Dr Xianyu Tan

Abstract:

Radiative-transfer (RT) is a fundamental part of modelling exoplanet atmospheres with general circulation models (GCMs). An accurate RT scheme is required for estimates of the atmospheric energy transport and for gaining physical insight from model spectra. We implement three RT schemes for Exo-FMS: semigrey, non-grey ‘picket fence’, and real gas with correlated-k. We benchmark the Exo-FMS GCM, using these RT schemes to hot Jupiter simulation results from the literature. We perform a HD 209458b-like simulation with the three schemes and compare their results. These simulations are then post-processed to compare their observable differences. The semigrey scheme results show qualitative agreement with previous studies in line with variations seen between GCM models. The real gas model reproduces well the temperature and dynamical structures from other studies. After post-processing our non-grey picket fence scheme compares very favourably with the real gas model, producing similar transmission spectra, emission spectra, and phase curve behaviours. Exo-FMS is able to reliably reproduce the essential features of contemporary GCM models in the hot gas giant regime. Our results suggest the picket fence approach offers a simple way to improve upon RT realism beyond semigrey schemes.

Abstract:

Photochemical hazes have been suggested as candidate for the high-altitude aerosols observed in the transmission spectra of many hot Jupiters. We present 3D simulations of the hot Jupiter HD 189733b to study how photochemical hazes are transported by atmospheric circulation. The model includes spherical, constant-size hazes particles that gravitationally settle and are transported by the winds as passive tracers, with particle radii ranging from 1 nm to 300 $\mu$m. We identify two general types of haze distribution based on particle size: In the small-particle regime (<30 nm), gravitational settling is unimportant, and hazes accumulate in two large mid-latitude vortices centered on the night side that extend across the morning terminator. Therefore, small hazes are more concentrated at the morning terminator than at the evening terminator. In the large-particle regime (>30 nm), hazes settle out quickly on the nightside, resulting in more hazes at the evening terminator. For small particles, terminator differences in haze mass mixing ratio and temperature considered individually can result in significant differences in the transit spectra of the terminators. When combining both effects for HD189733b, however, they largely cancel out each other, resulting in very small terminator differences in the spectra. Transit spectra based on the GCM-derived haze distribution fail to reproduce the steep spectral slope at short wavelengths in the current transit observations of HD 189733b. Differing optical properties of hazes, hotter temperatures at low pressures because of heating by hazes, enhanced sub-grid-scale mixing, or star spots might explain the mismatch between the model and observations.

Abstract:

Groundbased and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation leads to a novel regime of strong day-night radiative forcing. Circulation models predict large day-night temperature differences, global-scale eddies, patchy clouds, and, in most cases, a fast eastward jet at the equator—equatorial superrotation. The warm Jupiters lie farther from their stars and are not generally tidally locked, so they may exhibit a wide range of rotation rates, obliquities, and orbital eccentricities, which, along with the weaker irradiation, leads to circulation patterns and observable signatures predicted to differ substantially from hot Jupiters. Brown dwarfs are typically isolated, rapidly rotating worlds; they radiate enormous energy fluxes into space and convect vigorously in their interiors. Their atmospheres exhibit patchiness in clouds and temperature on regional to global scales—the result of modulation by large-scale atmospheric circulation. Despite the lack of irradiation, such circulations can be driven by interaction of the interior convection with the overlying atmosphere, as well as self-organization of patchiness due to cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to bridge the gap between these classes of objects, experiencing intense external irradiation as well as vigorous interior convection. Collectively, these diverse objects span over six orders of magnitude in intrinsic heat flux and incident stellar flux, and two orders of magnitude in rotation rate—thereby placing strong constraints on how the circulation of giant planets (broadly defined) depend on these parameters. A hierarchy of modeling approaches have yielded major new insights into the dynamics governing these phenomena.

Abstract:

Tidally locked gas giants, which exhibit a novel regime of day–night thermal forcing and extreme stellar irradiation, are typically in several-day orbits, implying a modest role for rotation in the atmospheric circulation. Nevertheless, there exist a class of gas-giant, highly irradiated objects—brown dwarfs orbiting white dwarfs in extremely tight orbits—whose orbital and hence rotation periods are as short as 1–2 hr. Phase curves and other observations have already been obtained for this class of objects, raising fundamental questions about the role of an increasing planetary rotation rate in controlling the circulation. So far, most modeling studies have investigated rotation periods exceeding a day, as appropriate for typical hot Jupiters. In this work, we investigate atmospheric circulation of tidally locked atmospheres with decreasing rotation periods (increasing rotation rate) down to 2.5 hr. With a decreasing rotation period, we show that the width of the equatorial eastward jet decreases, consistent with the narrowing of the equatorial waveguide due to a decrease of the equatorial deformation radius. The eastward-shifted equatorial hot-spot offset decreases accordingly, and the off-equatorial westward-shifted hot areas become increasingly distinctive. At high latitudes, winds become weaker and more rotationally dominated. The day–night temperature contrast becomes larger due to the stronger influence of rotation. Our simulated atmospheres exhibit variability, presumably caused by instabilities and wave interactions. Unlike typical hot Jupiter models, the thermal phase curves of rapidly rotating models show a near alignment of peak flux to secondary eclipse. This result helps to explain why, unlike hot Jupiters, brown dwarfs closely orbiting white dwarfs tend to exhibit IR flux peaks nearly aligned with secondary eclipse. Our results have important implications for understanding fast-rotating, tidally locked atmospheres.