The ALMA-PILS survey: Gas dynamics in IRAS 16293$-$2422 and the
  connection between its two protostars
  [Abridged] The majority of stars form in binary or higher order systems. The
Class 0 protostellar system IRAS16293-2422 contains two protostars, 'A' and
'B', separated by ~600 au and embedded in a single, 10^4 au scale envelope.
Their relative evolutionary stages have been debated. We aim to study the
relation and interplay between the two protostars A and B at spatial scales of
60 to ~1000 au. We selected molecular gas line transitions of CO, H2CO, HCN,
CS, SiO, and CCH from the ALMA-PILS spectral imaging survey (329-363 GHz) and
used them as tracers of kinematics, density, and temperature in the
IRAS16293-2422 system. The angular resolution of the PILS data set allows us to
study these quantities at a resolution of 0.5 arcsec (60 au [..]).
Line-of-sight velocity maps of both optically thick and optically thin
molecular lines reveal: (i) new manifestations of previously known outflows
emanating from protostar A; (ii) a kinematically quiescent bridge of dust and
gas spanning between the two protostars, with an inferred density between 4
10^4 and 3 10^7 cm^-3; and (iii) a separate, straight filament seemingly
connected to protostar B seen only in CCH, with a flat kinematic signature.
Signs of various outflows, all emanating from source A, are evidence of
high-density and warmer gas; none of them coincide spatially and kinematically
with the bridge. We hypothesize that the bridge arc is a remnant of filamentary
substructure in the protostellar envelope material from which protostellar
sources A and B have formed. One particular morphological structure appears to
be due to outflowing gas impacting the quiescent bridge material. The
continuing lack of clear outflow signatures unambiguously associated to
protostar B and the vertically extended shape derived for its disk-like
structure lead us to conclude that source B may be in an earlier evolutionary
stage than source A.
