Measuring turbulent motion in planet-forming disks with ALMA: A
  detection around DM Tau and non-detections around MWC 480 and V4046 Sgr
  Turbulence is a crucial factor in many models of planet formation, but it has
only been directly constrained among a small number of planet forming disks.
Building on the upper limits on turbulence placed in disks around HD 163296 and
TW Hya, we present ALMA CO J=2-1 line observations at $\sim0.3"$ (20-50 au)
resolution and 80 ms$^{-1}$ channel spacing of the disks around DM Tau, MWC
480, and V4046 Sgr. Using parametric models of disk structure, we robustly
detect non-thermal gas motions around DM Tau of between 0.25 c$_s$ and 0.33
c$_s$, with the range dominated by systematic effects, making this one of the
only systems with directly measured non-zero turbulence. Using the same
methodology, we place stringent upper limits on the non-thermal gas motion
around MWC 480 ($<$0.08c$_s$) and V4046 Sgr ($<$0.12 c$_s$). The preponderance
of upper limits in this small sample, and the modest turbulence levels
consistent with dust studies, suggest that weak turbulence
($\alpha\lesssim10^{-3}$) may be a common, albeit not universal, feature of
planet-forming disks. We explore the particular physical conditions around DM
Tau that could lead this system to be more turbulent than the others.
