The JDISC Survey: Inner Disk Chemistry of Class I/FS Disks and Tentative Evidence for Early Pebble Drift

We present the first chemical survey of Class I and flat-spectrum (I/FS) disks using the JWST Mid-Infrared Instrument/Medium-Resolution Spectrometer, targeting 16 sources in the Ophiuchus star-forming region. Through empirical line luminosity measurements and multicomponent slab modeling, we characterize the molecular reservoir of these young systems and compare them to 12 Class II disks of similar stellar mass. Water, HCN, C 2 H 2 , and CO 2 are frequently detected in I/FS sources with inclinations i < 70 ∘ , whereas edge-on systems show significantly suppressed emission. Compared to Class II disks, I/FS sources show suggestive—though not yet statistically significant—evidence for elevated cold water (∼200 K) mass and lower CO 2 excitation temperatures. Statistical analyses identify accretion luminosity as the primary correlate of molecular mass across both evolutionary stages. Once this dependence is removed, cold water and CO 2 masses anticorrelate with millimeter-dust disk radius, while hot water remains insensitive to disk size. These patterns are qualitatively consistent with pebble drift models that predict early water enrichment followed by delayed CO 2 delivery, suggesting an evolutionary progression from molecular-poor Class 0 sources, through water-rich Class I/FS disks, to Class II disks with reduced cold water excess. This work provides an initial evolutionary framework for disk chemistry that requires larger, multiregion samples to confirm.