1,990 research outputs found
Limb Darkening and Planetary Transits: Testing Center-to-limb Intensity Variations and Limb-Darkening Directly from Model Stellar Atmospheres
The transit method, employed by MOST, \emph{Kepler}, and various ground-based
surveys has enabled the characterization of extrasolar planets to unprecedented
precision. These results are precise enough to begin to measure planet
atmosphere composition, planetary oblateness, star spots, and other phenomena
at the level of a few hundred parts-per-million. However, these results depend
on our understanding of stellar limb darkening, that is, the intensity
distribution across the stellar disk that is sequentially blocked as the planet
transits. Typically, stellar limb darkening is assumed to be a simple
parameterization with two coefficients that are derived from stellar atmosphere
models or fit directly. In this work, we revisit this assumption and compute
synthetic planetary transit light curves directly from model stellar atmosphere
center-to-limb intensity variations (CLIV) using the plane-parallel
\textsc{Atlas} and spherically symmetric \textsc{SAtlas} codes. We compare
these light curves to those constructed using best-fit limb-darkening
parameterizations. We find that adopting parametric stellar limb-darkening laws
lead to systematic differences from the more geometrically realistic model
stellar atmosphere CLIV of about 50 -- 100 ppm at the transit center and up to
300 ppm at ingress/egress. While these errors are small they are systematic,
and appear to limit the precision necessary to measure secondary effects. Our
results may also have a significant impact on transit spectra.Comment: 12 pages, 14 figures, accepted for publication in ApJ after revision
Beaver Dam Influences on Streamflow Hydraulic Properties and Thermal Regimes
Beaver dams alter channel hydraulics which in turn change the geomorphic templates of streams. Variability in geomorphic units, the building blocks of stream systems, and water temperature, critical to stream ecological function, define habitat heterogeneity and availability. While prior research has shown the impact of beaver dams on stream hydraulics, geomorphic template, or temperature, the connections or feedbacks between these habitat measures are not well understood. This has left questions regarding relationships between temperature variability at different spatial scales to hydraulic properties such as flow depth and velocity that are dependent on the geomorphology. We combine detailed predicted hydraulic properties, field-based maps with an additional classification scheme of geomorphic units, and detailed water temperature observations throughout a study reach to demonstrate the relationship between these factors at different spatial scales (reach, beaver dam complexes, and geomorphic units). Over a three-week, low flow period we found temperature to vary 2 °C between the upstream and downstream extents of the reach with a net warming of 1 °C during the day and a net cooling of 0.5 °C at night. At the beaver dam complex scale, net warming of 1.15 °C occurred during the day with variable cooling at night. Regardless of limited temperature changes at these larger scales, the temperature variability within a beaver dam complex reached up to 10.5 °C due to the diversity of geomorphic units. At the geomorphic unit scale, the highly altered flow velocity and depth distributions within primary geomorphic units provide an explanation of the temperature variability within the dam complex and insight regarding increases in habitat heterogeneity
Analysis of the Effects of Dam Release Properties and Ambient Groundwater Flow on Surface WaterâGroundwater Exchange Over a 100âkmâLong Reach
Hydroelectric dams often create highly dynamic downstream flows that promote surface waterâgroundwater (SWâGW) interactions including bank storage, the temporary storage of river water in the riverbank. Previous research on SWâGW exchanges in dammed rivers has primarily been at single study sites, which has limited the understanding of how these exchanges evolve as dam releases travel downstream. This study evaluates how dam releases affect SWâGW exchange continuously over a 100âkm distance. This is accomplished by longitudinally routing water releases through a synthetic river and modeling bed and bank fluid and solute exchange across transverse transects spaced along the reach. Peak and square dam release hydrograph shapes with three magnitudes (0.5, 1.0, and 1.5 m) were considered. The effect of four ambient groundwater flow conditions (very slightly losing, neutral, and two gaining from the perspective of the river) was evaluated for each dam release scenario. Both types of dam release shapes cause SWâGW interaction over the entire 100âkm distance, and our results show that square type releases cause bank storage exchange well beyond this distance. Strongly gaining conditions reduce the amount of exchange and allow flushing of riverâsourced solute out of the bank after the dam pulse has passed. Both neutral and losing conditions have larger fluid and solute flux into the bank and limit the amount of solute that returns to the river. Our results support that river corridors downstream of dams have increased riverâaquifer connectivity and that this enhanced connectivity can extend at least 100 km downstream
Empirical Models for Predicting Water and Heat Flow Properties of Permafrost Soils
Warming and thawing in the Arctic are promoting biogeochemical processing and hydrologic transport in carbonârich permafrost and soils that transfer carbon to surface waters or the atmosphere. Hydrologic and biogeochemical impacts of thawing are challenging to predict with sparse information on arctic soil hydraulic and thermal properties. We developed empirical and statistical models of soil properties for three main strata in the shallow, seasonally thawed soils above permafrost in a study area of ~7,500 km2 in Alaska. The models show that soil vertical stratification and hydraulic properties are predictable based on vegetation cover and slope. We also show that the distinct hydraulic and thermal properties of each soil stratum can be predicted solely from bulk density. These findings fill the gap for a sparsely mapped region of the Arctic and enable regional interpolation of soil properties critical for determining future hydrologic responses and the fate of carbon in thawing permafrost
Comparative Modelling of the Spectra of Cool Giants
Our ability to extract information from the spectra of stars depends on
reliable models of stellar atmospheres and appropriate techniques for spectral
synthesis. Various model codes and strategies for the analysis of stellar
spectra are available today. We aim to compare the results of deriving stellar
parameters using different atmosphere models and different analysis strategies.
The focus is set on high-resolution spectroscopy of cool giant stars. Spectra
representing four cool giant stars were made available to various groups and
individuals working in the area of spectral synthesis, asking them to derive
stellar parameters from the data provided. The results were discussed at a
workshop in Vienna in 2010. Most of the major codes currently used in the
astronomical community for analyses of stellar spectra were included in this
experiment. We present the results from the different groups, as well as an
additional experiment comparing the synthetic spectra produced by various codes
for a given set of stellar parameters. Similarities and differences of the
results are discussed. Several valid approaches to analyze a given spectrum of
a star result in quite a wide range of solutions. The main causes for the
differences in parameters derived by different groups seem to lie in the
physical input data and in the details of the analysis method. This clearly
shows how far from a definitive abundance analysis we still are.Comment: accepted for publication in A&A. This version includes also the
online tables. Reference spectra will later be available via the CD
Estimating Increased Transient Water Storage With Increases in Beaver Dam Activity
Dam building by beaver (Castor spp.) slows water movement through montane valleys, increasing transient water storage and the diversity of residence times. In some cases, water storage created by beaver dam construction is correlated to changes in streamflow magnitude and timing. However, the total amount of additional surface and groundwater storage that beaver dams may create (and, thus, their maximum potential impact on streamflow) has not been contextualized in the water balance of larger river basins. We estimate the potential transient water storage increases that could be created at 5, 25, 50, and 100% of maximum modeled beaver dam capacity in the Bear River basin, USA, by adapting the height above nearest drainage (HAND) algorithm to spatially estimate surface water storage. Surface water storage estimates were combined with the MODFLOW groundwater model to estimate potential increases in groundwater storage throughout the basin. We tested four scenarios to estimate potential transient water storage increases resulting from the construction of 1179 to 34,897 beaver dams, and estimated surface water storage to range from 57.5 to 72.8 m3 per dam and groundwater storage to range from 182.2 to 313.3 m3 per dam. Overall, we estimate that beaver dam construction could increase transient water storage by up to 10.38 million m3 in the Bear River basin. We further contextualize beaver dam-related water storage increases with streamflow, reservoir, and snowpack volumes
Dynamic correlations in symmetric electron-electron and electron-hole bilayers
The ground-state behavior of the symmetric electron-electron and
electron-hole bilayers is studied by including dynamic correlation effects
within the quantum version of Singwi, Tosi, Land, and Sjolander (qSTLS) theory.
The static pair-correlation functions, the local-field correction factors, and
the ground-state energy are calculated over a wide range of carrier density and
layer spacing. The possibility of a phase transition into a density-modulated
ground state is also investigated. Results for both the electron-electron and
electron-hole bilayers are compared with those of recent diffusion Monte Carlo
(DMC) simulation studies. We find that the qSTLS results differ markedly from
those of the conventional STLS approach and compare in the overall more
favorably with the DMC predictions. An important result is that the qSTLS
theory signals a phase transition from the liquid to the coupled Wigner crystal
ground state, in both the electron-electron and electron-hole bilayers, below a
critical density and in the close proximity of layers (d <~ r_sa_0^*), in
qualitative agreement with the findings of the DMC simulations.Comment: 13 pages, 11 figures, 2 table
Superconductivity in correlated disordered two-dimensional electron gas
We calculate the dynamic effective electron-electron interaction potential
for a low density disordered two-dimensional electron gas. The disordered
response function is used to calculate the effective potential where the
scattering rate is taken from typical mobilities from recent experiments. We
investigate the development of an effective attractive pair potential for both
disordered and disorder free systems with correlations determined from existing
numerical simulation data. The effect of disorder and correlations on the
superconducting critical temperature Tc is discussed.Comment: 4 pages, RevTeX + epsf, 4 figure
Herbivore biocontrol and manual removal successfully reduce invasive macroalgae on coral reefs
Invasive macroalgae pose a serious threat to coral reef biodiversity by monopolizing reef habitats, competing with native species, and directly overgrowing, and smothering reef corals. Several invasive macroalgae (Eucheuma clade E, Kappaphycus clade A and B, Gracilaria salicornia, and Acanthophora spicifera) are established within KÄneâohe Bay (Oâahu, Hawaiâi, USA), and reducing invasive macroalgae cover is a coral reef conservation and management priority. Invasive macroalgae control techniques, however, are limited and few successful large-scale applications exist. Therefore, a two-tiered invasive macroalgae control approach was designed, where first, divers manually remove invasive macroalgae (Eucheuma and Kappaphycus) aided by an underwater vacuum system (âThe Super Suckerâ). Second, hatchery-raised juvenile sea urchins (Tripneustes gratilla), were outplanted to graze and control invasive macroalgae regrowth. To test the effectiveness of this approach in a natural reef ecosystem, four discrete patch reefs with high invasive macroalgae cover (15â26%) were selected, and macroalgae removal plus urchin biocontrol (treatment reefs, n = 2), or no treatment (control reefs, n = 2), was applied at the patch reef-scale. In applying the invasive macroalgae treatment, the control effort manually removed âŒ19,000 kg of invasive macroalgae and âŒ99,000 juvenile sea urchins were outplanted across to two patch reefs, totaling âŒ24,000 m2 of reef area. Changes in benthic cover were monitored over 2 years (five sampling periods) before-and-after the treatment was applied. Over the study period, removal and biocontrol reduced invasive macroalgae cover by 85% at treatment reefs. Our results show manual removal in combination with hatchery raised urchin biocontrol to be an effective management approach in controlling invasive macroalgae at reef-wide spatial scales and temporal scales of months to years
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