Transition disk chemistry and future prospects with ALMA

TL;DR

This paper models the chemical structure of transition disks with large gaps, predicting that ALMA observations can directly probe the chemically active midplane region at the disk's inner edge.

Contribution

It introduces a chemical model of transition disks with large gaps, highlighting the potential for ALMA to observe and analyze the active chemistry at the disk's inner edge.

Findings

Midplane becomes observable in molecular emission due to irradiation effects.

High-J rotational transitions can effectively probe the midplane gas.

ALMA can resolve the chemically active transition region in disks.

Abstract

We explore the chemical structure of a disk that contains a large central gap of R ~ 45 AU, as is commonly seen in transitional disk systems. In our chemical model of a disk with a cleared inner void, the midplane becomes revealed to the central star so that it is directly irradiated. The midplane material at the truncation radius is permissive to reprocessed optical heating radiation, but opaque to the photo-dissociating ultraviolet, creating an environment abundant in gas-phase molecules. Thus the disk midplane, which would otherwise for a full disk be dominated by near complete heavy element freeze-out, should become observable in molecular emission. If this prediction is correct this has exciting prospects for observations with the Atacama Large Millimeter/Submillimeter Array (ALMA), as the inner transition region should thus be readily detected and resolved, especially using high-J rotational transitions excited in the high density midplane gas. Therefore such observations will potentially provide us with a direct probe of the physics and chemistry at this actively evolving interface.