A cold accretion flow onto one component of a multiple protostellar system

Abstract

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 more evenly distributed among the components. Aims. The distribution of gas accretion flows within the cloud core of the deeply embedded, chemically rich, low-mass multiple protostellar system IRAS 16293−2422 is explored out to 6000 AU. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations of low-J transitions of various molecules such as HNC, cyanopolyynes (HC 3 N, HC 5 N), and N 2 H + are used to probe the cloud core structure of IRAS 16293−2422 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 IRAS 16293−2422 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 driven by the source. The two outflows driven by IRAS 16293−2422 A present different molecular gas distributions. Conclusions. Accretion of cold gas is seen from 6000 AU scales onto IRAS 16293−2422 A, but not onto source B, indicating 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.