We have analyzed HCN ( 1–0 ) and CS ( 2–1 ) line profiles obtained with high signal-to-noise ratios toward distinct positions in three selected objects in order to search for small-scale structure in molecular cloud cores associated with regions of high-mass star formation .
In some cases , ripples were detected in the line profiles , which could be due to the presence of a large number of unresolved small clumps in the telescope beam .
The number of clumps for regions with linear scales of \sim 0.2 - 0.5 Â pc is determined using an analytical model and detailed calculations for a clumpy cloud model ; this number varies in the range : \sim 2 \times 10 ^ { 4 } -3 \times 10 ^ { 5 } , depending on the source .
The clump densities range from \sim 3 \times 10 ^ { 5 } -10 ^ { 6 } Â cm ^ { -3 } , and the sizes and volume filling factors of the clumps are \sim ( 1 - 3 ) \times 10 ^ { -3 } Â pc and \sim 0.03 - 0.12 .
The clumps are surrounded by inter-clump gas with densities not lower than \sim ( 2 - 7 ) \times 10 ^ { 4 } Â cm { -3 } .
The internal thermal energy of the gas in the model clumps is much higher than their gravitational energy .
Their mean lifetimes can depend on the inter-clump collisional rates , and vary in the range \sim 10 ^ { 4 } -10 ^ { 5 } Â yr .
These structures are probably connected with density fluctuations due to turbulence in high-mass star-forming regions .

Key words : interstellar medium , molecular clouds , interstellar molecules , radio lines .