1,494 research outputs found
Modeling the Formation of Clouds in Brown Dwarf Atmospheres
Because the opacity of clouds in substellar mass object (SMO) atmospheres
depends on the composition and distribution of particle sizes within the cloud,
a credible cloud model is essential for accurately modeling SMO spectra and
colors. We present a one--dimensional model of cloud particle formation and
subsequent growth based on a consideration of basic cloud microphysics. We
apply this microphysical cloud model to a set of synthetic brown dwarf
atmospheres spanning a broad range of surface gravities and effective
temperatures (g_surf = 1.78 * 10^3 -- 3 * 10^5 cm/s^2 and T_eff = 600 -- 1600
K) to obtain plausible particle sizes for several abundant species (Fe,
Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are
largest, the particle sizes thus computed range from ~5 microns to over 300
microns in radius over the full range of atmospheric conditions considered. We
show that average particle sizes decrease significantly with increasing brown
dwarf surface gravity. We also find that brown dwarfs with higher effective
temperatures have characteristically larger cloud particles than those with
lower effective temperatures. We therefore conclude that it is unrealistic when
modeling SMO spectra to apply a single particle size distribution to the entire
class of objects.Comment: 25 pages; 8 figures. We have added considerable detail describing the
physics of the cloud model. We have also added discussions of the issues of
rainout and the self-consistent coupling of clouds with brown dwarf
atmospheric models. We have updated figures 1, 3, and 4 with new vertical
axis labels and new particle sizes for forsterite and gehlenite. Accepted to
the Astrophysical Journal, Dec. 2, 200
A map of the day-night contrast of the extrasolar planet HD 189733b
"Hot Jupiter" extrasolar planets are expected to be tidally locked because
they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth
distance) to their parent stars, resulting in permanent daysides and
nightsides. By observing systems where the planet and star periodically eclipse
each other, several groups have been able to estimate the temperatures of the
daysides of these planets. A key question is whether the atmosphere is able to
transport the energy incident upon the dayside to the nightside, which will
determine the temperature at different points on the planet's surface. Here we
report observations of HD 189733, the closest of these eclipsing planetary
systems, over half an orbital period, from which we can construct a 'map' of
the distribution of temperatures. We detected the increase in brightness as the
dayside of the planet rotated into view. We estimate a minimum brightness
temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11
K at a wavelength of 8 microns, indicating that energy from the irradiated
dayside is efficiently redistributed throughout the atmosphere, in contrast to
a recent claim for another hot Jupiter. Our data indicate that the peak
hemisphere-integrated brightness occurs 166 degrees before opposition,
corresponding to a hot spot shifted east of the substellar point. The secondary
eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than
predicted, which may indicate a slightly eccentric orbit.Comment: To appear in the May 10 2007 issue of Nature, 10 pages, 2 black and
white figures, 1 colo
Life expectancy of persons receiving combination antiretroviral therapy in low-income countries: a cohort analysis from Uganda
Little is known about the effect of combination antiretroviral therapy (cART) on life expectancy in sub-Saharan Africa
Resolving the Surfaces of Extrasolar Planets With Secondary Eclipse Light Curves
We present a method that employs the secondary eclipse light curves of
transiting extrasolar planets to probe the spatial variation of their thermal
emission. This technique permits an observer to resolve the surface of the
planet without the need to spatially resolve its central star. We evaluate the
feasibility of this technique for the HD 209458 system [..]. We consider two
representations of the planetary thermal emission; a simple model parameterized
by a sinusoidal dependence on longitude and latitude, as well as the results of
a three-dimensional dynamical simulation of the planetary atmosphere previously
published by Cooper & Showman. We find that observations of the secondary
eclipse light curve are most sensitive to a longitudinal offset in the
geometric and photometric centroids of the hemisphere of the planet visible
near opposition. To quantify this signal, we define a new parameter, the
``uniform time offset,'' which measures the time lag between the observed
secondary eclipse and that predicted by a planet with a uniform surface flux
distribution. We compare the predicted amplitude of this parameter for HD
209458 with the precision with which it could be measured with IRAC. We find
that IRAC observations at 3.6um a single secondary eclipse should permit
sufficient precision to confirm or reject the Cooper & Showman model of the
surface flux distribution for this planet. We quantify the signal-to-noise
ratio for this offset in the remaining IRAC bands (4.5um, 5.8um, and 8.0um),
and find that a modest improvement in photometric precision (as might be
realized through observations of several eclipse events) should permit a
similarly robust detection.Comment: AASTeX 5.2, 24 pages, 5 figures, accepted for publication in ApJ; v2:
clarifications, updated to version accepted by ApJ; v3: try to reduce spacin
Atmospheric Circulation of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing
We present global, three-dimensional numerical simulations of the atmospheric
circulation on HD 209458b and HD 189733b and calculate the infrared spectra and
light curves predicted by these simulations, which we compare with available
observations. Radiative heating/cooling is parameterized with a simplified
Newtonian relaxation scheme. Our simulations develop day-night temperature
contrasts that vary strongly with pressure. At low pressure (<10 mbar), air
flows from the substellar point toward the antistellar point, both along the
equator and over the poles. At deeper levels, the flow develops an eastward
equatorial jet with speeds of 3-4 km/sec, with weaker westward flows at high
latitudes. This basic flow pattern is robust to variations in model resolution,
gravity, radiative time constant, and initial temperature structure. Nightside
spectra show deep absorption bands of H2O, CO, and/or CH4, whereas on the
dayside these absorption bands flatten out or even flip into emission. This
results from the strong effect of dynamics on the vertical temperature-pressure
structure; the temperature decreases strongly with altitude on the nightside
but becomes almost isothermal on the dayside. In Spitzer bandpasses, our
predicted planet-to-star flux ratios vary by a factor of 2-10 with orbital
phase, depending on the wavelength and chemistry. For HD 189733b, where a
detailed 8-micron light curve has been obtained, we correctly produce the
observed phase offset of the flux maximum, but we do not explain the flux
minimum and we overpredict the total flux variation. This discrepancy likely
results from the simplifications inherent in the Newtonian relaxation scheme
and provides motivation for incorporating realistic radiative transfer in
future studies.Comment: 17 pages, 14 figures, submitted for publication in Ap
Rapid translation of clinical guidelines into executable knowledge : a case study of COVID-19 and online demonstration
Introduction:
We report a pathfinder study of AI/knowledge engineering methods to rapidly formalise COVIDâ19 guidelines into an executable model of decision making and care pathways. The knowledge source for the study was material published by BMJ Best Practice in March 2020.
Methods:
The PROforma guideline modelling language and OpenClinical.net authoring and publishing platform were used to create a data model for care of COVIDâ19 patients together with executable models of rules, decisions and plans that interpret patient data and give personalised care advice.
Results:
PROforma and OpenClinical.net proved to be an effective combination for rapidly creating the COVIDâ19 model; the Pathfinder 1 demonstrator is available for assessment at https://www.openclinical.net/index.php?id=746.
Conclusions:
This is believed to be the first use of AI/knowledge engineering methods for disseminating bestâpractice in COVIDâ19 care. It demonstrates a novel and promising approach to the rapid translation of clinical guidelines into point of care services, and a foundation for rapid learning systems in many areas of healthcare
Maternal pregnancy vitamin D supplementation increases offspring bone formation in response to mechanical loading: Findings from the MAVIDOS trial
The Maternal Vitamin D Osteoporosis (MAVIDOS) trial reported higher total body bone mineral content in winter-born infants of mothers receiving vitamin D supplementation [1000 IU/day cholecalciferol] compared with placebo from 14 weeks gestation until delivery. This sub-study aimed to determine whether antenatal vitamin D supplementation altered postnatal bone formation in response to mechanical stimulation. Thirty-one children born to MAVIDOS participants randomised to either placebo (n=19) or cholecalciferol (n=12) were recruited at age 4-5 years. Children received whole body vibration (WBV) for 10 minutes on 5 consecutive days. Fasting blood samples for bone homeostasis, 25 hydroxyvitamin D (25OHD), parathyroid hormone (PTH), and bone turnover markers (Pro-collagen Type 1 N-terminal propeptide, P1NP; Cross-linked C-telopeptide of Type I Collagen, CTX) were collected pre-WBV and on day 8 (D8). Mean changes (D) in P1NP (ng/ml) between baseline and D8 in the vitamin-D intervention and placebo groups were 40.6 and -92.6 respectively and mean changes (Î) in CTX (ng/ml) were 0.034 (intervention) and -0.084 (placebo) respectively. Between-group DP1NP difference was 133.2ng/ml [95% CI 0.4, 266.0; p=0.049] and ÎCTX 0.05ng/ml (95% CI -0.159, 0.26ng/mL; p=0.62). Antenatal vitamin-D supplementation resulted in increased P1NP in response to WBV, suggesting early life vitamin D supplementation increases the anabolic response of bone to mechanical loading in children
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