  We report molecular line and dust continuum observations, made with the SEST
telescope, towards four young high-mass star forming regions associated with
highly luminous (L> 6x10^5 Lsun) IRAS sources (15290-5546, 15502-5302,
15567-5236 and 16060-5146). Molecular emission was mapped in lines of CS
(J=2-1, 3-2 and 5-4), SiO (J=2-1 and 3-2), CH3OH (Jk=3k-2k and 2k-1k), and C34S
(J=3-2). In addition, single spectra at the peak position were taken in the CO,
13CO and C18O (J=1-0) lines. We find that the luminous star forming regions are
associated with molecular gas and dust structures with radii of typically 0.5
pc, masses of ~5x10^3 Msun, column densities of ~5x10^{23} cm^{-2}, molecular
hydrogen densities of typically ~2x10^5 cm^{-3} and dust temperatures of ~40 K.
The 1.2 mm dust continuum observations further indicate that the cores are
centrally condensed, having radial density profiles with power-law indices in
the range 1.6-1.9. We find that under these conditions dynamical friction by
the gas plays an important role in the migration of high-mass stars towards the
central core region, providing an explanation for the observed stellar mass
segregation within the cores.
