Rotationally Driven Fragmentation in the Formation of the Binary Protostellar System L1551 Irs 5

Abstract

Both bulk rotation and local turbulence have been widely suggested to drive the fragmentation in collapsing cores that produces multiple star systems. Even when the two mechanisms predict different alignments for stellar spins and orbits, subsequent internal or external interactions can drive multiple systems toward or away from alignment, thus masking their formation processes. Here, we demonstrate that the geometrical and dynamical relationship between a binary system and its surrounding bulk envelope provide the crucial distinction between fragmentation models. We find that the circumstellar disks of the binary protostellar system L1551 IRS 5 are closely parallel, not just with each other but also with their surrounding flattened envelope. Measurements of the relative proper motion of the binary components spanning nearly 30 years indicate an orbital motion related to that of the envelope rotation. Eliminating orbital solutions whereby the circumstellar disks would be tidally truncated to sizes smaller than observed, the remaining solutions favor a circular or low-eccentricity orbit tilted by up to ∼25°from the circumstellar disks. Turbulence-driven fragmentation can generate local angular momentum to produce a coplanar binary system, but this would have no particular relationship to the system's surrounding envelope. Instead, the observed properties conform with predictions for rotationally driven fragmentation. If the fragments were produced at different heights or on opposite sides of the mid-plane in the flattened central region of a rotating core, the resulting protostars would then exhibit circumstellar disks parallel with the surrounding envelope but tilted from the orbital plane, as is observed.

FAUST I. The hot corino at the heart of the prototypical Class I protostar L1551 IRS5

“…1. IRS5 S: Similarly to N, the lines are double-peaked towards S, with a central dip at +4.5 km s −1 , namely ∼ 3 km s −1 red-shifted with respect to N. Going south (positions P7 and P8) the red peak disappears and only the blue one remains, suggesting again emission from a rotating inner envelope or a disk, assuming that the red peak is mainly associated with S. 1983, 1998(Rodríguez et al 2003aLim et al 2016), while the cyan stars refer to 2018 (this work). The magenta arrows indicate the jet directions (Rodríguez et al 2003b).…”

“…ALMA observations also suggest the presence of two dusty disks (M disk >0.006M ⊙ ) towards N and S, with radii between 8 and 14 au. The protostellar disks inclination is expected to be ∼ 35-45 • for N and ∼ 24-44 • for S (Cruz-Sáenz de Miera et al 2019; Lim et al 2016). Proper motion measurements show an orbital rotation of N and S with a period of ∼260 yr and an eccentricity orbit tilted by up to 25 • from the circumbinary disk (Rodríguez et al 2003b;Lim et al 2016).…”

A Near-infrared Spectroscopic Survey of FU Orionis Objects

“…The source is a binary with projected separation of 0. ′′ 36 (Bieging & Cohen 1985, Lim et al 2016, with each component driving a jet (Fridlund & Liseau 1998, Rodríguez et al 2003. The near-infrared spectrum in Reipurth & Aspin (1997) shows H 2 emission and CO band absorption.…”

Disks around FUor-type young eruptive stars with ALMA

“…The circumstellar disks have radii of 9.4 au and 6.9 au for N and S, respectively. 370 F. Cruz-Sáenz de Miera et al Lim et al (2016). N shows extension towards the north which appears to connect it to the CBR.…”