44,473 research outputs found
On the Superradiance of Spin-1 Waves in an Equatorial Wedge around a Kerr Hole
Recently Van Putten has suggested that superradiance of magnetosonic waves in
a toroidal magnetosphere around a Kerr black hole may play a role in the
central engine of gamma-ray bursts. In this context, he computed (in the WKB
approximation) the superradiant amplification of scalar waves confined to a
thin equatorial wedge around a Kerr hole and found that the superradiance is
higher than for radiation incident over all angles. This paper presents
calculations of both spin-0 (scalar) superradiance (integrating the radial
equation rather than using the WKB method) and and spin-1
(electromagnetic/magnetosonic) superradiance, in Van Putten's wedge geometry.
In contrast to the scalar case, spin-1 superradiance decreases in the wedge
geometry, decreasing the likelihood of its astrophysical importance.Comment: Submitted to The Astrophysical Journal Letter
A review of knowledge: inter-row hoeing & its associated agronomy in organic cereal & pulse crops
The aim of this project was to establish the âstate of the artâ for inter-row hoeing and its associated agronomic practices in organic cereal and pulse crops. To achieve this a detailed review of literature was undertaken.
⢠To facilitate inter-row cultivation in cereal and pulse crops, some adjustment of row spacing may be required. For cereals, drilling crops in 25 cm rows can reduce yield compared with normal drilling practice, primarily due to greater intra-specific competition amongst the crop (i.e. competition between crop plants).
⢠The yield penalty resulting from widely spaced crop rows can be minimised using a number of approaches, depending on the drill:
1. Reducing the seed rate in widely spaced crop rows can help to minimise excessive intra-specific competition.
2. Band sowing the crop in wide rows can also help to minimise intra-specific competition as the seed is distributed over a greater area.
3. Using a twin-row arrangement can completely overcome the yield penalty.
⢠The recommended row spacing for peas (up to 20 cm) and beans (up to 35 cm) does not require any further adjustment for inter-row hoeing.
⢠Recent developments in automated guidance of inter-row hoeing equipment mean that weeding operations can now be conducted a much higher speeds (10 km h-1). This has highlighted the limitations of some of the cultivators currently used (e.g. âAâ blades), as excessive soil throw can occur at this high speed. Rolling cultivators may prove to be the most suitable at high forward speeds. For manually guided hoes working at slower speeds (5 km h-1), âAâ and âLâ blades offer an effective low cost solution.
⢠In terms of the timing of inter-row hoeing, it is suggested that weeding operations should be conducted at an early stage in the growing season, as the weeds that emerge with or shortly after the crop are the ones that pose the most significant threat for crop yield. Weeding on two occasions can provide better levels of weed control than weeding once, but weeding more frequently offered little additional benefit. Reductions of weed biomass of up to 99 % have been reported as a result of inter-row hoeing, although this has not always resulted in a positive crop yield response. This is probably due to crop damage resulting from inaccurate hoeing, a problem that can be overcome with automated guidance.
⢠There is some evidence to suggest that mechanical weeding operations can mineralise soil bound nitrogen.
⢠The impact of inter-row hoeing on ground nesting birds is uncertain. Early indications suggest that skylarks prefer to nest directly adjacent to or in the crop row rather than between rows.
The information contained within this review should enable farmers to make best use of inter-row hoeing in their arable crops.
There are a number of areas that require further research and development:
⢠The interaction of seed rate and row spacing needs to be confirmed in organic systems.
⢠Relatively little is known about the mechanisms of weed kill and the detailed interaction between the cultivator blade, the weed and the soil. This is particularly important with the new automated guidance equipment that allows weeding at high forward speeds.
⢠The timing and frequency of inter-row hoeing has received very little attention. The optimum weed control timings are based on small-plot crop:weed competition studies and need to be verified under field scale management with inter-row hoeing equipment.
⢠Finally, the impact of inter-row hoeing and widely spaced crop rows on ground-nesting birds has not been looked at directly, but is of importance.
Please see the main report for a more detailed summary before the full text
Triple mode Cepheid masses
Unconventional composition structures are proposed to explain the periods of the triple mode Cepheid aC And. A strong Cepheid wind appears to enrich helium in the convection zones down to about 60,000 K or 70,000 K. Then some downward partial mixing occurs to the bottom of a layer with about 1-q = .0005 of the stellar mass. It was found that AC And was not unlike anomalous Cepheids. However, masses of betwen one and two solar masses are suggested and the population is more likely a type two
Why Low-Mass Black-Hole Binaries Are Transient
We consider transient behavior in low-mass X-ray binaries. In short-period
neutron-star systems (orbital period less than ~ 1d) irradiation of the
accretion disk by the central source suppresses this except at very low mass
transfer rates. Formation constraints however imply that a significant fraction
of these neutron star systems have nuclear-evolved main-sequence secondaries
and thus mass transfer rates low enough to be transient. But most short-period
low-mass black-hole systems will form with unevolved main-sequence companions
and have much higher mass transfer rates. The fact that essentially all of them
are nevertheless transient shows that irradiation is weaker, as a direct
consequence of the fundamental black-hole property - the lack of a hard stellar
surface.Comment: 13 pages (including 3 figures); accepted for publication in Ap
A new instability of accretion disks around compact magnetic stars
Aperiodic variability and Quasi Periodic Oscillations (QPOs) are observed
from accretion disks orbiting white dwarfs, neutron stars, and black holes,
suggesting that the flow is universally broken up into discrete blobs. We
consider the interaction of these blobs with the magnetic field of a compact,
accreting star, where diamagnetic blobs suffer a drag. We show that when the
magnetic moment is not aligned with the spin axis, the resulting force is
pulsed, and this can lead to resonance with the oscillation of the blobs around
the equatorial plane; a resonance condition where energy is effectively pumped
into non--equatorial motions is then derived. We show that the same resonance
condition applies for the quadrupolar component of the magnetic field. We
discuss the conditions of applicability of this result, showing that they are
quite wide. We also show that realistic complications, such as chaotic magnetic
fields, buoyancy, radiation pressure, evaporation, Kelvin--Helmholtz
instability, and shear stresses due to differential rotation do not affect our
results. In accreting neutron stars with millisecond periods, we show that this
instability leads to Lense-Thirring precession of the blobs, and that damping
by viscosity can be neglected.Comment: Accepted for publication in the Astrophysical Journal. AASTeX LateX
needed. Two figure
The experiences of patients and carers in the daily management of care at the end of life
Background
Home is the preferred location for most people with an advanced disease and at the end of life. A variety of care professionals work in community settings to provide support to this population. Patients and their spouses, who also care for them (spouse-carers), are rarely accompanied by these sources of support at all times, and have to manage independently between their contact with care professionals.
Aim
To explore how patients and spouse-carers manage their involvement with care professionals in the community setting.
Method
Interpretive phenomenology informs the design of the research, whereby 16 interviews were conducted with the patients and spouse-carers. Interviews were recorded and transcribed verbatim. Data were analysed using phenomenological techniques including template analysis.
Findings
Patients and spouse-carers were interdependent and both parties played a role in co-ordinating care and managing relationships with professional care providers. The patients and spouse-carers actively made choices about how to manage their situation, and develop and modify managing strategies based on their experiences.
Conclusions
When daily management is effective and care professionals acknowledge the dyadic nature of the patient and spouse-carer relationship, people have confidence in living with advanced disease
Doing it differently: Engaging interview participants with imaginative variation
Imaginative variation was identified by Husserl (1936/1970) as a phenomenological technique for the purpose of elucidating the manner in which phenomena appear to consciousness. Briefly, by engaging in the phenomenological reduction and using imaginative variation, phenomenologists are able to describe the experience of consciousness, having stepped outside of the natural attitude through the epochÄ. Imaginative variation is a stage aimed at explicating the structures of experience, and is best described as a mental experiment. Features of the experience are imaginatively altered in order to view the phenomenon under investigation from varying perspectives. Husserl argued that this process will reveal the essences of an experience, as only those aspects that are invariant to the experience of the phenomenon will not be able to change through the variation.
Often in qualitative research interviews, participants struggle to articulate or verbalise their experiences. The purpose of this article is to detail a radical and novel way of using imaginative variation with interview participants, by asking the participants to engage with imaginative variation, in order to produce a rich and insightful experiential account of a phenomenon. We will discuss how the first author successfully used imaginative variation in this way in her study of the erotic experience of bondage, discipline, dominance & submission, and sadism & masochism (BDSM), before considering the usefulness of this technique when applied to areas of study beyond sexuality
Low angular momentum accretion in the collapsar: how long can a long GRB be?
The collapsar model is the most promising scenario to explain the huge
release of energy associated with long duration gamma-ray-bursts (GRBs). Within
this scenario GRBs are believed to be powered by accretion through a
rotationally support torus or by fast rotation of a compact object. In both
cases then, rotation of the progenitor star is one of the key properties
because it must be high enough for the torus to form, the compact object to
rotate very fast, or both. Here, we check what rotational properties a
progenitor star must have in order to sustain torus accretion over relatively
long activity periods as observed in most GRBs. We show that simple, often
cited, estimates of the total mass available for torus formation and
consequently the duration of a GRB are only upper limits. We revise these
estimates by taking into account the long term effect that as the compact
object accretes the minimum specific angular momentum needed for torus
formation increases. This in turn leads to a smaller fraction of the stellar
envelope that can form a torus. We demostrate that this effect can lead to a
significant, an order of magnidute, reduction of the total energy and overall
duration of a GRB event. This of course can be mitigated by assuming that the
progenitor star rotates faster then we assumed. However, our assumed rotation
is already high compared to observational and theoretical constraints. We also
discuss implications of our result.Comment: 29 pages, 10 figures, including 1 color fig., revised version
accepted by Ap
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