231 research outputs found
Clouds in the atmospheres of extrasolar planets. IV. On the scattering greenhouse effect of CO2 ice particles: Numerical radiative transfer studies
Owing to their wavelengths dependent absorption and scattering properties,
clouds have a strong impact on the climate of planetary atmospheres.
Especially, the potential greenhouse effect of CO2 ice clouds in the
atmospheres of terrestrial extrasolar planets is of particular interest because
it might influence the position and thus the extension of the outer boundary of
the classic habitable zone around main sequence stars.
We study the radiative effects of CO2 ice particles obtained by different
numerical treatments to solve the radiative transfer equation. The comparison
between the results of a high-order discrete ordinate method and simpler
two-stream approaches reveals large deviations in terms of a potential
scattering efficiency of the greenhouse effect. The two-stream methods
overestimate the transmitted and reflected radiation, thereby yielding a higher
scattering greenhouse effect. For the particular case of a cool M-type dwarf
the CO2 ice particles show no strong effective scattering greenhouse effect by
using the high-order discrete ordinate method, whereas a positive net
greenhouse effect was found in case of the two-stream radiative transfer
schemes. As a result, previous studies on the effects of CO2 ice clouds using
two-stream approximations overrated the atmospheric warming caused by the
scattering greenhouse effect. Consequently, the scattering greenhouse effect of
CO2 ice particles seems to be less effective than previously estimated. In
general, higher order radiative transfer methods are necessary to describe the
effects of CO2 ice clouds accurately as indicated by our numerical radiative
transfer studies.Comment: accepted for publication in A&
On the climatic impact of CO2 ice particles in atmospheres of terrestrial exoplanets
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Clouds play a significant role for the energy budget in planetary atmospheres. They can scatter incident stellar radiation back to space, effectively cooling the surface of terrestrial planets. On the other hand, they may contribute to the atmospheric greenhouse effect by trapping outgoing thermal radiation. For exoplanets near the outer boundary of the habitable zone, condensation of CO2 can occur due to the low atmospheric temperatures. These CO2 ice clouds may play an important role for the surface temperature and, therefore, for the question of habitability of those planets. However, the optical properties of CO2 ice crystals differ significantly from those of water droplets or water ice particles. Except for a small number of strong absorption bands, they are almost transparent with respect to absorption. Instead, they are highly effective scatterers at long and short wavelengths. Therefore, the climatic effect of a CO2 ice cloud will depend on how much incident stellar radiation is scattered to space in comparison to the amount of thermal radiation scattered back towards the planetary surface. This contribution aims at the potential greenhouse effect of CO2 ice particles. Their scattering and absorption properties are calculated for assumed particle size distributions with different effective radii and particle densities. An accurate radiative transfer model is used to determine the atmospheric radiation field affected by such CO2 particles. These results are compared to less detailed radiative transfer schemes employed in previous studies
Low-temperature nucleation in a kinetic Ising model under different stochastic dynamics with local energy barriers
Using both analytical and simulational methods, we study low-temperature
nucleation rates in kinetic Ising lattice-gas models that evolve under two
different Arrhenius dynamics that interpose between the Ising states a
transition state representing a local energy barrier. The two dynamics are the
transition-state approximation [T. Ala-Nissila, J. Kjoll, and S. C. Ying, Phys.
Rev. B 46, 846 (1992)] and the one-step dynamic [H. C. Kang and W. H. Weinberg,
J. Chem. Phys. 90, 2824 (1989)]. Even though they both obey detailed balance
and are here applied to a situation that does not conserve the order parameter,
we find significant differences between the nucleation rates observed with the
two dynamics, and between them and the standard Glauber dynamic [R. J. Glauber,
J. Math. Phys. 4, 294 (1963)], which does not contain transition states. Our
results show that great care must be exercised when devising kinetic Monte
Carlo transition rates for specific physical or chemical systems.Comment: 14 pages RevTex, 6 embedded figures. Minor revisions. J. Chem. Phys.,
in pres
The extrasolar planet Gliese 581 d: a potentially habitable planet? (Corrigendum to arXiv:1009.5814)
We report here that the equation for H2O Rayleigh scattering was incorrectly
stated in the original paper [arXiv:1009.5814]. Instead of a quadratic
dependence on refractivity r, we accidentally quoted an r^4 dependence. Since
the correct form of the equation was implemented into the model, scientific
results are not affected.Comment: accepted to Astronomy&Astrophysic
Quantum approach to nucleation times of kinetic Ising ferromagnets
Low temperature dynamics of Ising ferromagnets under finite magnetic fields
are studied in terms of quantum spin representations of stochastic evolution
operators. These are constructed for the Glauber dynamic as well as for a
modification of this latter, introduced by K. Park {\it et al.} in Phys. Rev.
Lett. {\bf 92}, 015701 (2004). In both cases the relaxation time after a field
quench is evaluated both numerically and analytically using the spectrum gap of
the corresponding operators. The numerical work employs standard recursive
techniques following a symmetrization of the evolution operator accomplished by
a non-unitary spin rotation. The analytical approach uses low temperature
limits to identify dominant terms in the eigenvalue problem. It is argued that
the relaxation times already provide a measure of actual nucleation lifetimes
under finite fields. The approach is applied to square, triangular and
honeycomb lattices.Comment: 14 pages, 6 figure
A spectral survey of an ultra-hot Jupiter: Detection of metals in the transmission spectrum of KELT-9 b
Context: KELT-9 b exemplifies a newly emerging class of short-period gaseous
exoplanets that tend to orbit hot, early type stars - termed ultra-hot
Jupiters. The severe stellar irradiation heats their atmospheres to
temperatures of K, similar to the photospheres of dwarf stars. Due
to the absence of aerosols and complex molecular chemistry at such
temperatures, these planets offer the potential of detailed chemical
characterisation through transit and day-side spectroscopy. Studies of their
chemical inventories may provide crucial constraints on their formation process
and evolution history.
Aims: To search the optical transmission spectrum of KELT-9 b for absorption
lines by metals using the cross-correlation technique.
Methods: We analyse 2 transits observed with the HARPS-N spectrograph. We use
an isothermal equilibrium chemistry model to predict the transmission spectrum
for each of the neutral and singly-ionized atoms with atomic numbers between 3
and 78. Of these, we identify the elements that are expected to have spectral
lines in the visible wavelength range and use those as cross-correlation
templates.
Results: We detect absorption of Na I, Cr II, Sc II and Y II, and confirm
previous detections of Mg I, Fe I, Fe II and Ti II. In addition, we find
evidence of Ca I, Cr I, Co I, and Sr II that will require further observations
to verify. The detected absorption lines are significantly deeper than model
predictions, suggesting that material is transported to higher altitudes where
the density is enhanced compared to a hydrostatic profile. There appears to be
no significant blue-shift of the absorption spectrum due to a net day-to-night
side wind. In particular, the strong Fe II feature is shifted by km~s, consistent with zero. Using the orbital velocity of the
planet we revise the steller and planetary masses and radii.Comment: Submitted to Astronomy and Astrophysics on January 18, 2019. Accepted
on May 3, 2019. 26 pages, 11 figure
Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics
We study low-temperature nucleation in kinetic Ising models by analytical and
simulational methods, confirming the general result for the average metastable
lifetime, = A*exp(beta*Gamma) (beta = 1/kT) [E. Jordao Neves and R.H.
Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we
find that both A and Gamma depend significantly on the stochastic dynamic. In
particular, for a ``soft'' dynamic, in which the effects of the interactions
and the applied field factorize in the transition rates, Gamma does NOT simply
equal the energy barrier against nucleation, as it does for the standard
Glauber dynamic, which does not have this factorization property.Comment: 4 pages RevTex4, 2 figures. Phys. Rev. Lett., in pres
The unstable CO2 feedback cycle on ocean planets
Ocean planets are volatile-rich planets, not present in our Solar system, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet's surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilizing carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilizing effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planet
Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
We study the impact of multi-layered clouds (low-level water and high-level
ice clouds) on the thermal emission spectra of Earth-like planets orbiting
different types of stars. Clouds have an important influence on such planetary
emission spectra due to their wavelength dependent absorption and scattering
properties. We also investigate the influence of clouds on the ability to
derive information about planetary surface temperatures from low-resolution
spectra.Comment: accepted for publication in A&
Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones
The effects of multi-layered clouds in the atmospheres of Earth-like planets
orbiting different types of stars are studied. The radiative effects of cloud
particles are directly correlated with their wavelength-dependent optical
properties. Therefore the incident stellar spectra may play an important role
for the climatic effect of clouds. We discuss the influence of clouds with mean
properties measured in the Earth's atmosphere on the surface temperatures and
Bond albedos of Earth-like planets orbiting different types of main sequence
dwarf stars.Comment: accepted for publication in A&
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