1,140 research outputs found
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
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
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
Melt-band instabilities with two-phase damage
Deformation experiments on partially molten rocks in simple shear form melt bands at 20â—¦ to
the shear plane instead of at the expected 45â—¦ principal compressive stress direction. Thesemelt
bands may play an important role in melt focusing in mid-ocean ridges. Such shallow bands
are known to form for two-phase media under shear if strongly non-Newtonian power-law
creep is employed for the solid phase, or anisotropy imposed. However laboratory experiments
show that shallow bands occur regardless of creep mechanism, even in diffusion creep, which
is nominally Newtonian. Here we propose that a couple of forms of two-phase damage allow
for shallow melt bands even in diffusion creep.Support was provided by the National Science Foundation (NSF,
grant EAR-1015229), the Natural Environment Research Council
(NERC, grant NE/I023929/1) and Trinity College.This is the final published version. This article has been accepted for publication in Geophysical Journal International ©: 2015 the Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved
A mechanism for mode selection in melt band instabilities
The deformation of partially molten mantle in tectonic environments can lead to exotic structures, which potentially affect both melt and plate-boundary focussing. Examples of such structures are found in laboratory deformation experiments on partially molten rocks. Simple-shear and torsion experiments demonstrate the formation of concentrated melt bands at angles of around 20° to the shear plane. The melt bands form in the experiments with widths between a few to tens of microns, and a band spacing roughly an order of magnitude larger. Existing compaction theories, however, cannot predict this band width structure, let alone any mode selection, since they infer the fastest growing instability to occur for wavelengths or bands of vanishing width. Here, we propose that surface tension in the mixture, especially on a diffuse interface in the limit of sharp melt-fraction gradients, can mitigate the instability at vanishing wavelength and thus permit mode selection for finite-width bands. Indeed, the expected weak capillary forces on the diffuse interface lead to predicted mode selection at the melt-band widths observed in the experiments.The authors sincerely thank Sam Butler and Ben Holtzman for thoughtful reviews. Support was provided by the National Science Foundation (NSF, grant EAR-1344538), the Natural Environment Research Council (NERC, grant NE/I023929/1) and Trinity College.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.epsl.2015.10.05
Gravity, Topography, and Melt Generation Rates From Simple 3-D Models of Mantle Convection
Convection in fluid layers at high Rayleigh number (Ra ) have a
spoke pattern planform. Instabilities in the bottom thermal boundary layer
develop into hot rising sheets of fluid, with a component of radial flow
towards a central upwelling plume. The sheets form the "spokes" of the pattern,
and the plumes the "hubs". Such a pattern of flow is expected to occur beneath
plate interiors on Earth, but it remains a challenge to use observations to
place constraints on the convective planform of the mantle. Here we present
predictions of key surface observables (gravity, topography, and rates of melt
generation) from simple 3D numerical models of convection in a fluid layer.
These models demonstrate that gravity and topography have only limited
sensitivity to the spokes, and mostly reflect the hubs (the rising and sinking
plumes). By contrast, patterns of melt generation are more sensitive to short
wavelength features in the flow. There is the potential to have melt generation
along the spokes, but at a rate which is relatively small compared with that at
the hubs. Such melting of spokes can only occur when the lithosphere is
sufficiently thin ( km) and mantle water contents are sufficiently
high ( ppm). The distribution of volcanism across the Middle East,
Arabia and Africa north of equator suggests that it results from such spoke
pattern convection.Leverhulme Trus
Changing teaching techniques and adapting new technologies to improve student learning in an introductory meteorology and climate course
Responding to the call for reform in science education, changes were made in an introductory meteorology and climate course offered at a large public university. These changes were a part of a larger project aimed at deepening and extending a program of science content courses that model effective teaching strategies for prospective middle school science teachers. Therefore, revisions were made to address misconceptions about meteorological phenomena, foster deeper understanding of key concepts, encourage engagement with the text, and promote inquiry-based learning. Techniques introduced include: use of a flash cards, student reflection questionnaires, writing assignments, and interactive discussions on weather and forecast data using computer technology such as Integrated Data Viewer (IDV). The revision process is described in a case study format. Preliminary results (self-reflection by the instructor, surveys of student opinion, and measurements of student achievement), suggest student learning has been positively influenced. This study is supported by three grants: NSF grant No. 0202923, the Unidata Equipment Award, and the Lucia Harrison Endowment Fund
Estimating fuel channel bore from fuel grab load trace data
Detailed measurements of the graphite core fuel channels are made by specialist inspection equipment during planned outages, typically every 18 months to 3 years. The bores of the graphite fuel bricks are obtained during these inspections and are used to provide important information about the health of the core. Additionally, less detailed online monitoring data is obtained much more frequently during refuelling events, called the fuel grab load trace (FGLT), which can be also used to infer the health of the graphite core. This paper describes the process of creating a model which isolates a component of the refuelling data and maps it directly to dimensional measurements of fuel channel bore. The model is created from a combination of the theoretical understanding of the physical interactions of the fuel stringer during refuelling events and several years of refuelling and inspection data to estimate suitable model parameters. Initially the model created was a coarse estimation of FGLT to fuel bore dimension but through refinements a much more accurate model has been created. An application of this model is shown through a case study of a recent outage where estimations were made on refuelling data and were compared to previously unseen inspection data
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