HCN as a probe of the inner disk in a candidate proto-brown dwarf

TL;DR

This study uses ALMA observations of HCN to probe the inner disk of a candidate proto-brown dwarf, revealing dense, compact structures and suggesting HCN as a useful tracer for small-scale disk features in faint objects.

Contribution

It demonstrates that HCN emission traces the innermost dense regions of a proto-brown dwarf, providing a new method to study small disks in faint, compact objects.

Findings

HCN emission arises from a compact, dense region near the proto-brown dwarf.

HCO+ traces the outer envelope and is depleted in the center.

HCN can survive in the inner dense regions, making it a useful probe.

Abstract

The detection of Keplerian rotation is rare among Class 0 protostellar systems. We have investigated the high-density tracer HCN as a probe of the inner disk in a Class 0 proto-brown dwarf candidate. Our ALMA high angular resolution observations show the peak in the HCN (3-2) line emission arises from a compact component near the proto-brown dwarf with a small bar-like structure and a deconvolved size of $\sim$50 au. Radiative transfer modelling indicates that this HCN feature is tracing the innermost, dense regions in the proto-brown dwarf where a small Keplerian disk is expected to be present. The limited velocity resolution of the observations, however, makes it difficult to confirm the rotational kinematics of this feature. A brightening in the HCN emission towards the core center suggests that HCN can survive in the gas phase in the inner, dense regions of the proto-brown dwarf. In contrast, modelling of the HCO$^{+}$ (3-2) line emission indicates that it originates from the outer pseudo-disk/envelope region and is centrally depleted. HCN line emission can reveal the small-scale structures and can be an efficient observational tool to study the inner disk properties in such faint compact objects where spatially resolving the disk is nearly impossible.