A cold accretion flow onto one component of a multiple protostellar
  system
  Context: Gas accretion flows transport material from the cloud core onto the
protostar. In multiple protostellar systems, it is not clear if the delivery
mechanism is preferential or evenly distributed among the components.
  Aims: Gas accretion flows within IRAS16293 is explored out to 6000 AU.
  Methods: ALMA Band 3 observations of low-$J$ transitions of HNC,
cyanopolyynes (HC$_3$N, HC$_5$N), and N$_2$H$^+$ are used to probe the cloud
core structure at ~100 AU resolution. Additional Band 3 archival data provide
low-$J$ HCN and SiO lines. These data are compared with the corresponding
higher-$J$ lines from the PILS Band 7 data for excitation analysis. The HNC/HCN
ratio is used as a temperature tracer.
  Results: The low-$J$ transitions of HC$_3$N, HC$_5$N, HNC and N$_2$H$^+$
trace extended and elongated structures from 6000 AU down to ~100 AU, without
accompanying dust continuum emission. Two structures are identified: one traces
a flow that is likely accreting toward the most luminous component of the
system IRAS16293 A. Temperatures inferred from the HCN/HNC ratio suggest that
the gas in this flow is cold, between 10 and 30 K. The other structure is part
of an UV-irradiated cavity wall entrained by one of the outflows. The two
outflows driven by IRAS16293 A present different molecular gas distributions.
  Conclusions: Accretion of cold gas is seen from 6000 AU scales onto IRAS16293
A, but not onto source B, indicates that cloud core material accretion is
competitive due to feedback onto a dominant component in an embedded multiple
protostellar system. The preferential delivery of material could explain the
higher luminosity and multiplicity of source A compared to source B. The
results of this work demonstrate that several different molecular species, and
multiple transitions of each species, are needed to confirm and characterize
accretion flows in protostellar cloud cores.
