None of the 1D RCTE models capture the 4 µm absorption feature seen in the data. We searched for several candidate gas species that could produce this feature if their abundances differ from the expected abundances from thermochemical equilibrium. The list of searched chemical species include C-bearing gases such as C2H2, CS, CS2, C2H6, C2H4, CH3, CH, C2, CH3Cl, CH3F, CN and CP. Various metal hydrides, bromides, flourides and chlorides such as LiH, AlH, FeH, CrH, BeH, TiH, CaH, HBr, LiCl, HCl, HF, AlCl, NaF and AlF were also searched as potential candidates to explain the feature. SO2, SO3, SO and SH are among the sulfur-based gases that were considered. Other species that were considered include gases such as PH3, H2S, HCN, N2O, GeH4, SiH4, SiO, AsH3, H2CO, H+3, OH+, KOH, Brα-H, AlO, CN, CP, CaF, H2O2, H3O+, HNO3, KF, MgO, PN, PO, PS, SiH, SiO2, SiS, TiO and VO.
Among all these gases, SO2 was the most promising candidate in terms of its spectral shape and chemical plausibility, although the expected chemical equilibrium abundance of SO2 is too low to produce the absorption signal seen in the data. However, previous work exploring photochemistry in exoplanetary atmospheres25 (link),26 (link) have shown that higher amounts of SO2 can be created in the upper atmospheres of irradiated planets through photochemical processes. Therefore, we postprocess the PICASO 3.0 and ScCHIMERA chemical equilibrium models with varying amounts of SO2 in a Bayesian framework to estimate the SO2 abundance required to explain the strength of the 4-µm feature. The required volume mixing ratio of SO2 was found to be roughly 10−5–10−6. Note that in obtaining this estimate we assumed that the SO2 volume mixing ratio does not vary with pressure for simplicity. In a photochemical scenario this assumption is probably not realistic, although the pressure range probed by SO2 is also limited. Whether photochemical models can produce this amount of SO2 in the atmospheric conditions of WASP-39b is a pressing question that the ERS team is now exploring (Welbanks et al. (in prep), Tsai et al. (submitted)). Whether this feature can be better explained by any other gaseous absorber is also at present under investigation by the ERS team.
Among all these gases, SO2 was the most promising candidate in terms of its spectral shape and chemical plausibility, although the expected chemical equilibrium abundance of SO2 is too low to produce the absorption signal seen in the data. However, previous work exploring photochemistry in exoplanetary atmospheres25 (link),26 (link) have shown that higher amounts of SO2 can be created in the upper atmospheres of irradiated planets through photochemical processes. Therefore, we postprocess the PICASO 3.0 and ScCHIMERA chemical equilibrium models with varying amounts of SO2 in a Bayesian framework to estimate the SO2 abundance required to explain the strength of the 4-µm feature. The required volume mixing ratio of SO2 was found to be roughly 10−5–10−6. Note that in obtaining this estimate we assumed that the SO2 volume mixing ratio does not vary with pressure for simplicity. In a photochemical scenario this assumption is probably not realistic, although the pressure range probed by SO2 is also limited. Whether photochemical models can produce this amount of SO2 in the atmospheric conditions of WASP-39b is a pressing question that the ERS team is now exploring (Welbanks et al. (in prep), Tsai et al. (submitted)). Whether this feature can be better explained by any other gaseous absorber is also at present under investigation by the ERS team.
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