Anatomy of the massive star-forming region S106

Abstract

The central area (40″ × 40″) of the bipolar nebula S106 was mapped in the [O I] line at 63.2 μm (4.74 THz) with high angular (6″) and spectral (0.24 MHz) resolution, using the GREAT heterodyne receiver on board SOFIA. The spatial and spectral emission distribution of [O I] is compared to emission in the CO 16 →15, [C II] 158 μm, and CO 11 →10 lines, mm-molecular lines, and continuum. The [O I] emission is composed of several velocity components in the range from –30 to 25 km s−1. The high-velocity blue- and red-shifted emission (v = −30 to –9 km s−1 and 8 to 25 km s−1) can be explained as arising from accelerated photodissociated 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−1 and from 0.5 to 8 km s−1 line wings are observed in most of the lines that we attribute to cooling in photodissociation regions (PDRs) created by the ionizing radiation impinging on the cavity walls. The velocity range from –4 to 0.5 km s−1 is dominated by emission from the clumpy molecular cloud, and the [O I], [C II], and high-J CO lines are excited in PDRs on clump surfaces that are illuminated by the central stars. Modelling the line emission in the different velocity ranges with the KOSMA-τ code constrains a radiation field χ of a few times 104 and densities n of a few times 104 cm−3. Considering self-absorption of the [O I] line results in higher densities (up to 106 cm−3) only for the gas component seen at high blue- and red velocities. We thus confirm the scenario found in other studies that the emission of these lines can be explained by a two-phase PDR, but attribute the high-density gas to the high-velocity component only. The dark lane has a mass of ~275 M⊙ and shows a velocity difference of ~1.4 km s−1 along its projected length of ~1 pc, determined from H13CO+ 1 →0 mapping. Its nature depends on the geometry and can be interpreted as a massive accretion flow (infall rate of ~2.5 × 10−4 M⊙ yr−1), 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.

Globules and pillars in Cygnus X

“…Globule a notorious problem that we attribute to an absorbing foreground layer (between the clumpy PDR and the observer), resulting in a significant optical depth along the line of sight. This is in agreement with recent SOFIA observations of star-forming regions were the [O I] 63 µm line was found to be heavily affected by foreground absorption while the upper [O I] 145 µm line is mostly unaffected (Schneider et al 2018;Guevara et al 2020). Figure 11 also shows that the [O I] 63 µm line arising from the PDR of the globule head suffers from significant self-absorption.…”

“…Our observations are, thus, fit best with an Herbig Be Type III star since we observe that the star is located in a cavity and not associated with a dense clump, that there is no CO outflow, but high-velocity [C II] emission, tracing the PDR surfaces of the inner cavity walls, namely, the interface between the H II region and the molecular gas. This sort of [C II] dynamics was also observed -and interpreted in a similar way -for the bipolar nebula S106 (Schneider et al 2018). We note that we exclude shock excitation as a significant origin for the outflow because firstly, [C II] is not a good shock tracer and its origin is mostly PDRs, and secondly, the [O I] 63 µm line does not show prominent high-velocity wings, which would be the case if there were shocks.…”

The role of Galactic H II regions in the formation of filaments

“…However, they point out that the lack of detected expansion is roughly in agreement with models for the timeevolution of an H II region like RCW 120, and this is consistent with an expansion speed of 1.5 km s −1 . Forbidden lines, such as [OI] and [CII], are well suited to reveal the complex dynamics of H II regions (Schneider et al 2018) and will help to shed light on the internal dynamics of RCW 120 (Luisi et al, in prep. ).…”

Gas and dust cooling along the major axis of M 33 (HerM33es)

“…The [O i](63µm) line is expected to have a higher opacity than the [C ii] line and hence may be affected by self-absorption caused by cold foreground clouds of atomic oxygen, as has been seen in velocity resolved spectra and with narrow beams toward bright background sources in the Milky Way (Schneider et al 2018;Gerin et al 2015;Leurini et al 2015;Karska et al 2014;Lis et al 2001;Timmermann et al 1996). Spectra 5).…”