Atmospheric Circulation of Brown Dwarfs and Giant Planets

Characterizing the Orbital and Dynamical State of Extrasolar Multiple-Planet Systems Using Radial Velocity Measurements

Tan et al. (2013)

This is the topic back to my M.S. thesis working with Dr. Man Hoi Lee in the University of Hong Kong.

Based on high-precision radial velocity measurements, we perform dynamical fitting which takes gravitational interactions between planets into account for the HD 82943 system (Tan et al. 2013). We conclude that the two giant planets in the system is in 2:1 mean-motion resonance. The inclination of their orbital plane is about 20 degrees with respect to the sky plane and both planets have about 4.8 Jovian mass. Interestingly, our deduced inclination is consistent with that (27 degrees) inferred from observations of its debris disk (Kennedy et al. 2013). Our results can help constrain the orbital evolution history of the HD 82943 planetary system.

Our dynamical fitting method also helps to characterize other two multiple-planetary systems, the HD 73526 system (Wittenmyer et al. 2014) and the η Ceti system (Trifonov et al. 2014).

  1. 1.Effects of latent heating on atmospheres of brown dwarfs and directly imaged giant planets.

    Tan & Showman (2017)

Latent heating associated with condensation can be important in shaping atmospheric circulation and influencing cloud patchiness. We demonstrate this effect using an idealized general circulation model that includes a condensation cycle of silicates with latent heating and molecular weight effect due to rainout of condensate. Simulations with conditions appropriate for typical T dwarfs exhibit the development of localized storms and east-west jets. The storms are spatially inhomogeneous, evolving on timescale of hours to days and extending vertically from the condensation level to the tropopause. The fractional area of the brown dwarf covered by active storms is small. Based on a simple analytic model, we demonstrate the dependence of area fraction of storms on the radiative timescale and convective available potential energy. We predict that if latent heating dominates cloud formation processes, the fractional coverage area by clouds decreases as the spectral type goes through the L/T transition from high to lower effective temperature. This is a natural consequence of the variation of radiative timescale and convective available potential energy with spectral type.

Growing observations of brown dwarfs (BDs) have shown evidence for strong atmospheric circulation and cloud cycles on these objects. Directly imaged extrasolar giant planets (EGPs) share similar observations and can be viewed as low-gravity version of BDs. We plan to build up a series of general circulation models to explore the fundamental properties of their global circulation and the implications for observations.

  1. 2.Atmospheric circulation of brown dwarfs and giant planets under global thermal perturbations.

    Showman, Tan & Zhang (In prep);    Tan & Showman (In prep)

Vigorous convection in BDs and directly imaged EGPs can exert strong thermal perturbations beneath the stratified weather layers, pumping energy upward and driving a global circulation. We explore the circulation pattern and the dependence on properties of the forcing.

Tan & Showman (2017)

3. Atmospheric variability of brown dwarfs and directly imaged giant planets driven by radiative cloud feedback.

    Tan & Showman (Submitted to ApJ)

We show that spontaneous atmospheric variability of temperature and cloud structure can be driven by cloud radiative feedback in a simple one-dimensional context. Typical oscillation periods are one to tens of hours, with typical amplitude of the variability up to several hundreds of Kelvin in effective temperature. The oscillations are totally natural without any injected forcing.

Atmospheric Circulation of Hot Jupiters

I contribute to a paper led by Tad Komacek (Komacek, Showman & Tan 2017) that explores day-night temperature difference using a semi-grey general circulation model. Theory (Komacek & Showman 2016) demonstrates that the competition between radiative damping, wave adjustment process and atmospheric drag determines the day-night temperature difference.

Using the same model but with a simple cloud cycle that allows radiative feedbacks, I am working on understanding how cloud feedbacks affect the global circulation in the context of an idealized model. Several fancy GCMs have been published but yet no study seriously think about this issue. Idealized model has the advantage of running longer, exploring more parameter space, and most importantly, easier to tease out key mechanisms in play.

Recent observations and theoretical work point to a new emerging class of exoplanets - the ultra-hot Jupiters - whose equilibrium temperature can excess 2000 Kelvin. Many interesting physics can occur in their atmospheres, including dissociation of molecules like H2 and water. Tad Komacek and I are interested in the dynamical effects of H2 dissociation/recombination in there atmospheres. We have carried out a scaling theory to predict the day-night temperature contrasts of these atmospheres, and later will perform GCM simulations to test the theory and to understand the detailed circulation which is useful to understand the phase-curve observations.

Tan & Showman (2018)

Showman, Tan & Zhang (2018)