Chemical Divergence and Water Depletion: Gas Properties of Evolved Upper Scorpius Disks Revealed by JWST/MIRI

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Tracing the chemical evolution of protoplanetary disks over time requires observations of disks at differentages. However, most JWST/MIRI surveys published to date have targeted younger (∼1-3 Myr) rather thanolder systems. We present the results of a JWST/MIRI MRS survey of the inner regions of 10 protoplanetarydisks (ages ∼2-6 Myr, spectral types M0-M4.5) in the Upper Scorpius region previously characterized by theALMA AGE-PRO large program. Using MCMC slab modeling, we fit to a wide variety of detected molecules,including H2O, CO, C2H2, 13CCH2, HCN, HC3N, CO2, 13CO2, C2H6, C4H2, and OH, as well as C6H6, CH3,and H2 visually. We classify each disk along two independent axes—a Water Classification based on H2Oline luminosity (Water-Rich, Water-Poor, or Water-Absent) and a Chemotype based on the dominant non-waterchemistry (Organic-Rich, CO2-Dominated, or Molecule-Absent)—and find an unexpectedly high diversity ofdistinct chemical compositions within our population. We leverage the heterogeneity of detected moleculesin our sample to present new characteristic “diagnostic” wavelength regions for most species. We find thatcarbon-based molecules consistently exhibit markedly lower excitation temperatures (≲300 K) compared toyounger (∼1–3 Myr) star-forming regions (∼600-1000 K), hinting at relatively colder molecular reservoirs. Wealso determine that Upper Scorpius disks show systematically lower water luminosities by factors of 10-1000.In particular, disks with strong carbon-based molecular features but no observed H2O defy expectations of aninner-disk dust cavity or a low (≲ 3) Rgas/Rdust ratio, instead suggesting that the presence of a strong outer-diskdust trap largely controls the chemical outcome of the terrestrial planet-forming region.

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