Toward understanding the formation of multiple systems - A pilot
  IRAM-PdBI survey of Class 0 objects
  The formation process of binary stars and multiple systems is poorly
understood. Here, we seek to determine the typical outcome of protostellar
collapse and to constrain models of binary formation by core fragmentation
during collapse, using high-resolution millimeter continuum imaging of very
young (Class 0) protostars observed at the beginning of the main accretion
phase. We carried out a pilot high-resolution study of 5 Class 0 objects, using
the most extended (A) configuration of the IRAM PdBI at 1.3 mm, which allow us
to probe the multiplicity of Class 0 protostars down to separations a ~50 AU
and circumstellar mass ratios q ~0.07. We show that our PdBI observations
revealed only wide (>1500 AU) protobinary systems and/or outflow-generated
features. When combined with previous millimeter interferometric observations
of Class 0 protostars, our pilot PdBI study tentatively suggests that the
binary fraction in the ~ 75-1000 AU range increases from the Class 0 to the
Class I stage. It also seems to argue against purely hydrodynamic models of
binary star formation. We briefly discuss possible alternative scenarios to
reconcile the low multiplicity rate of Class 0 protostars on small scales with
the higher binary fraction observed at later evolutionary stages.
