The results are presented of a molecular line survey to search for the spectral signature of infall towards 77 850 \mu m continuum sources believed to be candidate high mass protostellar objects .
Up to six different transitions , HCO ^ { + } J = 1 \rightarrow 0 , J = 3 \rightarrow 2 and J = 4 \rightarrow 3 , H _ { 2 } CO 2 _ { 12 } -1 _ { 11 } , N _ { 2 } H ^ { + } J = 1 \rightarrow 0 and H ^ { 13 } CO ^ { + } J = 3 \rightarrow 2 , were observed towards each source .
Towards the peak of the 850 \mu m emission , N _ { 2 } H ^ { + } was typically strong , with a peak antenna temperature of \sim 1.5 K , with a typical linewidth of \sim 2 km s ^ { -1 } .
The good agreement between the velocity and velocity width of the N _ { 2 } H ^ { + } and H ^ { 13 } CO ^ { + } emission suggests that both species are tracing similar material in the sources .
With respect to the velocity of the N _ { 2 } H ^ { + } , there is a statistically significant excess of blue asymmetric line profiles in both the HCO ^ { + } J = 1 \rightarrow 0 and H _ { 2 } CO transitions .
This excess reaches levels similar to that seen towards samples of low mass protostars , and suggests that the material around these high mass sources is infalling .
We identify 22 promising candidate infall sources which show at least one blue asymmetric line profile and no red asymmetric profiles .
The infall velocity is estimated to be in the range of 0.1 km s ^ { -1 } to 1 km s ^ { -1 } with an implied mass accretion rate of between 2 \times 10 ^ { -4 } M _ { \odot } /yr and 10 ^ { -3 } M _ { \odot } /yr .