HST/WFC3 Constraints on the Abundances of OH and FeH in the Atmosphere of the Ultra-Hot Neptune LTT-9779 b

Planets residing within the hot-Neptune Desert are rare, and studying their atmospheres can provide valuable insights into their formation and evolutionary processes. We present the atmospheric characterization of the first known ultra-hot Neptune, LTT-9779 b, using transmission spectroscopic observations obtained with the HST/WFC3 G141 and G102 grisms. Using the Iraclis pipeline and TauREx3 retrieval code, we find that LTT-9779 b likely possesses a H/He-dominated primary atmosphere with an opaque aerosol layer and the pure cloudy, flat-line model is rejected with approximately 2.7-$σ$ confidence. Although we do not find conclusive evidence supporting the presence of any molecular species, we place 95% confidence level upper limits on the volume mixing ratios (VMRs) of hydroxyl radical (OH) and iron hydride (FeH) at $7.18\times10^{-2}$ and $1.52\times10^{-8}$, respectively. Notably, the retrieval results are inconsistent with predictions from equilibrium chemistry models, which favor higher $\rm H_2O$ abundances over OH. This discrepancy suggests that disequilibrium processes, such as photochemistry or vertical mixing, may have altered the atmospheric composition. Comparisons between HST, Spitzer and JWST data reveal no evidence of temporal variations in the atmospheric composition of the terminator region. Our results highlight the need for higher-resolution spectroscopy and secondary eclipse observations to resolve LTT-9779 b's temperature-pressure (T-P) profile and chemical inventory definitively.