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 ( { \cal L } > 6 \times 10 ^ { 5 } { \cal L } _ { \odot } ) IRAS sources ( 15290-5546 , 15502-5302 , 15567-5236 and 16060-5146 ) .
Molecular emission was mapped in three lines of CS ( J=2 \rightarrow 1 , 3 \rightarrow 2 and 5 \rightarrow 4 ) , two lines of SiO ( J=2 \rightarrow 1 and 3 \rightarrow 2 ) , two rotational transitions of CH _ { 3 } OH ( J _ { k } =3 _ { k } \rightarrow 2 _ { k } and 2 _ { k } \rightarrow 1 _ { k } ) , and in the C ^ { 34 } S ( J=3 \rightarrow 2 ) line .
In addition , single spectra at the peak position were taken in the CO ( J=1 \rightarrow 0 ) , ^ { 13 } CO ( J=1 \rightarrow 0 ) and C ^ { 18 } O ( J=1 \rightarrow 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 \sim 5 \times 10 ^ { 3 } M _ { \odot } , column densities of \sim 5 \times 10 ^ { 23 } cm ^ { -2 } , molecular hydrogen densities of typically \sim 2 \times 10 ^ { 5 } cm ^ { -3 } and dust temperatures of \sim 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 .

The CS profiles show two distinct emission components : a bright component , with line widths of typically 5 km s ^ { -1 } ~ { } ( FWHM ) , and a weaker and wider velocity component , which extends up to typically \pm 13 km s ^ { -1 } ~ { } from the ambient cloud velocity .
The SiO profiles also show emission from both components , but the intensity of the pedestal feature relative to that of the bright component is stronger than for CS .
The narrow SiO component is likely to trace warm ambient gas close to the recently formed massive stars , whereas the high velocity emission indicates mass outflows produced by either the expansion of the HII regions , stellar winds , and/or collimated outflows .
We find that the abundances of CS , CH _ { 3 } OH and SiO , relative to H _ { 2 } , in the warm ambient gas of the massive cores are typically 4 \times 10 ^ { -8 } , 6 \times 10 ^ { -9 } , and 5 \times 10 ^ { -11 } , respectively .