Modelling Mineral Snowflakes in the Atmospheres of Gas-Giant Exoplanets

<p>Exoplanets provide excellent laboratories to explore novel atmospheric regimes; using observations coupled with microphysical models we can probe our understanding of the formation and evolution of planets beyond those in the Solar System. However, clouds remain a key challenge in observation of exoplanet atmospheres, both altering the local atmospheric composition and obscuring deeper atmospheric layers. Currently, most observed exoplanet atmospheres are tidally locked gas-giants in close orbit around their host star. These hot and ultra-hot Jupiters have day-side temperatures in excess of 2500 K, and still above 400 K on the night-side, thus they form solid clouds made of minerals, metal oxides and metals. These clouds may form snowflake like structures, either through condensation or by constructive collisions (coagulation).</p><p>We explore the effects of non-compact, non-spherical cloud particles in gas-giant exoplanet atmospheres by expanding our kinetic non-equilibrium cloud formation model, to include parameterised porous cloud particles as well as cloud particle growth and fragmentation through collisions. We apply this model to prescribed 1D temperature - pressure Drift-Phoenix atmospheric profiles, using Mie theory and effective medium theory to study cloud optical depths, representing the effects of the non-spherical cloud particles through a statistical distribution of hollow spheres.</p><p>Finally, we apply our cloud formation model to a sample of gas-giants as well as ultra-hot Jupiters, using 1D profiles extracted from the 3D SPARC/MITgcm general circulation model. In particular, we take the example cases of gas-giant WASP-43b and the ultra-hot Jupiter HAT-P-7b, where we find dramatic differences in the day-/night-side distribution of clouds between these types of exoplanets due to the intensity of stellar irradiation for HAT-P-7b. Further an asymmetry in cloud coverage at the terminators of ultra-hot Jupiters is observable in the optical depth of the clouds, which affects the observable atmospheric column and thus has implication for detection of key gas phase species. Clouds also enhance the gas phase C/O which is often used as an indicator of formation history. With next-generation instruments such as the James Webb Space Telescope (JWST) such details will begin to be examined, but we find that a detailed understanding of cloud formation processes will be required to interpret observations.</p>

Chandrasekhar's integral stability criterion for an equilibrium spherical cloud of a protostar, modified in the framework of non-Gaussian kappa-statistics

Within the framework of the non-extensive statistical mechanics of Kanyadakis, a generalization of the integral stability theorem of Chandrasekhar for the spherically symmetric distribution of matter and black radiation in an exoplanetary cloud in a state of gravitational equilibrium is obtained. For this purpose, the elements of deformed thermodynamics for an ideal gas, deformed canonical Gibbs distribution, as well as the effective gravitational constant, calculated in the formalisms of Kanyadakis and Verlinde, are used. In this, the deformation parameter κ (kappa) measures the so-called degree of nonextensiveness of the cloud system. In addition, the modified thermodynamic properties of blackbody radiation, in particular, the analogue of Stefan's law for radiation energy and generalized expressions for the entropy, heat capacity and radiation pressure, are discussed in the context of κ -statistics. The presented method of combining the indicated anomalous physical processes provides an alternative to the classical procedure of Chandrasekhar's derivation of the well-known integral theorems for gas configurations in gravitational equilibrium, and restores all standard expressions in the limit κ → 0. The results obtained will be able, according to the author, to explain some astrophysical problems of stellar-planetary cosmogony, associated, in particular, with modeling the processes of joint formation and evolution of a protosun and an exoplanetary cloud from a single nebula.

Striations, integrals, hourglasses, and collapse – thermal instability driven magnetic simulations of molecular clouds

ABSTRACT The MHD version of the adaptive mesh refinement (AMR) code, MG, has been employed to study the interaction of thermal instability, magnetic fields, and gravity through 3D simulations of the formation of collapsing cold clumps on the scale of a few parsecs, inside a larger molecular cloud. The diffuse atomic initial condition consists of a stationary, thermally unstable, spherical cloud in pressure equilibrium with lower density surroundings and threaded by a uniform magnetic field. This cloud was seeded with 10 per cent density perturbations at the finest initial grid level around n = 1.1 cm−3 and evolved with self-gravity included from the outset. Several cloud diameters were considered (100, 200, and 400 pc) equating to several cloud masses (17 000, 136 000, and 1.1 × 106 M⊙). Low-density magnetic-field-aligned striations were observed as the clouds collapse along the field lines into disc-like structures. The induced flow along field lines leads to oscillations of the sheet about the gravitational minimum and an integral-shaped appearance. When magnetically supercritical, the clouds then collapse and generate hourglass magnetic field configurations with strongly intensified magnetic fields, reproducing observational behaviour. Resimulation of a region of the highest mass cloud at higher resolution forms gravitationally bound collapsing clumps within the sheet that contain clump-frame supersonic (M ∼ 5) and super-Alfvénic (MA ∼ 4) velocities. Observationally realistic density and velocity power spectra of the cloud and densest clump are obtained. Future work will use these realistic initial conditions to study individual star and cluster feedback.

Droplet Fate in a Cough Puff

The dynamic and thermodynamic evolution of droplets, in a size range characterizing a cough, has been analysed using basic equations of motion and coupled to the evolution of a spherical cloud puff in which they are supposed to be expired. It has been found that the maximum contamination range of the emitted droplets is controlled by two different mechanisms: surface evaporation and inertia-gravitational settling—with a switching threshold between them for a radius around a few tens of microns. For the smallest droplets, the environmental conditions (the temperature and humidity) are found to be very effective in determining the contamination range, even for weak entrainment in the cloud puff. This last fact could be of some relevance in the seasonal behaviour of air-borne epidemics.

Mineral snowflakes on exoplanets and brown dwarfs

Context. Exoplanet atmosphere characterisation has become an important tool in understanding exoplanet formation, evolution, and it also is a window into potential habitability. However, clouds remain a key challenge for characterisation: upcoming space telescopes (e.g. the James Webb Space Telescope, JWST, and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey) and ground-based high-resolution spectrographs (e.g. the next-generation CRyogenic high-resolution InfraRed Echelle Spectrograph) will produce data requiring detailed understanding of cloud formation and cloud effects for a variety of exoplanets and brown dwarfs. Aims. We aim to understand how the micro-porosity of cloud particles affects the cloud structure, particle size, and material composition on exoplanets and brown dwarfs. We further examine the spectroscopic effects of micro-porous particles, the particle size distribution, and non-spherical cloud particles. Methods. We expanded our kinetic non-equilibrium cloud formation model to study the effect of micro-porosity on the cloud structure using prescribed 1D (Tgas–pgas) profiles from the DRIFT-PHOENIX model atmosphere grid. We applied the effective medium theory and the Mie theory to model the spectroscopic properties of cloud particles with micro-porosity and a derived particle size distribution. In addition, we used a statistical distribution of hollow spheres to represent the effects of non-spherical cloud particles. Results. Highly micro-porous cloud particles (90% vacuum) have a larger surface area, enabling efficient bulk growth higher in the atmosphere than for compact particles. Increases in single scattering albedo and cross-sectional area for these mineral snowflakes cause the cloud deck to become optically thin only at a wavelength of ~100 μm instead of at the ~20 μm for compact cloud particles. A significant enhancement in albedo is also seen when cloud particles occur with a locally changing Gaussian size distribution. Non-spherical particles increase the opacity of silicate spectral features, which further increases the wavelength at which the clouds become optically thin. Conclusions. Retrievals of cloud properties, particularly particle size and mass of clouds, are biased by the assumption of compact spherical particles. The JWST mid-infrared instrument will be sensitive to signatures of micro-porous and non-spherical cloud particles based on the wavelength at which clouds are optically thin. Details of spectral features are also dependent on particle shape, and greater care must be taken in modelling clouds as observational data improves.

Statistical mass function of prestellar cores from the density distribution of their natal clouds

The mass function of clumps observed in molecular clouds raises interesting theoretical issues, especially in its relation to the stellar initial mass function (IMF). We propose a statistical model of the mass function of prestellar cores (CMF), formed in self-gravitating isothermal clouds at a given stage of their evolution. The latter is characterized by the mass-density probability distribution function (ρ-PDF), which is a power-law with slope q. The different molecular clouds are divided into ensembles according to the PDF slope and each ensemble is represented by a single spherical cloud. The cores are considered as elements of self-similar structure typical for fractal clouds and are modeled by spherical objects populating each cloud shell. Our model assumes relations between size, mass, and density of the statistical cores. Out of these, a core mass-density relationship ρ ∝ mx is derived where x = 1∕(1 + q). We find that q determines the existence or nonexistence of a threshold density for core collapse. The derived general CMF is a power law of slope − 1 while the CMF of gravitationally unstable cores has a slope (−1 + x∕2), comparable with the slopes of the high-mass part of the stellar IMF and of observational CMFs.

Asteroid triple-system 2001 SN263: surface characteristics and dynamical environment

ABSTRACT The (153591) 2001 SN263 asteroid system, a target of the first Brazilian interplanetary space mission, is one of the known three triple systems within the population of near-Earth asteroids. One of the mission objectives is to collect data about the formation of this system. The analysis of these data will help in the investigation of the physical and dynamical structures of the components (Alpha, Beta, and Gamma) of this system, in order to find vestiges related to its origin. In this work, we assume the irregular shape of the 2001 SN263 system components as uniform-density polyhedra and computationally investigate the gravitational field generated by these bodies. The goal is to explore the dynamical characteristics of the surface and environment around each component. Then, taking into account the rotational speed, we analyse their topographic features through the quantities geometric altitude, tilt, geopotential, slope, and surface accelerations among others. Additionally, the investigation of the environment around the bodies made it possible to construct zero-velocity curves, which delimit the location of equilibrium points. The Alpha component has a peculiar number of 12 equilibrium points, all of them located very close to its surface. In the cases of Beta and Gamma, we found four equilibrium points not so close to their surfaces. Then, performing numerical experiments around their equilibrium points, we identified the location and size of just one stable region, which is associated with an equilibrium point around Beta. Finally, we integrated a spherical cloud of particles around Alpha and identified the location on the surface of Alpha where the particles have fallen.

The evolution of the gaseous envelope around a newborn protostar

In this paper, we investigate the influence of nonisothermal processes on the evolution of the cloud's envelope around a newborn protostar. For this purpose, we study the evolution of a spherical cloud harboring a central hydrostatic newborn protostar. This model includes thermal effects due to heating of the cosmic rays and cooling of the gas and gas–dust energy transfer. We have ignored the effects of the magnetic field and rotation. The semianalytical Adomian decomposition method (ADM) is used to solve the system of nonlinear dynamical equations for different initial conditions. In this paper, the ADM allows us to follow the time evolution of the cloud's envelope and its mass accretion rate onto the newborn protostar. We find that the mass accretion rates of the envelope are increasing functions of time and highly depend on the choice of initial conditions. Moreover, we find that the nonisothermal processes affect the evolution of the mass accretion rates compared with the isothermal processes for different initial conditions.

Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538−66

Context. In 1981, the Be/X-ray binary A0538−66 showed outbursts characterized by high peak luminosities in the X-ray (Lx ≈ 1039 erg s−1) and optical (Lopt ≈ 3 × 1038 erg s−1) bands. The bright optical outbursts were qualitatively explained as X-ray reprocessing in a gas cloud surrounding the binary system. Aims. Since then, further important information about the properties of A0538−66 have been obtained, and sophisticated photoionization codes have been developed to calculate the radiation emerging from a gas nebula illuminated by a central X-ray source. In the light of the new information and tools available, we considered it was worth studying again the enhanced optical emission displayed by A0538−66 to understand the mechanisms responsible for these unique events among the class of Be/X-ray binaries. Methods. We performed about 105 simulations of a gas envelope surrounding the binary system photoionized by an X-ray source. We assumed for the shape of the gas cloud either a sphere or a circumstellar disc observed edge-on. We studied the effects of varying the main properties of the envelope (shape, density, slope of the power law density profile, size) and the influence of different input X-ray spectra and X-ray luminosity on the optical/UV emission emerging from the photoionized cloud. We determined the properties of the cloud and the input X-ray emission by comparing the computed spectra with the IUE spectrum and photometric UBV measurements obtained during the outburst of 29 April 1981. We also explored the role played by the X-ray heating of the surface of the donor star and the accretion disc irradiated by the X-ray emission of the neutron star. Results. We found that reprocessing in a spherical cloud with a shallow radial density distribution and size of about 3 × 1012 cm can reproduce the optical/UV emission observed on 29 April 1981. To our knowledge, this configuration has never been observed either in A0538−66 during other epochs or in other Be/X-ray binaries. We found, contrary to the case of most other Be/X-ray binaries, that the optical/UV radiation produced by the X-ray heating of the surface of the donor star irradiated by the neutron star is non-negligible, due to the particular orbital parameters of this system that bring the neutron star very close to its companion.