We present the results of N _ { 2 } H ^ { + } ( 1–0 ) observations of 35 dense molecular cloud cores from the northern and southern hemispheres where massive stars and star clusters are formed .
Line emission has been detected in 33 sources , for 28 sources detailed maps have been obtained .
Peak N _ { 2 } H ^ { + } column densities lie in the range : 3.6 \cdot 10 ^ { 12 } -1.5 \cdot 10 ^ { 14 } cm ^ { -2 } .
Intensity ratios of ( 01–12 ) to ( 23–12 ) hyperfine components are slightly higher than the LTE value .
The optical depth of ( 23–12 ) component toward peak intensity positions of 10 sources is \sim 0.2 - 1 .
In many cases the cores have elongated or more complex structures with several emission peaks .
In total , 47 clumps have been revealed in 26 sources .
Their sizes lie in the range 0.3–2.1 pc , the range of virial masses is \sim 30 - 3000 ~ { } M _ { \odot } .
Mean N _ { 2 } H ^ { + } abundance for 36 clumps is 5 \cdot 10 ^ { -10 } .
Integrated intensity maps with aspect ratios < 2 
have been fitted with a power-law radial distribution \sim r ^ { - p } 
convolved with the telescope beam .
Mean power-law index for 25 clumps is close to 1.3 .
For reduced maps where positions of low intensity are rejected mean power-law index is close to unity corresponding to the \sim r ^ { -2 } 
density profile provided N _ { 2 } H ^ { + } excitation conditions do not vary inside these regions .
In those cases where we have relatively extensive and high quality maps , line widths of the cores either decrease or stay constant with distance from the center , implying an enhanced dynamical activity in the center .
There is a correlation between total velocity gradient direction and elongation angle of the cores .
However , the ratio of rotational to gravitational energy is too low ( 4 \cdot 10 ^ { -4 } –7.1 \cdot 10 ^ { -2 } ) for rotation to play a significant role in the dynamics of the cores .
A correlation between mean line widths and sizes of clumps has been found .
A comparison with physical parameters of low-mass cores is given .