  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 4.9 GHz, 12CO(1-0) observations, and
IRAS and MSX infrared data. Adopting a distance of 3 kpc for the HII region and
the ionizing cluster, we derived an electron density of 33+/-3 cm^-3 and an
ionized mass of (3.3+/-1.1)x10^3 Mo based on the radio continuum data at 4.9
GHz. The HI 21-cm line images revealed an HI shell surrounding the HII region.
The HI structure is 10.0+/-1.7 pc in radius, has a neutral mass of 1500+/-500
Mo, and is expanding at 11 km/s. The associated molecular gas amounts to
1.1+/-0.5)x10^5 Mo, being its volume density of about 350 cm^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
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.
