2,017 research outputs found
The Influence of Dust Formation Modelling on Na I and K I Line Profiles in Substellar Atmospheres
We aim to understand the correlation between cloud formation and alkali line
formation in substellar atmospheres.We perform line profile calculations for Na
I and K I based on the coupling of our kinetic model for the formation and
composition of dust grains with 1D radiative transfer calculations in
atmosphere models for brown dwarfs and giant gas planets. The Na I and K I line
profiles sensibly depend on the way clouds are treated in substellar atmosphere
simulations. The kinetic dust formation model results in the highest
pseudo-continuum compared to the limiting cases.Comment: 5 pages, Accepted for publication in MNRA
Benzene formation in the inner regions of protostellar disks
Benzene (c-C6H6) formation in the inner 3 AU of a protostellar disk can be
efficient, resulting in high abundances of benzene in the midplane region. The
formation mechanism is different to that found in interstellar clouds and in
protoplanetary nebulae, and proceeds mainly through the reaction between allene
(C3H4) and its ion. This has implications for PAH formation, in that some
fraction of PAHs seen in the solar system could be native rather than inherited
from the interstellar medium.Comment: 9 pages, 2 colour figures, to be published in the Astrophysical
Journal Letter
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
Dust in brown dwarfs and extra-solar planets IV. Assessing TiO2 and SiO nucleation for cloud formation modeling
Clouds form in atmospheres of brown dwarfs and planets. The cloud particle
formation processes are similar to the dust formation process studied in
circumstellar shells of AGB stars and in Supernovae. Cloud formation modelling
in substellar objects requires gravitational settling and element replenishment
in addition to element depletion. All processes depend on the local conditions,
and a simultaneous treatment is required. We apply new material data in order
to assess our cloud formation model results regarding the treatment of the
formation of condensation seeds. We re-address the question of the primary
nucleation species in view of new (TiO2)_N-cluster data and new SiO vapour
pressure data. We apply the density functional theory using the computational
chemistry package Gaussian 09 to derive updated thermodynamical data for
(TiO2)_N-clusters as input for our TiO2 seed formation model. We test different
nucleation treatments and their effect on the overall cloud structure by
solving a system of dust moment equations and element conservation or a
pre-scribed Drift-Phoenix atmosphere structure. Updated Gibbs free energies for
the (TiO2)_N-clusters are presented, and a slightly temperature dependent
surface tension for T=500 ... 2000K with an average value of sigma_infty =
480.6 erg 1/cm2. The TiO2-seed formation rate changes only slightly with the
updated cluster data. A considerably larger effect on the rate of seed
formation, and hence on grain size and dust number density, results from a
switch to SiO-nucleation. Despite the higher abundance of SiO over TiO2 in the
gas phase, TiO2 remains considerably more efficient in forming condensation
seeds by homogeneous nucleation followed by heterogeneous grain growth. The
paper discussed the effect on the cloud structure in more detail.Comment: accepted for publication in A&A (abstract abridged
Hamiltonian and physical Hilbert space in polymer quantum mechanics
In this paper, a version of polymer quantum mechanics, which is inspired by
loop quantum gravity, is considered and shown to be equivalent, in a precise
sense, to the standard, experimentally tested, Schroedinger quantum mechanics.
The kinematical cornerstone of our framework is the so called polymer
representation of the Heisenberg-Weyl (H-W) algebra, which is the starting
point of the construction. The dynamics is constructed as a continuum limit of
effective theories characterized by a scale, and requires a renormalization of
the inner product. The result is a physical Hilbert space in which the
continuum Hamiltonian can be represented and that is unitarily equivalent to
the Schroedinger representation of quantum mechanics. As a concrete
implementation of our formalism, the simple harmonic oscillator is fully
developed.Comment: 19 pages, 2 figures. Comments and references added. Version to be
published in CQ
Polymer state approximations of Schroedinger wave functions
It is shown how states of a quantum mechanical particle in the Schroedinger
representation can be approximated by states in the so-called polymer
representation. The result may shed some light on the semiclassical limit of
loop quantum gravity.Comment: 11 pages, 1 figure, Conclusions section adde
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
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