Dr Xianyu Tan

Abstract:

Planets with nonzero obliquity and/or orbital eccentricity experience seasonal variations of stellar irradiation at local latitudes. The extent of the atmospheric response can be crudely estimated by the ratio of the orbital timescale to the atmospheric radiative timescale. Given a set of atmospheric parameters, we show that this ratio depends mostly on the stellar properties and is independent of orbital distance and planetary equilibrium temperature. For Jupiter-like atmospheres, this ratio is ≪1 for planets around very low mass M dwarfs and ≳1 when the stellar mass is greater than about 0.6 solar mass. Complications can arise from various factors, including varying atmospheric metallicity, clouds, and atmospheric dynamics. Given the eccentricity and obliquity, the seasonal response is expected to be systematically weaker for gaseous exoplanets around low-mass stars and stronger for those around more massive stars. The amplitude and phase lag of atmospheric seasonal variations as a function of host stellar mass are quantified by idealized analytic models. At the infrared emission level in the photosphere, the relative amplitudes of thermal flux and temperature perturbations are negligible, and their phase lags are closed to −90° for Jupiter-like planets around very low mass stars. The relative amplitudes and phase lags increase gradually with increasing stellar mass. With a particular stellar mass, the relative amplitude and phase lag decrease from low- to high-infrared optical depth. We also present numerical calculations for a better illustration of the seasonal behaviors. Last, we discuss implications for the atmospheric circulation and future atmospheric characterization of exoplanets in systems with different stellar masses.

Abstract:

Hot Jupiters have temperature gradients of several hundreds of degrees between their permanent daysides and nightsides. Such a strong gradient creates winds with speeds of the order of kilometres per second, which advect chemical species over the whole planet. When this transport is faster than the time needed for chemical species to react, it holds back the chemical equilibration of the atmospheric carbon reservoir, which would otherwise transition from CO on the dayside to CH4 on the nightside. Direct evidence of this process has remained elusive so far, as it is often degenerate with other atmospheric processes, such as vertical mixing or non-stellar elemental abundances. Here we present observational evidence for such a fast day-to-night horizontal transport of chemical species by observing the full 18-h orbit of the exoplanet NGTS-10A b with the JWST/NIRSpec instrument. We show that the carbon chemistry is dominated by CO in both the dayside and the nightside of the planet, with a strong depletion of CH4 on the nightside compared with expectations from chemical equilibrium. By measuring the atmospheric abundances of all the main carbon and oxygen molecules, we further demonstrate that the lack of CH4 on the planetary nightside cannot be attributed to non-solar elemental abundances or to vertical mixing mechanisms and must, therefore, be due to fast horizontal transport. Our study shows the fundamental role that atmospheric transport plays in shaping the distribution of chemical species on exoplanet atmospheres.

Abstract:

We present a new analysis of the spectroscopic variability of WISE J104915.57−531906.1AB (WISE 1049AB, L7.5+T0.5), observed using the NIRSpec instrument on board the James Webb Space Telescope (GO 2965; PI: Biller). We explore the variability of the dominant molecular bands present in their 0.6–5.3 μm spectra (H2O, CH4, and CO), finding that the B component exhibits a higher maximum deviation than the A component in all the wavelength ranges tested. The light curves reveal wavelength-dependent (atmospheric depth) and possibly chemistry-dependent variability. In particular, for the A component, the variability in the light curves at the wavelengths traced by the CH4 and CO molecular absorption features is higher than that for of H2O, even when both trace similar pressure levels. We conclude that clouds alone are unlikely to explain the increased variability of CO and CH4 with respect to H2O, suggesting that an additional physical mechanism is needed to explain the observed variability. This mechanism is probably due to thermochemical instabilities. Finally, we provide visual representations of the 3D atmospheric maps reconstructed for both components using the molecular band contributions at different pressure levels and the fits of planetary-scale waves.

Abstract:

Vertical mixing disrupts the thermochemical equilibrium and introduces additional heat flux that alters exoplanetary atmospheric temperatures. We investigate how this mixing-induced heat flux affects atmospheric chemistry. Temperature increase in the lower atmosphere by the mixing-induced heat flux alters species abundances there and modifies those in the upper atmosphere through vertical transport. In the lower atmosphere, most species follow thermodynamic equilibrium with temperature changes. In the upper layers, species mixing ratios depend on the positions of quenching levels relative to the regions exhibiting significant mixing-induced temperature variations. When the quenching level resides within such a region (e.g., CO, CH4, and H2O with strong mixing), the mixing ratios in the upper atmosphere are modified due to changes in the quenched ratios affected by the temperature variation in the lower atmosphere. This alters the mixing ratio of other species (e.g., NO and CO2) through the chemical reaction network, whose quenching occurs in the region without much temperature change. The mixing ratios of CH4, H2O, and NH3 decrease in the lower atmosphere with increasing mixing heat flux, similarly reducing these ratios in the upper atmosphere. Conversely, the mixing ratios of CO, CO2, and NO rise in the lower atmosphere, with CO and CO2 also increasing in the upper levels, although NO decreases. Weaker host star irradiation lowers the overall temperature of the planet, allowing a smaller mixing to have a similar effect. We conclude that understanding the vertical mixing heat flux is essential for accurate atmospheric chemistry modeling and retrieval.

Abstract:

We present JWST NIRSpec/PRISM integral field unit time-resolved observations of 2M1207 A and b (TWA 27), an ∼10 Myr binary system consisting of an ∼2500 K substellar primary hosting an ∼1300 K companion. Our data provide 20 time-resolved spectra over an observation spanning 12.56 hr. We provide an empirical characterization for the spectra of both objects across time. For 2M1207 A, nonlinear trend models are statistically favored within the ranges 0.6-2.3 μm and 3.8-5.3 μm. However, most of the periods constrained from sinusoidal models exceed the observing window, setting a lower limit of 12.56 hr. We find the data at Hα and beyond 4.35 μm show a moderate time correlation, as well as a pair of light curves at 0.73-0.80 μm and 3.36-3.38 μm. For 2M1207 b, light curves integrated across 0.86-1.77 μm and 3.29-4.34 μm support linear trend models. Following the interpretation of Z. Zhang et al., we model the 2M1207 b data with two 1D atmospheric components, both with silicate and iron condensates. The model of time variability due to changes in the cloud filling factor shows broad consistency with the variability amplitudes derived from our data. Our amplitudes, however, disagree with the models at ≈0.86-1 μm. While an additional model component such as rainout chemistry may be considered here, our analysis is limited by low signal-to-noise ratio. Our results demonstrate the capability of JWST to simultaneously monitor the spectral variability of a planetary-mass companion and host at low contrast.