2025A&A...701A.296F

Context. Recent JWST measurements allow access to the near-infrared spectrum of the sub-Neptune TOI-270 d, for which two different interpretations, a high-metallicity miscible envelope and a lower metallicity hycean world, are currently in conflict.
Aims. Here, we reanalyze published data and reproduce previously retrieved molecular abundances based on an independent data reduction and a different retrieval framework. The aim of this study is to refine the understanding of TOI-270 d and highlight the impact of various choices during JWST data analysis. Particularly, we test the impact of data resolution on atmospheric retrieval calculations.
Methods. We reduced one JWST NIRSpec G395H and one NIRISS SOSS GR700XD transit dataset using the Eureka! pipeline and a custom Markov Chain Monte Carlo-based light curve fitting algorithm at the instruments' native resolutions. The atmospheric composition was estimated with the updated BEAR retrieval code across a grid of retrieval setups and spectral resolutions.
Results. Our transit spectrum is consistent with previous studies except at the red end of the NIRISS data. Our retrievals support a H₂/He-dominated atmosphere with high mean molecular weight for TOI-270 d. We provide refined abundance constraints and find statistically favored model extensions indicating either sulfur-rich chemistry with species such as CS₂, CS, and H₂CS or the possible presence of CH₃Cl or CH₃F. However, Bayesian inference cannot distinguish between these scenarios due to similar opacities below 4 µm. To obtain physically plausible atmospheric solutions at native resolution, accounting for the instrument's line spread function is essential.
Conclusions. Our analysis reinforces TOI-270 d as a highly interesting warm sub-Neptune for atmospheric studies, with a complex chemistry in a cloud-free upper atmosphere. However, its exact nature remains uncertain and warrants further detailed photochemical modeling and observations.