9,863 research outputs found
From cellular properties to population asymptotics in the Population Balance Equation
Proliferating cell populations at steady state growth often exhibit broad
protein distributions with exponential tails. The sources of this variation and
its universality are of much theoretical interest. Here we address the problem
by asymptotic analysis of the Population Balance Equation. We show that the
steady state distribution tail is determined by a combination of protein
production and cell division and is insensitive to other model details. Under
general conditions this tail is exponential with a dependence on parameters
consistent with experiment. We discuss the conditions for this effect to be
dominant over other sources of variation and the relation to experiments.Comment: Exact solution of Eq. 9 is adde
The volume change due to F centres in alkali halides
The change in volume due to F centres in alkali halides is calculated using the ion-size correction method of Bartram, Stoneham and Gash (1968). In all cases the volume change Delta V is positive (expansion of the crystal) and satisfactory agreement with experiment is found in the two cases where measurements are available. The fractional volume change ( Delta V/a 3 ) decreases systematically with increasing anion or cation size
The methanol lines and hot core of OMC2-FIR4, an intermediate-mass protostar, with Herschel/HIFI
In contrast with numerous studies on the physical and chemical structure of low- and high-mass protostars, much less is known about their intermediate-mass counterparts, a class of objects that could help to elucidate the mechanisms of star formation on both ends of the mass range. We present the first results from a rich HIFI spectral dataset on an intermediate-mass protostar, OMC2-FIR4, obtained in the CHESS (Chemical HErschel Survey of Star forming regions) key programme. The more than 100 methanol lines detected between 554 and 961 GHz cover a range in upper level energy of 40 to 540 K. Our physical interpretation focusses on the hot core, but likely the cold envelope and shocked regions also play a role in reality, because an analysis of the line profiles suggests the presence of multiple emission components. An upper limit of 10^(-6) is placed on the methanol abundance in the hot core, using a population diagram, large-scale source model and other considerations. This value is consistent with abundances previously seen in low-mass hot cores. Furthermore, the highest energy lines at the highest frequencies display asymmetric profiles, which may arise from infall around the hot core
Herschel/HIFI discovery of interstellar chloronium (H_2Cl^+)
We report the first detection of chloronium, H_2Cl^+, in the interstellar medium, using the HIFI instrument aboard the Herschel Space Observatory.
The 2_(12)−1_(01) lines of ortho-H^(35)_2 Cl^+ and ortho-H^(37)_2 Cl^+ are detected in absorption towards NGC 6334I, and the 1_(11)−0_(00) transition of para-H^(35)_2 Cl^+ is
detected in absorption towards NGC 6334I and Sgr B2(S). The H_2Cl^+ column densities are compared to those of the chemically-related species
HCl. The derived HCl/H_2Cl^+ column density ratios, ~1–10, are within the range predicted by models of diffuse and dense photon dominated
regions (PDRs). However, the observed H_2Cl^+ column densities, in excess of 10^(13) cm^(−2), are significantly higher than the model predictions. Our
observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints
for astrochemical models
Nitrogen hydrides in interstellar gas: Herschel/HIFI observations towards G10.6-0.4 (W31C)
The HIFI instrument on board the Herschel Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards
G10.6−0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH N = 1 ← 0,
J = 2 ← 1 and ortho-NH_2 1_(1,1) ← 0_(0,0). We also observed ortho-NH_3 1_0 ← 0_0, and 2_0 ← 1_0, para-NH_3 2_1 ← 1_1, and searched unsuccessfully for
NH^+. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material
over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total
column densities over the velocity range 11−54 km s^(−1) are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen,
approximately 6 × 10^(−9), 3 × 10^(−9), and 3 × 10^(−9) for NH, NH_2, and NH_3, respectively. These abundances are discussed with reference to models of
gas-phase and surface chemistry
Reversible and non-reversible Markov Chain Monte Carlo algorithms for reservoir simulation problems
We compare numerically the performance of reversible and non-reversible
Markov Chain Monte Carlo algorithms for high dimensional oil reservoir
problems; because of the nature of the problem at hand, the target measures
from which we sample are supported on bounded domains. We compare two
strategies to deal with bounded domains, namely reflecting proposals off the
boundary and rejecting them when they fall outside of the domain. We observe
that for complex high dimensional problems reflection mechanisms outperform
rejection approaches and that the advantage of introducing non-reversibility in
the Markov Chain employed for sampling is more and more visible as the
dimension of the parameter space increases
Detection of interstellar oxidaniumyl: Abundant H_2O^+ towards the star-forming regions DR21, Sgr B2, and NGC6334
Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin.
Methods. The characteristic hyperfine pattern of the H_2O^+ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H_2O^+ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation.
Results. In DR21, the velocity distribution of H_2O^+ matches that of the [C_(II)] line at 158 μm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H_(II)-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H_2O^+ column density of 7.2 × 10^(12) cm^(−2) in NGC 6334, 2.3 × 10^(13) cm^(−2) in DR21, and 1.1 × 10^(15) cm^(−2) in Sgr B2
Orbital assembly and maintenance study
The requirements, conceptual design, tradeoffs, procedures, and techniques for orbital assembly of the support structure of the microwave power transmission system and the radio astronomy telescope are described. Thermal and stress analyses, packaging, alignment, and subsystems requirements are included along with manned vs. automated and transportation tradeoffs. Technical and operational concepts for the manned and automated maintenance of satellites were investigated and further developed results are presented
Orbital assembly and maintenance study. Executive summary
A sound, practical approach for the assembly and maintenance of very large structures in space is presented. The methods and approaches for assembling two large structures are examined. The maintenance objectives include the investigation of methods to maintain five geosynchronous satellites. The two assembly examples are a 200-meter-diameter radio astronomy telescope and a 1,000-meter-diameter microwave power transmission system. The radio astronomy telescope operates at an 8,000-mile altitude and receives RF signals from space. The microwave power transmission system is part of a solar power satellite that will be used to transmit converted solar energy to microwave ground receivers. Illustrations are included
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