The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

Vol. 677
6. Interstellar and circumstellar matter

The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

Massive stars erode and photo-evaporate their surrounding cloud with their strong UV radiation field, which can either trigger or quench the formation of new stars. Understanding this star-cloud interaction requires studying in detail the structure of a cloud exposed to a UV radiation field, which is expected to contain an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized component. To study this transition zone, Claudio Hernández-Vera and collaborators observed, with a resolution of ~200 au, the iconic Horsehead Nebula, which is viewed nearly edge-on with respect to the illuminating source σ Ori. The ALMA observations presented by these authors reveal a very sharp transition between the molecular and ionized components, which suggests that the atomic layer in the nebula is very thin (< 650 au). The new ALMA observations also reveal a web of molecular gas filaments and the presence of a steep density gradient at the cloud edge that can be explained by a stationary isobaric photo-dissociation model, although dynamical effects cannot be completely ruled out. Even higher angular observations will be needed to unveil their role.