The observational anatomy of externally photoevaporating planet-forming discs – I. Atomic carbon.

We demonstrate the utility of C  i as a tracer of photoevaporative winds that are being driven from discs by their ambient UV environment. Commonly observed CO lines only trace these winds in relatively weak UV environments and are otherwise dissociated in the wind at the intermediate to high UV fields that most young stars experience. However, C  i traces unsubtle kinematic signatures of a wind in intermediate UV environments (∼1000 G0) and can be used to place constraints on the kinematics and temperature of the wind. In C  i position–velocity (PV) diagrams external photoevaporation results in velocities that are faster than those from Keplerian rotation alone, as well as emission from quadrants of PV space in which there would be no Keplerian emission. This is independent of viewing angle because the wind has components that are perpendicular to the azimuthal rotation of the disc. At intermediate viewing angles (∼30–60°) moment 1 maps also exhibit a twisted morphology over large scales (unlike other processes that result in twists, which are typically towards the inner disc). C  i is readily observable with ALMA, which means that it is now possible to identify and characterize the effect of external photoevaporation on planet-forming discs in intermediate UV environments. [ABSTRACT FROM AUTHOR]