalyst synthesised by a modified polyol reduction method. From XRD patterns, the PtâSn/C peaks shifted slightly to lower 2Ξ angles when compared with commercial Pt/C catalyst, suggesting that Sn formed alloy with Pt. Based on HR-TEM images, the PtâSn/C nanoparticles showed small particle sizes and well dispersed onto the carbon support with a narrow particle distribution. The methanol oxidation reaction on the as-prepared PtâSn/C catalyst appeared at lower currents (+7.08 mA at +480 mV vs. Ag/AgCl) compared to the commercial Pt/C (+8.25 mA at +480 mV vs. Ag/AgCl) suggesting that the PtâSn/C catalyst has âmethanol tolerance capabilitiesâ. PtâSn/C HA Slurry pH3 catalysts showed better activity towards the oxygen-reduction reaction (ORR) than commercial Pt/C which could be attributed to smaller particle sizes. In our study, the PtâSn/C catalyst appears to be a promising methanol-tolerant catalyst with activity towards the ORR in the DMFC.Web of Scienc
Recommended from our members
A Search for MeV to TeV Neutrinos from Fast Radio Bursts with IceCube
We present two searches for IceCube neutrino events coincident with 28 fast radio bursts (FRBs) and 1 repeating FRB. The first improves on a previous IceCube analysis - searching for spatial and temporal correlation of events with FRBs at energies greater than roughly 50 GeV - by increasing the effective area by an order of magnitude. The second is a search for temporal correlation of MeV neutrino events with FRBs. No significant correlation is found in either search; therefore, we set upper limits on the time-integrated neutrino flux emitted by FRBs for a range of emission timescales less than one day. These are the first limits on FRB neutrino emission at the MeV scale, and the limits set at higher energies are an order-of-magnitude improvement over those set by any neutrino telescope
Recommended from our members
Efficient propagation of systematic uncertainties from calibration to analysis with the SnowStorm method in IceCube
Efficient treatment of systematic uncertainties that depend on a large number of nuisance parameters is a persistent difficulty in particle physics and astrophysics experiments. Where low-level effects are not amenable to simple parameterization or re-weighting, analyses often rely on discrete simulation sets to quantify the effects of nuisance parameters on key analysis observables. Such methods may become computationally untenable for analyses requiring high statistics Monte Carlo with a large number of nuisance degrees of freedom, especially in cases where these degrees of freedom parameterize the shape of a continuous distribution. In this paper we present a method for treating systematic uncertainties in a computationally efficient and comprehensive manner using a single simulation set with multiple and continuously varied nuisance parameters. This method is demonstrated for the case of the depth-dependent effective dust distribution within the IceCube Neutrino Telescope
Recommended from our members
Search for sources of astrophysical neutrinos using seven years of icecube cascade events
Low-background searches for astrophysical neutrino sources anywhere in the sky can be performed using cascade events induced by neutrinos of all flavors interacting in IceCube with energies as low as âŒ1 TeV. Previously we showed that, even with just two years of data, the resulting sensitivity to sources in the southern sky is competitive with IceCube and ANTARES analyses using muon tracks induced by charge current muon neutrino interactions - especially if the neutrino emission follows a soft energy spectrum or originates from an extended angular region. Here, we extend that work by adding five more years of data, significantly improving the cascade angular resolution, and including tests for point-like or diffuse Galactic emission to which this data set is particularly well suited. For many of the signal candidates considered, this analysis is the most sensitive of any experiment to date. No significant clustering was observed, and thus many of the resulting constraints are the most stringent to date. In this paper we will describe the improvements introduced in this analysis and discuss our results in the context of other recent work in neutrino astronomy
- âŠ