The HCN-Water Ratio in the Planet Formation Region of Disks

TL;DR

This study reveals a correlation between the HCN/H2O flux ratio in the inner regions of T Tauri disks and the outer disk mass, suggesting planetesimal formation influences inner disk chemistry.

Contribution

It proposes a novel interpretation linking molecular emission ratios to planetesimal formation and water sequestration beyond the snow line in protoplanetary disks.

Findings

A trend between HCN/H2O ratio and disk mass in Taurus T Tauri stars.

Potential link between planetesimal formation and inner disk molecular composition.

Discussion of alternative explanations and future research directions.

Abstract

We find a trend between the mid-infrared HCN/H2O flux ratio and submillimeter disk mass among T Tauri stars in Taurus. While it may seem puzzling that the molecular emission properties of the inner disk (< few AU) are related to the properties of the outer disk (beyond ~20 AU) probed by the submillimeter continuum, an interesting possible interpretation is that the trend is a result of planetesimal and protoplanet formation. Because objects this large are decoupled from the accretion flow, when they form, they can lock up water (and oxygen) beyond the snow line, thereby enhancing the C/O ratio in the inner disk and altering the molecular abundances there. We discuss the assumptions that underlie this interpretation, a possible alternative explanation, and related open questions that motivate future work. Whatever its origin, understanding the meaning of the relation between the HCN/H2O ratio and disk mass is of interest as trends like this among T Tauri disk properties are relatively rare.