Massive 70 micron quiet clumps II: non-thermal motions driven by gravity
  in massive star formation?
  The dynamic activity in massive star forming regions prior to the formation
of bright protostars is still not fully investigated. In this work we present
observations of HCO+ J=1-0 and N2H+ J=1-0 made with the IRAM 30m telescope
towards a sample of 16 Herschel-identified massive 70 micron quiet clumps
associated with infrared dark clouds. The clumps span a mass range from 300
M_sun to 2000 M_sun. The N2H+ data show that the regions have significant
non-thermal motions with velocity dispersion between 0.28 km s^-1 and 1.5 km
s^-1, corresponding to Mach numbers between 2.6 and 11.5. The majority of the
70 micron quiet clumps have asymmetric HCO+ line profiles, indicative of
significant dynamical activity. We show that there is a correlation between the
degree of line asymmetry and the surface density Sigma of the clumps, with
clumps of Sigma>=0.1 g cm^-2 having more asymmetric line profiles, and so are
more dynamically active, than clumps with lower Sigma. We explore the
relationship between velocity dispersion, radius and Sigma and show how it can
be interpreted as a relationship between an acceleration generated by the
gravitational field a_G, and the measured kinetic acceleration, a_k, consistent
with the majority of the non-thermal motions originating from self-gravity.
Finally, we consider the role of external pressure and magnetic fields in the
interplay of forces.
