IRAM-PdBI OBSERVATIONS OF BINARY PROTOSTARS. I. THE HIERARCHICAL SYSTEM SVS 13 in NGC 1333

We present millimeter (mm) interferometric observations of the young stellar object SVS 13 in NCG 1333 in the N 2 H + (1–0) line and at 1.4 mm and 3 mm dust continuum, using the IRAM Plateau de Bure interferometer. The results are complemented by infrared data from the Spitzer Space Telescope . The mm dust continuum images resolve four sources (A, B, C, and VLA 3) in SVS 13. With the dust continuum images, we derive gas masses of 0.2–1.1 M ☉ for the sources. N 2 H + (1–0) line emission is detected and spatially associated with the dust continuum sources B and VLA 3. The observed mean line width is ∼ 0.48 km s −1 and the estimated virial mass is ∼ 0.7 M ☉ . By simultaneously fitting the seven hyperfine line components of N 2 H + , we derive the velocity field and find a symmetric velocity gradient of ∼ 28 km s −1 pc −1 across sources B and VLA 3, which could be explained by core rotation. The velocity field suggests that sources B and VLA 3 are forming a physically bound protobinary system embedded in a common N 2 H + core. Spitzer images show mid-infrared emission from sources A and C, which is spatially associated with the mm dust continuum emission. No infrared emission is detected from source B, implying that the source is deeply embedded. Based on the morphologies and velocity structure, we propose a hierarchical fragmentation picture for SVS 13 where the three sources (A, B, and C) were formed by initial fragmentation of a filamentary prestellar core, while the protobinary system (sources B and VLA 3) was formed by rotational fragmentation of a single collapsing subcore.