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 (3;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.