Ambipolar Diffusion in the Magnetorotational Instability

The effects of ambipolar diffusion on the linear stability of weakly ionised accretion discs are examined. Earlier work on this topic has focused on axial magnetic fields and perturbation wavenumbers. We consider here more general field and wavenumber geometries, and find that qualitatively new results are obtained. Provided a radial wavenumber and azimuthal field are present along with their axial counterparts, ambipolar diffusion will always be destabilising, with unstable local modes appearing at well-defined wavenumber bands. The wavenumber corresponding to the maximum growth rate need not, in general, lie along the vertical axis. Growth rates become small relative to the local angular velocity when the ion-neutral collision time exceeds the orbital time. In common with Hall electromotive forces, ambipolar diffusion destabilises both positive and negative angular velocity gradients. In at least some cases, therefore, uniformly rotating molecular cloud cores may reflect the marginally stable state of the ambipolar magnetorotational instability.Comment: Submitted to MN, 6 pages, 3 figs, MN style file v2.

The dynamics of inner dead-zone boundaries in protoplanetary disks

In protoplanetary disks, the inner radial boundary between the MRI turbulent (`active') and MRI quiescent (`dead') zones plays an important role in models of the disk evolution and in some planet formation scenarios. In reality, this boundary is not well-defined: thermal heating from the star in a passive disk yields a transition radius close to the star (<0.1 au), whereas if the disk is already MRI active, it can self-consistently maintain the requisite temperatures out to a transition radius of roughly 1 au. Moreover, the interface may not be static; it may be highly fluctuating or else unstable. In this paper, we study a reduced model of the dynamics of the active/dead zone interface that mimics several important aspects of a real disk system. We find that MRI-transition fronts propagate inward (a `dead front' suppressing the MRI) if they are initially at the larger transition radius, or propagate outward (an `active front' igniting the MRI) if starting from the smaller transition radius. In both cases, the front stalls at a well-defined intermediate radius, where it remains in a quasi-static equilibrium. We propose that it is this new, intermediate stalling radius that functions as the true boundary between the active and dead zones in protoplanetary disks. These dynamics are likely implicated in observations of variable accretion, such as FU Ori outbursts, as well as in those planet formation theories that require the accumulation of solid material at the dead/active interface.Comment: 16 pages, 10 figures; MNRAS accepted; v3 final correction

Resistive double-diffusive instability in the dead-zones of protostellar disks

We outline a novel linear instability that may arise in the dead-zones of protostellar disks, and possibly the fluid interiors of planets and protoplanets. In essence it is an axisymmetric buoyancy instability, but one that would not be present in a purely hydrodynamical gas. The necessary ingredients for growth include a negative radial entropy gradient (of any magnitude), weak magnetic fields, and efficient resistive diffusion (in comparison with thermal diffusion). The character of the instability is local, axisymmetric, and double-diffusive, and it attacks lengths much shorter than the resistive scale. Like the axisymmetric convective instability, it draws its energy from the negative radial entropy gradient; but by utilising the diffusing magnetic field, it can negate the stabilising influence of rotation. Its nonlinear saturated state, while not transporting appreciable angular momentum, could drive radial and vertical mixing, which may influence the temperature structure of the disk, dust dynamics and, potentially, planet formation.Comment: 16 pages, 5 figures. MNRAS Accepted. V2: cosmetic changes to bring in line with MNRAS versio

A Self-Consistent Reduced Model for Dusty Magnetorotationally Unstable Discs

The interaction between settling of dust grains and magnetorotational instability (MRI) turbulence in protoplanetary disks is analyzed. We use a reduced system of coupled ordinary differential equations to represent the interaction between the diffusion of grains and the inhibition of the MRI. The coupled equations are styled on a Landau equation for the turbulence and a Fokker-Planck equation for the diffusion. The turbulence-grain interaction is probably most relevant near the outer edge of the disk's quiescent, or "dead" zone. Settling is most pronounced near the midplane, where a high dust concentration can self-consistently suppress the MRI. Under certain conditions, however, grains can reach high altitudes, a result of some observational interest. Finally, we show that the equilibrium solutions are linearly stable.Comment: 8 pages, 3 figures, accepted to MNRA

Gender relations and couple negotiations of British men's food practice changes after prostate cancer.

Nutrition plays an important role in the health of men diagnosed with prostate cancer and dietary interventions can therefore be a significant part of prostate cancer survivorship supportive care. Family food provision, however, involves complex social interactions, which shape how men engage with their diets and dietary interventions. The role that gender plays in shaping prostate cancer couples' food practices and men's diets after a prostate cancer diagnosis is thought to be important but is little understood. This study explored couples' accounts of nutrition information seeking and diet change to gain a better understanding of how gender relations shaped men's food practices after prostate cancer diagnosis. Qualitative health interviews with men and their partners were conducted and analysed using interpretive descriptive methods. Findings demonstrated how couples navigated food change journeys that involved seeking information, deciding what changes were warranted and implementing and regulating diet changes. Two overarching themes that illustrated couples' food negotiations were called 'Seeking information and deciding on food changes' and 'Monitoring food changes'. Additional sub-themes described who led food changes, women's filtering of information, and moderation or 'treats'. Throughout these food change journeys interactions between men and women were at play, demonstrating how gender relations and dynamics acted to shape couples food negotiations and men's food practices. Findings reveal that attention to gender relations and the men's family food dynamics should inform diet interventions for men with prostate cancer in order to improve uptake

The ionization fraction in alpha-models of protoplanetary disks

We calculate the ionization fraction of protostellar alpha disks, taking into account vertical temperature structure, and the possible presence of trace metal atoms. Both thermal and X-ray ionization are considered. Previous investigations of layered disks used radial power-law models with isothermal vertical structure. But alpha models are used to model accretion, and the present work is a step towards a self-consistent treatment. The extent of the magnetically uncoupled (``dead'') zone depends sensitively on alpha, on the assumed accretion rate, and on the critical magnetic Reynolds number, below which MHD turbulence cannot be self-sustained. Its extent is extremely model-dependent. It is also shown that a tiny fraction of the cosmic abundance of metal atoms can dramatically affect the ionization balance. Gravitational instabilities are an unpromising source of transport, except in the early stages of disk formation.Comment: 25 pages including 8 figures, Latex in the MN style - Accepted by MNRA

Nutrition and Physical Activity in British Army Officer Cadet Training Part 2 - Daily Distribution of Energy and Macronutrient Intake

Dietary intake and physical activity impact performance and adaptation during training. The aims of this study were to compare energy and macronutrient intake during British Army Officer Cadet training with dietary guidelines and describe daily distribution of energy and macronutrient intake and estimated energy expenditure (EE). Thirteen participants (seven women) were monitored during three discrete periods of military training for nine days on-camp (CAMP), five days’ field exercise (FEX) and nine days of a mixture of the two (MIX). Dietary intake was measured using researcher-led food weighing and food diaries and EE was estimated from wrist-worn accelerometers. Energy intake was below guidelines for men (4600kcal·d-1) and women (3500kcal·d-1) during CAMP (men: -16%; women -9%), FEX (men: -33%; women: -42%) and MIX (men and women both -34%). Carbohydrate intake of men and women were below guidelines (6g·kg·d-1) during CAMP (men: -10%; women: -9%), FEX (men: -18%; women: -37%), and MIX (men: -3%; women: -39%), respectively. Protein intake was above guidelines (1.2kcal·kg·d-1) for men and women during CAMP (men: 48%; women: 39%) and MIX (men: 9%; women: 3%), but below guidelines during FEX (men: -13%; women: -27%). Energy and macronutrient intake during CAMP centred around mealtimes with a discernible sleep/wake cycle for EE. During FEX, energy and macronutrient intake were individually variable and EE was high throughout the day and night. These findings could be used to inform evidenced-based interventions to change the amount and timing of energy and macronutrient intake around physical activity to optimise performance and adaptations during military training

Reconstructing Disturbances and Their Biogeochemical Consequences over Multiple Timescales

Ongoing changes in disturbance regimes are predicted to cause acute changes in ecosystem structure and function in the coming decades, but many aspects of these predictions are uncertain. A key challenge is to improve the predictability of postdisturbance biogeochemical trajectories at the ecosystem level. Ecosystem ecologists and paleoecologists have generated complementary data sets about disturbance (type, severity, frequency) and ecosystem response (net primary productivity, nutrient cycling) spanning decadal to millennial timescales. Here, we take the first steps toward a full integration of these data sets by reviewing how disturbances are reconstructed using dendrochronological and sedimentary archives and by summarizing the conceptual frameworks for carbon, nitrogen, and hydrologic responses to disturbances. Key research priorities include further development of paleoecological techniques that reconstruct both disturbances and terrestrial ecosystem dynamics. In addition, mechanistic detail from disturbance experiments, long-term observations, and chronosequences can help increase the understanding of ecosystem resilienc

Reconstructing Disturbances and Their Biogeochemical Consequences over Multiple Timescales

Ongoing changes in disturbance regimes are predicted to cause acute changes in ecosystem structure and function in the coming decades, but many aspects of these predictions are uncertain. A key challenge is to improve the predictability of postdisturbance biogeochemical trajectories at the ecosystem level. Ecosystem ecologists and paleoecologists have generated complementary data sets about disturbance (type, severity, frequency) and ecosystem response (net primary productivity, nutrient cycling) spanning decadal to millennial timescales. Here, we take the first steps toward a full integration of these data sets by reviewing how disturbances are reconstructed using dendrochronological and sedimentary archives and by summarizing the conceptual frameworks for carbon, nitrogen, and hydrologic responses to disturbances. Key research priorities include further development of paleoecological techniques that reconstruct both disturbances and terrestrial ecosystem dynamics. In addition, mechanistic detail from disturbance experiments, long-term observations, and chronosequences can help increase the understanding of ecosystem resilience

Unusual shock-excited oh maser emission in a young Planetary Nebula

© 2016. The American Astronomical Society. All rights reserved. We report on OH maser emission toward G336.644-0.695 (IRAS 16333-4807), which is a H2O maser-emitting Planetary Nebula (PN). We have detected 1612, 1667, and 1720 MHz OH masers at two epochs using the Australia Telescope Compact Array, hereby confirming it as the seventh known case of an OH-maser-emitting PN. This is only the second known PN showing 1720 MHz OH masers after K 3-35 and the only evolved stellar object with 1720 MHz OH masers as the strongest transition. This PN is one of a group of very young PNe. The 1612 MHz and 1667 MHz masers are at a similar velocity to the 22 GHz H2O masers, whereas the 1720 MHz masers show a variable spectrum, with several components spread over a higher velocity range (up to 36 km s-1). We also detect Zeeman splitting in the 1720 MHz transition at two epochs (with field strengths of ~2 to ~10 mG), which suggests the OH emission at 1720 MHz is formed in a magnetized environment. These 1720 MHz OH masers may trace short-lived equatorial ejections during the formation of the PN