Molecular cloud structure is regulated by stellar feedback in various forms. Two of the most important feedback processes are UV photoionization and supernovae from massive stars. However, the ...precise response of the cloud to these processes, and the interaction between them, remains an open question. In particular, we wish to know under which conditions the cloud can be dispersed by feedback, which, in turn, can give us hints as to how feedback regulates the star formation inside the cloud. We perform a suite of radiative magnetohydrodynamic simulations of a 10 super( 5) solar mass cloud with embedded sources of ionizing radiation and supernovae, including multiple supernovae and a hypernova model. A UV source corresponding to 10 per cent of the mass of the cloud is required to disperse the cloud, suggesting that the star formation efficiency should be of the order of 10 per cent. A single supernova is unable to significantly affect the evolution of the cloud. However, energetic hypernovae and multiple supernovae are able to add significant quantities of momentum to the cloud, approximately 10 super( 43) g cm s super( -1) of momentum per 10 super( 51) erg of supernova energy. We argue that supernovae alone are unable to regulate star formation in molecular clouds. We stress the importance of ram pressure from turbulence in regulating feedback in molecular clouds.
When transiting their host stars, hot Jupiters absorb about 10 per cent of the light in the wings of the stellar Lyman α emission line. The absorption occurs at wavelengths Doppler-shifted from line ...centre by ±100 km s−1 - larger than the thermal speeds with which partially neutral, ∼104 K hydrogen escapes from hot Jupiter atmospheres. It has been proposed that the absorption arises from ∼106 K hydrogen from the host stellar wind, made momentarily neutral by charge exchange with planetary H i. The ±100 km s−1 velocities would then be attributed to the typical velocity dispersions of protons in the stellar wind - as inferred from spacecraft measurements of the solar wind. To test this proposal, we perform 2D hydrodynamic simulations of colliding hot Jupiter and stellar winds, augmented by a chemistry module to compute the amount of hot neutral hydrogen produced by charge exchange. We observe the contact discontinuity where the two winds meet to be Kelvin-Helmholtz unstable. The Kelvin-Helmholtz instability mixes the two winds; in the mixing layer, charge exchange reactions establish, within tens of seconds, a chemical equilibrium in which the neutral fraction of hot stellar hydrogen equals the neutral fraction of cold planetary hydrogen (about 20 per cent). In our simulations, enough hot neutral hydrogen is generated to reproduce the transit observations, and the amount of absorption converges with both spatial resolution and time. Our calculations support the idea that charge transfer between colliding winds correctly explains the Lyman α transit observations - modulo the effects of magnetic fields, which we do not model but which may suppress mixing. Other neglected effects include, in order of decreasing importance, rotational forces related to orbital motion, gravity and stellar radiation pressure; we discuss quantitatively the errors introduced by our approximations. How hot stellar hydrogen cools when it collides with cold planetary hydrogen is also considered; a more careful treatment of how the mixing layer thermally equilibrates might explain the recent detection of Balmer Hα absorption in transiting hot Jupiters.
We present a new set of analytic models for the expansion of H ii regions powered by ultraviolet (UV) photoionization from massive stars and compare them to a new suite of radiative ...magnetohydrodynamic simulations of turbulent, self-gravitating molecular clouds. To perform these simulations, we use ramses-rt, a Eulerian adaptive mesh magnetohydrodynamics code with radiative transfer of UV photons. Our analytic models successfully predict the global behaviour of the H ii region provided the density and velocity structure of the cloud are known. We give estimates for the H ii region behaviour based on a power-law fit to the density field assuming that the system is virialized. We give a radius at which the ionization front should stop expanding (‘stall’). If this radius is smaller than the distance to the edge of the cloud, the H ii region will be trapped by the cloud. This effect is more severe in collapsing clouds than in virialized clouds, since the density in the former increases dramatically over time, with much larger photon emission rates needed for the H ii region to escape a collapsing cloud. We also measure the response of Jeans unstable gas to the H ii regions to predict the impact of UV radiation on star formation in the cloud. We find that the mass in unstable gas can be explained by a model in which the clouds are evaporated by UV photons, suggesting that the net feedback on star formation should be negative.
Abstract
The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest known brown dwarfs, with effective temperatures
T
eff
≲ 475 K. The first published observations provide important ...information: not only is the atmospheric chemistry out of equilibrium, as previously known, but the pressure–temperature profile is not in the standard adiabatic form. The rapid rotation of these Jupiter-size, isolated, brown dwarfs dominates the atmospheric dynamics, and thermal and compositional changes disrupt convection. These processes produce a colder lower atmosphere, and a warmer upper atmosphere, compared to a standard adiabatic profile. Leggett et al. presented empirical models where the pressure–temperature profile was adjusted so that synthetic spectra reproduced the 1 ≲
λ
(
μ
m) ≲ 20 spectral energy distributions of brown dwarfs with 260 ≤
T
eff
(K) ≤ 540. We show that spectra generated by these models fit the first JWST Y dwarf spectrum better than standard-adiabat models. Unexpectedly, there is no 4.3
μ
m PH
3
feature in the JWST spectrum and atmospheres without phosphorus better reproduce the 4
μ
m flux peak. Our analysis of new JWST photometry indicates that the recently discovered faint secondary of the WISE J033605.05-014350AB system has
T
eff
≈ 295 K, making it the first dwarf in the significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is publicly available. Photometric color transformations are provided in an appendix.
Given the forthcoming launch of the James Webb Space Telescope (JWST), which will allow observing exoplanet atmospheres with unprecedented signal-to-noise ratio, spectral coverage, and spatial ...resolution, the uncertainties in the atmosphere modeling used to interpret the data need to be assessed. As the first step, we compare three independent 1D radiative-convective models: ATMO, Exo-REM, and petitCODE. We identify differences in physical and chemical processes that are taken into account thanks to a benchmark protocol we have developed. We study the impact of these differences on the analysis of observable spectra. We show the importance of selecting carefully relevant molecular linelists to compute the atmospheric opacity. Indeed, differences between spectra calculated with Hitran and ExoMol exceed the expected uncertainties of future JWST observations. We also show the limits of the precision of the models due to uncertainties on alkali and molecule lineshape, which induce spectral effects that are also larger than the expected JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical Code, which do not lead to significant differences in the emission or transmission spectra. We discuss the observational consequences of using equilibrium or out-of-equilibrium chemistry and the major impact of phosphine, detectable with the JWST. Each of the models has benefited from the benchmarking activity and has been updated. The protocol developed in this paper and the online results can constitute a test case for other models.
We present results from a set of simulations using a fully coupled three-dimensional (3D) chemistry-radiation-hydrodynamics model and investigate the effect of transport of chemical species by the ...large-scale atmospheric flow in hot Jupiter atmospheres. We coupled a flexible chemical kinetics scheme to the Met Office Unified Model, which enables the study of the interaction of chemistry, radiative transfer, and fluid dynamics. We used a newly-released “reduced” chemical network, comprising 30 chemical species, that was specifically developed for its application in 3D atmosphere models. We simulated the atmospheres of the well-studied hot Jupiters HD 209458b and HD 189733b which both have dayside–nightside temperature contrasts of several hundred Kelvin and superrotating equatorial jets. We find qualitatively quite different chemical structures between the two planets, particularly for methane (CH
4
), when advection of chemical species is included. Our results show that consideration of 3D chemical transport is vital in understanding the chemical composition of hot Jupiter atmospheres. Three-dimensional mixing leads to significant changes in the abundances of absorbing gas-phase species compared with what would be expected by assuming local chemical equilibrium, or from models including 1D – and even 2D – chemical mixing. We find that CH
4
, carbon dioxide (CO
2
), and ammonia (NH
3
) are particularly interesting as 3D mixing of these species leads to prominent signatures of out-of-equilibrium chemistry in the transmission and emission spectra, which are detectable with near-future instruments.
Using two-dimensional (2D) thermal structure models and pseudo-2D chemical kinetics models, we explore how atmospheric temperatures and composition change as a function of altitude and longitude ...within the equatorial regions of close-in transiting Neptune-class exoplanets at different distances from their host stars. Our models predict that the day-night stratospheric temperature contrasts increase with increasing planetary effective temperatures
T
eff
and that the atmospheric composition changes significantly with
T
eff
. We find that horizontal transport-induced quenching is very effective in our simulated exo-Neptune atmospheres, acting to homogenize the vertical profiles of species abundances with longitude at stratospheric pressures where infrared observations are sensitive. Our models have important implications for planetary emission observations as a function of orbital phase with the
Ariel
mission. Cooler solar-composition exo-Neptunes with
T
eff
= 500–700 K are strongly affected by photochemistry and other disequilibrium chemical processes, but their predicted variations in infrared emission spectra with orbital phase are relatively small, making them less robust phase-curve targets for
Ariel
observations. Hot solar-composition exo-Neptunes with
T
eff
≥ 1300 K exhibit strong variations in infrared emission with orbital phase, making them great targets for constraining global temperatures, energy-balance details, atmospheric dynamics, and the presence of certain high-temperature atmospheric constituents. However, such high-temperature exo-Neptunes are arguably less interesting from an atmospheric chemistry standpoint, with spectral signatures being dominated by a small number of species whose abundances are expected to be constant with longitude and consistent with thermochemical equilibrium. Solar-composition exo-Neptunes with
T
eff
= 900–1100 K reside in an interesting intermediate regime, with infrared phase curve variations being affected by both temperature and composition variations, albeit at smaller predicted phase-curve amplitudes than for the hotter planets. This interesting intermediate regime shifts to smaller temperatures as atmospheric metallicity is increased, making cool higher-metallicity Neptune-class planets appropriate targets for
Ariel
phase-curve observations.
In this paper we present 3D atmospheric simulations of the hot Jupiter HD 189733b under two different scenarios: local chemical equilibrium and including advection of the chemistry by the resolved ...wind. Our model consistently couples the treatment of dynamics, radiative transfer, and chemistry, completing the feedback cycle between these three important processes. The effect of wind-driven advection on the chemical composition is qualitatively similar to our previous results for the warmer atmosphere of HD 209458b, found using the same model. However, we find more significant alterations to both the thermal and dynamical structure for the cooler atmosphere of HD 189733b, with changes in both the temperature and wind velocities reaching ∼10%. We also present the contribution function, diagnosed from our simulations, and show that wind-driven chemistry has a significant impact on its 3D structure, particularly for regions where methane is an important absorber. Finally, we present emission phase curves from our simulations and show the significant effect of wind-driven chemistry on the thermal emission, particularly within the 3.6 m Spitzer/IRAC channel.
The treatment of radiation transport in global circulation models (GCMs) is crucial for correctly describing Earth and exoplanet atmospheric dynamics processes. The two-stream approximation and ...correlated-k method are currently state-of-the-art approximations applied in both Earth and hot Jupiter GCM radiation schemes to facilitate the rapid calculation of fluxes and heating rates. Their accuracy have been tested extensively for Earth-like conditions, but verification of the methods’ applicability to hot Jupiter-like conditions is lacking in the literature. We are adapting the UK Met Office GCM, the Unified Model (UM), for the study of hot Jupiters, and present in this work the adaptation of the Edwards-Slingo radiation scheme based on the two-stream approximation and the correlated-k method. We discuss the calculation of absorption coefficients from high-temperature line lists and highlight the large uncertainty in the pressure-broadened line widths. We compare fluxes and heating rates obtained with our adapted scheme to more accurate discrete ordinate (DO) line-by-line (LbL) calculations ignoring scattering effects. We find that, in most cases, errors stay below 10% for both heating rates and fluxes using ~10 k-coefficients in each band and a diffusivity factor D = 1.66. The two-stream approximation and the correlated-k method both contribute non-negligibly to the total error. We also find that using band-averaged absorption coefficients, which have previously been used in radiative-hydrodynamical simulations of a hot Jupiter, may yield errors of ~100%, and should thus be used with caution.
The James Webb Space Telescope (JWST) is expected to revolutionize the field of exoplanets. The broad wavelength coverage and the high sensitivity of its instruments will allow characterization of ...exoplanetary atmospheres with unprecedented precision. Following the Call for the Cycle 1 Early Release Science Program, the Transiting Exoplanet Community was awarded time to observe several targets, including WASP-43b. The atmosphere of this hot Jupiter has been intensively observed but still harbors some mysteries, especially concerning the day-night temperature gradient, the efficiency of the atmospheric circulation, and the presence of nightside clouds. We will constrain these properties by observing a full orbit of the planet and extracting its spectroscopic phase curve in the 5-12 m range with JWST/MIRI. To prepare for these observations, we performed extensive modeling work with various codes: radiative transfer, chemical kinetics, cloud microphysics, global circulation models, JWST simulators, and spectral retrieval. Our JWST simulations show that we should achieve a precision of 210 ppm per 0.1 m spectral bin on average, which will allow us to measure the variations of the spectrum in longitude and measure the nightside emission spectrum for the first time. If the atmosphere of WASP-43b is clear, our observations will permit us to determine if its atmosphere has an equilibrium or disequilibrium chemical composition, eventually providing the first conclusive evidence of chemical quenching in a hot Jupiter atmosphere. If the atmosphere is cloudy, a careful retrieval analysis will allow us to identify the cloud composition.
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