Water-rich Disks around Late M Stars Unveiled: Exploring the Remarkable Case of Sz 114

We present an analysis of the JDISCS JWST/MIRI-MRS spectrum of Sz 114, an accreting M5 star surrounded by a large dust disk with a shallow gap at ∼39 au. The spectrum is molecule-rich; we report the detection of water, CO, CO₂, HCN, C₂H₂, and H₂. The only identified atomic/ionic transition is from [Ne ii] at 12.81 μm. A distinct feature of this spectrum is the forest of water lines with the 17.22 μm emission surpassing that of most mid-to-late M star disks by an order of magnitude in flux and aligning instead with disks of earlier-type stars. Moreover, the flux ratios of C₂H₂/H₂O and HCN/H₂O in Sz 114 also resemble those of earlier-type disks, with a slightly elevated CO₂/H₂O ratio. While accretional heating can boost all infrared lines, the unusual properties of Sz 114 could be explained by the young age of the source, its formation under unusual initial conditions (a large massive disk), and the presence of dust substructures. The latter delays the inward drift of icy pebbles and helps preserve a lower C/O ratio over an extended period. In contrast, mid-to-late M-star disks—which are typically faint, small in size, and likely lack significant substructures—may have more quickly depleted the outer icy reservoir and already evolved out of a water-rich inner disk phase. Our findings underscore the unexpected diversity within mid-infrared spectra of mid-to-late M-star disks, highlighting the need to expand the observational sample for a comprehensive understanding of their variations and thoroughly test pebble drift and planet formation models.