1,046 research outputs found
The Nature of Scientific Proof in the Age of Simulations
Is numerical mimicry a third way of establishing truth?Comment: Published in American Scientist: Volume 102, Number 3, Pages 174 to
177
(http://www.americanscientist.org/issues/pub/2014/3/the-nature-of-scientific-proof-in-the-age-of-simulations
A Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines: Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths
We present a dimensionless index that quantifies the degree of cloudiness of
the atmosphere of a transiting exoplanet. Our cloudiness index is based on
measuring the transit radii associated with the line center and wing of the
sodium or potassium line. In deriving this index, we revisited the algebraic
formulae for inferring the isothermal pressure scale height from transit
measurements. We demonstrate that the formulae of Lecavelier et al. and Benneke
& Seager are identical: the former is inferring the temperature while assuming
a value for the mean molecular mass and the latter is inferring the mean
molecular mass while assuming a value for the temperature. More importantly,
these formulae cannot be used to distinguish between cloudy and cloudfree
atmospheres. We derive values of our cloudiness index for a small sample of 7
hot Saturns/Jupiters taken from Sing et al. We show that WASP-17b, WASP-31b and
HAT-P-1b are nearly cloudfree at visible wavelengths. We find the tentative
trend that more irradiated atmospheres tend to have less clouds consisting of
sub-micron-sized particles. We also derive absolute sodium and/or potassium
abundances cm for WASP-17b, WASP-31b and HAT-P-1b (and upper
limits for the other objects). Higher-resolution measurements of both the
sodium and potassium lines, for a larger sample of exoplanetary atmospheres,
are needed to confirm or refute this trend.Comment: Accepted by ApJL. 6 pages, 1 figure, 2 table
On the Existence of Shocks in Irradiated Exoplanetary Atmospheres
Supersonic flows are expected to exist in the atmospheres of irradiated
exoplanets, but the question of whether shocks develop lingers. Specifically,
it reduces to whether continuous flow in a closed loop may become supersonic
and if some portions of the supersonic flow steepen into shocks. We first
demonstrate that continuous, supersonic flow may exist in two flavors:
isentropic and non-isentropic, with shocks being included in the latter class
of solutions. Supersonic flow is a necessary but insufficient condition for
shocks to develop. The development of a shock requires the characteristics of
neighboring points in a flow to intersect. We demonstrate that the intersection
of characteristics may be quantified via knowledge of the Mach number. Finally,
we examine 3D simulations of hot Jovian atmospheres and demonstrate that shock
formation is expected to occur mostly on the dayside hemisphere, upstream of
the substellar point, because the enhanced temperatures near the substellar
point provide a natural pressure barrier for the returning flow. Understanding
the role of shocks in irradiated exoplanetary atmospheres is relevant to
correctly modeling observables such as the peak offsets of infrared phase
curves.Comment: Accepted by ApJ Letters. 5 pages, 4 figure
The Next Great Exoplanet Hunt
What strange new worlds will our next-generation telescopes find?Comment: Published in American Scientist: Volume 103, Number 3, Pages 196 to
203
(http://www.americanscientist.org/issues/feature/2015/3/the-next-great-exoplanet-hunt).
Error concerning liquid helium correcte
Analytical Models of Exoplanetary Atmospheres. I. Atmospheric Dynamics via the Shallow Water System
Within the context of exoplanetary atmospheres, we present a comprehensive
linear analysis of forced, damped, magnetized shallow water systems, exploring
the effects of dimensionality, geometry (Cartesian, pseudo-spherical and
spherical), rotation, magnetic tension and hydrodynamic and magnetic sources of
friction. Across a broad range of conditions, we find that the key governing
equation for atmospheres and quantum harmonic oscillators are identical, even
when forcing (stellar irradiation), sources of friction (molecular viscosity,
Rayleigh drag and magnetic drag) and magnetic tension are included. The global
atmospheric structure is largely controlled by a single, key parameter that
involves the Rossby and Prandtl numbers. This near-universality breaks down
when either molecular viscosity or magnetic drag acts non-uniformly across
latitude or a poloidal magnetic field is present, suggesting that these effects
will introduce qualitative changes to the familiar chevron-shaped feature
witnessed in simulations of atmospheric circulation. We also find that
hydrodynamic and magnetic sources of friction have dissimilar phase signatures
and affect the flow in fundamentally different ways, implying that using
Rayleigh drag to mimic magnetic drag is inaccurate. We exhaustively lay down
the theoretical formalism (dispersion relations, governing equations and
time-dependent wave solutions) for a broad suite of models. In all situations,
we derive the steady state of an atmosphere, which is relevant to interpreting
infrared phase and eclipse maps of exoplanetary atmospheres. We elucidate a
pinching effect that confines the atmospheric structure to be near the equator.
Our suite of analytical models may be used to decisively develop physical
intuition and as a reference point for three-dimensional, magnetohydrodynamic
(MHD) simulations of atmospheric circulation.Comment: Accepted by ApJS, 36 pages, 6 figures, 3 tables, 273 equation
On the Stability of Super-Earth Atmospheres
We investigate the stability of super Earth atmospheres around M stars using
a 7-parameter, analytical framework. We construct stability diagrams in the
parameter space of exoplanetary radius versus semi-major axis and elucidate the
regions in which the atmospheres are stable against the condensation of their
major constituents, out of the gas phase, on their permanent nightside
hemispheres. We find that super Earth atmospheres which are nitrogen-dominated
("Earth-like") occupy a smaller region of allowed parameter space, compared to
hydrogen-dominated atmospheres, because of the dual effects of diminished
advection and enhanced radiative cooling. Furthermore, some super Earths which
reside within the habitable zones of M stars may not possess stable
atmospheres, depending on the mean molecular weight and infrared photospheric
pressure of their atmospheres. We apply our stability diagrams to GJ 436b and
GJ 1214b, and demonstrate that atmospheric compositions with high mean
molecular weights are disfavoured if these exoplanets possess solid surfaces
and shallow atmospheres. Finally, we construct stability diagrams tailored to
the Kepler dataset, for G and K stars, and predict that about half of the
exoplanet candidates are expected to habour stable atmospheres if Earth-like
conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability
diagram for G stars.Comment: Accepted by ApJ. 10 pages, 6 figures. No changes from previous
version, except for added hypen in titl
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
Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalisation degeneracy
A comprehensive analysis of 38 previously published Wide Field Camera 3
(WFC3) transmission spectra is performed using a hierarchy of nested-sampling
retrievals: with versus without clouds, grey versus non-grey clouds, isothermal
versus non-isothermal transit chords and with water, hydrogen cyanide and/or
ammonia. We revisit the "normalisation degeneracy": the relative abundances of
molecules are degenerate at the order-of-magnitude level with the absolute
normalisation of the transmission spectrum. Using a suite of mock retrievals,
we demonstrate that the normalisation degeneracy may be partially broken using
WFC3 data alone, even in the absence of optical/visible data and without
appealing to the presence of patchy clouds, although lower limits to the mixing
ratios may be prior-dominated depending on the measurement uncertainties. With
James Webb Space Telescope-like spectral resolutions, the normalisation
degeneracy may be completely broken from infrared spectra alone. We find no
trend in the retrieved water abundances across nearly two orders of magnitude
in exoplanet mass and a factor of 5 in retrieved temperature (about 500 to 2500
K). We further show that there is a general lack of strong Bayesian evidence to
support interpretations of non-grey over grey clouds (only for WASP-69b and
WASP-76b) and non-isothermal over isothermal atmospheres (no objects). 35 out
of 38 WFC3 transmission spectra are well-fitted by an isothermal transit chord
with grey clouds and water only, while 8 are adequately explained by flat
lines. Generally, the cloud composition is unconstrained.Comment: Accepted by MNRAS. 33 pages, 29 figures, 3 table
Debris discs around M stars: non-existence versus non-detection
Motivated by the reported dearth of debris discs around M stars, we use
survival models to study the occurrence of planetesimal discs around them.
These survival models describe a planetesimal disc with a small number of
parameters, determine if it may survive a series of dynamical processes and
compute the associated infrared excess. For the WISE satellite, we demonstrate
that the dearth of debris discs around M stars may be attributed to the small
semi-major axes generally probed if either: 1. the dust grains behave like
blackbodies emitting at a peak wavelength coincident with the observed one; 2.
or the grains are hotter than predicted by their blackbody temperatures and
emit at peak wavelengths that are shorter than the observed one. At these small
distances from the M star, planetesimals are unlikely to survive or persist for
time scales of 300 Myr or longer if the disc is too massive. Conversely, our
survival models allow for the existence of a large population of low-mass
debris discs that are too faint to be detected with current instruments.
However, our interpretation becomes less clear and large infrared excesses are
allowed if only one of these scenarios holds: 3. the dust grains are hotter
than blackbody and predominantly emit at the observed wavelength; 4. or are
blackbody in nature and emit at peak wavelengths longer than the observed one.
Both scenarios imply that the parent planetesimals reside at larger distances
from the star than inferred if the dust grains behaved like blackbodies. In all
scenarios, we show that the infrared excesses detected at 22 and 70 microns
from AU Mic are easily reconciled with its young age. We elucidate the
conditions under which stellar wind drag may be neglected when considering dust
populations around M stars. The WISE satellite should be capable of detecting
debris discs around young M stars with ages on the order of 10 Myr.Comment: Accepted by MNRAS. 11 pages, 9 figure
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