The environs of the HII region Gum31

We analyze the distribution of the interstellar matter in the environs of the \hii region Gum 31, excited by the open cluster NGC 3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material. We use 21cm-line data, radio continuum images at 0.843, 2.4 and 4.9 GHz, ${}^{12}${\bf CO(1-0)} observations, and IRAS and MSX infrared data. Adopting a distance of 3 kpc for the \hii
region and the ionizing cluster, we have derived an electron density of 33$\pm$3 cm${}^{-3}$ and an ionized mass of (3.3$\pm$1.1)$\times {10}^3$ M${}_{\odot }$ based on the radio continuum data at 4.9 GHz. The \hi 21-cm line images revealed an \hi shell surrounding the H {\sc ii} region. The \hi structure is 10.0$\pm$1.7 pc in radius, has a neutral mass of 1500$\pm$500 M${}_{\odot }$, and is expanding at 11 km s${}^{-1}$. The associated molecular gas amounts to (1.5$\pm$0.5)$\times {10}^5$ M${}_{\odot }$, being its volume density of about 500 cm^{-3}. This molecular material probably represents the remains of the cloud where the young open cluster NGC 3324 was born. 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. We conclude that either the massive stars in the open cluster have weak stellar winds or the stellar winds have blown during a very short period of time to create an interstellar bubble in an interstellar medium as dense as observed. IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence star forming regions. The expansion of the \hii region has triggered stellar formation in the molecular shell.