Resurgence of CO in a warm bubble around accreting protoplanets and its observability

Abstract

The cold outer regions of protoplanetary disks are expected to contain a midplane-centered layer where gas-phase CO molecules freeze out and their overall abundance is low. The layer then manifests itself as a void in the channel maps of CO rotational emission lines. We explore whether the frozen-out layer can expose the circumplanetary environment of embedded accreting protoplanets to observations. To this end, we performed 3D radiative gas-dust hydrodynamic simulations with opacities determined by the redistribution of submicron- and millimeter-sized dust grains. A Jupiter-mass planet with an accretion luminosity of $\sim$$10^{-3}\,L_{\odot}$ was considered as the nominal case. The accretion heating sustains a warm bubble around the planet, which locally increases the abundance of gas-phase CO molecules. Radiative transfer predictions of the emergent sky images show that the bubble becomes a conspicuous CO emission source in channel maps. It appears as a low-intensity optically thick spot located in between the so-called dragonfly wings that trace the fore- and backside line-forming surfaces. The emission intensity of the bubble is nearly independent of the tracing isotopolog, suggesting a very rich observable chemistry, as long as its signal can be deblended from the extended disk emission. This can be achieved with isotopologs that are optically thin or weakly thermally stratified across the planet-induced gap, such as C$^{18}$O. For these, the bubble stands out as the brightest residual in synthetic ALMA observations after subtraction of axially averaged channel maps inferred from the disk kinematics, enabling new automatic detections of forming protoplanets. By contrast, the horseshoe flow steadily depletes large dust grains from the circumplanetary environment, which becomes unobservable in the submillimeter continuum, in accordance with the scarcity of ALMA detections.