147 research outputs found
Evolutionary Models of Super-Earths and Mini-Neptunes Incorporating Cooling and Mass Loss
We construct models of the structural evolution of super-Earth- and
mini-Neptune-type exoplanets with hydrogen-helium envelopes, incorporating
radiative cooling and XUV-driven mass loss. We conduct a parameter study of
these models, focusing on initial mass, radius, and envelope mass fractions, as
well as orbital distance, metallicity, and the specific prescription for mass
loss. From these calculations, we investigate how the observed masses and radii
of exoplanets today relate to the distribution of their initial conditions.
Orbital distance and initial envelope mass fraction are the most important
factors determining planetary evolution, particular radius evolution. Initial
mass also becomes important below a "turnoff mass," which varies with orbital
distance, with mass-radius curves being approximately flat for higher masses.
Initial radius is the least important parameter we study, with very little
difference between the hot start and cold start limits after an age of 100 Myr.
Model sets with no mass loss fail to produce results consistent with
observations, but a plausible range of mass loss scenarios is allowed. In
addition, we present scenarios for the formation of the Kepler-11 planets. Our
best fit to observations Kepler-11b and Kepler-11c involves formation beyond
the snow line, after which they moved inward, circularized, and underwent a
reduced degree mass loss.Comment: 17 pages, 18 figures, 1 table, Accepted to Ap
An Information-Theoretic Approach to Optimize JWST Observations and Retrievals of Transiting Exoplanet Atmospheres
We provide an example of an analysis to explore the optimization of
observations of transiting hot jupiters with JWST to characterize their
atmospheres, based on a simple three-parameter forward model. We construct
expansive forward model sets for eleven hot jupiters, ten of which are
relatively well-characterized, exploring a range of parameters such as
equilibrium temperature and metallicity, as well as considering host stars over
a wide range in brightness. We compute posterior distributions of our model
parameters for each planet with all of the available JWST spectroscopic modes
and several programs of combined observations and compute their effectiveness
using the metric of estimated mutual information per degree of freedom. From
these simulations, clear trends emerge that provide guidelines for designing a
JWST observing program. We demonstrate that these guidelines apply over a wide
range of planet parameters and target brightnesses for our simple forward
model.Comment: Accepted to ApJ, 25 pages, 14 figures, 8 table
Mass-Radius Relations and Core-Envelope Decompositions of Super-Earths and Sub-Neptunes
Many exoplanets have been discovered with radii of 1-4 Earth radii, between
that of Earth and Neptune. A number of these are known to have densities
consistent with solid compositions, while others are "sub-Neptunes" likely to
have significant hydrogen-helium envelopes. Future surveys will no doubt
significantly expand these populations. In order to understand how the measured
masses and radii of such planets can inform their structures and compositions,
we construct models both for solid layered planets and for planets with solid
cores and gaseous envelopes, exploring a range of core masses, hydrogen-helium
envelope masses, and associated envelope entropies. For planets in the
super-Earth/sub-Neptune regime for which both radius and mass are measured, we
estimate how each is partitioned into a solid core and gaseous envelope,
associating a specific core mass and envelope mass with a given exoplanet. We
perform this decomposition for both "Earth-like" rock-iron cores and pure ice
cores, and find that the necessary gaseous envelope masses for this important
sub-class of exoplanets must range very widely from zero to many Earth masses,
even for a given core mass. This result bears importantly on exoplanet
formation and envelope evaporation processes.Comment: 26 pages, 21 figures, 16 tables, accepted to Ap
Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age
Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and morphologically more conservative: a division of labour between graptoloid zooids for such functions as feeding, breeding and rhabdosome construction may have been the key to their remarkable evolutionary success
Theoretical Transit Spectra for GJ 1214b and Other "Super-Earths"
We present new calculations of transit spectra of super-Earths that allow for
atmospheres with arbitrary proportions of common molecular species and haze. We
test this method with generic spectra, reproducing the expected systematics and
absorption features, then apply it to the nearby super-Earth GJ 1214b, which
has produced conflicting observational data, leaving the questions of a
hydrogen-rich versus hydrogen-poor atmosphere and the water content of the
atmosphere ambiguous. We present representative transit spectra for a range of
classes of atmosphere models for GJ 1214b. Our analysis supports a
hydrogen-rich atmosphere with a cloud or haze layer, although a hydrogen-poor
model with less than 10% water is not ruled out. Several classes of models are
ruled out, however, including hydrogen-rich atmospheres with no haze,
hydrogen-rich atmospheres with a haze of about 0.01-micron tholin particles,
and hydrogen-poor atmospheres with major sources of absorption other than
water. We propose an observational test to distinguish hydrogen-rich from
hydrogen-poor atmospheres. Finally, we provide a library of theoretical transit
spectra for super-Earths with a broad range of parameters to facilitate future
comparison with anticipated data.Comment: 33 pages, 21 figures, 3 table
Atmospheric Retrieval of L Dwarfs: Benchmarking Results and Characterizing the Young Planetary Mass Companion HD 106906 b in the Near-infrared
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0We present model constraints on the atmospheric structure of HD 106906 b, a planetary-mass companion orbiting at a ∼700 au projected separation around a 15 Myr old stellar binary, using the APOLLO retrieval code on spectral data spanning 1.1–2.5 μm. C/O ratios can provide evidence for companion formation pathways, as such pathways are ambiguous both at wide separations and at star-to-companion mass ratios in the overlap between the distributions of planets and brown dwarfs. We benchmark our code against an existing retrieval of the field L dwarf 2MASSW J2224–0158, returning a C/O ratio consistent with previous fits to the same JHK s data, but disagreeing in the thermal structure, cloud properties, and atmospheric scale height. For HD 106906 b, we retrieve C/O =0.53−0.25+0.15 , consistent with the C/O ratios expected for HD 106906's stellar association and therefore consistent with a stellar-like formation for the companion. We find abundances of H2O and CO near chemical equilibrium values for a solar metallicity but a surface gravity lower than expected, as well as a thermal profile with sharp transitions in the temperature gradient. Despite high signal-to-noise ratio and spectral resolution, more accurate constraints necessitate data across a broader wavelength range. This work serves as preparation for subsequent retrievals in the era of JWST, as JWST's spectral range provides a promising opportunity to resolve difficulties in fitting low-gravity L dwarfs and also underscores the need for simultaneous comparative retrievals on L-dwarf companions with multiple retrieval codes.Peer reviewe
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