As part of the large-scale mapping program (PI A. Tielens) the Orion A molecular cloud was observed in [CII] emission using the upGREAT instrument (PI J. Stutzki) on board SOFIA. These observations produced a one-square-degree map, the largest map of ionized carbon emission to date.
In the frame of the Orion-Legacy program (PI S. Kabanovic), multiple regions along Orion A—encompassing NGC 1977, M43, M42, and parts of the Orion Veil—were observed in [13CII]. Combined with the broader Orion A map, this dataset represents an unparalleled contribution to the SOFIA heritage. The deep [13CII] integration yielded unprecedented spectral resolution of the three hyperfine transition lines, challenging laboratory observations, enabling us to refine the hyperfine frequency shifts of [13CII] lines.
The high spectral resolution enables us to determine the ionized carbon ratio along Orion A, leveraging the wings of the [12CII] line, which are unaffected by self-absorption effects. Deep [CII] observations of isolated globules, swept up by the Orion Veil, indicate that their cold and isolated environments lead to strong fractionation effects, where [13CII] is bound into molecular carbon (13CO) altering the observable carbon ratio.
A direct comparison between [12CII] and [13CII] indicates significant optical depth effects across multiple regions of Orion A. By solving radiative transfer equations for multiple components distributed between two layers, we disentangle the contribution of warm emitting gas from cold absorbing material. Our model results indicate that a substantial fraction of ionized carbon resides in the cold foreground layer. To explore whether this extensive cold material arises from a diffuse atomic layer or a dense clumpy medium, we analyze the spatial column density variations of ionized carbon along Orion A, providing new insights into the puzzling nature of this cold gas.