No TiO detected in the hot Neptune-desert planet LTT-9779 b in reflected light at high spectral resolution

Abstract

LTT-9779 b is an inhabitant of the hot Neptune desert and one of only a few planets with a measured high albedo. Characterising the atmosphere of this world is the key to understanding what processes dominate in creating the hot Neptune desert. We aim to characterise the reflected light of LTT-9779 b at high spectral resolution to break the degeneracy between clouds and atmospheric metallicity. This is key to interpreting its mass loss history which may illuminate how it kept its place in the desert. We use the high resolution cross-correlation spectroscopy technique on four half-nights of ESPRESSO observations in 4-UT mode (16.4-m effective mirror) to constrain the reflected light spectrum of LTT-9779 b. We do not detect the reflected light spectrum of LTT-9779 b despite these data having the expected sensitivity at the level 100 ppm. Injection tests on the post-eclipse data indicate that TiO should have been detected for a range of different equilibrium chemistry models. Therefore this non-detection suggests TiO depletion in the western hemisphere however, this conclusion is sensitive to temperature which impacts the chemistry in the upper atmosphere and the reliability of the line list. Additionally, we are able to constrain the top of the western cloud deck to $P_{\text{top, western}}<10^{-2.0}$ bar and the top of the eastern cloud deck $P_{\text{top, eastern}}<10^{-0.5}$ bar, which is consistent with the predicted altitude of MgSiO$_3$ and Mg$_2$SiO$_4$ clouds from JWST NIRISS/SOSS. While we do not detect the reflected light spectrum of LTT-9779 b, we have verified that this technique can be used in practice to characterise the high spectral resolution reflected light of exoplanets so long as their spectra contain a sufficient number of deep spectral lines. Therefore this technique may become an important cornerstone of exoplanet characterisation with the ELT and beyond.