Hydrodynamic modelling of accretion flows

In the proceedings of this, and of several recent close binary conferences, there have been several contributions describing smoothed particle hydrodynamics simulations of accretion disks. It is apposite therefore to review the numerical scheme itself with emphasis on its advantages for disk modelling, and the methods used for modelling viscous processes.Comment: 3 pages, to appear in proceedings of IAU Colloquium 194: Compact binaries in the galaxy and beyon

The effect of quantization on the FCIQMC sign problem

The sign problem in Full Configuration Interaction Quantum Monte Carlo (FCIQMC) without annihilation can be understood as an instability of the psi-particle population to the ground state of the matrix obtained by making all off-diagonal elements of the Hamiltonian negative. Such a matrix, and hence the sign problem, is basis dependent. In this paper we discuss the properties of a physically important basis choice: first versus second quantization. For a given choice of single-particle orbitals, we identify the conditions under which the fermion sign problem in the second quantized basis of antisymmetric Slater determinants is identical to the sign problem in the first quantized basis of unsymmetrized Hartree products. We also show that, when the two differ, the fermion sign problem is always less severe in the second quantized basis. This supports the idea that FCIQMC, even in the absence of annihilation, improves the sign problem relative to first quantized methods. Finally, we point out some theoretically interesting classes of Hamiltonians where first and second quantized sign problems differ, and others where they do not.Comment: 4 pages w/ 2 page appendix, 2 figures, 1 tabl

Discovery of disc precession in the M31 dipping X-ray binary Bo 158

We present results from three XMM-Newton observations of the M31 low mass X-ray binary XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004, July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-hr period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anti-correlated with source intensity. However, our new observations do not favour a strict intensity dependance, but rather suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio <~ 0.3 (period <~ 4 hr), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of ~11 P_orb, and prograde disc precession on a period of ~29 P_orb. This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on a 81+/-3 hr cycle.Comment: 9 pages, 6 figures, accepted for publication by MNRAS, changed conten

Riding a Spiral Wave: Numerical Simulation of Spiral Waves in a Co-Moving Frame of Reference

We describe an approach to numerical simulation of spiral waves dynamics of large spatial extent, using small computational grids.Comment: 15 pages, 14 figures, as accepted by Phys Rev E 2010/03/2

On the Growth of Al_2 O_3 Scales

Understanding the growth of Al2O3 scales requires knowledge of the details of the chemical reactions at the scale–gas and scale–metal interfaces, which in turn requires specifying how the creation/annihilation of O and Al vacancies occurs at these interfaces. The availability of the necessary electrons and holes to allow for such creation/annihilation is a crucial aspect of the scaling reaction. The electronic band structure of polycrystalline Al2O3 thus plays a decisive role in scale formation and is considered in detail, including the implications of a density functional theory (DFT) calculation of the band structure of a Σ7 View the MathML source bicrystal boundary, for which the atomic structure of the boundary was known from an independent DFT energy-minimization calculation and comparisons with an atomic-resolution transmission electron micrograph of the same boundary. DFT calculations of the formation energy of O and Al vacancies in bulk Al2O3 in various charge states as a function of the Fermi energy suggested that electronic conduction in Al2O3 scales most likely involves excitation of both electrons and holes, which are localized on singly charged O vacancies, View the MathML source and doubly charged Al vacancies, View the MathML source, respectively. We also consider the variation of the Fermi level across the scale and bending (“tilting”) of the conduction band minimum and valence band maximum due to the electric field developed during the scaling reaction. The band structure calculations suggest a new mechanism for the “reactive element” effect—a consequence of segregation of Y, Hf, etc., to grain boundaries in Al2O3 scales, which results in improved oxidation resistance—namely, that the effect is due to the modification of the near-band edge grain-boundary defect states rather than any blocking of diffusion pathways, as previously postulated. Secondly, Al2O3 scale formation is dominated by grain boundary as opposed to lattice diffusion, and there is unambiguous evidence for both O and Al countercurrent transport in Al2O3 scale-forming alloys. We postulate that such transport is mediated by migration of grain boundary disconnections containing charged jogs, rather than by jumping of isolated point defects in random high-angle grain boundaries

On the dynamics of a twisted disc immersed in a radiation field

We study the dynamics of a twisted tilted disc under the influence of an external radiation field. Assuming the effect of absorption and reemission/scattering is that a pressure is applied to the disc surface where the local optical depth is of order unity, we determine the response of the vertical structure and the influence it has on the possibility of instability to warping. We derive a pair of equations describing the evolution of a small tilt as a function of radius in the small amplitude regime that applies to both the diffusive and bending wave regimes. We also study the non linear vertical response of the disc numerically using an analogous one dimensional slab model. For global warps, we find that in order for the disc vertical structure to respond as a quasi uniform shift or tilt, as has been assumed in previous work, the product of the ratio of the external radiation momentum flux to the local disc mid plane pressure, where it is absorbed, with the disc aspect ratio should be significantly less than unity. Namely, this quantity should be of the order of or smaller than the ratio of the disc gas density corresponding to the layer intercepting radiation to the mid plane density, $\lambda \ll 1$. When this condition is not satisfied the disc surface tends to adjust so that the local normal becomes perpendicular to the radiation propagation direction. In this case dynamical quantities determined by the disc twist and warp tend to oscillate with a large characteristic period $T_{*}\sim \lambda^{-1}T_{K}$, where $T_{K}$ is some 'typical' orbital period of a gas element in the disc. The possibility of warping instability then becomes significantly reduced. In addition, when the vertical response is non uniform, the possible production of shocks may lead to an important dissipation mechanism.Comment: submitted to MNRA

Nitrogen partitioning and remobilization in relation to leaf senescence, grain yield and protein concentration in Indian wheat cultivars

© 2020 The Authors Nitrogen (N) fertilizer represents a significant cost for the grower and may also have environmental impacts through nitrate leaching and N2O (a greenhouse gas) emissions associated with denitrification. The objectives of this study were to quantify the genetic variability in N partitioning and N remobilization in Indian spring wheat cultivars and identify traits for improved grain yield and grain protein content for application in breeding N-efficient cultivars. Twenty-eight bread wheat cultivars and two durum wheat cultivars were tested in field experiments in two years in Maharashtra, India. Growth analysis was conducted at anthesis and harvest to assess above-ground dry matter (DM) and dry matter and N partitioning. Flag-leaf photosynthesis rate (Amax), flag-leaf senescence rate and canopy normalized difference vegetation index (NDVI) were also assessed. Significant N × genotype level interaction was observed for grain yield and N-use efficiency. There was a positive linear association between post-anthesis flag-leaf Amax and grain yield amongst the 30 genotypes under high N (HN) conditions. Flag-leaf Amax was positively associated with N uptake at anthesis (AGNA). Under both HN and low N (LN) conditions, higher N uptake at anthesis was associated with delayed onset of flag-leaf senescence and higher grain yield. Under N limitation, there was a genetic negative correlation between grain yield and grain protein concentration. Deviation from this negative relationship (grain protein deviation or GPD) was related to genotypic differences in post-anthesis N uptake. It is concluded that N uptake at anthesis was an important determinant of flag-leaf photosynthesis rate and grain yield under high N conditions; while post-anthesis N uptake was an important determinant of GPD of wheat grown under low to moderate N conditions in India

Discovery and modelling of disc precession in the M31 X-ray binary Bo 158?

The low mass X-ray binary (LMXB) associated with the M31 globular cluster Bo 158 is known to exhibit intensity dips on a ~2.78 hr period. This is due to obscuration of the X-ray source on the orbital period by material on the outer edge of the accretion disc. However, the depth of dipping varied from <10% to \~83% in three archival XMM-Newton observations of Bo 158. Previous work suggested that the dip depth was anticorrelated with the X-ray luminosity. However, we present results from three new XMM-Newton observations that suggest that the evolution of dipping is instead due to precession of the accretion disc. Such precession is expected in neutron star LMXBs with mass ratios <0.3 (i.e. with orbital periods <4 hr), such as the Galactic dipping LMXB 4U 1916-053. We simulated the accretion disc of Bo 158 using cutting-edge 3D smoothed particle hydrodynamics (SPH), and using the observed parameters. Our results show disc variability on two time-scales. The disc precesses in a prograde direction on a period of 81+/-3 hr. Also, a radiatively-driven disc warp is present in the inner disc, which undergoes retrograde precesson on a \~31 hr period. From the system geometry, we conclude that the dipping evolution is driven by the disc precession. Hence we predict that the dipping behaviour repeats on a ~81 hr cycle.Comment: 6 pages, 4 figures, to appear in proceedings "The X-ray Universe 2005", San Lorenzo de El Escoriale (Madrid, Spain), 26-30 September 200

The Effect of Drought on Phytochemical active Compounds Content in Chamomile and Yarrow

The effect of drought stress conditions on the phytochemical compounds content of German chamomile (Matricaia chamomilla L.), common yarrow and summer berries yarrow (Achillea millefolium L.) was investigated. Treatments contained drought stress 70% field capacity (FC) pots and control (well-watered 100% FC) pots under controlled glasshouse conditions. Freeze dried and ground leaves were extracted at room temperature using Shaking extraction with methanol (99.9%) as an efficient solvent in order to determine the phytochemical compositions of phenols and flavonoids from dried chamomile and yarrow leaves. Samples were then analysed using High performance liquid chromatography with ultra-violet detection (HPLC-UV). Five times sample collections were applied over leaf production period for each plant species. Results confirmed that these two plant species can produce a good mass production of phytochemical compounds content even when there is insufficient water in the soil. More phytochemically active compounds were identified in chamomile compared with yarrow. The main phenol and flavonoid compounds found were chlorogenic acid, caffeic acid, apig-7-glucoside, umbelliferon and luteolin. Total yields of chlorogenic acid, apig-7-glucoside and umbelliferon dominated over other compounds in chamomile, common yarrow and summer berries yarrow under both well-watered and drought stress conditions