525 research outputs found
Alkaline Exospheres of Exoplanet Systems: Evaporative Transmission Spectra
Hydrostatic equilibrium is an excellent approximation for the dense layers of
planetary atmospheres where it has been canonically used to interpret
transmission spectra of exoplanets. Here we exploit the ability of
high-resolution spectrographs to probe tenuous layers of sodium and potassium
gas due to their formidable absorption cross-sections. We present an
atmosphere-exosphere degeneracy between optically thick and optically thin
mediums, raising the question of whether hydrostatic equilibrium is appropriate
for Na I lines observed at exoplanets. To this end we simulate three
non-hydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and
(iii) torus to examine their imprint on an alkaline exosphere in transmission.
By analyzing an evaporative curve of growth we find that equivalent widths of
mA are naturally driven by evaporation rates
kg/s of pure atomic Na. To break the degeneracy between
atmospheric and exospheric absorption, we suggest that if the line ratio is
the gas is optically thin on average and roughly
indicating a non-hydrostatic structure of the atmosphere/exosphere. We show
this is the case for Na I observations at hot Jupiters WASP-49b and HD189733b
and also simulate their K I spectra. Lastly, motivated by the slew of metal
detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b
and WASP-121b is consistent with the Na I data at present.Comment: 23 pages, 21 figures, accepted by MNRA
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
Continuous Uniform Finite Time Stabilization of Planar Controllable Systems
Continuous homogeneous controllers are utilized in a full state feedback setting for the uniform finite time stabilization of a perturbed double integrator in the presence of uniformly decaying piecewise continuous disturbances. Semiglobal strong Lyapunov functions are identified to establish uniform asymptotic stability of the closed-loop planar system. Uniform finite time stability is then proved by extending the homogeneity principle of discontinuous systems to the continuous case with uniformly decaying piecewise continuous nonhomogeneous disturbances. A finite upper bound on the settling time is also computed. The results extend the existing literature on homogeneity and finite time stability by both presenting uniform finite time stabilization and dealing with a broader class of nonhomogeneous disturbances for planar controllable systems while also proposing a new class of homogeneous continuous controllers
Direct observation of 4+ to 2+ gamma transition in 8Be
The low lying states in Be are believed to have a two-alpha cluster
structure and hence a large intrinsic quadrupole deformation. An earlier
calculation showed a large collective enhancement in gamma transition
probability between the low lying states leading to a 4 to 2 gamma
branch of and a resonant radiative cross section of 134 nb for
the entrance channel. We report here the first experimental
evidence for this transition through a coincidence
measurement in the reaction He()He using a gas
target. The measured cross sections on and off the 4 resonance are 165
41 (stat) 35 (sys) nb and 39 25 (stat) 7 (sys) nb,
respectively.Comment: Total 4 pages, 4 figures, in RevTeX format, submitted to PR
Efficient Classification of Satellite Image with Hybrid Approach Using CNN-CA
Today, satellite imagery is being utilized to help repair and restore societal issues caused by habitats for a variety of scientific studies. Water resource search, environmental protection simulations, meteorological analysis, and soil class analysis may all benefit from the satellite images. The categorization algorithms were used generally and the most appropriate strategies are also be used for analyzing the Satellite image. There are several normal classification mechanisms, such as optimum likelihood, parallel piping or minimum distance classification that have presented in some other existing technologies. But the traditional classification algorithm has some disadvantages. Convolutional neural network (CNN) classification based on CA was implemented in this article. Using the gray level Satellite image as the target and CNN image classification by the CA’s selfiteration mechanism and eventually explores the efficacy and viability of the proposed method in long-term satellite remote sensing image water body classification. Our findings indicate that the proposed method not only has rapid convergence speed, reliability but can also efficiently classify satellite remote sensing images with long-term sequence and reasonable applicability. The proposed technique acquires an accuracy of 91% which is maximum than conventional methods
Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape
The Kepler satellite has revealed a gap between sub-Neptunes and super-Earths
that atmospheric escape models had predicted as an evaporation valley. We seek
to contrast results from a simple XUV-driven energy-limited (ELIM) escape model
against those from a direct hydrodynamic (HYDRO) model. Besides XUV-driven
escape, the latter also includes the boil-off regime. We couple the two models
to an internal structure model and follow the planets' temporal evolution over
Gyr. To see the population-wide imprint of the two models, we first employ a
rectangular grid in initial conditions. We then study the slope of the valley
also for initial conditions derived from the Kepler planets. For the
rectangular grid, we find that the power-law slope of the valley with respect
to orbital period is -0.18 and -0.11 in the ELIM and HYDRO model, respectively.
For the initial conditions derived from the Kepler planets, the results are
similar (-0.16 and -0.10). While the slope found with the ELIM model is steeper
than observed, the one of the HYDRO model is in excellent agreement with
observations. The reason for the shallower slope is caused by the two regimes
in which the ELIM model fails: First, puffy planets at low stellar irradiation.
For them, boil-off dominates mass loss. However, boil-off is absent in the ELIM
model, thus it underestimates escape relative to HYDRO. Second, massive compact
planets at high XUV irradiation. For them, the ELIM approximation overestimates
escape relative to the HYDRO case because of cooling by thermal conduction,
neglected in the ELIM model. The two effects act together in concert to yield
in the HYDRO model a shallower slope of the valley that agrees very well with
observations. We conclude that an escape model that includes boil-off and a
more realistic treatment of cooling mechanisms can reproduce one of the most
important constraints, the valley slope.Comment: 20 pages, 11 figures, accepted to A&
Continuous second order sliding mode based finite time tracking of a fully actuated biped robot
International audienceA second order sliding mode controller is modified to form a continuous homogeneous controller. Uniform finite time stability is proved by extending the homogeneity principle of discontinuous systems to the continuous case with uniformly decaying piece-wise continuous nonhomogeneous disturbances. The modified controller is then utilised to track reference trajectories for all the joints of a fully actuated biped robot where the joint torque is modeled as the control input. The modified controller ensures the attainment of a finite settling time between two successive impacts. The main contribution of the paper is to provide straightforward and realizable engineering guidelines for reference trajectory generation and for tuning a robust finite time controller in order to achieve stable gait of a biped in the presence of an external force disturbance. Such a disturbance has destabilising effects in both continuous and impact phases. Numerical simulations of a biped robot are shown to support the theoretical results
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