We present an implementation of the Hall term in the non-ideal magnetohydrodynamics equations into the adaptive-mesh-refinement code RAMSES to study its impact on star formation. Recent works show ...that the Hall effect heavily influences the regulation of the angular momentum in collapsing dense cores, strengthening or weakening the magnetic braking. Our method consists of a modification of the two-dimensional constrained transport scheme. Our scheme shows convergence of second order in space and the frequency of the propagation of whistler waves is accurate. We confirm previous results, namely that during the collapse, the Hall effect generates a rotation of the fluid with a direction in the mid-plane that depends on the sign of the Hall resistivity, while counter-rotating envelopes develop on each side of the mid-plane. However, we find that the predictability of our numerical results is severely limited. The angular momentum is not conserved in any of our dense core-collapse simulations with the Hall effect: a large amount of angular momentum is generated within the first Larson core, a few hundred years after its formation, without compensation by the surrounding gas. This issue is not mentioned in previous studies and may be correlated to the formation of the accretion shock on the Larson core. We expect that this numerical effect could be a serious issue in star formation simulations.
We develop a detailed chemical network relevant to calculate the conditions that are characteristic of prestellar core collapse. We solve the system of time-dependent differential equations to ...calculate the equilibrium abundances of molecules and dust grains, with a size distribution given by size-bins for these latter. These abundances are used to compute the different non-ideal magneto-hydrodynamics resistivities (ambipolar, Ohmic and Hall), needed to carry out simulations of protostellar collapse. For the first time in this context, we take into account the evaporation of the grains, the thermal ionisation of potassium, sodium, and hydrogen at high temperature, and the thermionic emission of grains in the chemical network, and we explore the impact of various cosmic ray ionisation rates. All these processes significantly affect the non-ideal magneto-hydrodynamics resistivities, which will modify the dynamics of the collapse. Ambipolar diffusion and Hall effect dominate at low densities, up to nH = 1012 cm-3, after which Ohmic diffusion takes over. We find that the time-scale needed to reach chemical equilibrium is always shorter than the typical dynamical (free fall) one. This allows us to build a large, multi-dimensional multi-species equilibrium abundance table over a large temperature, density and ionisation rate ranges. This table, which we make accessible to the community, is used during first and second prestellar core collapse calculations to compute the non-ideal magneto-hydrodynamics resistivities, yielding a consistent dynamical-chemical description of this process.
We present here a minor modification of our numerical implementation of the Hall effect for the 2D Riemann solver used in Constrained Transport schemes, as described in a former paper. In the ...previous work, the tests showed that the angular momentum was not conserved during protostellar collapse simulations, with significant impact. By removing the whistler waves speed from the characteristic speeds of non-magnetic variables in the 1D Riemann solver, we were able to improve the angular momentum conservation in our test-case by one order of magnitude, while keeping the second-order numerical convergence of the scheme. We also reproduce the simulations of a previous study with consistent resistivities, the three non-ideal magnetohydrodynamic effects and initial rotation, and agree with their results. In this case, the violation of angular momentum conservation is negligible in regard to the total angular momentum and the angular momentum of the disk.
Context.
The degree of coupling between the gas and the magnetic field during the collapse of a core and the subsequent formation of a disk depends on the assumed dust size distribution.
Aims.
We ...study the impact of grain–grain coagulation on the evolution of magnetohydrodynamic (MHD) resistivities during the collapse of a prestellar core.
Methods.
We use a 1D model to follow the evolution of the dust size distribution, out-of-equilibrium ionisation state, and gas chemistry during the collapse of a prestellar core. To compute the grain–grain collisional rate, we consider models for both random and systematic, size-dependent, velocities. We include grain growth through grain–grain coagulation and ice accretion, but ignore grain fragmentation.
Results.
Starting with a Mathis-Rumpl-Nordsieck (MRN) size distribution (Mathis et al. 1977, ApJ, 217, 425), we find that coagulation in grain–grain collisions generated by hydrodynamical turbulence is not efficient at removing the smallest grains and, as a consequence, does not have a large effect on the evolution of the Hall and ambipolar diffusion MHD resistivities, which still drop significantly during the collapse like in models without coagulation. The inclusion of systematic velocities, possibly induced by the presence of ambipolar diffusion, increases the coagulation rate between small and large grains, removing small grains earlier in the collapse and therefore limiting the drop in the Hall and ambipolar diffusion resistivities. At intermediate densities (
n
H
~ 10
8
cm
−3
), the Hall and ambipolar diffusion resistivities are found to be higher by 1 to 2 orders of magnitude in models with coagulation than in models where coagulation is ignored, and also higher than in a toy model without coagulation where all grains smaller than 0.1
μ
m would have been removed in the parent cloud before the collapse.
Conclusions.
When grain drift velocities induced by ambipolar diffusion are included, dust coagulation happening during the collapse of a prestellar core starting from an initial MRN dust size distribution appears to be efficient enough to increase the MHD resistivities to the values necessary to strongly modify the magnetically regulated formation of a planet-forming disk. A consistent treatment of the competition between fragmentation and coagulation is, however, necessary before reaching firm conclusions.
A single Anopheles dirus mosquito carrying sporozoites of Plasmodium knowlesi, P. falciparum, and P. vivax was recently discovered in Khanh Phu, southern Vietnam. Further sampling of humans and ...mosquitoes in this area during 2009-2010 showed P. knowlesi infections in 32 (26%) persons with malaria (n = 125) and in 31 (43%) sporozoite-positive An. dirus mosquitoes (n = 73). Co-infections of P. knowlesi and P. vivax were predominant in mosquitoes and humans, while single P. knowlesi infections were found only in mosquitoes. P. knowlesi-co-infected patients were largely asymptomatic and were concentrated among ethnic minority families who commonly spend nights in the forest. P. knowlesi carriers were significantly younger than those infected with other malaria parasite species. These results imply that even if human malaria could be eliminated, forests that harbor An. dirus mosquitoes and macaque monkeys will remain a reservoir for the zoonotic transmission of P. knowlesi.
In the present contribution, we consider Helmholtz equation with material coefficients being constant in each subdomain of a geometric partition of the propagation medium (discarding the presence of ...junctions), and we are interested in the numerical solution of such a problem by means of local multi-trace boundary integral formulations (local-MTF). For a one dimensional problem and configurations with two subdomains, it has been recently established that applying a Jacobi iterative solver to local-MTF is exactly equivalent to an Optimised Schwarz Method (OSM) with a non-local impedance. In the present contribution, we show that this correspondence still holds in the case where the subdomain partition involves an arbitrary number of subdomains. From this, we deduce that the depth of the adjacency graph of the subdomain partition plays a critical role in the convergence of linear solvers applied to local-MTF: we prove it for the case of homogeneous propagation medium and show, through numerical evidences, that this conclusion still holds for heterogeneous media. Our study also shows that, considering variants of local-MTF involving a relaxation parameter, there is a fixed value of this relaxation parameter that systematically leads to optimal speed of convergence for linear solvers.
Current contamination levels of selected legacy, currently-used and emerging halogenated contaminants were monitored in marine shellfish along French coastlines. The studied contaminants included ...polybrominated diphenyl ethers (PBDEs), hexabromocyclododecanes (HBCDDs), 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE), decabromodiphenylethane (DBDPE), hexabromobenzene (HBB), 2,2′,4,4′,5,5′-hexabromobiphenyl (BB-153) and perfluorinated compounds (PFCs).BDE-47, BDE-209, BTBPE, HBB and α-HBCDD were detected in 100% of the analyzed samples, whereas BB-153, DBDPE and PFOS were detected at frequencies of 97%, 90% and 55%, respectively. Concentrations were in the pg g−1 ww range and varied as follows: PFOS > BDE-47 ∼ α-HBCDD > BDE-209 > BTBPE ∼ DBDPE > HBB ∼ BB-153. Overall, non-PBDE Brominated Flame Retardants (BFRs) revealed concentrations between 3 and 59 times lower than those of PBDEs.PBDE pattern was dominated by BDE-47, followed by BDE-99 > BDE-100 > BDE-49 > BDE-209 > BDE-154; these 6 congeners represented 94% of the summed ten PBDEs. PFC pattern determination revealed PFOS as the predominant PFC in samples from the English Channel and Atlantic, whereas perfluorocarboxylic acids (PFCAs) prevailed in Mediterranean samples. Temporal trend investigations on archived samples from the Mediterranean coast collected between 1981 and 2012 showed a prevalence of PFOS until 1998; PFCAs subsequently increased and became more abundant than PFOS. High levels of PFCAs were observed until 2008, followed by a decrease and stabilization in 2010–2012. Amongst PFCAs, perfluorotridecanoic acid (PFTrDA) and perfluoroundecanoic acid (PFUnA) were predominant and exhibited similar time trends, suggesting similar sources at the investigated site, home to major industrial activity.
Monoclinic VO
2
nanoparticles are of interest due to the material's thermochromic properties, however, direct synthesis routes to VO
2
nanoparticles are often inaccessible due to the high synthesis ...temperatures or long reaction times required. Herein, we present a two-step synthesis route for the preparation of monoclinic VO
2
nanoparticles using Continuous Hydrothermal Flow Synthesis (CHFS) followed by a short post heat treatment step. A range of particle sizes, dependent on synthesis conditions, were produced from 50 to 200 nm by varying reaction temperatures and the residence times in the process. The nanoparticles were characterised by powder X-ray diffraction, Raman and UV/Vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The nanoparticles were highly crystalline with rod and sphere-like morphologies present in TEM micrographs, with the size of both the rod and spherical particles being highly dependent on both reaction temperature and residence time. SEM micrographs showed the surface of the powders produced from the CHFS process to be highly uniform. The samples were given a short post synthesis heat treatment to ensure that they were phase pure monoclinic VO
2
, which led to them exhibiting a large and reversible switch in optical properties (at near-IR wavelengths), which suggests that if such materials can be incorporated into coatings or in composites, they could be used for fenestration in architectural applications.
The synthesis of thermochromic VO
2
nanoparticles, with the ability to tailor the particle size and shape, is reported from a continuous hydrothermal synthetic route.
We consider GMRES applied to discretisations of the high-frequency Helmholtz equation with strong trapping; recall that in this situation the problem is exponentially ill-conditioned through an ...increasing sequence of frequencies. Our main focus is on boundary-integral-equation formulations of the exterior Dirichlet and Neumann obstacle problems in 2- and 3-d. Under certain assumptions about the distribution of the eigenvalues of the integral operators, we prove upper bounds on how the number of GMRES iterations grows with the frequency; we then investigate numerically the sharpness (in terms of dependence on frequency) of
both
our bounds
and
various quantities entering our bounds. This paper is therefore the first comprehensive study of the frequency-dependence of the number of GMRES iterations for Helmholtz boundary-integral equations under trapping.
► Levels and trends of HBCDs and PFCs were studied in the French coastal environment. ► Results show widespread contamination of shellfish by α-HBCD and PFOS. ► Contamination by α-HBCD increased ...exponentially between 1981 and 2011. ► PFOS levels underwent a significant linear decrease over time.
The levels and congener patterns of HBCDs (hexabromocyclododecanes) and PFCs (perfluorinated compounds) were determined in filter-feeding molluscs collected in 2008 and 2010 along the coasts of mainland France. α-HBCD and PFOS (perfluorooctane sulfonate) were detected in all samples, revealing widespread contamination of the coastal environment by these emerging contaminants. The spatial distribution of Σ-HBCD concentrations showed higher median levels in samples from the Mediterranean Sea and English Channel respectively, i.e. 0.19ngg−1 wet weight (ww) and 0.08ngg−1 ww, related to high anthropogenic pressure from urban and industrial activities, while the median concentration was 0.05ngg−1 ww in samples from the Atlantic coast. Among PFCs, PFOS was the only compound detected in all samples and PFDA (perfluorodecanoic acid) was the second most frequently-detected compound. PFOS median concentrations were 0.18ngg−1 ww, 0.09ngg−1 ww and 0.04ngg−1 ww in samples from the English Channel, the Atlantic coast and the Mediterranean coast respectively. The highest PFOS concentration was found in the Loire estuary, possibly related to local industrial activities. The Mediterranean samples showed a different pattern, with predominant long-chain PFCAs (perfluorocarboxylic acids), suggesting the presence of alternative sources on the Mediterranean coast.
The temporal trends studied in archived samples from the Seine estuary site showed a significant exponential increase in HBCD concentrations between 1981 and 2011, with a doubling time of 7years, while PFOS levels underwent a significant linear decrease over time. These trends are coherent with current regulations on the use of these compounds. The results presented in this paper provide the first data on the contamination of the French coastal marine environment by the selected emerging compounds, and constitute a reference for the future monitoring of French coastal contamination by emerging contaminants.
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