An analysis is presented of VLT-FLAMES spectroscopy for three Galactic clusters , NGC 3293 , NGC 4755 and NGC 6611 .
Non-LTE model atmosphere calculations have been used to estimate effective temperatures ( from either the helium spectrum or the silicon ionization equilibrium ) and gravities ( from the hydrogen spectrum ) .
Projected rotational velocities have been deduced from the helium spectrum ( for fast and moderate rotators ) or the metal line spectrum ( for slow rotators ) .
The origin of the low gravity estimates for apparently near main sequence objects is discussed and is related to the stellar rotational velocity .
The atmospheric parameters have been used to estimate cluster distances ( which are generally in good agreement with previous determinations ) and these have been used to estimate stellar luminosities and evolutionary masses .
The observed Hertzsprung-Russell diagrams are compared with theoretical predictions and some discrepancies including differences in the main sequence luminosities are discussed .
Cluster ages have been deduced and evidence for non-coeval star formation is found for all three of the clusters .
Projected rotational velocities for targets in the older clusters , NGC 3293 and NGC 4755 , have been found to be systematically larger than those for the field , confirming recent results in other similar age clusters .
The distribution of projected rotational velocities are consistent with a Gaussian distribution of intrinsic rotational velocities .
For the relatively unevolved targets in the older clusters , NGC 3293 and NGC 4755 , the peak of the velocity distribution would be 250 \mbox { km s } ^ { -1 } with a full-width-half-maximum of approximately 180 \mbox { km s } ^ { -1 } .
For NGC 6611 , the sample size is relatively small but implies a lower mean rotational velocity .
This may be evidence for the spin-down effect due to angular momentum loss through stellar winds , although our results are consistent with those found for very young high mass stars .
For all three clusters we deduce present day mass functions with \Gamma -values in the range of -1.5 to -1.8 , which are similar to other young stellar clusters in the Milky Way .