Supersonic Expansion of the Bipolar Hii Region Sh2-106: A 3,500 Year-Old Explosion?

TL;DR

This study uses multi-epoch HST imaging to reveal highly supersonic motions in the bipolar Hii region Sh2-106, proposing a 3,500-year-old explosion possibly caused by a major accretion event or stellar encounter.

Contribution

It presents evidence of a historic explosion in Sh2-106 and links nebular motions to a potential stellar event, offering new insights into the region's dynamic history.

Findings

Nebular proper motions reach over 150 km/s.

Evidence suggests a 3,500-year-old explosion.

Asymmetric structures indicate complex interactions.

Abstract

Multi-epoch narrow-band HST images of the bipolar Hii region Sh2-106 reveal highly supersonic nebular proper motions which increase with projected distance from the massive young stellar object S106~IR, reaching over ~30 mas/year (~150 km/s at D=1.09 kpc) at a projected separation of ~1.4' (0.44 pc) from S106~IR. We propose that S106~IR experienced a $\sim10^{47}$ erg explosion ~3,500 years ago. The explosion may be the result of a major accretion burst, a recent encounter with another star, or a consequence of the interaction of a companion with the bloated photosphere of S106~IR as it grew from ~10 through ~15 Solar masses at a high accretion rate. Near-IR images reveal fingers of molecular hydrogen emission pointing away from S106~IR and an asymmetric photon-dominated region surrounding the ionized nebula. Radio continuum and Brackett-gamma emission reveal a C-shaped bend in the plasma, either indicating motion of S106~IR toward the east, or deflection of plasma toward the west by the surrounding cloud. The Hii region bends around a ~1' diameter dark bay west of S106~IR that may be shielded from direct illumination by a dense molecular clump. Herbig-Haro (HH) and Molecular Hydrogen Objects (MHOs) tracing outflows powered by stars in the Sh2-106 proto-cluster such as the Class 0 source S106 FIR are discussed.