Constraint on ion-neutral drift velocity in the Class 0 protostar B335
  from ALMA observations
  Ambipolar diffusion can cause a velocity drift between ions and neutrals.
This is one of the non-ideal MHD effects proposed to enable the formation of
Keplerian disks with sizes of tens of au. To observationally study ambipolar
diffusion in collapsing protostellar envelopes, we compare gas kinematics
traced by the H13CO+ (3-2) and C18O (2-1) lines in the Class 0 protostar B335
obtained with our ALMA observations. A central compact (~1"-2") component that
is elongated perpendicular to the outflow direction and exhibits a clear
velocity gradient along the outflow direction is observed in both lines and
most likely traces the infalling flattened envelope. We constructed kinematical
models to fit the observed velocity structures and to measure the infalling
velocities of the ionized and neutral gas on a 100 au scale in B335. The
infalling velocities in the H13CO+ and C18O emission are both measured to be
0.85+/-0.2 km/s at a radius of 100 au, suggesting that the velocity drift
between the ionized and neutral gas is at most 0.3 km/s at a radius of 100 au.
The Hall parameter for H13CO+ is estimated to be >>1 on a 100 au scale in B335,
so that H13CO+ is expected to be attached to the magnetic field. Our
non-detection or upper limit of the velocity drift between ions and neutrals
could suggest that the magnetic field remains rather well coupled to the bulk
neutral material on a 100 au scale in this source, and that any significant
field-matter decoupling, if present, likely occurs only on a smaller scale,
leading to an accumulation of magnetic flux and thus efficient magnetic braking
in the inner envelope. However, because of our limited angular resolution, we
cannot rule out a significant ambipolar drift only in the midplane of the
infalling envelope. Future observations with higher angular resolutions (~0.1")
are needed to examine this possibility and ambipolar diffusion on a smaller
scale.
