16,680 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
Tolerance and confidence limits for classes of distributions based on failure rate
Tolerance and confidence limits for classes of distributions based on failure rate
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
A Quantitative Sequencing Framework for Absolute Abundance Measurements of Mucosal and Lumenal Microbial Communities
A fundamental goal in microbiome studies is determining which microbes affect host physiology. Standard methods for determining changes in microbial taxa measure relative, rather than absolute abundances. Moreover, studies often analyze only stool, despite microbial diversity differing substantially among gastrointestinal (GI) locations. Here, we develop a quantitative framework to measure absolute abundances of individual bacterial taxa by combining the precision of digital PCR with the high-throughput nature of 16S rRNA gene amplicon sequencing. In a murine ketogenic-diet study, we compare microbial loads in lumenal and mucosal samples along the GI tract. Quantitative measurements of absolute (but not relative) abundances reveal decreases in total microbial loads on the ketogenic diet and enable us to determine the differential effects of diet on each taxon in stool and small-intestine mucosa samples. This rigorous quantitative microbial analysis framework, appropriate for diverse GI locations enables mapping microbial biogeography of the mammalian GI tract and more accurate analyses of changes in microbial taxa in microbiome studies
Brownian motion on the Sierpinski carpet
We prove that, up to scalar multiples, there exists only one local regular
Dirichlet form on a generalized Sierpinski carpet that is invariant with
respect to the local symmetries of the carpet. Consequently for each such
fractal the law of Brownian motion is uniquely determined and the Laplacian is
well defined
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