13,367 research outputs found
OH emission from cometary knots in planetary nebulae
We model the molecular emission from cometary knots in planetary nebulae
(PNe) using a combination of photoionization and photodissociation region (PDR)
codes, for a range of central star properties and gas densities. Without the
inclusion of ionizing extreme ultraviolet (EUV) radiation, our models require
central star temperatures to be near the upper limit of the range
investigated in order to match observed H and OH surface brightnesses
consistent with observations - with the addition of EUV flux, our models
reproduce observed OH surface brightnesses for .
For , the predicted OH surface brightness is much
lower, consistent with the non-detection of this molecule in PNe with such
central star temperatures. Our predicted level of H emission is somewhat
weaker than commonly observed in PNe, which may be resolved by the inclusion of
shock heating or fluorescence due to UV photons. Some of our models also
predict ArH and HeH rotational line emission above detection
thresholds, despite neither molecule having been detected in PNe, although the
inclusion of photodissociation by EUV photons, which is neglected by our
models, would be expected to reduce their detectability.Comment: Accepted by MNRAS, 11 pages, 15 figures. Author accepted manuscript.
Accepted on 24/04/18. Deposited on 27/04/1
Modelling the ArH emission from the Crab Nebula
We have performed combined photoionization and photodissociation region (PDR)
modelling of a Crab Nebula filament subjected to the synchrotron radiation from
the central pulsar wind nebula, and to a high flux of charged particles; a
greatly enhanced cosmic ray ionization rate over the standard interstellar
value, , is required to account for the lack of detected [C I]
emission in published Herschel SPIRE FTS observations of the Crab Nebula. The
observed line surface brightness ratios of the OH and ArH transitions
seen in the SPIRE FTS frequency range can only be explained with both a high
cosmic ray ionization rate and a reduced ArH dissociative recombination
rate compared to that used by previous authors, although consistent with
experimental upper limits. We find that the ArH/OH line strengths and
the observed H vibration-rotation emission can be reproduced by model
filaments with cm,
and visual extinctions within the range found for dusty globules in the Crab
Nebula, although far-infrared emission from [O I] and [C II] is higher than the
observational constraints. Models with cm
underpredict the H surface brightness, but agree with the ArH and
OH surface brightnesses and predict [O I] and [C II] line ratios consistent
with observations. These models predict HeH rotational emission above
detection thresholds, but consideration of the formation timescale suggests
that the abundance of this molecule in the Crab Nebula should be lower than the
equilibrium values obtained in our analysis.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on
05/09/2017. Deposited on 05/09/1
Development of an integrated cognitive behavioral therapy for anxiety and opioid use disorder: Study protocol and methods
Opioid use disorder is a highly disabling psychiatric disorder, and is associated with both significant functional disruption and risk for negative health outcomes such as infectious disease and fatal overdose. Even among those who receive evidence-based pharmacotherapy for opioid use disorder, many drop out of treatment or relapse, highlighting the importance of novel treatment strategies for this population. Over 60% of those with opioid use disorder also meet diagnostic criteria for an anxiety disorder; however, efficacious treatments for this common co-occurrence have not be established. This manuscript describes the rationale and methods for a behavioral treatment development study designed to develop and test an integrated cognitive-behavioral therapy for those with co-occurring opioid use disorder and anxiety disorders. The aims of the study are (1) to develop and pilot test a new manualized cognitive behavioral therapy for co-occurring opioid use disorder and anxiety disorders, (2) to test the efficacy of this treatment relative to an active comparison treatment that targets opioid use disorder alone, and (3) to investigate the role of stress reactivity in both prognosis and recovery from opioid use disorder and anxiety disorders. Our overarching aim is to investigate whether this new treatment improves both anxiety and opioid use disorder outcomes relative to standard treatment. Identifying optimal treatment strategies for this population are needed to improve outcomes among those with this highly disabling and life-threatening disorder.This study was funded by NIDA grant DA035297. The funding source had no involvement in the study design, analysis and interpretation of data, writing of the report, or the decision to submit the article for publication. (DA035297 - NIDA)Accepted manuscrip
The dimension of loop-erased random walk in 3D
We measure the fractal dimension of loop-erased random walk (LERW) in 3
dimensions, and estimate that it is 1.62400 +- 0.00005. LERW is closely related
to the uniform spanning tree and the abelian sandpile model. We simulated LERW
on both the cubic and face-centered cubic lattices; the corrections to scaling
are slightly smaller for the face-centered cubic lattice.Comment: 4 pages, 4 figures. v2 has more data, minor additional change
The mass, location and heating of the dust in the Cassiopeia A supernova remnant
We model the thermal dust emission from dust grains heated by synchrotron
radiation and by particle collisions, under conditions appropriate for four
different shocked and unshocked gas components of the Cassiopeia A (Cas A)
supernova remnant (SNR). By fitting the resulting spectral energy distributions
(SEDs) to the observed SNR dust fluxes, we determine the required mass of dust
in each component. We find the observed SED can be reproduced by of silicate grains, the majority of which is in the unshocked
ejecta and heated by the synchrotron radiation field. Warmer dust, located in
the X-ray emitting reverse shock and blastwave regions, contribute to the
shorter wavelength infrared emission but make only a small fraction of the
total dust mass. Carbon grains can at most make up of the total
dust mass. Combined with estimates for the gas masses, we obtain dust-to-gas
mass ratios for each component, which suggest that the condensation efficiency
in the ejecta is high, and that dust in the shocked ejecta clumps is well
protected from destruction by sputtering in the reverse shock.Comment: Accepted by MNRAS, 14 pages, 6 figures. Author accepted manuscript.
Accepted on 05/02/2019. Deposited on 05/02/201
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Tracer concentration profiles measured in central London as part of the REPARTEE campaign
There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent’s Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London
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Remarkable dynamics of nanoparticles in the urban atmosphere
Nanoparticles emitted from road traffic are the largest source of respiratory exposure for the general public living in urban areas. It has been suggested that the adverse health effects of airborne particles may scale with the airborne particle number, which if correct, focuses attention on the nanoparticle (less than 100 nm) size range which dominates the number count in urban areas. Urban measurements of particle size distributions have tended to show a broadly similar pattern dominated by a mode centred on 20–30 nm diameter particles emitted by diesel engine exhaust. In this paper we report the results of measurements of particle number concentration and size distribution made in a major London park as well as on the BT Tower, 160 m high. These measurements taken during the REPARTEE project (Regents Park and BT Tower experiment) show a remarkable shift in particle size distributions with major losses of the smallest particle class as particles are advected away from the traffic source. In the Park, the traffic related mode at 20–30 nm diameter is much reduced with a new mode at <10 nm. Size distribution measurements also revealed higher number concentrations of sub-50 nm particles at the BT Tower during days affected by higher turbulence as determined by Doppler Lidar measurements and indicate a loss of nanoparticles from air aged during less turbulent conditions. These results suggest that nanoparticles are lost by evaporation, rather than coagulation processes. The results have major implications for understanding the impacts of traffic-generated particulate matter on human health
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