1,818 research outputs found
3D mixing in hot Jupiter atmospheres. I. application to the day/night cold trap in HD 209458b
Hot Jupiters exhibit atmospheric temperatures ranging from hundreds to
thousands of Kelvin. Because of their large day-night temperature differences,
condensable species that are stable in the gas phase on the dayside, such as
TiO and silicates, may condense and gravitationally settle on the nightside.
Atmospheric circulation may counterbalance this tendency to gravitationally
settle. This three dimensional (3D) mixing of chemical species has not
previously been studied for hot Jupiters, yet it is crucial to assess the
existence and distribution of TiO and silicates in the atmospheres of these
planets. We perform 3D global circulation models of HD209458b including passive
tracers that advect with the 3D flow, including a source/sink on the nightside
to represent condensation and gravitational settling of haze particles. We show
that global advection patterns produce strong vertical mixing that can keep
condensable species lofted as long as they are trapped in particles of sizes of
a few microns or less on the night side. We show that vertical mixing results
not from small-scale convection but from the large-scale circulation driven by
the day-night heating contrast. Although this vertical mixing is not diffusive
in any rigorous sense, a comparison of our results with idealized diffusion
models allows a rough estimate of the vertical diffusion coefficient.
Kzz=5x10**4/Sqrt(Pbar) m2/s can be used in 1D models of HD 209458b. Moreover,
our models exhibit strong spatial and temporal variability in the tracer
concentration that could result in observable variations during transit or
secondary eclipse measurements. Finally, we apply our model to the case of TiO
in HD209458b and show that the day-night cold trap would deplete TiO if it
condenses into particles bigger than a few microns on the planet's night side,
making it unable to create the observed stratosphere of the planet.Comment: Accepted in A&A in August 2013
http://dx.doi.org/10.1051/0004-6361/20132113
Vertical Tracer Mixing in Hot Jupiter Atmospheres
Aerosols appear to be ubiquitous in close-in gas giant atmospheres, and
disequilibrium chemistry likely impacts the emergent spectra of these planets.
Lofted aerosols and disequilibrium chemistry are caused by vigorous vertical
transport in these heavily irradiated atmospheres. Here we numerically and
analytically investigate how vertical transport should change over the
parameter space of spin-synchronized gas giants. In order to understand how
tracer transport depends on planetary parameters, we develop an analytic theory
to predict vertical velocities and mixing rates () and compare
the results to our numerical experiments. We find that both our theory and
numerical simulations predict that, if the vertical mixing rate is described by
an eddy diffusivity, then this eddy diffusivity should increase
with increasing equilibrium temperature, decreasing frictional drag strength,
and increasing chemical loss timescales. We find that the transition in our
numerical simulations between circulation dominated by a superrotating jet and
that with solely day-to-night flow causes a marked change in the vertical
velocity structure and tracer distribution. The mixing ratio of passive tracers
is greatest for intermediate drag strengths that corresponds to this transition
between a superrotating jet with columnar vertical velocity structure and
day-to-night flow with upwelling on the dayside and downwelling on the
nightside. Lastly, we present analytic solutions for as a
function of planetary effective temperature, chemical loss timescales, and
other parameters, for use as input to one-dimensional chemistry models of
spin-synchronized gas giant atmospheres.Comment: 25 pages, 12 figures, Accepted at Ap
Influence of structure on the optical limiting properties of nanotubes
We investigate the role of carbon nanotubes structure on their optical
limiting properties. Samples of different and well-characterized structural
features are studied by optical limiting and pump-probe experiments. The
influence of the diameter's size on the nano-object is demonstrated. Indeed,
both nucleation and growth of gas bubbles are expected to be sensitive to
diameter
Experiences of people taking opioid medication for chronic non-malignant pain : a qualitative evidence synthesis using meta-ethnography
Objective To review qualitative studies on the experience of taking opioid medication for chronic non-malignant pain (CNMP) or coming off them.
Design This is a qualitative evidence synthesis using a seven-step approach from the methods of meta-ethnography.
Data sources and eligibility criteria We searched selected databases—Medline, Embase, AMED, Cumulative Index to Nursing and Allied Health Literature, PsycINFO, Web of Science and Scopus (Science Citation Index and Social Science Citation Index)—for qualitative studies which provide patients’ views of taking opioid medication for CNMP or of coming off them (June 2017, updated September 2018).
Data extraction and synthesis Papers were quality appraised using the Critical Appraisal Skills Programme tool, and the GRADE-CERQual (Grading of Recommendations Assessment, Development and Evaluation working group - Confidence in Evidence from Reviews of Qualitative research) guidelines were applied. We identified concepts and iteratively abstracted these concepts into a line of argument.
Results We screened 2994 unique citations and checked 153 full texts, and 31 met our review criteria. We identified five themes: (1) reluctant users with little choice; (2) understanding opioids: the good and the bad; (3) a therapeutic alliance: not always on the same page; (4) stigma: feeling scared and secretive but needing support; and (5) the challenge of tapering or withdrawal. A new overarching theme of ‘constantly balancing’ emerged from the data.
Conclusions People taking opioids were constantly balancing tensions, not always wanting to take opioids, and weighing the pros and cons of opioids but feeling they had no choice because of the pain. They frequently felt stigmatised, were not always ‘on the same page’ as their healthcare professional and felt changes in opioid use were often challenging
SVA retrotransposons as modulators of gene expression.
Endogenous mobile genetic elements can give rise to de novo germline or somatic mutations that can have dramatic consequences for genome regulation both local and possibly more globally based on the site of integration. However if we consider them as "normal genetic" components of the reference genome then they are likely to modify local chromatin structure which would have an effect on gene regulation irrelevant of their ability to further transpose. As such they can be treated as any other domain involved in a gene × environment interaction. Similarly their evolutionary appearance in the reference genome would supply a driver for species specific responses/traits. Our recent data would suggest the hominid specific subset of retrotransposons, SINE-VNTR-Alu (SVA), can function as transcriptional regulatory domains both in vivo and in vitro when analyzed in reporter gene constructs. Of particular interest in the SVA element, were the variable number tandem repeat (VNTR) domains which as their name suggests can be polymorphic. We and others have previously shown that VNTRs can be both differential regulators and biomarkers of disease based on the genotype of the repeat. Here, we provide an overview of why polymorphism in the SVA elements, in particular the VNTRs, could alter gene expression patterns that could be mechanistically associated with different traits in evolution or disease progression in humans
Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs
Ground-based and spacecraft telescopic observations, combined with an
intensive modeling effort, have greatly enhanced our understanding of hot giant
planets and brown dwarfs over the past ten years. Although these objects are
all fluid, hydrogen worlds with stratified atmospheres overlying convective
interiors, they exhibit an impressive diversity of atmospheric behavior. Hot
Jupiters are strongly irradiated, and a wealth of observations constrain the
day-night temperature differences, circulation, and cloudiness. The intense
stellar irradiation, presumed tidal locking and modest rotation leads to a
novel regime of strong day-night radiative forcing. Circulation models predict
large day-night temperature differences, global-scale eddies, patchy clouds,
and, in most cases, a fast eastward jet at the equator-equatorial
superrotation. The warm Jupiters may exhibit a wide range of rotation rates,
obliquities, and orbital eccentricities, which, along with the weaker
irradiation, leads to circulation patterns and observable signatures predicted
to differ substantially from hot Jupiters. Brown dwarfs are typically isolated,
rapidly rotating worlds; they radiate enormous energy fluxes into space and
convect vigorously in their interiors. Their atmospheres exhibit patchiness in
clouds and temperature on regional to global scales-the result of modulation by
large-scale atmospheric circulation. Despite the lack of irradiation, such
circulations can be driven by interaction of the interior convection with the
overlying atmosphere, as well as self-organization of patchiness due to
cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to
bridge the gap between these classes of objects, experiencing intense external
irradiation as well as vigorous interior convection. A hierarchy of modeling
approaches have yielded major new insights into the dynamics governing these
phenomena.Comment: Invited review for Space Science Reviews special issue on
"Understanding the Diversity of Planetary Atmospheres". Accepted. 52 pages,
33 figures. This may be the last published work led by Adam Showman, whose
sudden death during the revision process of this manuscript deprived the
field of one of his giant. He will be missed by al
Inactive dynamical phase of a symmetric exclusion process on a ring
International audienceWe investigate the nature of the dynamically inactive phase of a simple symmetric exclusion process on a ring. We find that as the system's activity is tuned to a lower-than-average value the particles progressively lump into a single cluster, thereby forming a kink in the density profile. All dynamical regimes, and their finite size range of validity, are explicitly determined
Unveiling the atmospheres of giant exoplanets with an EChO-class mission
More than a thousand exoplanets have been discovered over the last decade. Perhaps more excitingly, probing their atmospheres has become possible. With current data we have glimpsed the diversity of exoplanet atmospheres that will be revealed over the coming decade. However, numerous questions concerning their chemical composition, thermal structure, and atmospheric dynamics remain to be answered. More observations of higher quality are needed. In the next years, the selection of a space-based mission dedicated to the spectroscopic characterization of exoplanets would revolutionize our understanding of the physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic vision program in 2014. Our paper is therefore based on the planned capabilities of the Exoplanet Characterization Observatory (EChO), but it should equally apply to any future mission with similar characteristics. With its large spectral coverage (0.4 − 16 μm), high spectral resolution (λ/Δλ > 300 below 5 μm and λ/Δλ > 30 above 5 μm) and 1.5m mirror, a future mission such as EChO will provide spectrally resolved transit lightcurves, secondary eclipses lightcurves, and full phase curves of numerous exoplanets with an unprecedented signal-to-noise ratio. In this paper, we review some of today’s main scientific questions about gas giant exoplanets atmospheres, for which a future mission such as EChO will bring a decisive contribution
Unveiling the atmospheres of giant exoplanets with an EChO-class mission
More than a thousand exoplanets have been discovered over the last decade.
Perhaps more excitingly, probing their atmospheres has become possible. With
current data we have glimpsed the diversity of exoplanet atmospheres that will
be revealed over the coming decade. However, numerous questions concerning
their chemical composition, thermal structure, and atmospheric dynamics remain
to be answered. More observations of higher quality are needed. In the next
years, the selection of a space-based mission dedicated to the spectroscopic
characterization of exoplanets would revolutionize our understanding of the
physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic
vision program in 2014. Our paper is therefore based on the planned
capabilities of the Exoplanet Characterization Observatory (EChO), but it
should equally apply to any future mission with similar characteristics. With
its large spectral coverage (), high spectral resolution
( below and
above ) and mirror, a
future mission such as EChO will provide spectrally resolved transit
lightcurves, secondary eclipses lightcurves, and full phase curves of numerous
exoplanets with an unprecedented signal-to-noise ratio. In this paper, we
review some of today's main scientific questions about gas giant exoplanets
atmospheres, for which a future mission such as EChO will bring a decisive
contribution.Comment: Accepted to the Experimental Journal of Astronom
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