# Revealing the chemical structure of the Class I disc Oph-IRS 67

## TL;DR

This study uses molecular line observations to analyze the structure and chemistry of the Class I protobinary IRS 67, revealing its intermediate characteristics between earlier and later star formation stages.

## Contribution

It provides a detailed molecular line analysis of IRS 67, highlighting its unique chemical and structural features that bridge Class 0 and Class II sources.

## Key findings

- Different molecular species trace distinct physical regions.
- The chemistry of IRS 67 shows similarities to both Class 0 and Class II sources.
- Temperature structure varies across the system, indicating diverse physical conditions.

## Abstract

(Abridged) The purpose of this paper is to explore the structure of a line-rich Class I protobinary source, Oph-IRS 67, and analyse the differences and similarities with Class 0 and Class II sources. We present a systematic molecular line study of IRS 67 with the Submillimeter Array (SMA) on 1 - 2" (150 - 300 AU) scales. The wide instantaneous band-width of the SMA observations (~30 GHz) provide detections of a range of molecular transitions that trace different physics, such as CO isotopologues, sulphur-bearing species, deuterated species, and carbon-chain molecules. We see significant differences between different groups of species. For example, the CO isotopologues and sulphur-bearing species show a rotational profile and are tracing the larger-scale circumbinary disc structure, while CN, DCN, and carbon-chain molecules peak at the southern edge of the disc at blue-shifted velocities. In addition, the cold gas tracer DCO+ is seen beyond the extent of the circumbinary disc. The detected molecular transitions can be grouped into three main components: cold regions far from the system, the circumbinary disc, and a UV-irradiated region likely associated with the surface layers of the disc that are reached by the UV radiation from the sources. The different components are consistent with the temperature structure derived from the ratio of two H2CO transitions, that is, warm temperatures are seen towards the outflow direction, lukewarm temperatures are associated with the UV-radiated region, and cold temperatures are related with the circumbinary disc structure. The chemistry towards IRS 67 shares similarities with both Class 0 and Class II sources, possibly due to the high gas column density and the strong UV radiation arising from the binary system. IRS 67 is, therefore, highlighting the intermediate chemistry between deeply embedded sources and T-Tauri discs.