Relative locations of PMS stars (orange contours) and massive MS stars (blue dots). The contour plots were derived after smoothing the distribution with a Gaussian beam whose size is indicated by the circle in the lower left corner (σ = 4″). The step between contour levels corresponds to a factor of 1.5. For reference, the blue dots denote the massive upper MS stars in the field (> 8 M_⊙). The massive stars exhibiting Hα excess emission are indicated by an additional cross. As discussed in Sect. 4.2, these are possibly Be stars. Unlike massive stars, PMS objects are rather uniformly distributed across the field and do not reveal any central concentration.