4,147 research outputs found
The standstill luminosity in Z Cam systems
We consider accretion discs in close binary systems. We show that heating of
a disc at the impact point of the accretion stream contributes significantly to
the local energy budget at its outer edge. As a result the thermal balance
relation between local accretion rate and surface density (the `S-curve')
changes; the critical mass transfer rate above which no dwarf nova outbursts
occur can be up to 40% smaller than without impact heating. Standstills in ZCam
systems thus occur at smaller mass transfer rates than otherwise expected, and
are rather fainter than the peak luminosity during the dwarf nova phase as a
result.Comment: 3 pages, 3 figures. Accepted for publication in MNRA
Line width distributions as evidence for axisymmetry in the broad line regions of active galaxies
The nuclei of a wide class of active galaxies emit broad emission lines with
widths at half maximum (FWHM) in the range km s. This
spread of widths is not solely a consequence of the range of the luminosities
of these sources since a plot of width versus luminosity shows a large scatter.
We propose that the broad line emission region (BLR) is axially symmetric and
that this scatter in line width arises from an additional dependence on the
angle of the line of sight to the axis of the emission region. Such a relation
is natural in unified models of active nuclei which link a variety of observed
properties to viewing angle. Adopting a simple form for the line width as a
function of luminosity and angle, and convolving this with the observed
luminosity function, allows us to predict a line width distribution consistent
with the available data. Furthermore, we use the relation between the
equivalent width of a line and the luminosity in the continuum (the `Baldwin
Effect') to predict an observed correlation between line width and equivalent
width. The scatter on this correlation is again provided by angular dependence.
The results have applications as diagnostics of models of the broad line
emission region and in cosmology.Comment: 8 pages including 4 figures. Accepted for publication in MNRAS
Letter
The luminosity dependence of opening angle in unified models of active galaxies
In unified models of active galaxies the direct line of sight to the nucleus
is unobscured only within a certain cone of directions. An opening angle for
this cone is usually estimated by methods such as the overall ratio of Seyfert
1s to Seyfert2s, the latter assumed to be obscured versions of the former. Here
we shall show, as has often been suspected, that the opening angle of the cone
depends on the luminosity of the central source, with higher luminosities
corresponding to larger opening angles. This conclusion depends only on the
assumption that the width of the broad emission lines at a given luminosity is
a measure of inclination angle, an assumption that is supported by observation
in radio-loud systems. On the other hand we show that the scatter in X-ray
spectral index is not primarily an effect of viewing angle, in contrast to what
might be expected if the scatter on the spectral index versus luminosity
relation were a consequence of absorption in the obscuring material. The
observed correlation between linewidth and spectral index appears to be a
further consequence of the dependence of opening angle on luminosity.Comment: 8 pages, 7 figures, uses mn.sty. Accepted for publication in MNRA
Counting quantum jumps: a summary and comparison of fixed-time and fluctuating-time statistics in electron transport
In quantum transport through nanoscale devices, fluctuations arise from
various sources: the discreteness of charge carriers, the statistical
non-equilibrium that is required for device operation, and unavoidable quantum
uncertainty. As experimental techniques have improved over the last decade,
measurements of these fluctuations have become available.} They have been
accompanied by a plethora of theoretical literature using many different
fluctuation statistics to describe the quantum transport. In this paper, we
overview three prominent fluctuation statistics: full counting, waiting time,
and first-passage time statistics. We discuss their weaknesses and strengths,
and explain connections between them in terms of renewal theory. In particular,
we discuss how different information can be encoded in different statistics
when the transport is non-renewal, and how this behavior manifests in the
measured physical quantities of open quantum systems. All theoretical results
are illustrated via a demonstrative transport scenario: a Markovian master
equation for a molecular electronic junction with electron-phonon interactions.
{{} We demonstrate that to obtain non-renewal behavior, and thus to have
temporal correlations between successive electron tunneling events, there must
be a strong coupling between tunneling electrons and out-of-equilibrium
quantized molecular vibrations.Comment: 24 pages, 8 figure
Determining the cosmological parameters from the linewidths of active galaxies
We have previously shown that the linewidth distribution in AGN can be
accounted for by an axisymmetric broad emission line region. In this paper we
show that the linewidth distribution changes with redshift and that these
changes are dependent on H_0 and q_0. We show that relatively small samples of
AGN at high redshift with measured linewidth at half maximum can be used to
distinguish between values of H_0 and q_0. Furthermore larger low redshift
samples can be used to distinguish between luminosity functions and hence
different models of quasar evolution.Comment: Accepted for publication in MNRAS. 8 pages LaTeX, uses mn.st
The energetics of melting fertile heterogeneities within the depleted mantle
To explore the consequences of mantle heterogeneity for primary melt production, we develop a
mathematical model of energy conservation for an upwelling, melting body of recycled oceanic crust
embedded in the depleted upper mantle. We consider the endâmember geometric cases of spherical blobs
and tabular veins. The model predicts that thermal diffusion into the heterogeneity can cause a factorâofâ
two increase in the degree of melting for bodies with minimum dimension smaller than âŒ1 km, yielding
melt fractions between 50 and 80%. The role of diffusion is quantified by an appropriately defined Peclet
number, which represents the balance of diffusionâdriven and adiabatic melting. At intermediate Peclet
number, we show that melting a heterogeneity can cool the ambient mantle by up to âŒ20 K (spherical)
or âŒ60 K (tabular) within a distance of two times the characteristic size of the body. At small Peclet
number, where heterogeneities are expected to be in thermal equilibrium with the ambient mantle, we
calculate the energetic effect of pyroxenite melting on the surrounding peridotite; we find that each 5%
of recycled oceanic crust diminishes the peridotite degree of melting by 1â2%. Injection of the magma from
highly molten bodies of recycled oceanic crust into a melting region of depleted upper mantle may nucleate
reactiveâdissolution channels that remain chemically isolated from the surrounding peridotite
Investigation into intermodulation distortion in HEMTs using a quasi-2-D physical model
The need for both linear and efficient pseudomorphic high electron-mobility transistors (pHEMTs) for modern wireless handsets necessitates a thorough understanding of the origins of intermodulation distortion at the device level. For the first time, the dynamic large-signal internal physical behavior of a pHEMT is examined using a quasi-two-dimensional physical device model.
The model accounts fully for device-circuit interaction and is validated experimentally for a two-tone experiment around 5 GHz
An investigation of the basement complex aquifer system in Lofa county, Liberia, for the purpose of siting boreholes
Liberia is recovering from a 14 year civil war and only 51% of the rural population has access to safe drinking water. Little hydrogeological knowledge survives in Liberia, increasing the difficulty in successfully siting new boreholes. An understanding of the local hydrogeological environment is therefore needed to improve borehole site selection and increase success rates. This research provides a semi-quantitative characterization of the hydrogeological environment of the basement aquifer in Lofa county, Liberia. Based on literature review and analysis of borehole logs, the study has developed a conceptual hydrogeological model for the local conditions, which is further characterized using 2D geoelectrical sections. Groundwater is predominantly obtained from the saprolite and underlying fractured bedrock, but specific capacities (median 281 l h-1 m-1; 25th and 75th percentile of 179 and 490 l h-1 m-1, respectively) are constrained by the limited thickness of the saturated saprolite. This study has shown that the groundwater resources in the crystalline basement in this part of Liberia conform to the general conceptual model, allowing standard techniques used elsewhere for siting and developing groundwater to be used
Analytical solutions of compacting flow past a sphere
AbstractA series of analytical solutions are presented for viscous compacting flow past a rigid impermeable sphere. The sphere is surrounded by a two-phase medium consisting of a viscously deformable solid matrix skeleton through which a low-viscosity liquid melt can percolate. The flow of the two-phase medium is described by McKenzieâs compaction equations, which combine Darcy flow of the liquid melt with Stokes flow of the solid matrix. The analytical solutions are found using an extension of the PapkovichâNeuber technique for Stokes flow. Solutions are presented for the three components of linear flow past a sphere: translation, rotation and straining flow. FaxĂ©n laws for the force, torque and stresslet on a rigid sphere in an arbitrary compacting flow are derived. The analytical solutions provide instantaneous solutions to the compaction equations in a uniform medium, but can also be used to numerically calculate an approximate evolution of the porosity over time whilst the porosity variations remain small. These solutions will be useful for interpreting the results of deformation experiments on partially molten rocks.This work was support by NERC standard grant NE/I023929/1.This is the author accepted manuscript. The final version is available from Cambridge University Press via https://doi.org/10.1017/jfm.2014.10
The ionization of hydrogen and of hydrogenic positive ions by electron impact
Ionization of hydrogen and hydrogenic positive ions by electron impac
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