Anatomy of the Massive Star-Forming Region S106: The [OI 63] Micron Line Observed with GREAT/SOFIA as a Versatile Diagnostic Tool for the Evolution of Massive Stars

Observations of the major cooling lines of the interstellar medium are needed to better understand the physical processes, such as mass accretion and ejection, and feedback (thermal heating, ionization, radiation pressure, stellar winds) involved in the formation and evolution of massive stars.  For that, we observed the well-known bipolar HII region S106 in the CII 158 and [OI] 63 micron finestructure lines, and the CO 16-15, 11-10 transitions with the upGREAT instrument on SOFIA. The [OI] 63 micron emission at 6” resolution is composed of several velocity components. High-velocity blue- and redshifted emission from accelerated photodissociated gas is associated with an accretion flow along the dark lane close to the massive binary system S106 IR.  We employed the observed line intensities and ratios using the KOSMA-tau photodissociation region (PDR) model and consistently found a high FUV field (10^4-5 G_o) and high densities (10^4-5 cm-3).  The model shows that the [OI] 63 micron line is strongly foreground-absorbed but KOSMA-tau can be used to constrain the OI foreground. We conclude that the binary system S106 IR is probably in an evolutionary stage where gas accretion is counteracted by the stellar winds and radiation, leading to the very complex spatial and kinematic emission of the various tracers. The jets and outflows are not well organized so that the emission distributions of the observed lines are more consistent with a competitive accretion or fragmentation-induced starvation star formation model than with the turbulent core model.