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