Jet streams and tracer mixing in the atmospheres of brown dwarfs and isolated young giant planets
Monthly Notices of the Royal Astronomical Society Oxford University Press 511:4 (2022) 4861-4881
Abstract:
Observations of brown dwarfs and relatively isolated young extrasolar giant planets have provided unprecedented details to probe atmospheric dynamics in a new regime. Questions about mechanisms governing the global circulation and its fundamental nature remain to be completely addressed. Previous studies have shown that small-scale randomly varying thermal perturbations resulting from interactions between convection and the overlying stratified layers can drive zonal jet streams, waves, and turbulence. In this work, we improve upon our previous work by using a general circulation model coupled with a two-stream grey radiative transfer scheme to represent more realistic heating and cooling rates. We examine the formation of zonal jets and their time evolution, and vertical mixing of passive tracers including clouds and chemical species. Under relatively weak radiative and frictional dissipation, robust zonal jets with speeds up to a few hundred m s−1 are typical outcomes. The off-equatorial jets tend to be pressure independent, while the equatorial jets exhibit significant vertical wind shear. On the other hand, models with strong dissipation inhibit the jet formation and leave isotropic turbulence in off-equatorial regions. Quasi-periodic oscillations of the equatorial flow with periods ranging from tens of days to months are prevalent at relatively low atmospheric temperatures. Submicron cloud particles can be easily transported to several scale heights above the condensation level, while larger particles form thinner layers. Cloud decks are significantly inhomogeneous near their cloud tops. Chemical tracers with chemical time-scales >105 s can be driven out of equilibrium. The equivalent vertical diffusion coefficients, Kzz, for the global-mean tracer transport are diagnosed from our models and are typically on the order of 1–102 m2 s−1. Finally, we derive an analytic estimation of Kzz for different types of tracers under relevant conditions.Indicate separate contributions of long-lived and short-lived greenhouse gases in emission targets
npj Climate and Atmospheric Science Springer Nature 5:1 (2022) 5
HST/WFC3 Complete Phase-resolved Spectroscopy of White-dwarf-brown-dwarf Binaries WD 0137 and EPIC 2122
The Astronomical Journal American Astronomical Society 163:1 (2022) 17
Mapping the Pressure-dependent Day-Night Temperature Contrast of a Strongly Irradiated Atmosphere with HST Spectroscopic Phase Curve
Astronomical Journal 163:1 (2022)
Abstract:
Many brown dwarfs are on ultrashort-period and tidally locked orbits around white dwarf hosts. Because of these small orbital separations, the brown dwarfs are irradiated at levels similar to hot Jupiters. Yet, they are easier to observe than hot Jupiters because white dwarfs are fainter than main-sequence stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore, ideal hot Jupiter analogs for studying the atmospheric response under strong irradiation and fast rotation. We present the 1.1-1.67 μm spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the SDSS J141126.20 + 200911.1 brown dwarf-white dwarf binary with the near-infrared G141 grism of the Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a 50M Jup brown dwarf with an irradiation temperature of 1300 K and has an orbital period of 2.02864 hr. Our best-fit model suggests a phase-curve amplitude of 1.4% and places an upper limit of 11 for the phase offset from the secondary eclipse. After fitting the white dwarf spectrum, we extract the phase-resolved brown dwarf emission spectra. We report a highly wavelength-dependent day-night spectral variation, with a water-band flux variation of about 360% 70% and a comparatively small J-band flux variation of 37% 2%. By combining the atmospheric modeling results and the day-night brightness temperature variations, we derive a pressure-dependent temperature contrast. We discuss the difference in the spectral features of SDSS1411-B and hot Jupiter WASP-43b, as well as the lower-than-predicted day-night temperature contrast of J4111-BD. Our study provides the high-precision observational constraints on the atmospheric structures of an irradiated brown dwarf at different orbital phases.
Blue marble, stagnant lid: Could dynamic topography avert a waterworld?
The Planetary Science Journal 3:3 (2022)
Abstract:
Topography on a wet rocky exoplanet could raise land above its sea level. Although land elevation is the product of many complex processes, the large-scale topographic features on any geodynamically-active planet are the expression of the convecting mantle beneath the surface. This so-called "dynamic topography" exists regardless of a planet's tectonic regime or volcanism; its amplitude, with a few assumptions, can be estimated via numerical simulations of convection as a function of the mantle Rayleigh number. We develop new scaling relationships for dynamic topography on stagnant lid planets using 2D convection models with temperature-dependent viscosity. These scalings are applied to 1D thermal history models to explore how dynamic topography varies with exoplanetary observables over a wide parameter space. Dynamic topography amplitudes are converted to an ocean basin capacity, the minimum water volume required to flood the entire surface. Basin capacity increases less steeply with planet mass than does the amount of water itself, assuming a water inventory that is a constant planetary mass fraction. We find that dynamically-supported topography alone could be sufficient to maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately 10−4 times their mass, in the most favourable thermal states. By considering only dynamic topography, which has ~1-km amplitudes on Earth, these results represent a lower limit to the true ocean basin capacity. Our work indicates that deterministic geophysical modelling could inform the variability of land propensity on low-mass planets.