The ALMA-PILS survey: gas dynamics in IRAS 16293−2422 and the connection between its two protostars.

Context. The majority of stars form in binary or higher order systems. The evolution of each protostar in a multiple system may start at different times and may progress differently. The Class 0 protostellar system IRAS 16293–2422 contains two protostars, "A" and "B", separated by ~600 au and embedded in a single, 10⁴ au scale envelope. Their relative evolutionary stages have been debated. Aims. We aim to study the relation and interplay between the two protostars A and B at spatial scales of 60 au up to ~10³ au. Methods. We selected molecular gas line transitions of the species CO, H₂CO, HCN, CS, SiO, and C₂H from the ALMA-PILS spectral imaging survey (329–363 GHz) and used them as tracers of kinematics, density, and temperature in the IRAS 16293–2422 system. The angular resolution of the PILS data set allows us to study these quantities at a resolution of 0.5′′ (60 au at the distance of the source). Results. 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 × 10⁴ cm⁻³ and ~3 × 10⁷ cm⁻³; and (iii) a separate, straight filament seemingly connected to protostar B seen only in C₂H, 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. Conclusions. 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. [ABSTRACT FROM AUTHOR]