Introduction
The bipolar HII region S106 in Cygnus X at a distance of 1.3 kpc, is an enigmatic, well studied object. S106 IR was always thought to be a single late O to early B star, but we recently (Comeron et al. 2017) showed that it is a close (distance possibly <0.2 AU), massive binary system, most likely consisting of a late-O and a late-B star. The system is responsible for the optical visible bipolar emission nebula (see Fig. right). Associated with this system is an IR-cluster with more than 160 members (Hodapp et al. 1991). A small (30 mas), edge-on disk-like feature around the two stars was discovered by cm-inteferometry (Hoare et al. 1996, Gibb et al. 2007), but its exact evolutionary status is not clear (Adams et al. 2015). The system already shows signatures of main sequence stars, emitting copiously in the UV and driving an ionized wind with a velocity of 100 to 200 km/s into the bipolar cavity. A cartoon of the region is shown in the Figure below.
We observed S106 in the OI atomic fine structure line at 4.74 THz (63.2 micron) and the CO 16-15 rotational line at 1.84 THz (162.8 micron) using the GREAT receiver onboard SOFIA. The total OI emission is shown in the Figure below, overlaid to a Subaru IR image.

Spectra
The OI emission is composed of several velocity components in the range -30 km/s to 25 km/s, indicated in the average spectrum in the Figure right. The high velocity blue- and redshifted emission (v=-30 to -9 km/s and 8 to 25 km/s) can be explained as arising from accelerated photodissociated gas, associated with the 'dark lane' close to the massive binary system S106 IR, shocks caused by the stellar wind and/or a disk-envelope interaction. At velocities -9 to -4 km/s and 0.5 to 8 km/s line wings are observed in most of the lines. We attribute this OI emission to cooling in PDRs created by the ionizing radiation impinging on the cavity walls. The velocity range -4 to 0.5 km/s is dominated by emission from the clumpy molecular cloud and the OI, CII, and high-J CO lines are excited in PDRs on clump surfaces that are illuminated by the central stars.

The dark lane of S106
The dark lane of S106 is a high-column density feature well visible in optical images of the region. It has a mass of 275 Msun, and shows a velocity difference of 1.4 km/s along its length of 1 pc, determined from H13CO+ 1-0 mapping (see Figure right where H13CO+ contours are overlaid on a Subaru image. The blue contour indicates the densest part of the dark lane). Its nature depends on the geometry and can be a massive accretion flow (infall rate of 2.5 10^-4 Msun/yr), or the remains of it, linked to S106 IR, or alternatively, the gas of the lane is being dispersed. Both scenarios are shown in the cartoons below. The two lobes of the HII region are shown in red and bright blue and the dark lane is indicated in green as an accretion flow and in blue as a gas filament that is dipsersed.