How does rotation rate influence the weather on hot Jupiters?

While hot Jupiters are exotic scorching gas giants that have no equivalent in our Solar System, they provide a good bechmarks for atmospheric modelling. They are in synchronous rotation with their host star on very low eccentricity orbit and with negligible axial tilt, so they don’t experience change in seasons. Therefore, their atmospheres shouldn’t vary greatly in time. But despite their size and proximity to the star, which makes them relatively easy to observe, directly resolving their atmospheres is still inacessible. Therefore, interpreting indirect observations is crucial to understanding their global atmospheric circulation.

Here, we aim to test the hypothesis that systematically varying the rotation rate of hot Jupiters influences their general atmospheric circulation, and the phase shift and amplitude of their phase curves. We used DYNAMICO (Dubos et al, 2015), a 3d General Circulation Model (GCM) that uses an energy-conserving Hamiltonian to solve fluid dynamics equations on a rotating sphere.

We modelled benchmark hot Jupiter HD209458b, WASP121b and WASP-34b. We used PICASO (Robbins-Blanch et al., 2022), a 3d state-of-the-art radiative transfer code, to model the phase curves in JWST NIRCam filters for 4.5 μm wavelength. To check if chemical species can condense into quartz and forsterite clouds, we used GGChem (Woitke et al., 2018).

Our simulations show that the general atmospheric circulation can be divided into two circulation regimes: rotational and divergent.

The rotational regime is characteristic of models with fast rotation rates (between one and 40 times the nominal rotation rate). They have a narrow and weak eastward jet due to their rotationally dominated circulation. Its speed drops from 3km/s to 0.3km/s and its width decreases with increasing rotation rate. These weak jets transport less heat from the substellar point, which sustains larger day and night temperature differences, and therefore the synthetic phase curves have a large amplitude (the ratio of the planetary to stellar flux is around 3 × 10‒5) and little to no offset.

The slow rotators have atmospheres dominated by divergent (overturning) circulation, which is more effective at heat transport. The temperature across the planet is more homogenised, therefore, the synthetic phase curves exhibit 3 times smaller amplitudes than in the rotational regime and offsets of the order of tens of degrees. Our results for HD209458b suggest that differences in the shapes and offsets of the phase curve are most prominent in the 4.5 μm band.

However, our model underestimates the amplitude of the phase curves by half (the synthetic value is around 5 × 10‒4 compared to 1 × 10‒3 for the observed one). Even though our model includes accounts for the complex interplay between a planet's physical characteristics (size, mass, period, etc.) and its chemical composition (metallicity and opacity), it is still idealised.

To improve the model, we have computed new models for HD 209458 b with different temperature-pressure (T-P) profiles and cooling timescales. We have models for extended the T-P profile to 10-6 bar, increased its temperature by 200 K, increased the day-to-night temperature contrast and modelled slight temperature inversion. We have also computed models for 10x longer and shorter cooling timescales.  The best model that will match the observed phase curve, will be a one with longer cooling timescale and greater day-to-night temperature contrast. The former increases the model’s phase shift closer to the observed one from 0.030 to 0.127 and the latter increases the amplitude from 5 * 10-4 to 10-3. The models are yet to include high-altitude clouds in phase curve modelling.

 

 Dubos, T., Dubey, S., Tort, M., Mittal, R., Meurdesoif, Y. and Hourdin, F. DYNAMICO-1.0, an icosahedral hydrostatic dynamical core designed for consistency and versatility. 2015. Geoscientific Model Development. 8: 3131-3150.Robbins-Blanch, N., Kataria, T., Batalha, N. and Adams, D. J. Cloudy and Cloud-free Thermal Phase Curves with PICASO: Applications to WASP-43b. 2022. The Astrophysical Journal. 930: 93-102. Woitke, P., Helling, Ch., Hunter, G. H., Millard, J. D., Turner, G. E., Worters, M., Blecic, J., and Stock, J. W. Equilibrium chemistry down to 100 K. Impact of silicates and phyllosilicates on carbon/oxygen ratio. 2018. Astronomy & Astrophysics. 614.