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