Context : A complete study of the molecular and ionized gas in the environs of the nebula RCW 78 around WR 55 is presented .

Aims : The aim of this work is to investigate the spatial distribution , physical characteristics , and kinematical properties of the molecular gas linked to the galactic nebula RCW 78 to achieve a better understanding of its interaction with the star and with the ionized gas .

Methods : This study was based on ^ { 12 } CO ( 1-0 ) fully sampled observations of a region of \sim 0 \aas@@fstack { \circ } 45 in size around the star WR 55 and the nebula RCW 78 obtained with the 4-m NANTEN telescope , radio continuum archival data at 1.4 and 4.85 GHz , obtained from SGPS and PMNRAO Southern Radio Survey , respectively , and available infrared MIPSGAL images at 24 \mu m .

Results : A molecular gas component in the velocity range from \sim –58 to –45 km s ^ { -1 } , compatible with the velocity of the ionized gas , was found to be associated with the optical nebula .
Adopting a distance of \sim 5 kpc , the mass of this molecular component is about 3.4 \times 10 ^ { 4 } M _ { \odot } .
The analysis of the molecular data revealed the presence of a velocity gradient , in agreement with the H \alpha line .

New radiocontinuum flux density determinations confirm the thermal nature of RCW 78 .
This indicates that the ionized gas in RCW 78 arises from photoionization of the molecular gas component in the velocity range from - 58 km s ^ { -1 } to - 45 km s ^ { -1 } .

A molecular concentration at a velocity of - 56.1 km s ^ { -1 } ( identified as C1 ) is very likely associated with the star HD 117797 and with a collection of candidate YSOs , lying at a distance of 3.9 kpc , while the rest of the molecular gas at velocities between –56 and -46 km s ^ { -1 } constitute an incomplete ring-like structure which expands around WR 55 at a velocity of about \sim 5 km s ^ { -1 } .
Mechanical energy and time requirements indicate that WR 55 is very capable of sustaining the expansion of the nebula .

Conclusions :