Toward understanding the formation of multiple systems - A pilot IRAM-PdBI survey of Class?0 objects

摘要

Context. The formation process of binary stars andmultiple systems is poorly understood. The multiplicity rate ofClassII pre-main-sequence stars and ClassIprotostars is well documented and known to be high (30% to 50% between 100and 4000AU). However, optical / near-infrared observations ofClassI/ClassII YSOs barely constrain the pristineproperties of multiple systems, since dynamical evolution can quicklyalter these properties during the protostellar phase. Aims. Here, we seek to determine the typical outcomeof protostellar collapse and to constrain models of binary formation bycore fragmentation during collapse, using high-resolution millimetercontinuum imaging of very young (Class0) protostars observedat the beginning of the main accretion phase. Methods. We carried out a pilot high-resolutionstudy of 5 Class0 objects, including 3 Taurus sources and 2Perseus sources, using the most extended (A) configuration of the IRAMPlateau de Bure Interferometer (PdBI) at 1.3mm. Our PdBIobservations have a typical HPBW resolution 0.3-0.5and rms continuumsensitivity 0.1-1mJy/beam,which allow us to probe the multiplicity of Class0 protostarsdown to separations AUand circumstellar mass ratios .Results. We detected all 5 primary Class0sources in the 1.3mm dust continuum. A single componentassociated with the primary Class0 object was detected in thecase of the three Taurus sources, while robust evidence of secondarycomponents was found toward the two Perseus sources: L1448-C andNGC1333-IR2A. We show that the secondary 1.3mm continuumcomponent detected 600AUsouth-east of L1448-C, at a position angle close to that of the CO(2-1)jet axis traced by our data, is an outflow feature directly associatedwith the powerful jet driven by L1448-C. The secondary 1.3mmcontinuum component detected 1900AU south-east ofNGC1333-IR2A may either be a genuine protostellar companion or tracethe edge of an outflow cavity. Therefore, our PdBI observationsrevealed only wide (>1500AU) protobinary systemsand/or outflow-generated features. Conclusions. When combined with previous millimeterinterferometric observations of Class0 protostars, our pilotPdBI study tentatively suggests that the binary fraction in the 75-1000AUrange increases from the Class0 to the ClassIstage. It also seems to argue against purely hydrodynamic models ofbinary star formation. We briefly discuss possible alternativescenarios to reconcile the low multiplicity rate of Class0protostars on small scales with the higher binary fraction observed atlater (e.g. ClassI) evolutionary stages. Key words: stars: formation - binaries:general - ISM: clouds - ISM: jets and outflows