  We present millimeter interferometric observations of the young stellar
object SVS13 in NCG1333 in the N2H+(1-0) line and at 1.4 and 3mm dust
continuum, using the IRAM Plateau de Bure interferometer. The results are
complemented by infrared data from the Spitzer Space Telescope. The millimeter
dust continuum images resolve four sources (A, B, C, and VLA3) in SVS13. With
the dust continuum images, we derive gas masses of 0.2-1.1 M_sun for the
sources. N2H+(1-0) line emission is detected and spatially associated with the
dust continuum sources B and VLA3. The observed mean line width is ~0.48 km/s
and the estimated virial mass is ~0.7 M_sun. By simultaneously fitting the
seven hyperfine line components of N2H+, we derive the velocity field and find
a symmetric velocity gradient of about 28 km/s/pc across sources B and VLA3,
which could be explained by core rotation. The velocity field suggests that
sources B and VLA3 are forming a physically bound protobinary system embedded
in a common N2H+ 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 SVS13 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 VLA3) was formed by rotational
fragmentation of a single collapsing sub-core.
