Competing chemical signatures in the atmosphere of TOI-270 d: Inference of sulfur and carbon chemistry

Abstract

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. Here, we reanalyze the 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 considerations for JWST data analysis. Additionally, we test the impact of data resolution on atmospheric retrieval calculations. We reduce one JWST NIRSpec G395H and one NIRISS SOSS GR700XD transit dataset using the Eureka! pipeline and a custom MCMC-based light curve fitting algorithm at the instruments' native resolutions. The atmospheric composition is estimated with the updated BeAR retrieval code across a grid of retrieval setups and spectral resolutions. Our transit spectrum is consistent with previous studies, except at the red end of the NIRISS data. Our retrievals support a higher mean molecular weight atmosphere 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 microns. 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.

arXiv commentPublished in A&A

References in corpus (49)

• emcee: The MCMC Hammer
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• The James Webb Space Telescope
• Multimodal nested sampling: an efficient and robust alternative to MCMC methods for astronomical data analysis
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Bayes in the sky: Bayesian inference and model selection in cosmology
• batman: BAsic Transit Model cAlculatioN in Python
• Importance Nested Sampling and the MultiNest Algorithm
• Efficient, uninformative sampling of limb darkening coefficients for two-parameter laws
• ExoMol molecular line lists XXX: a complete high-accuracy line list for water
• A Temperature and Abundance Retrieval Method for Exoplanet Atmospheres
• Growth Model Interpretation of Planet Size Distribution
• Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
• A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
• How to Distinguish between Cloudy Mini-Neptunes and Water/Volatile-Dominated Super-Earths
• A Framework for Characterizing the Atmospheres of Low-Mass Low-Density Transiting Planets
• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• Atmospheric Escape and the Evolution of Close-in Exoplanets
• Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H
• ExoMol molecular line lists XXXV: a rotation-vibration line list for hot ammonia
• ExoMol molecular line lists - XVI: The rotation-vibration spectrum of hot H${}_2$S
• An accurate, extensive, and practical line list of methane for the HITEMP database
• ExoMol line lists -- XXXIX. Ro-vibrational molecular line list for CO${}_2$
• HELIOS-K: An Ultrafast, Open-source Opacity Calculator for Radiative Transfer
• ExoMol molecular line lists - XIV: The rotation-vibration spectrum of hot SO${}_2$
• High Tide or Riptide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ~486b from JWST Observations
• Transit spectrophotometry of the exoplanet HD189733b. I. Searching for water but finding haze with HST NICMOS
• Habitability and Biosignatures of Hycean Worlds
• HELIOS-K 2.0 Opacity Calculator and Open-source Opacity Database for Exoplanetary Atmospheres
• Eureka!: An End-to-End Pipeline for JWST Time-Series Observations
• A hot Uranus transiting the nearby M dwarf GJ3470. Detected with HARPS velocimetry. Captured in transit with TRAPPIST photometry
• Irradiated ocean planets bridge super-Earth and sub-Neptune populations
• A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270
• Helios-r2 -- A new Bayesian, open-source retrieval model for brown dwarfs and exoplanet atmospheres
• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• Analysis of a JWST NIRSpec Lab Time Series: Characterizing Systematics, Recovering Exoplanet Transit Spectroscopy, and Constraining a Noise Floor
• Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry
• Clouds and Clarity: Revisiting Atmospheric Feature Trends in Neptune-size Exoplanets
• Hubble Space Telescope transmission spectroscopy for the temperate sub-Neptune TOI-270d: a possible hydrogen-rich atmosphere containing water vapour
• ExoMol molecular line lists XII: Line Lists for 8 isotopologues of CS
• ExoMol molecular line lists - XXVI: spectra of SH and NS
• Planetary Population Synthesis and the Emergence of Four Classes of Planetary System Architectures
• A Variationally Computed IR Line List for the Methyl Radical CH${}_3$
• Chemical mapping of temperate sub-Neptune atmospheres: Constraining the deep-interior H2O/H2 using the atmospheric CO2/CH4
• Efficient, uninformative sampling of limb-darkening coefficients for a three-parameter law
• Mass determination of the 1:3:5 near-resonant planets transiting GJ 9827 (K2-135)
• The rotation-vibration spectrum of methyl fluoride from first principles
• ExoMol line lists - XXIX. The rotation-vibration spectrum of methyl chloride up to 1200 K
• High-resolution absorption cross sections of C${}_2$H${}_6$ at elevated temperatures

Cited by in corpus (5)

• Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d
• Coupled Thermal-Chemical Evolution Models of Sub-Neptunes Reveal Atmospheric Signatures of Their Formation Location
• The Effects of Non-ideal Mixing in Planetary Magma Oceans and Atmospheres
• Silicate Clouds, SiO, and Molecular Trends across the L-T Sequence: A JWST/MIRI LRS Spectral Library of Brown Dwarfs
• Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures