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
  The central area (40"x40") of the bipolar nebula S106 was mapped in the OI
line at 63.2 micron with high angular (6") and spectral resolution, using GREAT
on board SOFIA. The OI emission distribution is compared to the CO 16-15, CII
158 micron, and CO 11-10 lines, mm-molecular lines, and continuum. It is
composed of several velocity components in the range from -30 km/s to 25 km/s.
The high-velocity blue- and redshifted emission can be explained as arising
from accelerated photodissociated (PDR) gas associated with a dark lane close
to the massive binary system S106 IR, and from shocks caused by the stellar
wind and/or a disk--envelope interaction. At velocities from -9 to -4 km/s and
0.5 to 8 km/s line wings are observed that we attribute to cooling in PDRs
created by the ionizing radiation impinging on the cavity walls. The bulk
velocity range is dominated by PDR emission from the clumpy molecular cloud.
Modelling the emission in the different velocity ranges with the KOSMA-tau code
constrains a radiation field chi of a few times 10^4 and densities n of a few
times 10^4 cm^-3. Considering self-absorption of the OI line results in higher
densities (up to 10^6 cm^-3) only for the gas component seen at high blue- and
red velocities. The dark lane has a mass of 275 Msun and shows a velocity
difference of 1.4 km/s along its projected length of 1 pc, determined from
H13CO+ 1-0 mapping. It can be interpreted as a massive accretion flow, or the
remains of it, linked to S106 IR/FIR. The most likely explanation is that the
binary system is at a stage of its evolution where gas accretion is
counteracted by the stellar winds and radiation, leading to the very complex
observed spatial and kinematic emission distribution of the various tracers.
