In our recent A&A paper (Dannhauer et al., subm.), we investigated the “Diamond Ring” in Cygnus X, a striking 6 pc wide  ring-like structure seen in [C II] 158 μm and dust emission. Unlike the three-dimensional [C II] shells discovered in recent years, the Diamond Ring reveals itself as the first pure ring, slowly expanding at only ~1.3 km/s. 

The [C II] data stem from velocity-resolved SOFIA/upGREAT maps from the FEEDBACK legacy program. Combined with archival molecular line data, dust continuum, radio observations, and new spectroscopy of the central star, we establish a picture of the nature of this object. The ring is powered by a B0.5e star located near its center which also creates an HII region in the ring cavity. Interestingly, the bright “Diamond” to the southeast of the ring is not part of the same structure, but  an unrelated clump of dense gas and young stars at a completely different velocity along the line of sight. 

We carried out dedicated 3D simulations of stellar feedback in a flat molecular slab, in order to follow the evolution of a ring-like structure. These simulations demonstrate that the Diamond Ring represents the terminal phase of a [C II] bubble evolving inside such a slab. In its early life, the bubble expanded in three dimensions. However, gas moving perpendicular to the plane of the slab quickly dispersed into the surrounding lower-density medium dropping below our detection limit, while the parts confined to the slab persisted as a coherent, slowly expanding ring.

From the observations and simulations, we derive an age of only ~400–500 kyr, much younger than classical estimates assuming spherical expansion, with cloud dispersal taking over after 100 kyr as also suggested by previous studies. The Diamond Ring highlights how the geometry of molecular clouds, especially flat clouds which might be common,  fundamentally shapes stellar feedback.