The CRIMSON survey I: super-stellar SiO in the directly imaged companion TWA 5 B from high-resolution M-band spectroscopy

Silicon is a key refractory element in giant planet atmospheres, which governs the formation of magnesium-silicate clouds, and reflects the quantity of silicates accreted during formation. While observations of directly imaged giant exoplanets have focused on the measurement of volatile species (e.g. CO, H 2 O), high-resolution spectroscopy with CRIRES+ M-band provides access to gas phase silicon chemistry in sub-stellar atmospheres, through the ro-vibrational band head of SiO at 4 mu m. Here, we present the first results of the CRIMSON survey of silicon chemistry in directly imaged companions with CRIRES+ M-band. We report the strong detection of gaseous SiO (S/N = 7.5) in the directly imaged companion TWA 5 B, with an atmospheric abundance of log(SiO) = -3.56+0.42-0.32 VMR, providing access to the refractory content of the atmosphere. The high retrieved SiO abundance implies the absence of significant magnesium-silicate cloud condensation, and thus the atmospheric silicon abundance is contained almost entirely within the observed gas phase SiO. Using the detection of refractory silicon, together with strong detections of the volatile species CO (S/N = 9.1) and H 2 O (S/N = 18.8), we measure a stellar C/O and a marginally sub-stellar O/Si and C/Si, but a super-stellar Si/H ([Si/H] = 1.41+0.42-0.32). Collectively, these volatile-to-refractory ratios are consistent with formation through core-accretion beyond the CO snowline, or gravitational instability followed by substantial solid enrichment. Finally, we discuss how gas phase SiO provides a unique diagnostic of the cloud properties in hot gas-giants, and can be used to probe the dominant cloud species forming across the directly imaged planet and isolated brown dwarf populations.