Abstract
We analyze the atomic neutral and molecular gas associated with the ring nebula RCW 78 around the Wolf-Rayet star HD 117688 (WN7). The CO(1-0) data were obtained with the SEST telescope of the ESO, located at La Silla, Chile, with an angular resolution of 45 arcsec. The neutral hydrogen data belong to the Southern Galactic Plane Survey with a synthesized beam of 2.1 arcmin.
The CO emission distribution reveals the presence of molecular gas with velocities in the range -66 to -32 km s^{-1} associated with the brightest part of the nebula.
The study of the HI gas emission distribution allowed the identification of a void and shell linked to the nebula, which was interpreted as the neutral gas counterpart of the optical ring nebula.
The analysis suggests that RCW 78 has probably developped at the surface of the molecular cloud, where the UV stellar photons have photodissociated and ionized the dense gas.