Widespread subsonic turbulence in Ophiuchus North 1

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.