A prominent feature in dust-emission south-west of the famous DR21 ridge, the ‘Diamond Ring’ is the first detection of a true ring-like structure in the main cooling line of the PDR, in contrast to recently detected bubbles. The [CII] observations of the Cygnus X region were done as part of the SOFIA legacy program FEEDBACK (PI: N. Schneider, A.G.G.M. Tielens). By fitting an ellipse model to the observed velocity gradient across the ring we can infer a low expansion speed, suggesting a late evolutionary stage. With our own NIR spectroscopy using the Nordic Optical Telescope in La Palma and GLOSTAR cm-observations we find that the ionising source that is driving the expansion is a B0.5e star.
With dedicated hydrodynamic simulations that follow the evolution of ionising feedback of a star of the same spectral type within a molecular slab we can reproduce the morphology, [CII] intensity, dynamics and HII region pressure for a molecular slab with 2pc diameter and a hydrogen density of nH=350cm-3. The high velocity but low density outflow perpendicular to the slab, remains below the detection limit in our synthetic SOFIA observations, giving rise to the ring-like appearance.
Furthermore, models that assume constant expansion speeds or use HII region pressure to infer the age of associated structures would overestimate timescales by a factor of 5–6. This work adds to the growing body of evidence for flat molecular clouds, which are most likely created by large-scale colliding flows.
Read here the accompanying press release of the UoC.
Kreck Observatory 