Dr Xianyu Tan

Abstract:

Brown dwarfs, planetary-mass objects and directly imaged giant planets exhibit significant observational evidence for active atmospheric circulation, raising critical questions about mechanisms driving the circulation, its fundamental nature and time variability. Our previous work has demonstrated the crucial role of cloud radiative feedback on driving a vigorous atmospheric circulation using local models that assume a Cartesian geometry and constant Coriolis parameters. In this study, we extend the models to a global geometry and explore properties of the global dynamics. We show that, under relatively strong dissipation in the bottom layers of the model, horizontally isotropic vortices are prevalent at mid-to-high latitudes while large-scale zonally propagating waves are dominant at low latitudes near the observable layers. The equatorial waves have both eastward and westward phase speeds, and the eastward components with typical velocities of a few hundred  m s−1 usually dominate the equatorial time variability. Lightcurves of the global simulations show variability with amplitudes from 0.5 per cent to a few percent depending on the rotation period and viewing angle. The time evolution of simulated lightcurves is critically affected by the equatorial waves, showing wave beating effects and differences in the lightcurve periodicity to the intrinsic rotation period. The vertical extent of clouds is the largest at the equator and decreases poleward due to the increasing influence of rotation with increasing latitude. Under weaker dissipation in the bottom layers, strong and broad zonal jets develop and modify wave propagation and lightcurve variability. Our modelling results help to qualitatively explain several features of observations of brown dwarfs and directly imaged giant planets, including puzzling time evolution of lightcurves, a slightly shorter period of variability in IR than in radio wavelengths, and the viewing angle dependence of variability amplitude and IR colors.

Abstract:

Observations of brown dwarfs (BDs), free-floating planetary-mass objects, and directly imaged extrasolar giant planets (EGPs) exhibit rich evidence of large-scale weather. Cloud radiative feedback has been proposed as a potential mechanism driving the vigorous atmospheric circulation on BDs and directly imaged EGPs, and yet it has not been demonstrated in three-dimensional dynamical models at relevant conditions. Here, we present a series of atmospheric circulation models that self-consistently couple dynamics with idealized cloud formation and its radiative effects. We demonstrate that vigorous atmospheric circulation can be triggered and self-maintained by cloud radiative feedback. Typical isobaric temperature variation could reach over 100 K and horizontally averaged wind speed could be several hundreds of $\, {\rm m\, s^{-1}}$. The circulation is dominated by cloud-forming and clear-sky vortices that evolve over time-scales from several to tens of hours. The typical horizontal length-scale of dominant vortices is closed to the Rossby deformation radius, showing a linear dependence on the inverse of rotation rate. Stronger rotation tends to weaken vertical transport of vapour and clouds, leading to overall thinner clouds. Domain-mean outgoing radiative flux exhibits variability over time-scales of tens of hours due to the statistical evolution of storms. Different bottom boundary conditions in the models could lead to qualitatively different circulation near the observable layer. The circulation driven by cloud radiative feedback represents a robust mechanism generating significant surface inhomogeneity as well as irregular flux time variability. Our results have important implications for near-infrared (IR) colours of dusty BDs and EGPs, including the scatter in the near-IR colour–magnitude diagram and the viewing-geometry-dependent near-IR colours.

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.

Abstract:

© 2020. The American Astronomical Society. All rights reserved. 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.