2,255 research outputs found
Comparison of cloud models for Brown Dwarfs
A test case comparison is presented for different dust cloud model approaches
applied in brown dwarfs and giant gas planets. We aim to achieve more
transparency in evaluating the uncertainty inherent to theoretical modelling.
We show in how far model results for characteristic dust quantities vary due to
different assumptions. We also demonstrate differences in the spectral energy
distributions resulting from our individual cloud modelling in 1D substellar
atmosphere simulationsComment: 5 pages, Proceeding to "Exoplantes: Detection, Formation, Dynamics",
eds. Ferraz-Mello et
Determination of Optical Constants for Titan Aerosol-, and Exoplanet and Brown Dwarf Cloud Particle Analogs from the Visible to the Far Infrared
Here we present optical constants covering a broad wavelength range, from the visible to the far infrared, for Titan aerosol analogs produced in the Titan Haze Simulation (THS) experiment at Ames COSmIC facility, as well as other exoplanet-relevant materials
Radiative Transfer for Exoplanet Atmospheres
Remote sensing of the atmospheres of distant worlds motivates a firm
understanding of radiative transfer. In this review, we provide a pedagogical
cookbook that describes the principal ingredients needed to perform a radiative
transfer calculation and predict the spectrum of an exoplanet atmosphere,
including solving the radiative transfer equation, calculating opacities (and
chemistry), iterating for radiative equilibrium (or not), and adapting the
output of the calculations to the astronomical observations. A review of the
state of the art is performed, focusing on selected milestone papers.
Outstanding issues, including the need to understand aerosols or clouds and
elucidating the assumptions and caveats behind inversion methods, are
discussed. A checklist is provided to assist referees/reviewers in their
scrutiny of works involving radiative transfer. A table summarizing the
methodology employed by past studies is provided.Comment: 7 pages, no figures, 1 table. Filled in missing information in
references, main text unchange
Detectability of dirty dust grains in brown dwarf atmospheres
Dust clouds influence the atmospheric structure of brown dwarfs, and they
affect the heat transfer and change the gas-phase chemistry. However, the
physics of their formation and evolution is not well understood. In this
letter, we predict dust signatures and propose a potential observational test
of the physics of dust formation in brown dwarf atmosphere based on the
spectral features of the different solid components predicted by dust formation
theory. A momentum method for the formation of dirty dust grains (nucleation,
growth, evaporation, drift) is used in application to a static brown dwarf
atmosphere structure to compute the dust grain properties, in particular the
heterogeneous grain composition and the grain size. Effective medium and Mie
theory are used to compute the extinction of these spherical grains. Dust
formation results in grains whose composition differs from that of grains
formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models
because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong
broad absorption feature centered at 8.7mum, while amorphous
Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at
longer wavelength. Those features at lambda < 15mum are detectable in
absorption if grains are small (radius < 0.2mum) in the upper atmosphere as
suggested by our model. We suggest that the detection of a feature at 8.7mum in
deep infrared spectra could provide evidence for non-equilibrium dust formation
that yields grains composed of metastable solids in brown dwarf atmospheres.
This feature will shift towards 10mum and broaden if silicates (e.g. fosterite)
are much more abundant.Comment: A&A Letter, accepte
Simulated Tractor Chassis Suspension System
The development of a chassis suspension system for an agricultural tractor has been described to improve the operator ride comfort. Computer simulation tech-niques were used to formulate the tractor models and to compute the natural frequencies and frequency response of the models, as well as the RMS acceleration response, to evaluate the effect of the suspension system and cab position on operator ride comfort
A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres
The atmospheres of substellar objects contain clouds of oxides, iron,
silicates, and other refractory condensates. Water clouds are expected in the
coolest objects. The opacity of these `dust' clouds strongly affects both the
atmospheric temperature-pressure profile and the emergent flux. Thus any
attempt to model the spectra of these atmospheres must incorporate a cloud
model. However the diversity of cloud models in atmospheric simulations is
large and it is not always clear how the underlying physics of the various
models compare. Likewise the observational consequences of different modeling
approaches can be masked by other model differences, making objective
comparisons challenging. In order to clarify the current state of the modeling
approaches, this paper compares five different cloud models in two sets of
tests. Test case 1 tests the dust cloud models for a prescribed L, L--T, and
T-dwarf atmospheric (temperature T, pressure p, convective velocity
vconv)-structures. Test case 2 compares complete model atmosphere results for
given (effective temperature Teff, surface gravity log g). All models agree on
the global cloud structure but differ in opacity-relevant details like grain
size, amount of dust, dust and gas-phase composition. Comparisons of synthetic
photometric fluxes translate into an modelling uncertainty in apparent
magnitudes for our L-dwarf (T-dwarf) test case of 0.25 < \Delta m < 0.875 (0.1
< \Delta m M 1.375) taking into account the 2MASS, the UKIRT WFCAM, the Spitzer
IRAC, and VLT VISIR filters with UKIRT WFCAM being the most challenging for the
models. (abr.)Comment: 22 pages, 17 figures, MNRAS 2008, accepted, (minor grammar/typo
corrections
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