ABSTRACT
The nature of cosmic ray (CR) transport in the Milky Way remains elusive. The predictions of current microphysical CR transport models in magnetohydrodynamic (MHD) turbulence are drastically ...different from what is observed. These models usually focus on MHD turbulence with a strong guide field and ignore the impact of turbulent intermittency on particle propagation. This motivates our studying the alternative regime of large-amplitude turbulence with δB/B0 ≫ 1, in which intermittent small-scale magnetic field reversals are ubiquitous. We study particle transport in such turbulence by integrating trajectories in stationary snapshots. To quantify spatial diffusion, we use a set-up with continuous particle injection and escape, which we term the turbulent leaky box. We find that particle transport is very different from the strong guide-field case. Low-energy particles are better confined than high-energy particles, despite less efficient pitch-angle isotropization at small energies. In the limit of weak guide field, energy-dependent confinement is driven by the energy-dependent (in)ability to follow reversing magnetic field lines exactly and by the scattering in regions of ‘resonant curvature’, where the field line bends on a scale that is of the order of the local particle gyro-radius. We derive a heuristic model of particle transport in magnetic folds that approximately reproduces the energy dependence of transport found numerically. We speculate that CR propagation in the Galaxy is regulated by the intermittent field reversals highlighted here and discuss the implications of our findings for CR transport in the Milky Way.
We present a study of the effect of wind-driven advection on the chemical composition of hot-Jupiter atmospheres using a fully consistent 3D hydrodynamics, chemistry, and radiative transfer code, the ...Met Office Unified Model (UM). Chemical modeling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work, we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure, and chemical composition. In this Letter, we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude, which is directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot-Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase nonequilibrium chemistry is unlikely to explain the model-observation discrepancy in the 4.5 m Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.
Genetic diversity within species is a potentially important, but poorly studied, determinant of plant community dynamics. Here we report experiments testing the influence of genotype identity and ...genotypic diversity both on the invasibility of a foundation, matrix-forming species (Kentucky bluegrass, Poa pratensis), and on the invasiveness of a colonizing species (dandelion, Taraxacum officinale). Genotypes of Kentucky bluegrass in monoculture showed significant variation in productivity and resistance to dandelion invasion, but the productivity and invasion resistance of genotypic mixtures were not significantly different from those of genotypic monocultures. Indirect evidence suggested temporal shifts in the genotypic composition of mixtures. Dandelion genotypes in monoculture showed striking and significant variation in productivity and seed production, but there was no significant tendency for these variables in mixtures to deviate from null expectations based on monocultures. However, productivity and seed production of dandelion mixtures were consistently greater than those of the two least productive genotypes, and statistically indistinguishable from those of the three most productive genotypes, suggesting the possibility of greater invasiveness of genotypically diverse populations in the long run due to dominance by highly productive genotypes. In both experiments, the identity of genotypes was far more important than genetic diversity per se.
ABSTRACT
We present results of 3D hydrodynamical simulations of HD209458b including a coupled, radiatively active cloud model (eddysed). We investigate the role of the mixing by replacing the default ...convective treatment used in previous works with a more physically relevant mixing treatment (Kzz) based on global circulation. We find that uncertainty in the efficiency of sedimentation through the sedimentation factor fsed plays a larger role in shaping cloud thickness and its radiative feedback on the local gas temperatures – e.g. hotspot shift and day-to-night side temperature gradient – than the switch in mixing treatment. We demonstrate using our new mixing treatments that simulations with cloud scales that are a fraction of the pressure scale height improve agreement with the observed transmission spectra, the emission spectra, and the Spitzer 4.5 µm phase curve, although our models are still unable to reproduce the optical and ultraviolet transmission spectra. We also find that the inclusion of cloud increases the transit asymmetry in the optical between the east and west limbs, although the difference remains small ($\lesssim 1{{\ \rm per\ cent}}$).
ABSTRACT
We present a method to characterize star-formation driven outflows from edge-on galaxies and apply this method to the metal-poor starburst galaxy, Mrk 1486. Our method uses the distribution ...of emission line flux (from H β and O iii 5007) to identify the location of the outflow and measure the extent above the disc, the opening angle, and the transverse kinematics. We show that this simple technique recovers a similar distribution of the outflow without requiring complex modelling of line-splitting or multi-Gaussian components, and is therefore applicable to lower spectral resolution data. In Mrk 1486 we observe an asymmetric outflow in both the location of the peak flux and total flux from each lobe. We estimate an opening angle of 17–37° depending on the method and assumptions adopted. Within the minor axis outflows, we estimate a total mass outflow rate of ∼2.5 M⊙ yr−1, which corresponds to a mass loading factor of η = 0.7. We observe a non-negligible amount of flux from ionized gas outflowing along the edge of the disc (perpendicular to the biconical components), with a mass outflow rate ∼0.9 M⊙ yr−1. Our results are intended to demonstrate a method that can be applied to high-throughput low spectral resolution observations, such as narrow-band filters or low spectral resolution integral field spectrographs that may be more able to recover the faint emission from outflows.
Abstract
We use hydrodynamical simulations of two Milky Way–mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with ...previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sight lines of the simulated galaxies’ CGM and use Voigt profile-fitting methods to extract ion column densities, Doppler-
b
parameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower O
vi
absorption features and broader Si
iii
absorption features, a quality that is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM.
By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth's oceans, fingering convection in ...stellar atmospheres, and moist convection in Earth's atmosphere are derived from the same general diabatic convective instability. We also show that "radiative convection" triggered by the CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analogous to moist and thermohaline convection. We derive a generalization of the mixing-length theory to include the effect of source terms in 1D codes. We show that CO/CH4 "radiative" convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth's atmosphere, thus possibly explaining the reddening in brown dwarf spectra. By using idealized 2D hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and seen as a giant cooling crisis: an analog of the boiling crisis in liquid/steam-water convective flows. This mechanism, with other chemical transitions, could be present in many giant and Earth-like exoplanets. The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility of using brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet.
ABSTRACT
Stellar flares present challenges to the potential habitability of terrestrial planets orbiting M dwarf stars through inducing changes in the atmospheric composition and irradiating the ...planet’s surface in large amounts of ultraviolet light. To examine their impact, we have coupled a general circulation model with a photochemical kinetics scheme to examine the response and changes of an Earth-like atmosphere to stellar flares and coronal mass ejections. We find that stellar flares increase the amount of ozone in the atmosphere by a factor of 20 compared to a quiescent star. We find that coronal mass ejections abiotically generate significant levels of potential bio-signatures such as N2O. The changes in atmospheric composition cause a moderate decrease in the amount of ultraviolet light that reaches the planets surface, suggesting that while flares are potentially harmful to life, the changes in the atmosphere due to a stellar flare act to reduce the impact of the next stellar flare.
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