2008A&A...477..173C

We analyze the distribution of the interstellar matter in the environs of the HII region Gum31, excited by the open cluster NGC3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material. We use neutral hydrogen 21-cm line data, radio continuum images at 0.843, 2.4 and 4.9GHz, ¹²CO(1-0) observations, and IRAS and MSX infrared data. Adopting a distance of 3kpc for the HII region and the ionizing cluster, we derived an electron density of 33±3cm^–3 and an ionized mass of (3.3±1.1)x10³M_☉ based on the radio continuum data at 4.9GHz. The HI 21-cm line images revealed an Hi shell surrounding the HII region. The HI structure is 10.0±1.7pc in radius, has a neutral mass of 1500±500 M_☉, and is expanding at 11km/s. The associated molecular gas amounts to (1.1±0.5)x10⁵M_☉, being its volume density of about 350cm^–3. This molecular shell could represent the remains of the cloud where the young open cluster NGC3324 was born or could have originated by the shock front associated with the HII region. The difference between the ambient density and the electron density of the HII region suggests that the HII region is expanding. The distributions of the ionized and molecular material, along with that of the emission in the MSX band A, suggest that a photodissociation region has developed at the interface between the ionized and molecular gas. The copious UV photon flux from the early type stars in NGC3324 keeps the HII region ionized. The characteristics of a relatively large number of the IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence of active star forming regions. Very probably, the expansion of the HII region has triggered stellar formation in the molecular shell.