1,078 research outputs found
Modeling the gas-phase chemistry of the transitional disk around HD 141569A
Aims: The chemistry, distribution and mass of the gas in the transitional
disk around the 5 Myr old B9.5 V star HD 141569A are constrained.
Methods: A quasi 2-dimensional (2D) chemistry code for photon dominated
regions (PDR) is used to calculate the chemistry and gas temperatures in the
disk. The calculations are performed for several gas distributions, PAH
abundances and values of the total gas mass. The resulting CO J=2-1 and J=3-2
emission lines are computed with a 2D radiative transfer code and are compared
to observations.
Results: The CO abundance is very sensitive to the total disk mass because
the disk is in a regime where self-shielding just sets in. The observed CO
emission lines are best fit by a power-law gas distribution of 80 M_earth
starting at 80 AU from the central star, indicating that there is some gas in
the inner hole. Predictions are made for intensities of atomic fine-structure
lines. [C I], which is the dominant form of carbon in large parts of the disk,
is found to be a good alternative tracer of the gas mass.Comment: 11 pages, 9 figures. Accepted for publication in A&
Higher analogues of the discrete-time Toda equation and the quotient-difference algorithm
The discrete-time Toda equation arises as a universal equation for the
relevant Hankel determinants associated with one-variable orthogonal
polynomials through the mechanism of adjacency, which amounts to the inclusion
of shifted weight functions in the orthogonality condition. In this paper we
extend this mechanism to a new class of two-variable orthogonal polynomials
where the variables are related via an elliptic curve. This leads to a `Higher
order Analogue of the Discrete-time Toda' (HADT) equation for the associated
Hankel determinants, together with its Lax pair, which is derived from the
relevant recurrence relations for the orthogonal polynomials. In a similar way
as the quotient-difference (QD) algorithm is related to the discrete-time Toda
equation, a novel quotient-quotient-difference (QQD) scheme is presented for
the HADT equation. We show that for both the HADT equation and the QQD scheme,
there exists well-posed -periodic initial value problems, for almost all
\s\in\Z^2. From the Lax-pairs we furthermore derive invariants for
corresponding reductions to dynamical mappings for some explicit examples.Comment: 38 page
Using discrete Darboux polynomials to detect and determine preserved measures and integrals of rational maps
In this Letter we propose a systematic approach for detecting and calculating
preserved measures and integrals of a rational map. The approach is based on
the use of cofactors and Discrete Darboux Polynomials and relies on the use of
symbolic algebra tools. Given sufficient computing power, all rational
preserved integrals can be found.
We show, in two examples, how to use this method to detect and determine
preserved measures and integrals of the considered rational maps.Comment: 8 pages, 1 Figur
CO ro-vibrational lines in HD100546: A search for disc asymmetries and the role of fluorescence
We have studied the emission of CO ro-vibrational lines in the disc around
the Herbig Be star HD100546 with the final goal of using these lines as a
diagnostic to understand inner disc structure in the context of planet
formation. High-resolution IR spectra of CO ro-vibrational emission at eight
different position angles were taken with CRIRES at the VLT. From these spectra
flux tables, CO ro-vibrational line profiles, and population diagrams were
produced. We have investigated variations in the line profile shapes and line
strengths as a function of slit position angle. We used the thermochemical disc
modelling code ProDiMo based on the chemistry, radiation field, and temperature
structure of a previously published model for HD100546. Comparing observations
and the model, we investigated the possibility of disc asymmetries, the
excitation mechanism (UV fluorescence), the geometry, and physical conditions
of the inner disc. The observed CO ro-vibrational lines are largely emitted
from the inner rim of the outer disc at 10-13 AU. The line shapes are similar
for all v levels and line fluxes from all vibrational levels vary only within
one order of magnitude. All line profile asymmetries and variations can be
explained with a symmetric disc model to which a slit correction and pointing
offset is applied. Because the angular size of the CO emitting region (10-13
AU) and the slit width are comparable the line profiles are very sensitive to
the placing of the slit. The model reproduces the line shapes and the fluxes of
the v=1-0 lines as well as the spatial extent of the CO ro-vibrational
emission. It does not reproduce the observed band ratios of 0.5-0.2 with higher
vibrational bands. We find that lower gas volume densities at the surface of
the inner rim of the outer disc can make the fluorescence pumping more effcient
and reproduce the observed band ratios.Comment: 20 pages, 21 figure
Neurobehavioral effects of transportation noise in primary schoolchildren: a cross-sectional study.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.BACKGROUND: Due to shortcomings in the design, no source-specific exposure-effect relations are as yet available describing the effects of noise on children's cognitive performance. This paper reports on a study investigating the effects of aircraft and road traffic noise exposure on the cognitive performance of primary schoolchildren in both the home and the school setting. METHODS: Participants were 553 children (age 9-11 years) attending 24 primary schools around Schiphol Amsterdam Airport. Cognitive performance was measured by the Neurobehavioral Evaluation System (NES), and a set of paper-and-pencil tests. Multilevel regression analyses were applied to estimate the association between noise exposure and cognitive performance, accounting for demographic and school related confounders. RESULTS: Effects of school noise exposure were observed in the more difficult parts of the Switching Attention Test (SAT): children attending schools with higher road or aircraft noise levels made significantly more errors. The correlational pattern and factor structure of the data indicate that the coherence between the neurobehavioral tests and paper-and-pencil tests is high. CONCLUSIONS: Based on this study and previous scientific literature it can be concluded that performance on simple tasks is less susceptible to the effects of noise than performance on more complex tasks.Funding was provided by the European Community (QLRT-2000-00197), the Department of Environment, Food and Rural Affairs in the UK, the Dutch Ministry of Spatial Planning, Housing and the Environment, the Dutch Ministry of Public Health, Welfare and Sports, and the Dutch Ministry of Transport, Public Works and Water Management
Line emission from circumstellar disks around A stars
The nature of the tenuous disks around A stars has raised considerable
controversy in the literature during the past decade. The debate whether or not
the disk around beta Pictoris contains gaseous molecular hydrogen is only the
most recent example. Since CO is in general a poor tracer for the gas content
of these low mass disks, we discuss here detailed emission line calculations
for alternative tracers like C and C+, based on recent optically thin disk
models by Kamp & van Zadelhoff (2001). The [CII] 157.7 mum line was searched
toward Vega and beta Pictoris -- the most prominent A stars with disks -- using
ISO LWS data, and a tentative detection is reported toward the latter object.
From a comparison with emission line observations as well as absorption line
studies of both stars, the gas-to-dust ratio is constrained to lie between 0.5
and 9 for beta Pictoris. For Vega the [CII] observations indicate an upper
limit of 0.2 M_Earth for the disk gas mass. Predicted line intensities of C+
and C are presented for a range of models and appear promising species to trace
the gas content in the disks around A stars with future instrumental
capabilities (SOFIA, Herschel, APEX and ALMA). Searches for CO emission should
focus on the J=3-2 line.Comment: 14 pages, to appear in A&A (accepted
Radiation thermo-chemical models of protoplanetary discs. III. Impact of inner rims on Spectral Energy Distributions
We study the hydrostatic density structure of the inner disc rim around
HerbigAe stars using the thermo-chemical hydrostatic code ProDiMo. We compare
the Spectral Energy Distributions (SEDs) and images from our hydrostatic disc
models to that from prescribed density structure discs. The 2D continuum
radiative transfer in ProDiMo includes isotropic scattering. The dust
temperature is set by the condition of radiative equilibrium. In the
thermal-decoupled case the gas temperature is governed by the balance between
various heating and cooling processes. The gas and dust interact thermally via
photoelectrons, radiatively, and via gas accommodation on grain surfaces. As a
result, the gas is much hotter than in the thermo-coupled case, where the gas
and dust temperatures are equal, reaching a few thousands K in the upper disc
layers and making the inner rim higher. A physically motivated density drop at
the inner radius ("soft-edge") results in rounded inner rims, which appear
ring-like in near-infrared images. The combination of lower gravity pull and
hot gas beyond ~1 AU results in a disc atmosphere that reaches a height over
radius ratio z/r of 0.1 while this ratio is 0.2 only in the thermo-coupled
case. This puffed-up disc atmosphere intercepts larger amount of stellar
radiation, which translates into enhanced continuum emission in the 3- 30
micron wavelength region from hotter grains at ~500 K. We also consider the
effect of disc mass and grain size distribution on the SEDs self-consistently
feeding those quantities back into the gas temperature, chemistry, and
hydrostatic equilibrium computation.Comment: Accepted to MNRA
Trees and superintegrable Lotka-Volterra families
To any tree on vertices we associate an -dimensional Lotka-Volterra
system with parameters and prove it is superintegrable, i.e. it admits
functionally independent integrals. We also show how these systems can be
reduced to an ()-dimensional system which is superintegrable and solvable
by quadratures.Comment: 13 pages, 2 figure
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