The Infall Motion in the Low-Mass Protostellar Binary NGC 1333 IRAS
  4A1/4A2
  We report ALMA observations of NGC 1333 IRAS 4A, a young low-mass
protostellar binary, referred as 4A1 and 4A2. With multiple H$_2$CO transitions
and HNC (4$-$3) observed at a resolution of 0.25" ($\sim$70 au), we investigate
the gas kinematics of 4A1 and 4A2. Our results show that, on the large angular
scale ($\sim$10"), 4A1 and 4A2 each display a well-collimated outflow along the
N-S direction, and an S-shaped morphology is discerned in the outflow powered
by 4A2. On the small scale ($\sim$0.3"), 4A1 and 4A2 exhibit distinct spectral
features toward the continuum centroid, with 4A1 showing simple symmetric
profiles predominantly in absorption and 4A2 demonstrating rather complicated
profiles in emission as well as in absorption. Based on radiative transfer
modeling exercises, we find that the physical parameters inferred from earlier
low-resolution observations cannot be directly extrapolated down to the 4A1
inner region. Possible reasons for the discrepancies between the observed and
modelled profiles are discussed. We constrain the mass infall rate in 4A1 to be
at most around 3$\times$10$^{-5}$ M$_{\odot}$ year$^{-1}$ at the layer of 75
au. For the kinematics of the 4A2 inner envelope, the absorbing dips in the
H$_2$CO spectra are skewed toward the redshifted side and likely signatures of
inward motion. These absorbing dips are relatively narrow. This is, like the
case for 4A1, significantly slower than the anticipated inflow speed. We
estimate a mass infall rate of 3.1$-$6.2 $\times$ 10$^{-5}$ M$_\odot$
year$^{-1}$ at the layer of 100 au in 4A2.
