6,305 research outputs found
Convectively driven shear and decreased heat flux
We report on direct numerical simulations of two-dimensional, horizontally
periodic Rayleigh-B\'enard convection, focusing on its ability to drive
large-scale horizontal flow that is vertically sheared. For the Prandtl numbers
() between 1 and 10 simulated here, this large-scale shear can be induced
by raising the Rayleigh number () sufficiently, and we explore the
resulting convection for up to . When present in our simulations,
the sheared mean flow accounts for a large fraction of the total kinetic
energy, and this fraction tends towards unity as . The shear helps
disperse convective structures, and it reduces vertical heat flux; in parameter
regimes where one state with large-scale shear and one without are both stable,
the Nusselt number of the state with shear is smaller and grows more slowly
with . When the large-scale shear is present with , the
convection undergoes strong global oscillations on long timescales, and heat
transport occurs in bursts. Nusselt numbers, time-averaged over these bursts,
vary non-monotonically with for . When the shear is present with
, the flow does not burst, and convective heat transport is
sustained at all times. Nusselt numbers then grow roughly as powers of ,
but the growth rates are slower than any previously reported for
Rayleigh-B\'enard convection without large-scale shear. We find the Nusselt
numbers grow proportionally to when and to when
. Analogies with tokamak plasmas are described.Comment: 25 pages, 12 figures, 5 video
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
A possible dearth of hot gas in galaxy groups at intermediate redshift
We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at
redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing
signal of the CNOC2 groups has shown that they are likely to be genuine,
gravitationally bound objects. Of the 21 groups in the field of view of the
EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of
observation, even though three of the them have velocity dispersions high
enough that they would easily be visible if their luminosities scaled with
their velocity dispersions in the same way as nearby groups' luminosities
scale. We consider the possibility that this is due to the reported velocity
dispersions being erroneously high, and conclude that this is unlikely. We
therefore find tentative evidence that groups at intermediate redshift are
underluminous relative to their local cousins.Comment: 16 pages, 5 figures, reference added in section 1, typos corrected,
published in Ap
Generation of political priority for global surgery: a qualitative policy analysis
Background Despite the high burden of surgical conditions, the provision of surgical services has been a low global
health priority. We examined factors that have shaped priority for global surgical care.
Methods We undertook semi-structured interviews by telephone with members of global surgical networks and
ministries of health to explore the challenges and opportunities surgeons, anaesthesiologists, and other proponents
face in increasing global priority for surgery. We did a literature review and collected information from reports from
organisations involved in surgery. We used a policy framework consisting of four categories—actor power, ideas,
political contexts, and characteristics of the issue itself—to analyse factors that have shaped global political priority for
surgery. We did a thematic analysis on the collected information.
Findings Several factors hinder the acquisition of attention and resources for global surgery. With respect to actor
power, the global surgery community is fragmented, does not have unifying leadership, and is missing guiding
institutions. Regarding ideas, community members disagree on how to address and publicly position the problem.
With respect to political contexts, the community has made insuffi cient eff orts to capitalise on political opportunities
such as the Millennium Development Goals. Regarding issue characteristics, data on the burden of surgical diseases
are limited and public misperceptions surrounding the cost and complexity of surgery are widespread. However, the
community has several strengths that portend well for the acquisition of political support. These include the existence
of networks deeply committed to the cause, the potential to link with global health priorities, and emerging research
on the cost-eff ectiveness of some procedures.
Interpretation To improve global priority for surgery, proponents will need to create an eff ective governance structure
that facilitates achievement of collective goals, generate consensus on solutions, and fi nd an eff ective public
positioning of the issue that attracts political support
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
Habitable Climates
According to the standard liquid-water definition, the Earth is only
partially habitable. We reconsider planetary habitability in the framework of
energy-balance models, the simplest seasonal models in physical climatology, to
assess the spatial and temporal habitability of Earth-like planets. We quantify
the degree of climatic habitability of our models with several metrics of
fractional habitability. Previous evaluations of habitable zones may have
omitted important climatic conditions by focusing on close Solar System
analogies. For example, we find that model pseudo-Earths with different
rotation rates or different land-ocean fractions have fractional habitabilities
that differ significantly from that of the Earth itself. Furthermore, the
stability of a planet's climate against albedo-feedback snowball events
strongly impacts its habitability. Therefore, issues of climate dynamics may be
central in assessing the habitability of discovered terrestrial exoplanets,
especially if astronomical forcing conditions are different from the moderate
Solar System cases.Comment: Accepted by ApJ. Several references added. 41 pages, 11 figures, 2
table
Antiretroviral therapy for refugees and internally displaced persons: a call for equity.
Available evidence suggests that refugees and internally displaced persons (IDPs) in stable settings can sustain high levels of adherence and viral suppression.
Moral, legal, and public health principles and recent evidence strongly suggest that refugees and IDPs should have equitable access to HIV treatment and support.
Exclusion of refugees and IDPs from HIV National Strategic Plans suggests that they may not be included in future national funding proposals to major donors.
Levels of viral suppression among refugees and nationals documented in a stable refugee camp suggest that some settings require more intensive support for all population groups.
Detailed recommendations are provided for refugees and IDPs accessing antiretroviral therapy in stable settings
Stellar Hydrodynamics in Radiative Regions
We present an analysis of the response of a radiative region to waves
generated by a convective region of the star; this wave treatment of the
classical problem of ``overshooting'' gives extra mixing relative to the
treatment traditionally used in stellar evolutionary codes. The interface
between convectively stable and unstable regions is dynamic and nonspherical,
so that the nonturbulent material is driven into motion, even in the absence of
``penetrative overshoot.'' These motions may be described by the theory of
nonspherical stellar pulsations, and are related to motion measured by
helioseismology. Multi-dimensional numerical simulations of convective flow
show puzzling features which we explain by this simplified physical model.
Gravity waves generated at the interface are dissipated, resulting in slow
circulation and mixing seen outside the formal convection zone. The approach
may be extended to deal with rotation and composition gradients. Tests of this
description in the stellar evolution code TYCHO produce carbon stars on the
asymptotic giant branch (AGB), an isochrone age for the Hyades and three young
clusters with lithium depletion ages from brown dwarfs, and lithium and
beryllium depletion consistent with observations of the Hyades and Pleiades,
all without tuning parameters. The insight into the different contributions of
rotational and hydrodynamic mixing processes could have important implications
for realistic simulation of supernovae and other questions in stellar
evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa
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