HNC in Protoplanetary Disks

TL;DR

This study presents the first spatially resolved observations of HNC in protoplanetary disks, revealing its distribution and linking its chemistry to disk physical conditions, thus enhancing understanding of disk chemistry and structure.

Contribution

It provides the first spatially resolved HNC observations in disks and analyzes its distribution, linking HNC chemistry to disk temperature and structure.

Findings

HNC detected in both disks, with a ring-like distribution in TW Hya.

HNC/HCN emission ratio is 0.1-0.2 in both disks.

HNC chemistry is strongly influenced by disk temperature and structure.

Abstract

The distributions and abundances of small organics in protoplanetary disks are potentially powerful probes of disk physics and chemistry. HNC is a common probe of dense interstellar regions and the target of this study. We use the Submillimeter Array (SMA) to observe HNC 3--2 towards the protoplanetary disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296. HNC is detected toward both disks, constituting the first spatially resolved observations of HNC in disks. We also present SMA observations of HCN 3--2, and IRAM 30m observations of HCN and HNC 1--0 toward HD 163296. The disk-averaged HNC/HCN emission ratio is 0.1--0.2 toward both disks. Toward TW Hya, the HNC emission is confined to a ring. The varying HNC abundance in the TW Hya disk demonstrates that HNC chemistry is strongly linked to the disk physical structure. In particular, the inner rim of the HNC ring can be explained by efficient destruction of HNC at elevated temperatures, similar to what is observed in the ISM. To realize the full potential of HNC as a disk tracer requires, however, a combination of high SNR spatially resolved observations of HNC and HCN, and disk specific HNC chemical modeling.