257 research outputs found
A comparison of design and model selection methods for supersaturated experiments
Various design and model selection methods are available for supersatu-rated designs having more factors than runs but little research is available ontheir comparison and evaluation. In this paper, simulated experiments areused to evaluate the use of E(s2)-optimal and Bayesian D-optimal designs,and to compare three analysis strategies representing regression, shrinkageand a novel model-averaging procedure. Suggestions are made for choosingthe values of the tuning constants for each approach. Findings include that(i) the preferred analysis is via shrinkage; (ii) designs with similar numbersof runs and factors can be effective for a considerable number of active effectsof only moderate size; and (iii) unbalanced designs can perform well. Somecomments are made on the performance of the design and analysis methodswhen effect sparsity does not hol
Cassini Ring Seismology as a Probe of Saturn's Interior I: Rigid Rotation
Seismology of the gas giants holds the potential to resolve long-standing
questions about their internal structure and rotation state. We construct a
family of Saturn interior models constrained by the gravity field and compute
their adiabatic mode eigenfrequencies and corresponding Lindblad and vertical
resonances in Saturn's C ring, where more than twenty waves with pattern speeds
faster than the ring mean motion have been detected and characterized using
high-resolution Cassini Visual and Infrared Mapping Spectrometer (VIMS) stellar
occultation data. We present identifications of the fundamental modes of Saturn
that appear to be the origin of these observed ring waves, and use their
observed pattern speeds and azimuthal wavenumbers to estimate the bulk rotation
period of Saturn's interior to be (median and 5%/95% quantiles),
significantly faster than Voyager and Cassini measurements of periods in
Saturn's kilometric radiation, the traditional proxy for Saturn's bulk rotation
period. The global fit does not exhibit any clear systematics indicating strong
differential rotation in Saturn's outer envelope.Comment: 19 pages, 6 figures, 3 tables, accepted to ApJ; a bug fix improves
the fit, predicts faster bulk spin periods (Figure 4) and virtually
eliminates evidence for strong radial differential rotation (Figure 5
The Sonora Brown Dwarf Atmosphere and Evolution Models. I. Model Description and Application to Cloudless Atmospheres in Rainout Chemical Equilibrium
We present a new generation of substellar atmosphere and evolution models , appropriate for application to studies of L-, T-, and Y-type brown dwarfs and self-luminous extrasolar planets. The models describe the expected temperature-pressure profiles and emergent spectra of atmospheres in radiative-convective equilibrium with effective temperatures and gravities within the ranges ≤ Teff ≤ 2400 K and . These ranges encompass masses from about 0.5 to 85 Jupiter masses for a set of metallicities ([M/H] = − 0.5 to + 0.5),C/O ratios (from 0.5 to 1.5 times that of solar), and ages. These models expand the diversity of model atmospheres currently available, notably to cooler effective temperatures and greater ranges in C/O. Notable improvements from past such models include updated opacities and atmospheric chemistry. Here we describe our modeling approach and present our initial tranche of models for cloudless, chemical equilibrium atmospheres. We compare the modeled spectra, photometry, and evolution to various data sets
Post-prandial hyperlipidaemia results in systemic nitrosative stress and impaired cerebrovascular function in the aged
Post-prandial hyperlipidaemia (PPH) acutely impairs systemic vascular endothelial function, potentially attributable to a free radical-mediated reduction in vascular nitric oxide (NO) bioavailability (oxidative-nitrosative stress). However, it remains to be determined whether this extends to the cerebrovasculature. To examine this, 38 (19 young (≤35 years) and 19 aged (≥60 years)) healthy males were recruited. Cerebrovascular function (middle cerebral artery velocity, MCAv) and cerebrovascular reactivity to hypercapnea (CVRCO2Hyper) and hypocapnea (CVRCO2Hypo) were determined via trans-cranial Doppler ultrasound and capnography. Venous blood samples were obtained for the assessment of triglycerides (photometry), glucose (photometry), insulin (radioimmunoassay), ascorbate free radical (A•-, electron paramagnetic resonance spectroscopy) and nitrite (NO2 -, ozone-based chemiluminescence) in the fasted state prior to and 4 h following consumption of a standardized high-fat meal (1362 kcal; 130 g of fat). Circulating triglycerides, glucose and insulin increased in both groups following the high-fat meal (P < 0.05), with triglycerides increasing by 1.37 ± 1.09 mmol/l in the young and 1.54 ± 1.00 mmol/l in the aged (P < 0.05). This resulted in an increased systemic formation of free radicals in the young (P < 0.05) but not the aged (P > 0.05) and corresponding reduction in NO2 - in both groups (P < 0.05). While the meal had no effect on MCAv in either age group, CVRCO2Hyper was selectively impaired in the aged (P < 0.05). These findings indicate that PPH causes acute cerebrovascular dysfunction in the aged subsequent to systemic nitrosative stress
A Study of the Diverse T Dwarf Population Revealed by WISE
We report the discovery of 87 new T dwarfs uncovered with the Wide-field
Infrared Survey Explorer (WISE) and three brown dwarfs with extremely red
near-infrared colors that exhibit characteristics of both L and T dwarfs. Two
of the new T dwarfs are likely binaries with L7+/-1 primaries and mid-type T
secondaries. In addition, our follow-up program has confirmed 10 previously
identified T dwarfs and four photometrically-selected L and T dwarf candidates
in the literature. This sample, along with the previous WISE discoveries,
triples the number of known brown dwarfs with spectral types later than T5.
Using the WISE All-Sky Source Catalog we present updated color-color and
color-type diagrams for all the WISE-discovered T and Y dwarfs. Near-infrared
spectra of the new discoveries are presented, along with spectral
classifications. To accommodate later T dwarfs we have modified the integrated
flux method of determining spectral indices to instead use the median flux.
Furthermore, a newly defined J-narrow index differentiates the early-type Y
dwarfs from late-type T dwarfs based on the J-band continuum slope. The K/J
indices for this expanded sample show that 32% of late-type T dwarfs have
suppressed K-band flux and are blue relative to the spectral standards, while
only 11% are redder than the standards. Comparison of the Y/J and K/J index to
models suggests diverse atmospheric conditions and supports the possible
re-emergence of clouds after the L/T transition. We also discuss peculiar brown
dwarfs and candidates that were found not to be substellar, including two Young
Stellar Objects and two Active Galactic Nuclei. The coolest WISE-discovered
brown dwarfs are the closest of their type and will remain the only sample of
their kind for many years to come.Comment: Accepted to ApJS on 15 January 2013; 99 pages in preprint format, 30
figures, 12 table
Probing the Substellar Regime with SIRTF
One of the main scientific drivers of the Space InfraRed Telescope Facility
(SIRTF) is the search for brown dwarfs and extrasolar superplanets. We discuss
observational strategies for identification of these objects, and conclude that
an optimal strategy is a wide IRAC survey (18 deg) with a 5
sensitivity of 3.9 Jy in channel 2 (M). For this sensitivity,
we provide estimates of the number of low mass brown dwarfs and isolated
planets detected per square degree for power-law mass functions with
=1.5, and 1.0. Shallower surveys covering a larger area are inefficient
because of large overheads and detector noise. Deeper surveys covering a
smaller area become more and more affected by crowding with galaxies. A survey
like the one that we propose would determine the field mass function down to a
few Jupiter masses through the identification of a large sample of brown dwarfs
and isolated planets. The proposed SIRTF survey would also allow the first
detection of ultracool substellar objects with temperatures between 700 K and
200 K. The cooling curves of substellar objects with masses less than 20
Jupiters imply that they should spend most of their lifetimes at temperatures
below 700 K. Preliminary models indicate that their atmospheres could be
dominated by water clouds, which would diminish their optical and near-infrared
fluxes. The properties of those objects are still completely unexplored.Comment: 7 pages, 6 figures, submitted to PAS
ELemental abundances of Planets and brown dwarfs Imaged around Stars (ELPIS): I. Potential Metal Enrichment of the Exoplanet AF Lep b and a Novel Retrieval Approach for Cloudy Self-luminous Atmospheres
AF Lep A+b is a remarkable planetary system hosting a gas-giant planet that
has the lowest dynamical mass among directly imaged exoplanets. We present an
in-depth analysis of the atmospheric composition of the star and planet to
probe the planet's formation pathway. Based on new high-resolution spectroscopy
of AF Lep A, we measure a uniform set of stellar parameters and elemental
abundances (e.g., [Fe/H] = dex). The planet's dynamical mass
( M) and orbit are also refined using published
radial velocities, relative astrometry, and absolute astrometry. We use
petitRADTRANS to perform chemically-consistent atmospheric retrievals for AF
Lep b. The radiative-convective equilibrium temperature profiles are
incorporated as parameterized priors on the planet's thermal structure, leading
to a robust characterization for cloudy self-luminous atmospheres. This novel
approach is enabled by constraining the temperature-pressure profiles via the
temperature gradient , a departure from previous studies
that solely modeled the temperature. Through multiple retrievals performed on
different portions of the m spectrophotometry, along with
different priors on the planet's mass and radius, we infer that AF Lep b likely
possesses a metal-enriched atmosphere ([Fe/H] dex). AF Lep b's
potential metal enrichment may be due to planetesimal accretion, giant impacts,
and/or core erosion. The first process coincides with the debris disk in the
system, which could be dynamically excited by AF Lep b and lead to planetesimal
bombardment. Our analysis also determines K,
dex, and the presence of silicate clouds and
dis-equilibrium chemistry in the atmosphere. Straddling the L/T transition, AF
Lep b is thus far the coldest exoplanet with suggested evidence of silicate
clouds.Comment: AJ, in press. Main text: Pages 1-32, Figures 1-15, Tables 1-6. All
figures and tables after References belong to the Appendix (Pages 32-58,
Figures 16-20, Table 7). For supplementary materials, please refer to the
Zenodo repository https://doi.org/10.5281/zenodo.826746
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