Toward understanding the formation of multiple systems

Abstract

Context. The formation process of binary stars and multiple systems is poorly understood. The multiplicity rate of Class II premain-sequence stars and Class I protostars is well documented and known to be high (∼ 30% to 50% between ∼100 and 4000 AU). However, optical / near-infrared observations of Class I/Class II YSOs barely constrain the pristine properties of multiple systems, since dynamical evolution can quickly alter these properties during the protostellar phase. Aims. 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. Methods. We carried out a pilot high-resolution study of 5 Class 0 objects, including 3 Taurus sources and 2 Perseus sources, using the most extended (A) configuration of the IRAM Plateau de Bure Interferometer (PdBI) at 1.3 mm. Our PdBI observations have a typical HPBW resolution ∼0.3 −0.5 and rms continuum sensitivity ∼0.1−1 mJy/beam, which allow us to probe the multiplicity of Class 0 protostars down to separations a ∼ 50 AU and circumstellar mass ratios q ∼ 0.07. Results. We detected all 5 primary Class 0 sources in the 1.3 mm dust continuum. A single component associated with the primary Class 0 object was detected in the case of the three Taurus sources, while robust evidence of secondary components was found toward the two Perseus sources: L1448-C and NGC1333-IR2A. We show that the secondary 1.3 mm continuum component detected ∼600 AU south-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 associated with the powerful jet driven by L1448-C. The secondary 1.3 mm continuum component detected ∼1900 AU southeast of NGC1333-IR2A may either be a genuine protostellar companion or trace the edge of an outflow cavity. Therefore, our PdBI observations revealed only wide (>1500 AU) protobinary systems and/or outflow-generated features. Conclusions. 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 (e.g. Class I) evolutionary stages.

Finding proto-spectroscopic binaries

“…On the other hand, Maury et al (2010) failed to detect companions in a small sample of five protostars at separations 50 < a < 5000 AU (a being the semimajor axis) in a millimeter A&A 539, A62 (2012) study of self-embedded, young (Class 0) objects (∼10 4 −10 5 yrs). Taken together with the observations of Looney et al (2000), the authors argue that multiplicity in the separation ranges from 100 to 600 AU would only be defined in a later stage of star formation (namely after the Class 0 phase) and that early multiplicity in pristine systems may not be as ubiquitous and primeval.…”

Protostellar disk formation and transport of angular momentum during magnetized core collapse

“…While observations of circumstellar disks around late young stellar objects (YSO), from Class I to T Tauri stars, are well-established (Watson et al 2007), it is still unclear when circumstellar disks form during the early collapse of prestellar dense cores and the early Class 0 phase, and what their initial properties are (mass, radius, magnetic flux, and temperature). For these embedded sources, direct observations are indeed more difficult than for YSOs, since disk emission is difficult to distinguish from the envelope signature (Belloche et al 2002), even with a relatively high spatial resolution (50 AU, Maury et al 2010). However, other studies observing at lower resolution (about 250 AU) and without resolving the disks, infer from detailed emission modeling the presence of disks as massive as one solar mass, corresponding to about 12% of the envelope mass.…”

First results from the CALYPSO IRAM-PdBI survey

“…So far, a quite limited number of Class 0 jets has been observed at sub-arcsecond angular resolution (needed to disentangle the jet and the outflow cavities): HH211 (Lee et al 2007(Lee et al , 2009(Lee et al , 2010, HH212 Lee et al 2008), IRAS04166+2706 (Tafalla et al 2010), and L1448-C (Maury et al 2010;Hirano et al 2010). The IRAM Plateau de Bure Interferometer (PdBI) large program CALYPSO 1 (Continuum and Lines from Young ProtoStellar Objects) is correcting this situation by providing the first sub-arcsecond statistical study of inner jet properties in nearby low-luminosity Class 0 sources in combination with studies of the envelopes, disks, and multiplicity structure.…”