The Musca cloud: A 6 pc-long velocity-coherent, sonic filament

TL;DR

The Musca cloud is a 6.5 pc-long, velocity-coherent filament with predominantly thermal motions, representing the longest known sonic-like structure in the ISM, challenging traditional turbulence models.

Contribution

This study provides the first observational evidence of a filament fully decoupled from turbulence over multi-parsec scales, highlighting its transonic nature.

Findings

Musca is the longest velocity-coherent, sonic-like filament in the ISM.

Internal motions are dominated by thermal rather than turbulent processes.

The filament's properties challenge Larson's velocity dispersion-size relationship.

Abstract

Filaments play a central role in the molecular clouds' evolution, but their internal dynamical properties remain poorly characterized. To further explore the physical state of these structures, we have investigated the kinematic properties of the Musca cloud. We have sampled the main axis of this filamentary cloud in $^{13}$CO and C$^{18}$O (2--1) lines using APEX observations. The different line profiles in Musca shows that this cloud presents a continuous and quiescent velocity field along its $\sim$6.5 pc of length. With an internal gas kinematics dominated by thermal motions (i.e., $\sigma_{NT}/c_s\lesssim1$) and large-scale velocity gradients, these results reveal Musca as the longest velocity-coherent, sonic-like object identified so far in the ISM. The transonic properties of Musca present a clear departure from the predicted supersonic velocity dispersions expected in the Larson's velocity dispersion-size relationship, and constitute the first observational evidence of a filament fully decoupled from the turbulent regime over multi-parsec scales.