Dr Xianyu Tan

Abstract:

We present high-precision chemical abundances for 25 FGK-type stars hosting exoplanets observed in JWST Cycle 3 programs and all Guaranteed Time Observations and Director’s Discretionary Time programs from Cycles 1–3, based on high-resolution, high-signal-to-noise ratio optical spectra from ground-based telescopes. Using a strictly differential, line-by-line analysis relative to the Sun, we derive homogeneous stellar parameters and abundances for 19 elements with an atomic number Z ≤ 30. The sample spans a wide range of stellar properties, with [Fe/H] = −0.6 to +0.4 dex and effective temperatures between 4700 and 6600 K, and includes hosts of terrestrial and giant planets as well as multiplanet systems. We refine carbon and sulfur abundances in cool dwarfs using a spectral synthesis, mitigating systematics from line blending. Several chemically interesting systems are identified, including mildly α-enhanced metal-poor stars and multiplanet hosts with elevated [C/O]. Using isochrone ages, we derive empirical Galactic chemical evolution (GCE) relations and examine condensation temperature (Tcond) trends before and after a GCE correction. The Tcond slopes show no dependence on stellar or planetary properties, indicating that they reflect a mixture of multiple mechanisms, with planet-related signatures entangled in GCE and stellar evolution effects. Thus, Tcond trends require careful interpretation. Several systems with significantly positive or negative Tcond slopes are identified. Together with forthcoming JWST atmospheric measurements, this homogeneous stellar abundance catalog provides a basis for probing star–planet chemical connections and planet formation pathways.

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:

Context. Close-in warm Jupiters orbiting M dwarf stars are expected to exhibit diverse atmospheric chemistry, with clouds playing a key role in shaping their albedo, heat distribution, and spectral properties. Aims. We study WASP-80b, a warm Jupiter orbiting an M dwarf star, using the latest JWST panchromatic emission and transmission spectra to comprehensively characterise its atmosphere, including cloud coverage, chemical composition, and particle sizes, and compare the observations with predictions from the general circulation models (GCMs). Methods. We used a GCM, ADAM (ADvanced Atmospheric MITgcm, formerly known as SPARC/MITgcm), combined with the latest JWST data to study the atmosphere of WASP-80b. A cloud module with radiatively active, tracer-based clouds was integrated with the GCM to study the effects on the atmosphere and the spectrum. Results. We find that the emission and transmission spectra of WASP-80b are only compatible with cloudless atmospheres or with clouds composed of sufficiently large particles, namely Na2S (≥10 μm), KCl (≥1 μm), and MgSiO3 (≥5 μm). For these large-particle cloud cases, efficient gravitational settling confines the clouds to deeper atmospheric layers, resulting in weak spectral signatures. Smaller particles are ruled out due to their strong radiative feedback on the atmospheric structure. Conclusions. Overall, our results suggest that WASP-80b’s atmosphere is either effectively cloud-free or contains clouds composed of large, settled particles whose opacity has little impact on the observable atmosphere. This underscores the importance of particle size and vertical cloud distribution in interpreting exoplanet spectra. Future observations at shorter wavelengths may help distinguish between large-particle cloud scenarios and a truly cloudless atmosphere.

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.