O-TYPE STARS IN THE YOUNG OPEN CLUSTER NGC 2244 AND THE MON OB2 ASSOCIATION

The O-type stars play a key role in the ecology of the galaxies. However, the 
knowledge of a great of their formation is still fragmented. The study of 
massive stars in clusters are thus interesting to make the highligths between 
the different formation scenarios proposed by the theoreticians. Indeed, these
massive stars constitute a homogenous population (same distance, age and 
chemical composition) and their study, in particular their multiplicity, is of 
major interest for the understandings of the physical properties of these 
objects.

Previous intensive works were performed by our team and have allowed us to 
seriously revise the results proposed by Garcia y Mermilliad for the O-stars
rich clusters. In this context, I will present the main results from our 
long-term spectroscopic campaign devoted to the O-type stars in the young open
cluster NGC 2244 and the Mon OB2 association. I will revise the spectral 
clasification, the rotational velocity and the multiplicity of the O-type stars
and I will put in perspective this investigation with the previous ones made 
by our team.