[Paper Review] Linking atmospheric chemistry of the hot Jupiter HD 209458b to its formation location through infrared transmission and emission spectra

Spandan Dash, Liton Majumdar+7 more|arXiv (Cornell University)|Apr 8, 2022

Chemical Thermodynamics and Molecular Structure73 references16 citations

TL;DR

This study links the atmospheric chemistry of the hot Jupiter HD 209458b to its formation location in the protostellar disc by simulating disequilibrium chemistry using radial abundance profiles from disc chemical kinetics models. It finds that CH4, NH3, HCN, and C2H2 are more prominent under realistic non-inverted P-T profiles, and JWST simulations show distinct spectral features that point to gas accretion between the CO2 and CH4 icelines with a super-solar C/O ratio.

ABSTRACT

The elemental ratios of carbon, nitrogen, and oxygen in the atmospheres of hot Jupiters may hold clues to their formation locations in the protostellar disc. In this work, we adopt gas phase chemical abundances of C, N and O from several locations in a disc chemical kinetics model as sources for the envelope of the hot Jupiter HD 209458b and evolve the planet's atmospheric composition using a 1D chemical kinetics model, treating both vertical mixing and photochemistry. We consider two atmospheric pressure-temperature profiles, one with and one without a thermal inversion. From each of the resulting 32 atmospheric composition profiles, we find that the molecules CH4, NH3, HCN, and C2H2 are more prominent in the atmospheres computed using a realistic non-inverted P-T profile in comparison to a prior equilibrium chemistry based work which used an analytical P-T profile. We also compute the synthetic transmission and emission spectra for these atmospheres and find that many spectral features vary with the location in the disc where the planet's envelope was accreted. By comparing with the species detected using the latest high-resolution ground-based observations, our model suggests HD 209458b could have accreted most of its gas between the CO2 and CH4 icelines with a super solar C/O ratio from its protostellar disc, which in turn directly inherited its chemical abundances from the protostellar cloud. Finally, we simulate observing the planet with the James Webb Space Telescope (JWST) and show that differences in spectral signatures of key species can be recognized. Our study demonstrates the enormous importance of JWST in providing new insights into hot Jupiter's formation environments.

Motivation & Objective

• To connect the atmospheric composition of HD 209458b to its formation location in the protostellar disc using chemical and radiative transfer modeling.
• To investigate how radial variations in C, N, and O abundances across the disc influence the planet's atmospheric chemistry.
• To assess the impact of disequilibrium chemistry (including vertical mixing and photochemistry) on spectral features compared to equilibrium models.
• To simulate JWST transmission and emission spectra to evaluate the observability of formation-location indicators.
• To quantify the potential contamination effects of disc solids on atmospheric C/O ratios and spectral signatures.

Proposed method

• Used gas-phase elemental abundances from Eistrup et al. (2016) disc chemical kinetics model as initial conditions for HD 209458b's envelope.
• Employed a 1D chemical kinetics model (VULCAN) coupled with radiative transfer (petitRADTRANS) to simulate atmospheric evolution with vertical mixing and photochemistry.
• Constructed two P-T profiles: one non-inverted (self-consistently from HELIOS) and one thermally inverted (from Moses et al. 2011), to assess thermal structure effects.
• Generated synthetic transmission and emission spectra using petitRADTRANS for all 32 atmospheric profiles (from 8 disc locations × 2 P-T profiles × 2 ionization states).
• Simulated JWST observations with NIRCam and MIRI LRS (2.4–13 µm) to evaluate detectability of spectral features.
• Assessed contamination effects by incorporating solid accretion from Turrini et al. (2021), varying initial abundances (atomic vs. molecular) and disc evolution models.

Experimental results

Research questions

• RQ1 Can the atmospheric chemistry of HD 209458b be linked to its formation location in the protostellar disc using radial elemental abundance profiles?
• RQ2 How do disequilibrium chemistry processes (vertical mixing, photochemistry) alter molecular abundances compared to equilibrium chemistry models?
• RQ3 Which P-T profile (inverted vs. non-inverted) better reproduces observed spectral features of HD 209458b?
• RQ4 Can JWST distinguish between atmospheric compositions arising from different disc formation locations based on spectral signatures?
• RQ5 To what extent does contamination from accreted solids alter the inferred C/O ratio and spectral features of the planet’s atmosphere?

Key findings

• CH4, NH3, HCN, and C2H2 are significantly more abundant in atmospheres simulated with a realistic non-inverted P-T profile compared to equilibrium chemistry models.
• Synthetic transmission and emission spectra show clear spectral differences depending on the disc location where the planet’s gas was accreted, particularly in H2O, CH4, HCN, and CO features.
• The best match to observed species (H2O, CH4, HCN, NH3) from high-resolution ground-based data is achieved for models where gas was accreted between the CO2 and CH4 icelines, with a super-solar C/O ratio.
• JWST simulations show that the three major spectral types—H2O-dominated, CH4-dominated, and HCN-dominated—can be distinguished in transmission spectra between 2.4–13 µm.
• Disc solid contamination can drastically alter the C/O ratio and spectral appearance, making formation location inference highly model-dependent.
• The combination of C/O and C/N ratios, when absolute abundances (C/H, O/H) are above ~10⁻⁶, serves as a robust indicator of the planet’s formation location in the disc.