13,881 research outputs found
Two-Stream Instability of Counter-Rotating Galaxies
The present study of the two-stream instability in stellar disks with
counter-rotating components of stars and/or gas is stimulated by recently
discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream
instability of tightly-wrapped spiral waves is found for one and two-armed
waves with the pattern angular speed of the unstable waves always intermediate
between the angular speed of the co-rotating matter () and that of the
counter-rotating matter (). The instability arises from the
interaction of positive and negative energy modes in the co- and
counter-rotating components. The unstable waves are in general convective -
they move in radius and radial wavenumber space - with the result that
amplification of the advected wave is more important than the local growth
rate. For a galaxy of co-rotating stars and counter-rotating stars of
mass-fraction , or of counter-rotating gas of mass-fraction
, the largest amplification is usually for the one-armed
leading waves (with respect to the co-rotating stars). For the case of both
counter-rotating stars and gas, the largest amplifications are for , also for one-armed leading waves. The two-armed trailing
waves usually have smaller amplifications. The growth rates and amplifications
all decrease as the velocity spreads of the stars and/or gas increase. It is
suggested that the spiral waves can provide an effective viscosity for the gas
causing its accretion.Comment: 14 pages, submitted to ApJ. One table and 17 figures can be obtained
by sending address to R. Lovelace at [email protected]
Blinded patient preference for morphine compared to placebo in the setting of chronic refractory breathlessness – an exploratory study
Context Patients’ preference for morphine therapy has received little attention in the setting of chronic refractory breathlessness. However, this is one important factor in considering longer term therapy. Objectives The aim of this secondary analysis is to explore blinded patient preference of morphine compared to placebo for this indication and to define any predictors of preference. Methods Data were pooled from three randomized, double-blind, crossover, placebo-controlled studies of morphine (four days each) in chronic refractory breathlessness. Blinded patient preferences were chosen at the end of each study. A multivariable regression model was used to establish patient predictors of preference. Results Sixty-five participants provided sufficient data (60 males; median age 74 years; heart failure 55%, chronic obstructive pulmonary disease 45%; median Eastern Cooperative Oncology Group performance status 2). Forty-three percent of participants preferred morphine (32% placebo and 25% no preference). Morphine preference and younger age were strongly associated: odds ratio = 0.85, 95% confidence interval 0.78, 0.93;
Staff experiences of formulating within a team setting
This study evaluates psychology-led formulation sessions within an assessment and treatment service. Five staff members completed interviews exploring their experiences of formulation and their perception of its usefulness to clients. Results suggested that they perceived formulation to be beneficial on a number of levels for themselves and their practice but were uncertain about the tangible benefits for clients
Plasma oscillations in a spherical, magnetized, cold cathode discharge
Plasma oscillations in spherical, magnetized, cold cathode discharge tube
The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is There a Problem?
It is well known that the mass function for_halos_ in CDM cosmology is a
relatively steep power law for low masses, possibly too steep to be consistent
with observations. But how steep is the_galaxy_ mass function? We have analyzed
the stellar and gas mass functions of the first massive luminous objects formed
in a \Lambda CDM universe, as calculated in the numerical simulation described
in Gnedin (2000ab). We found that while the dark matter mass function is steep,
the stellar and gas mass functions are flatter for low mass objects. The
stellar mass function is consistently flat at the low mass end. Moreover, while
the gas mass function follows the dark matter mass function until reionization
at z~7, between z=7 and z=4, the gas mass function also flattens considerably
at the low mass end. At z=4, the gas and stellar mass functions are fit by a
Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the
dark matter halo mass function and consistent with some recent observations.
The baryonic mass functions are shallower because (a) the dark matter halo mass
function is consistent with the Press-Schechter formulation at low masses n(M)
M^-2 and (b) heating/cooling and ionization processes appear to cause baryons
to collect in halos with the relationship M_b M_d^4 at low masses. Combining
(a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus,
the well known observational fact that low mass galaxies are underabundant as
compared to expectations from numerical dark matter simulations or
Press-Schechter modeling of CDM universes emerges naturally from these results,
implying that perhaps no ``new physics'' beyond the standard model is needed.Comment: Submitted to ApJ, 17 pages including 6 figure
Is there a link between agricultural land-use management and flooding?
International audienceOver the past fifty years, significant changes in UK land use and management practices have occurred, driven by UK and EU agricultural policies. There is substantial evidence that modern land-use management practices have enhanced surface runoff generation at the local scale, frequently creating impacts through "muddy floods". Such local impacts can be avoided or mitigated through the adoption of better land management practices and/or small scale surface runoff control measures. There is little evidence that local scale changes in runoff generation propagate downstream to create impacts at the larger catchment scale. This does not imply that impacts do not exist, but the very few studies in which evidence has been sought have not produced any conclusive findings. Multiscale catchment experimentation, linked to new developments in modelling, is needed which can lead to a better understanding of how small scale changes to runoff generation propagate to larger catchment scales. To facilitate the tracking of changes from the local to the catchment scale, a new modelling approach is demonstrated which allows a downstream flood hydrograph to be mapped back onto its source areas, thus presenting impact information to users in a useful and comprehensible form
Axiomatic approach to radiation reaction of scalar point particles in curved spacetime
Several different methods have recently been proposed for calculating the
motion of a point particle coupled to a linearized gravitational field on a
curved background. These proposals are motivated by the hope that the point
particle system will accurately model certain astrophysical systems which are
promising candidates for observation by the new generation of gravitational
wave detectors. Because of its mathematical simplicity, the analogous system
consisting of a point particle coupled to a scalar field provides a useful
context in which to investigate these proposed methods. In this paper, we
generalize the axiomatic approach of Quinn and Wald in order to produce a
general expression for the self force on a point particle coupled to a scalar
field following an arbitrary trajectory on a curved background. Our equation
includes the leading order effects of the particle's own fields, commonly
referred to as ``self force'' or ``radiation reaction'' effects. We then
explore the equations of motion which follow from this expression in the
absence of non-scalar forces.Comment: 17 pages, 1 figur
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