Widespread subsonic turbulence in Ophiuchus North 1

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: ygong@mpifr-bonn.mpg.de
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
e-mail: liushu@nao.cas.cn
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
⁴ School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
⁵ School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai campus, No. 2, Daxue Road, Zhuhai, Guangdong 519082, PR China
⁶ South-Western Institute for Astronomy Research, Yunnan University, Kunming, Yunnan 650500, PR China
⁷ Purple Mountain Observatory and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210034, PR China

Received: 22 November 2021
Accepted: 28 March 2022

Abstract

Context. 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 (≳1 pc) may be present in various environments.

Aims. Located at a distance of about 110 pc, Ophiuchus North 1 (Oph N1) is one of the nearest molecular clouds that would allow for an in-depth investigation of its turbulence properties via large-scale mapping observations of single-dish telescopes.

Methods. We carried out the ¹²CO (J = 1−0) and C¹⁸O (J = 1−0) imaging observations toward Oph N1 with the Purple Mountain Observatory 13.7 m telescope. The observations have an angular resolution of ~55″ (i.e., 0.03 pc).

Results. Most of the whole C¹⁸O emitting regions have Mach numbers of ≲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 ≳9 µG. We infer that Oph N1 is globally sub-Alfvénic, 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 HII region or the dissipative range of incompressible turbulence.

Conclusions. Our observations reveal a surprising case of clouds that are characterized by widespread subsonic turbulence and a steep relation between the size and the linewidth. This cloud is magnetized where ion-neutral friction is assumed to play an important role.