arXiv subjects

Nidhi Bangera

Publications and source records attributed to Nidhi Bangera.

3 recordsLinked to original sources

arXiv2026

Chemical Tracers for 3D Atmospheric Asymmetries on WASP-69 b

Warm giant exoplanets exhibit strong three-dimensional temperature contrasts that can significantly alter atmospheric chemistry through quenching and photochemistry, yet transmission spectra are commonly interpreted using one-dimensional, limb-averaged models. Such simplifications may bias inferred atmospheric properties, particularly metallicity and C/O ratio. In this work we investigate the relative influence of atmospheric composition and three-dimensional thermal structure on atmospheric chemistry and transmission spectra using WASP-69b as a test case. WASP-69b is a ~900K warm Saturn, residing in a thermal regime especially sensitive to disequilibrium chemistry. We use three-dimensional general circulation model derived pressure-temperature profiles as inputs for a one-dimensional photochemical-kinetics model to resolve longitudinal and latitudinal chemical asymmetries across the atmosphere. We find that CH4 exhibits strong latitudinal variations linked to deep quench temperatures, while SO2 shows longitudinal asymmetries driven by upper-atmospheric photochemistry and irradiation geometry. In contrast, CO2 remains comparatively insensitive to spatial thermal variations and emerges as a robust tracer of atmospheric metallicity. Synthethic transmission spectra reveal that three-dimensional chemical asymmetries can produce spectral variations comparable to those induced by metallicity itself, demonstrating that limb-averaged interpretations can mask substantial spatial structure in warm giant exoplanet atmospheres.

astro-ph.EP↗

arXiv2025

Kinetic and photochemical disequilibrium in the potentially carbon-rich atmosphere of the warm-Jupiter WASP-69b

High-resolution transmission spectroscopy of the warm gas-giant WASP-69b has revealed the presence of H2O, CO, CH4, NH3, and C2H2 in its atmosphere. This study investigates the impact of vertical diffusion and photochemistry on its atmospheric composition, with a focus on the detected species plus HCN and CO2, to constrain the atmospheric C/O ratio. We utilize non-equilibrium gas-phase simulations to conduct a parameter study for vertical diffusion strength, local gas temperature, and C/O ratio. Our results indicate that a carbon-rich atmosphere enhances CH4 and C2H2 concentrations, while NH3 undergoes chemical conversion into HCN in carbon-rich, high-temperature environments. Consequently, HCN is abundantly produced in such atmospheres, though its strong spectral features remain undetected in WASP-69b. Photochemical production of HCN and C2H2 is highly sensitive to vertical diffusion strength, with weaker diffusion resulting in higher concentrations. Cross-correlation of synthetic spectra with observed data shows that models with C/O=2 best match observations, but models with C/O=0.55 and 0.9 lead to statistically equivalent fits, leaving the C/O ratio unconstrained. We highlight the importance of accurately modeling NH3 quenching at pressures greater than 100 bars. Models for WASP-69b capped at 100 bars bias cross-correlation fits towards carbon-rich values. We suggest that if the atmosphere of WASP-69b is indeed carbon-rich with a solar metallicity, future observations should reveal the presence of HCN.

astro-ph.EP↗

arXiv2023

Fully time-dependent cloud formation from a non-equilibrium gas-phase in exoplanetary atmospheres

Recent observations suggest the presence of clouds in exoplanet atmospheres but have also shown that certain chemical species in the upper atmosphere might not be in chemical equilibrium. The goal of this work is to calculate the two main cloud formation processes, nucleation and bulk growth, consistently from a non-equilibrium gas-phase. The aim is further to explore the interaction between a kinetic gas-phase and cloud micro-physics. The cloud formation is modeled using the moment method and kinetic nucleation which are coupled to a gas-phase kinetic rate network. Specifically, the formation of cloud condensation nuclei is derived from cluster rates that include the thermochemical data of (TiO$_2$)$_N$ from N = 1 to 15. The surface growth of 9 bulk Al/Fe/Mg/O/Si/S/Ti binding materials considers the respective gas-phase species through condensation and surface reactions as derived from kinetic disequilibrium. The effect of completeness of rate networks and the time evolution of the cloud particle formation is studied for an example exoplanet HD 209458 b. A consistent, fully time-dependent cloud formation model in chemical disequilibrium with respect to nucleation, bulk growth and the gas-phase is presented and first test cases are studied. This model shows that cloud formation in exoplanet atmospheres is a fast process. This confirms previous findings that the formation of cloud particles is a local process. Tests on selected locations within the atmosphere of the gas-giant HD 209458 b show that the cloud particle number density and volume reach constant values within 1s. The complex kinetic polymer nucleation of TiO$_2$ confirms results from classical nucleation models. The surface reactions of SiO[s] and SiO$_2$[s] can create a catalytic cycle that dissociates H$_2$ to 2 H, resulting in a reduction of the CH$_4$ number densities.

astro-ph.EP↗