Formation of the Unequal-Mass Binary Protostars in L1551 NE by Rotationally-Driven Fragmentation

TL;DR

This study uses high-resolution observations to analyze the structure and dynamics of the binary protostellar system L1551 NE, revealing insights into disk formation, alignment, and the effects of rotationally-driven fragmentation.

Contribution

It provides detailed observational evidence supporting rotationally-driven fragmentation as the formation mechanism of the binary system, with measurements of disk structures and orbital motion.

Findings

Disks are better fit by a shallow inner and steep outer power-law.

Disks are aligned with each other and the circumbinary disk.

Source B shows orbital motion consistent with the system's angular momentum.

Abstract

We present observations at 7 mm that fully resolve the two circumstellar disks, and a reanalyses of archival observations at 3.5 cm that resolve along their major axes the two ionized jets, of the class I binary protostellar system L1551 NE. We show that the two circumstellar disks are better fit by a shallow inner and steep outer power-law than a truncated power-law. The two disks have very different transition radii between their inner and outer regions of $\sim$18.6 AU and $\sim$8.9 AU respectively. Assuming that they are intrinsically circular and geometrically thin, we find that the two circumstellar disks are parallel with each other and orthogonal in projection to their respective ionized jets. Furthermore, the two disks are closely aligned if not parallel with their circumbinary disk. Over an interval of $\sim$10 yr, source B (possessing the circumsecondary disk) has moved northwards with respect to and likely away from source A, indicating an orbital motion in the same direction as the rotational motion of their circumbinary disk. All the aforementioned elements therefore share the same axis for their angular momentum, indicating that L1551 NE is a product of rotationally-driven fragmentation of its parental core. Assuming a circular orbit, the relative disk sizes are compatible with theoretical predictions for tidal truncation by a binary system having a mass ratio of $\sim$0.2, in agreement with the reported relative separations of the two protostars from the center of their circumbinary disk. The transition radii of both disks, however, are a factor of $\gtrsim$1.5 smaller than their predicted tidally-truncated radii.