DOI: 10.5281/zenodo.17101614
title: Products and Models for "Mapping the SO2 Shoreline in Gas Giant Exoplanets"

description: JWST has revealed sulfur chemistry in the atmospheres of gas giant exoplanets. Molecules such as SO2 may be sensitive tracers of high-altitude photochemistry, atmospheric metallicity, and of planet formation & migration. To ascertain the conditions that determine whether (or how much) SO2, H2S, and other sulfur-bearing species are present in exoplanet atmospheres, we present a series of model grids of planetary atmospheres covering metallicities from 0.3–1000× Solar and temperatures from 250–2050 K. With these models we map out the “SO2 shoreline,” the region of metallicity and irradiation for which SO2 (the most easily detected sulfur tracer) may be sufficiently abundant to be detectable. A key conclusion is that for a given temperature, SO2 abundance is an extremely sensitive indicator of metallicity (robust to all the other factors; mixing, plausible C/O ratios, UV irradiation). Expected SO2 abundances also depend strongly on overall temperature and C/O ratio; the SO2 abundance depends surprisingly weakly on XUV irradiation, also weakly on Kzz (for Teq ≳ 600 K), and is essentially independent of internal temperature. Despite its detection in a growing number of giant planets, SO2 is never the dominant sulfur-bearing molecule: depending on temperature and metallicity, H2S, S2, NS, SO, SH, and even S8 or atomic S are frequently as common (or more so) as SO2. Nonetheless SO2 remains the most easily detectable sulfur-bearing species, followed by H2S, though perhaps SO and SH could be detectable in some gas giants. Aside from a pressing need for additional observational constraints on sulfur, we also identify the need for future work to account for the effects of clouds and hazes, fully self-consistent atmospheric models, 2D and 3D models, a wider range of planetary masses and radii, and studies to measure and refine reaction rates and molecular opacities of sulfur-bearing species.

The atmospheric grid and associated models used in the paper arepresented here. An example directory name isMTgrid_XUV1,00_Kzz7.0_Tint100_TP_helios_1000K_SNCHO_1.0x_z83_C1.0_N1.0_O1.0_S1.0_where the name is formatted as"MTgrid_XUV{xuv}_Kzz{kzz}_Tint{tint}_TP_helios_{teq}K_SNCHO_{met}_z83_C{cmet}_N{nmet}_O{omet}_S{smet}_"

Table 1 and Section 2 of the paper provide further details on therelevant parameters and how they are used in the subsequent modelingprocess. The values are:  xuv - scaling from nominal HAT-P-26 XUV stellar spectrum  kzz - log10 of Kzz value [cgs units: cm^2/s]  tint - internal temperature [K] (for T-P profile calculation)  teq - planet temperature [K]  met - metallicity used for T-P profile calculation, relative to Solar  cmet - enhancement of C/H relative to the Solar value (for chemistry calculation)  nmet - enhancement of N/H relative to the Solar value (for chemistry calculation)  omet - enhancement of O/H relative to the Solar value (for chemistry calculation)  smet - enhancement of S/H relative to the Solar value (for chemistry calculation)

Each directory contains several files:  (1) cfg_mtgrid_helios.txt  (2) mtgrid_helios.vul  (3) mtgrid_helios_vertical_profiles.csv  (4) mtgrid_helios_vertical_mmr_profiles.csv  (5) mtgrid_helios_transmission.csv

(1) cfg_mtgrid_helios.txt  The VULCAN control file used for the run. See  https://github.com/exoclime/VULCAN and Tsai et al. (2021) -  https://ui.adsabs.harvard.edu/abs/2021ApJ...923..264T - for  more details.

(2) mtgrid_helios.vul  The VULCAN output file. This is a Python pickle object. See  https://github.com/exoclime/VULCAN and Tsai et al. (2021) -  https://ui.adsabs.harvard.edu/abs/2021ApJ...923..264T - for  more details.

(3) mtgrid_helios_vertical_profiles.csv  A machine-readable CSV file with vertical profiles extracted from  the VULCAN output: pressure, temperature, Kzz, MMW (mean molecular  weight), and volume mixing ratios of a subset of the molecules used  in the VULCAN photochemistry calculation.

(4) mtgrid_helios_vertical_mmr_profiles.csv  A machine-readable CSV file with vertical profiles extracted from  the VULCAN output: pressure, temperature, Kzz, MMW (mean molecular  weight), and mass mixing ratios (MMR) of a subset of the molecules used  in the VULCAN photochemistry calculation.

(5) mtgrid_helios_transmission.csv  A machine-readable CSV file with the synthetic transmission  (transit) spectrum calculated using petitRadTrans from the  associated VULCAN output file. The first two colums are 'wave'  (wavelength in microns) and 'rprs2' (the transit depth,  i.e. [Rp/Rs]^2).  Subsequent columns such as 'rprs2_only_CO2' and  'rprs2_no_CO2' are transmission spectra containing (respectively)  only the specified absorber, and all other absorbers except the  specified absorber.