107 research outputs found
Towards 3D Retrieval of Exoplanet Atmospheres: Assessing Thermochemical Equilibrium Estimation Methods
Characterizing exoplanetary atmospheres via Bayesian retrievals requires
assuming some chemistry model, such as thermochemical equilibrium or
parameterized abundances. The higher-resolution data offered by upcoming
telescopes enables more complex chemistry models within retrieval frameworks.
Yet, many chemistry codes that model more complex processes like photochemistry
and vertical transport are computationally expensive, and directly
incorporating them into a 1D retrieval model can result in prohibitively long
execution times. Additionally, phase-curve observations with upcoming
telescopes motivate 2D and 3D retrieval models, further exacerbating the
lengthy runtime for retrieval frameworks with complex chemistry models. Here,
we compare thermochemical equilibrium approximation methods based on their
speed and accuracy with respect to a Gibbs energy-minimization code. We find
that, while all methods offer orders of magnitude reductions in computational
cost, neural network surrogate models perform more accurately than the other
approaches considered, achieving a median absolute dex error <0.03 for the
phase space considered. While our results are based on a 1D chemistry model,
our study suggests that higher dimensional chemistry models could be
incorporated into retrieval models via this surrogate modeling approach.Comment: 22 pages, 14 figures, submitted to PSJ 2022/11/22, revised 2023/3/7,
accepted 2023/3/23. Updated to add Zenodo link to Reproducible Research
Compendiu
An Ensemble of Bayesian Neural Networks for Exoplanetary Atmospheric Retrieval
Machine learning is now used in many areas of astrophysics, from detecting
exoplanets in Kepler transit signals to removing telescope systematics. Recent
work demonstrated the potential of using machine learning algorithms for
atmospheric retrieval by implementing a random forest to perform retrievals in
seconds that are consistent with the traditional, computationally-expensive
nested-sampling retrieval method. We expand upon their approach by presenting a
new machine learning model, \texttt{plan-net}, based on an ensemble of Bayesian
neural networks that yields more accurate inferences than the random forest for
the same data set of synthetic transmission spectra. We demonstrate that an
ensemble provides greater accuracy and more robust uncertainties than a single
model. In addition to being the first to use Bayesian neural networks for
atmospheric retrieval, we also introduce a new loss function for Bayesian
neural networks that learns correlations between the model outputs.
Importantly, we show that designing machine learning models to explicitly
incorporate domain-specific knowledge both improves performance and provides
additional insight by inferring the covariance of the retrieved atmospheric
parameters. We apply \texttt{plan-net} to the Hubble Space Telescope Wide Field
Camera 3 transmission spectrum for WASP-12b and retrieve an isothermal
temperature and water abundance consistent with the literature. We highlight
that our method is flexible and can be expanded to higher-resolution spectra
and a larger number of atmospheric parameters
Grid-Based Atmospheric Retrievals for Reflected-Light Spectra of Exoplanets using PSGnest
Techniques to retrieve the atmospheric properties of exoplanets via direct
observation of their reflected light have often been limited in scope due to
computational constraints imposed by the forward-model calculations. We have
developed a new set of techniques which significantly decreases the time
required to perform a retrieval while maintaining accurate results. We
constructed a grid of 1.4 million pre-computed geometric albedo spectra valued
at discrete sets of parameter points. Spectra from this grid are used to
produce models for a fast and efficient nested sampling routine called PSGnest.
Beyond the upfront time to construct a spectral grid, the amount of time to
complete a full retrieval using PSGnest is on the order of seconds to minutes
using a personal computer. An extensive evaluation of the error induced from
interpolating intermediate spectra from the grid indicates that this bias is
insignificant compared to other retrieval error sources, with an average
coefficient of determination between interpolated and true spectra of 0.998. We
apply these new retrieval techniques to help constrain the optimal bandpass
centers for retrieving various atmospheric and bulk parameters from a
LuvEx-type mission observing several planetary archetypes. We show that
spectral observations made using a 20\% bandpass centered at 0.73 microns can
be used alongside our new techniques to make detections of and
without the need to increase observing time beyond what is necessary for a
signal-to-noise ratio of 10. The methods introduced here will enable robust
studies of the capabilities of future observatories to characterize exoplanets.Comment: 32 pages, 17 figures. Accepted for publication in The Astronomical
Journa
Correlation of measurable serum markers of inflammation with lung levels following bilateral femur fracture in a rat model
Accurate Machine Learning Atmospheric Retrieval via a Neural Network Surrogate Model for Radiative Transfer
Atmospheric retrieval determines the properties of an atmosphere based on its
measured spectrum. The low signal-to-noise ratio of exoplanet observations
require a Bayesian approach to determine posterior probability distributions of
each model parameter, given observed spectra. This inference is computationally
expensive, as it requires many executions of a costly radiative transfer (RT)
simulation for each set of sampled model parameters. Machine learning (ML) has
recently been shown to provide a significant reduction in runtime for
retrievals, mainly by training inverse ML models that predict parameter
distributions, given observed spectra, albeit with reduced posterior accuracy.
Here we present a novel approach to retrieval by training a forward ML
surrogate model that predicts spectra given model parameters, providing a fast
approximate RT simulation that can be used in a conventional Bayesian retrieval
framework without significant loss of accuracy. We demonstrate our method on
the emission spectrum of HD 189733 b and find good agreement with a traditional
retrieval from the Bayesian Atmospheric Radiative Transfer (BART) code
(Bhattacharyya coefficients of 0.9843--0.9972, with a mean of 0.9925, between
1D marginalized posteriors). This accuracy comes while still offering
significant speed enhancements over traditional RT, albeit not as much as ML
methods with lower posterior accuracy. Our method is ~9x faster per parallel
chain than BART when run on an AMD EPYC 7402P central processing unit (CPU).
Neural-network computation using an NVIDIA Titan Xp graphics processing unit is
90--180x faster per chain than BART on that CPU.Comment: 16 pages, 4 figures, submitted to PSJ 3/4/2020, revised 1/22/2021.
Text restructured and updated for clarity, model updated and expanded to work
for range of hot Jupiters, results/plots updated, two new appendices to
further justify model selection and methodolog
Prey handling and diet of Louisiana pine snakes (Pituophis ruthveni) and black pine snakes (P. melanoleucus lodingi), with comparisons to other selected colubrid snakes
Diet and prey handling behavior were determined for Louisiana pine snakes (Pituophis ruthveni) and black pine snakes (P. melanoleucus lodingi). Louisiana pine snakes prey heavily on Baird\u27s pocket gophers (Geomys breviceps), with which they are sympatric, and exhibit specialized behaviors that facilitate handling this prey species within the confines of burrow systems. Black pine snakes, which are not sympatric with pocket gophers, did not exhibit these specialized behaviors. For comparative purposes, prey handling of P. sayi sayi and Elaphe obsoleta lindheimeri was also examined
Proxima Centauri b is not a transiting exoplanet
We report Spitzer Space Telescope observations during predicted transits of
the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone
planet we will ever discover, any potential transit of Proxima b would place
strong constraints on its radius, bulk density, and atmosphere. Subsequent
transmission spectroscopy and secondary-eclipse measurements could then probe
the atmospheric chemistry, physical processes, and orbit, including a search
for biosignatures. However, our photometric results rule out planetary transits
at the 200~ppm level at 4.5, yielding a 3 upper radius limit
of 0.4~R_\rm{\oplus} (Earth radii). Previous claims of possible transits from
optical ground- and space-based photometry were likely correlated noise in the
data from Proxima Centauri's frequent flaring. Follow-up observations should
focus on planetary radio emission, phase curves, and direct imaging. Our study
indicates dramatically reduced stellar activity at near-to-mid infrared
wavelengths, compared to the optical. Proxima b is an ideal target for
space-based infrared telescopes, if their instruments can be configured to
handle Proxima's brightness.Comment: 8 pages, 3 figures, 2 tables, accepted for publication in MNRA
Integrating Machine Learning for Planetary Science: Perspectives for the Next Decade
Machine learning (ML) methods can expand our ability to construct, and draw
insight from large datasets. Despite the increasing volume of planetary
observations, our field has seen few applications of ML in comparison to other
sciences. To support these methods, we propose ten recommendations for
bolstering a data-rich future in planetary science.Comment: 10 pages (expanded citations compared to 8 page submitted version for
decadal survey), 3 figures, white paper submitted to the Planetary Science
and Astrobiology Decadal Survey 2023-203
Impact of Obesity on Pediatric Acute Recurrent and Chronic Pancreatitis
OBJECTIVE:
The aim of this study was to assess the impact of obesity on pediatric acute recurrent pancreatitis or chronic pancreatitis (CP).
METHODS:
We determined body mass index (BMI) status at enrollment in INSPPIRE (INternational Study group of Pediatric Pancreatitis: In search for a cuRE) cohort using CDC criteria for pediatric-specific BMI percentiles. We used the Cochran-Armitage test to assess trends and the Jonckheere-Terpstra test to determine associations.
RESULTS:
Of 446 subjects (acute recurrent pancreatitis, n = 241; CP, n = 205), 22 were underweight, 258 normal weight, 75 overweight, and 91 were obese. The BMI groups were similar in sex, race, and age at presentation. Hypertriglyceridemia was more common in overweight or obese. Obese children were less likely to have CP and more likely to have acute inflammation on imaging. Compared with children with normal weight, obese or overweight children were older at first acute pancreatitis episode and diagnosed with CP at an older age. Obese or overweight children were less likely to undergo medical or endoscopic treatment, develop exocrine pancreatic insufficiency, and require total pancreatectomy with islet autotransplantation. Diabetes was similar among all groups.
CONCLUSIONS:
Obesity or overweight seems to delay the initial acute pancreatitis episode and diagnosis of CP compared with normal weight or underweight. The impact of obesity on pediatric CP progression and severity deserves further study
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