  Massive stars are mainly found in stellar associations. These massive star
clusters occur in the heart of giant molecular clouds. The strong stellar wind
activity in these objects generates large bubbles and induces collective
effects that could accelerate particles up to high energy and produce gamma
rays. The best way to input an acceleration origin to the stellar wind
interaction in massive stellar cluster is to observe young massive star
clusters in which no supernova explosion has occurred yet.
  This work aims to constrain the part of stellar wind mechanical energy that
is converted into energetic particles using the sensitivity of the ongoing
Fermi/LAT instrument. This work further provides detailed predictions of
expected gamma-ray fluxes in the view of the on-set of the next generation of
imaging atmospheric Cherenkov telescopes.
  A one-zone model where energetic particles are accelerated by repeated
interactions with strong supersonic shocks occurring in massive star clusters
was developed. The particle escape from the star cluster and subsequent
interaction with the surrounding dense material and magnetic fields of the HII
region was computed. We applied this model to a selection of eight embedded
star clusters constricted by existing observations. We evaluated the gamma-ray
signal from each object, combining both leptonic and hadronic contributions. We
searched for these emissions in the Fermi/LAT observations in the energy range
from 3 to 300 GeV and compared them to the sensitivity of the Cherenkov
Telescope Array.
  No significant gamma-ray emission from these star clusters has been found.
Less than 10% of stellar wind luminosities are supplied to the relativistic
particles. Some clusters even show acceleration efficiency of less than 1%. The
CTA would be able to detect gamma-ray emission from several clusters in the
case of an acceleration efficiency of close to 1%.
