Widespread subsonic turbulence in Ophiuchus North 1

TL;DR

This study reveals that the Ophiuchus North 1 molecular cloud exhibits widespread subsonic turbulence at large scales, supported mainly by magnetic fields, contrasting with the common supersonic motions in molecular clouds.

Contribution

It provides the first large-scale observational evidence of widespread subsonic turbulence in a nearby molecular cloud, highlighting the role of magnetic fields in cloud support.

Findings

Most C$^{18}$O regions have Mach numbers less than 1.

Magnetic field strength estimated to be greater than 9 μG.

Cloud is globally sub-Alfvénic and magnetically supported.

Abstract

Supersonic motions are common in molecular clouds. (Sub)sonic turbulence is usually detected toward dense cores and filaments. However, it remains unknown whether (sub)sonic motions at larger scales ($\gtrsim$1~pc) can be present in different environments or not. Located at a distance of about 110 pc, Ophiuchus North 1 (Oph N1) is one of the nearest molecular clouds that allows in-depth investigation of its turbulence properties by large-scale mapping observations of single-dish telescopes. We carried out the $^{12}$CO ($J=1-0$) and C$^{18}$O ($J=1-0$) imaging observations toward Oph N1 with the Purple Mountain Observatory 13.7 m telescope. The observations have an angular resolution of $\sim$55\arcsec (i.e., 0.03~pc). Most of the whole C$^{18}$O emitting regions have Mach numbers of $\lesssim$1, demonstrating the large-scale (sub)sonic turbulence across Oph N1. Based on the polarization measurements, we estimate the magnetic field strength of the plane-of-sky component to be $\gtrsim$9~$\mu$G. We infer that Oph N1 is globally sub-Alfv{\'e}nic, and is supported against gravity mainly by the magnetic field. The steep velocity structure function can be caused by the expansion of the Sh~2-27 H{\scriptsize II} region or the dissipative range of incompressible turbulence. Our observations reveal a surprising case of clouds characterised by widespread subsonic turbulence and steep size-linewidth relationship. This cloud is magnetized where ion-neutral friction should play an important role.