[O i] 6300 Å emission as a probe of external photoevaporation of protoplanetary discs

We study the utility of the [O i] 6300 angstrom forbidden line for identifying and interpreting externally driven photoevaporative winds in different environments and at a range of distances. Thermally excited [O i] 6300 angstrom is a well-known tracer of inner disc winds, so any external contribution needs to be distinguishable. In external winds, the line is not thermally excited and instead results from the dissociation of OH, and we study how the line luminosity resulting from that process scales with the disc/environmental parameters. We find that the line luminosity increases dramatically with FUV radiation field strength above around 5000 G(0). The predicted luminosities from our models are consistent with measurements of the line luminosity of proplyds in the Orion Nebula Cluster. The high luminosity in strong UV environments alone may act as a diagnostic, but a rise in the [O i]-to-accretion luminosity ratio is predicted to better separate the two contributions. This could provide a means of identifying external photoevaporation in distant clusters where the proplyd morphology of evaporating discs cannot be spatially resolved.