Protoplanetary disk cavities with JWST-MIRI: a dichotomy in molecular emission

TL;DR

This study uses JWST-MIRI observations to reveal a clear dichotomy in molecular emission from protoplanetary disk cavities, linking dust depletion with changes in gas density and molecular composition, advancing understanding of disk evolution.

Contribution

It presents the first homogeneous JWST-MIRI spectral analysis of 12 disks with cavities, identifying a molecular emission dichotomy and linking it to dust and gas evolution in protoplanetary disks.

Findings

Disks show a dichotomy: molecule-rich vs. molecule-poor cavities.

Molecule-poor cavities have lower water column density and higher OH/H2O ratios.

Gas density decreases in cavities, affecting molecular excitation and dissociation.

Abstract

The evolution of planet-forming regions in protoplanetary disks is of fundamental importance to understanding planet formation. Disks with a central deficit in dust emission, a "cavity", have long attracted interest as potential evidence for advanced disk clearing by protoplanets and/or winds. Before JWST, infrared spectra showed that these disks typically lack the strong molecular emission observed in full disks. In this work, we combine a sample of 12 disks with millimeter cavities of a range of sizes ($\sim2$-70 au) and different levels of millimeter and infrared continuum deficits. We analyze their molecular spectra as observed with MIRI on JWST, homogeneously reduced with the new JDISCS pipeline. This analysis demonstrates a stark dichotomy in molecular emission where "molecule-rich" (MR) cavities follow global trends between water, CO, and OH luminosity and accretion luminosity as in full disks, while "molecule-poor" (MP) cavities are significantly sub-luminous in all molecules except sometimes OH. Disk cavities generally show sub-luminous organic emission, higher OH/H$_2$O ratios, and suggest a lower water column density. The sub-thermal excitation of CO and water vibrational lines suggests a decreased gas density in the emitting layer in all cavities, supporting model expectations for C$_2$H$_2$ photodissociation. We discover a bifurcation in infrared index (lower in MR cavities) suggesting that the molecular dichotomy is linked to residual $\mu$m-size dust within millimeter disk cavities. Put together, these results suggest a feedback process between dust depletion, gas density decrease, and molecule dissociation. Disk cavities may have a common evolutionary sequence where MR switch into MP over time.