Cloudlet Capture Model for Asymmetric Molecular Emission Lines Observed
  in TMC-1A with ALMA
  TMC-1A is a protostellar source harboring a young protostar, IRAS 04365+2353,
and shows a highly asymmetric features of a few 100 au scale in the molecular
emission lines. Blue-shifted emission is much stronger in the CS ($J=5$-4) line
than red-shifted one. The asymmetry can be explained if the gas accretion is
episodic and takes the form of cloudlet capture, given the cloudlet approached
toward us. The gravity of the protostar transforms the cloudlet into a stream
and changes its velocity along the flow. The emission from the cloudlet should
be blue-shifted before the periastron, while it should be red-shifted after the
periastron. If a major part of cloudlet has not reached the periastron, the
former should be dominant. We perform hydrodynamical simulations to examine the
validity of the scenario. Our numerical simulations can reproduce the observed
asymmetry if the orbit of the cloudlet is inclined to the disk plane. The
inclination can explain the slow infall velocity observed in the C$^{18}$O
($J$=2-1) line emission. Such episodic accretion may occur in various
protostellar cores since actual clouds could have inhomogeneous density
distribution. We also discuss the implication of the cloudlet capture on
observations of related objects.
