155 research outputs found
Cloud formation in giant planets
We calculate the formation of dust clouds in atmospheres of giant
gas-planets. The chemical structure and the evolution of the grain size
distribution in the dust cloud layer is discussed based on a consistent
treatment of seed formation, growth/evaporation and gravitational settling.
Future developments are shortly addressed.Comment: 4 pages, Proceeding to "Extreme solar systems", eds. Fischer, Rasio,
Thorsett, Wolszcza
Expect the unexpected: non-equilibrium processes in brown dwarf atmospheres
Brown Dwarf atmosphere are a chemically extremely rich, one example being the
formation of clouds driven by the phase-non-equilibrium of the atmospheric gas.
Cloud formation modelling is an integral part of any atmosphere simulation used
to interpret spectral observations of ultra-cool objects and to determine
fundamental parameters like log(g) and Teff. This proceeding to the workshop
'GAIA and the Unseen: The Brown Dwarf Question' first summarizes what a model
atmosphere simulation is, and then advocates two ideas: A) The use of a
multitude of model families to determine fundamental parameters with realistic
confidence interval. B) To keep an eye on the unexpected, like for example,
ionisation signatures resulting plasma processesComment: 5 pages, proceeding to the workshop 'GAIA and the Unseen: The Brown
Dwarf Question
Lightning and charge processes in brown dwarf and exoplanet atmospheres
The study of the composition of brown dwarf atmospheres helped to understand
their formation and evolution. Similarly, the study of exoplanet atmospheres is
expected to constrain their formation and evolutionary states. We use results
from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry
to investigate ionisation processes which will affect their atmosphere
chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen,
and not HO. Such planetary atmospheres exhibit a substantial degree of
thermal ionisation and clouds only form on the nightside where lightning leaves
chemical tracers (e.g. HCN) for possibly long enough to be detectable. External
radiation may cause exoplanets to be enshrouded in a shell of highly ionised,
H-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable
us to study the role of electron beams for the emergence of an extrasolar,
weather-system driven aurora-like chemistry, and the effect of strong magnetic
fields on cold atmospheric gases. Electron beams trigger the formation of
H in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may
react with it to form hydronium, HO, as a longer lived chemical tracer.
Brown dwarfs and super-hot gas giants may be excellent candidates to search for
HO as an H product.Comment: 16 pages, accepted for publication in the Philosophical Transactions
A of the Royal Society, (some typos corrected
Small hydrocarbon molecules in cloud-forming Brown Dwarf and giant gas planet atmospheres
We study the abundances of complex carbon-bearing molecules in the
oxygen-rich dust- forming atmospheres of Brown Dwarfs and giant gas planets.
The inner atmospheric re- gions that form the inner boundary for thermochemical
gas-phase models are investigated. Results from Drift-phoenix atmosphere
simulations, which include the feedback of phase- non-equilibrium dust cloud
formation on the atmospheric structure and the gas-phase abun- dances, are
utilised. The resulting element depletion leads to a shift in the
carbon-to-oxygen ratio such that several hydrocarbon molecules and
cyanopolycyanopolyynene molecules can be present. An increase in surface
gravity and/or a decrease in metallicity support the increase in the partial
pressures of these species. CO, CO2, CH4, and HCN contain the largest fraction
of carbon. In the upper atmosphere of low-metallicity objects, more carbon is
contained in C4H than in CO, and also CH3 and C2H2 play an increasingly
important role as carbon-sink. We determine chemical relaxation time-scales to
evaluate if hydrocarbon molecules can be affected by transport-induced
quenching. Our results suggest that a considerable amount of C2H6 and C2H2
could be expected in the upper atmospheres not only of giant gas planets, but
also of Brown Dwarfs. However, the exact quenching height strongly depends on
the data source used. These results will have an impact on future
thermo-kinetic studies, as they change the inner boundary condition for those
simulations.Comment: 17 pages, 13 figures, 1 table, accepted to MNRA
The need for small-scale turbulence in atmospheres of substellar objects
Brown dwarfs and giant gas planets are substellar objects whose spectral
appearance is determined by the chemical composition of the gas and the
solids/liquids in the atmosphere. Atmospheres of substellar objects possess two
major scale regimes: large-scale convective motions + gravitational settling
and small-scale turbulence + dust formation. Turbulence initiates dust
formation spot-like on small scale, while the dust feeds back into the
turbulent fluid field by its strong radiative cooling. Small, imploding dust
containing areas result which eventually become isothermal. Multi-dimensional
simulations show that these small-scale dust structures gather into large-scale
structures, suggesting the formation of clouds made of dirty dust grains. The
chemical composition of the grains, and thereby the chemical evolution of the
gas phase, is a function of temperature and depends on the grain's history.Comment: 7 pages, 2 figues, contribution to the Workshop on Interdisciplinary
Aspects of Turbulence, April 18 - 22, 2005, Castle Ringberg, German
The multi-scale dust formation in substellar atmospheres
Substellar atmospheres are observed to be irregularly variable for which the
formation of dust clouds is the most promising candidate explanation. The
atmospheric gas is convectively unstable and, last but not least, colliding
convective cells are seen as cause for a turbulent fluid field. Since dust
formation depends on the local properties of the fluid, turbulence influences
the dust formation process and may even allow the dust formation in an
initially dust-hostile gas. A regime-wise investigation of dust forming
substellar atmospheric situations reveals that the largest scales are
determined by the interplay between gravitational settling and convective
replenishment which results in a dust-stratified atmosphere. The regime of
small scales is determined by the interaction of turbulent fluctuations.
Resulting lane-like and curled dust distributions combine to larger and larger
structures. We compile necessary criteria for a subgrid model in the frame of
large scale simulations as result of our study on small scale turbulence in
dust forming gases.Comment: 22 Pages, 5 Figures, to appear in "Analysis and Numerics of
Conservation Laws", ed. G. Warnecke (Springer-Verlag
Exoplanet Clouds
Clouds also form in atmospheres of planets that orbit other stars than our
Sun, in so-called extrasolar planets or exoplanets. Exoplanet atmospheres can
be chemically extremely rich. Exoplanet clouds are therefor made of a mix of
materials that changes throughout the atmosphere. They affect the atmospheres
through element depletion and through absorption and scattering, hence, they
have a profound impact on the atmosphere's energy budget. While astronomical
observations point us to the presence of extrasolar clouds and make first
suggestions on particle sizes and material compositions, we require fundamental
and complex modelling work to merge the individual observations into a coherent
picture. Part of this is to develop an understanding for cloud formation in
non-terrestrial environments.Comment: Review paper for Annual Review of Earth and Planetary Sciences (26
pages), accepted for publicatio
Exo-lightning radio emission: the case study of HAT-P-11b
Lightning induced radio emission has been observed on solar system planets.
Lecavelier des Etangs et al. [2013] carried out radio transit observations of
the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal.
Here, we explore the possibility of the radio emission having been produced by
lightning activity on the exoplanet, following and expanding the work of
Hodos\'an et al. [2016a]. After a summary of our previous work [Hodos\'an et
al. 2016a], we extend it with a parameter study. The lightning activity of the
hypothetical storm is largely dependent on the radio spectral roll-off, ,
and the flash duration, . The best-case scenario would
require a flash density of the same order of magnitude as can be found during
volcanic eruptions on Earth. On average, times larger flash
densities than the Earth-storms with the largest lightning activity is needed
to produce the observed signal from HAT-P-11b. Combined with the results of
Hodos\'an et al. [2016a] regarding the chemical effects of planet-wide
thunderstorms, we conclude that future radio and infrared observations may lead
to lightning detection on planets outside the solar system.Comment: Accepted to the Conference Proceedings of the 8th International
Workshop on Planetary, Solar and Heliospheric Radio Emissions (PRE 8), held
in Seggauberg near Leibnitz/Graz, Austria, October 25-27, 2016. 12 pages, 2
figure
Jupiter as a Giant Cosmic Ray Detector
We explore the feasibility of using the atmosphere of Jupiter to detect
Ultra-High-Energy Cosmic Rays (UHECR's). The large surface area of Jupiter
allows us to probe cosmic rays of higher energies than previously accessible.
Cosmic ray extensive air showers in Jupiter's atmosphere could in principle be
detected by the Large Area Telescope (LAT) on the Fermi observatory. In order
to be observed, these air showers would need to be oriented toward the Earth,
and would need to occur sufficiently high in the atmosphere that the gamma rays
can penetrate. We demonstrate that, under these assumptions, Jupiter provides
an effective cosmic ray "detector" area of km. We predict
that Fermi-LAT should be able to detect events of energy eV with
fluence erg cm at a rate of about one per month. The observed
number of air showers may provide an indirect measure of the flux of cosmic
rays eV. Extensive air showers also produce a synchrotron
signature that may be measurable by ALMA. Simultaneous observations of Jupiter
with ALMA and Fermi-LAT could be used to provide broad constraints on the
energies of the initiating cosmic rays.Comment: 8 pages, 5 figures. Accepted for publication in the Astrophysical
Journal Letter
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