Spectroscopic analysis of early-type stars in young stellar groups .1. Differential analysis of the B1V stars in NGC 2244

We explore the potential of analysing fast rotating early-type stars differentially with respect to templates derived from a slowly rotating star of the same spectral type. The analysis is successfully applied to the normal B 1 V stars in NGC 2244, which have projected rotational velocities ranging from 22 to 260 km s(-1). A classical non-LTE analysis of the slow rotator #201 results in chemical abundances similar to those of early B-type stars in the field and in Orion. We argue that a differential analysis of the fast rotators relative to a broadened template derive;I from #201 can still reveal abundance differences of greater than or similar to 0.2 dex, but no evidence was found for differences in metal composition at this level. A differential analysis of H and He line profiles reveals the need to further explore rotation models. Neither the classical approach to stellar rotation, nor the published models that include the influence of rotation on the star's atmosphere can bring the profiles of the slow rotator into complete agreement with those of the fast rotators. An attempt to extend the analysis to a Be star failed. The Be spectrum is described qualitatively and quantitative measurements are made for the emission in the core of the Balmer lines.