124 research outputs found
Optical properties of potential condensates in exoplanetary atmospheres
The prevalence of clouds in currently observable exoplanetary atmospheres
motivates the compilation and calculation of their optical properties. First,
we present a new open-source Mie scattering code known as LX-MIE, which is able
to consider large size parameters () using a single computational
treatment. We validate LX-MIE against the classical MIEV0 code as well as
previous studies. Second, we embark on an expanded survey of the published
literature for both the real and imaginary components of the refractive indices
of 32 condensate species. As much as possible, we rely on experimental
measurements of the refractive indices and resort to obtaining the real from
the imaginary component (or vice versa), via the Kramers-Kronig relation, only
in the absence of data. We use these refractive indices as input for LX-MIE to
compute the absorption, scattering and extinction efficiencies of all 32
condensate species. Finally, we use a three-parameter function to provide
convenient fits to the shape of the extinction efficiency curve. We show that
the errors associated with these simple fits in the Wide Field Camera 3 (WFC3),
J, H and K wavebands are . These fits allow for the extinction cross
section or opacity of the condensate species to be easily included in retrieval
analyses of transmission spectra. We discuss prospects for future experimental
work. The compilation of the optical constants and LX-MIE are publicly
available as part of the open-source Exoclime Simulation Platform
(http://www.exoclime.org).Comment: accepted version; 15 pages, 5 figures, 3 table
The theory of transmission spectra revisited: a semi-analytical method for interpreting WFC3 data and an unresolved challenge
The computation of transmission spectra is a central ingredient in the study
of exoplanetary atmospheres. First, we revisit the theory of transmission
spectra, unifying ideas from several workers in the literature. Transmission
spectra lack an absolute normalization due to the a priori unknown value of a
reference transit radius, which is tied to an unknown reference pressure. We
show that there is a degeneracy between the uncertainty in the transit radius,
the assumed value of the reference pressure (typically set to 10 bar) and the
inferred value of the water abundance when interpreting a WFC3 transmission
spectrum. Second, we show that the transmission spectra of isothermal
atmospheres are nearly isobaric. We validate the isothermal, isobaric
analytical formula for the transmission spectrum against full numerical
calculations and show that the typical errors are ~0.1% (~10 ppm) within the
WFC3 range of wavelengths for temperatures of 1500 K (or higher). Third, we
generalize the previous expression for the transit radius to include a small
temperature gradient. Finally, we analyze the measured WFC3 transmission
spectrum of WASP-12b and demonstrate that we obtain consistent results with the
retrieval approach of Kreidberg et al. (2015) if the reference transit radius
and reference pressure are fixed to assumed values. The unknown functional
relationship between the reference transit radius and reference pressure
implies that it is the product of the water abundance and reference pressure
that is being retrieved from the data, and not just the water abundance alone.
This degeneracy leads to a limitation on how accurately we may extract
molecular abundances from transmission spectra using WFC3 data alone. Finally,
we compare our study to that of Griffith (2014) and discuss why the degeneracy
was missed in previous retrieval studies. [abridged]Comment: Accepted by MNRAS. 11 pages, 7 figures, 2 table
Analytical Models of Exoplanetary Atmospheres. IV. Improved Two-stream Radiative Transfer for the Treatment of Aerosols
We present a novel generalization of the two-stream method of radiative
transfer, which allows for the accurate treatment of radiative transfer in the
presence of strong infrared scattering by aerosols. We prove that this
generalization involves only a simple modification of the coupling coefficients
and transmission functions in the hemispheric two-stream method. This
modification originates from allowing the ratio of the first Eddington
coefficients to depart from unity. At the heart of the method is the fact that
this ratio may be computed once and for all over the entire range of values of
the single-scattering albedo and scattering asymmetry factor. We benchmark our
improved two-stream method by calculating the fraction of flux reflected by a
single atmospheric layer (the reflectivity) and comparing these calculations to
those performed using a 32-stream discrete-ordinates method. We further compare
our improved two-stream method to the two-stream source function (16 streams)
and delta-Eddington methods, demonstrating that it is often more accurate at
the order-of-magnitude level. Finally, we illustrate its accuracy using a toy
model of the early Martian atmosphere hosting a cloud layer composed of
carbon-dioxide ice particles. The simplicity of implementation and accuracy of
our improved two-stream method renders it suitable for implementation in
three-dimensional general circulation models. In other words, our improved
two-stream method has the ease of implementation of a standard two-stream
method, but the accuracy of a 32-stream method.Comment: Accepted by ApJS. 7 pages, 5 figure
Revisiting the Scattering Greenhouse Effect of CO2 Ice Clouds
Carbon dioxide ice clouds are thought to play an important role for cold terrestrial planets with thick CO2 dominated atmospheres. Various previous studies showed that a scattering greenhouse effect by carbon dioxide ice clouds could result in a massive warming of the planetary surface. However, all of these studies only employed simplified two-stream radiative transfer schemes to describe the anisotropic scattering. Using accurate radiative transfer models with a general discrete ordinate method, this study revisits this important effect and shows that the positive climatic impact of carbon dioxide clouds was strongly overestimated in the past. The revised scattering greenhouse effect can have important implications for the early Mars, but also for planets like the early Earth or the position of the outer boundary of the habitable zone
Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations
A terrestrial planet is molten during formation and may remain so if subject
to intense insolation or tidal forces. Observations continue to favour the
detection and characterisation of hot planets, potentially with large outgassed
atmospheres. We aim to determine the radius of hot Earth-like planets with
large outgassed atmospheres and explore differences between molten and solid
silicate planets and their influence on the mass-radius relationship and
transmission and emission spectra. An interior-atmosphere model, combined with
static structure calculations, tracks the evolving radius of a rocky mantle
that is outgassing CO and HO. Synthetic emission and transmission
spectra are generated for CO and HO dominated atmospheres. Atmospheres
dominated by CO suppress the outgassing of HO to a greater extent than
previously realised, as previous studies have applied an erroneous relationship
between volatile mass and partial pressure. We therefore predict more HO
can be retained by the interior during the later stages of magma ocean
crystallisation. Furthermore, formation of a lid at the surface can tie
outgassing of HO to the efficiency of heat transport through the lid,
rather than the atmosphere's radiative timescale. Contraction of the mantle as
it solidifies gives radius decrease, which can partly be offset by
addition of a relatively light species to the atmosphere. We conclude that a
molten silicate mantle can increase the radius of a terrestrial planet by
around compared to its solid counterpart, or equivalently account for a
decrease in bulk density. An outgassing atmosphere can perturb the total
radius according to its speciation. Atmospheres of terrestrial planets around
M-stars that are dominated by CO or HO can be distinguished by
observing facilities with extended wavelength coverage (e.g., JWST).Comment: 19 pages, published in A&A, abstract shortene
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
A ground-based NUV secondary eclipse observation of KELT-9b
KELT-9b is a recently discovered exoplanet with a 1.49 d orbit around a
B9.5/A0-type star. The unparalleled levels of UV irradiation it receives from
its host star put KELT-9b in its own unique class of ultra-hot Jupiters, with
an equilibrium temperature > 4000 K. The high quantities of dissociated
hydrogen and atomic metals present in the dayside atmosphere of KELT-9b bear
more resemblance to a K-type star than a gas giant. We present a single
observation of KELT-9b during its secondary eclipse, taken with the Wide Field
Camera on the Isaac Newton Telescope (INT). This observation was taken in the
U-band, a window particularly sensitive to Rayleigh scattering. We do not
detect a secondary eclipse signal, but our 3 upper limit of 181 ppm on
the depth allows us to constrain the dayside temperature of KELT-9b at
pressures of ~30 mbar to 4995 K (3). Although we can place an
observational constraint of 0.14, our models suggest that the actual
value is considerably lower than this due to H opacity. This places KELT-9b
squarely in the albedo regime populated by its cooler cousins, almost all of
which reflect very small components of the light incident on their daysides.
This work demonstrates the ability of ground-based 2m-class telescopes like the
INT to perform secondary eclipse studies in the NUV, which have previously only
been conducted from space-based facilities.Comment: Accepted in ApJL. 7 pages, 3 figure
FastChem Cond: Equilibrium chemistry with condensation and rainout for cool planetary and stellar environments
Cool astrophysical objects, such as (exo)planets, brown dwarfs, or asymptotic
giant branch stars, can be strongly affected by condensation. Condensation does
not only directly affect the chemical composition of the gas phase by removing
elements but the condensed material also influences other chemical and physical
processes in these object. This includes, for example, the formation of clouds
in planetary atmospheres and brown dwarfs or the dust-driven winds of evolved
stars. In this study we introduce FastChem Cond, a new version of the FastChem
equilibrium chemistry code that adds a treatment of equilibrium condensation.
Determining the equilibrium composition under the impact of condensation is
complicated by the fact that the number of condensates that can exist in
equilibrium with the gas phase is limited by a phase rule. However, this phase
rule does not directly provide information on which condensates are stable. As
a major advantage of FastChem Cond is able to automatically select the set
stable condensates satisfying the phase rule. Besides the normal equilibrium
condensation, FastChem Cond can also be used with the rainout approximation
that is commonly employed in atmospheres of brown dwarfs or (exo)planets.
FastChem Cond is available as open-source code, released under the GPLv3
licence. In addition to the C++ code, FastChem Cond also offers a Python
interface. Together with the code update we also add about 290 liquid and solid
condensate species to FastChem.Comment: submitted to MNRAS, code available at
https://github.com/exoclime/FastChe
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