Arcsecond-resolution submillimeter HCN imaging of the binary protostar IRAS 16293-2422

With the Submillimeter Array (SMA) we have made high angular resolution (∼1″ = 160 AU ) observations of the protobinary system IRAS 16293-2422 in the HCN (4-3), HC¹⁵N (4-3), and 354.5 GHz continuum emission. The HCN (4-3) line was also observed using the JCMT to supply missing short-spacing information. The submillimeter continuum emission is detected from the individual binary components of source A in the southeast and source B in the northwest, with a separation of ∼5″. The optically thin HC ¹⁵N (4-3) emission taken with the SMA has revealed a compact (∼500 AU) flattened structure (P.A. = -16°) at source A. This compact structure shows a velocity gradient along the projected minor axis, which can be interpreted as an infalling gas motion. Our HCN image including the short-spacing information shows an extended (∼3000 AU ) circumbinary envelope, as well as the compact structure at source A. A toy model consisting of a flattened structure with radial infall toward a 1 M_∞ central star reproduces the HCN/HC¹⁵N position-velocity diagram along the minor axis of the HC¹⁵N emission. In the extended envelope there is also a northeast (blue) to southwest (red) velocity gradient across the binary alignment, which is likely to reflect gas motion in the swept-up dense gas associated with the molecular outflow from source A. Only a weak and narrow (∼2 km s^-1) compact HC¹⁵N emission is associated with source B, where no clear molecular outflow is identified, suggesting the different evolutionary starges between sources A and B. Our study demonstrates the importance of adding short-spacing data to interferometer data in order to probe the detailed structure and kinematics of low-mass protostellar envelopes.