Massive 70 µm quiet clumps - II. Non-thermal motions driven by gravity in massive star formation?

Abstract

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 30 m telescope towards a sample of 16 Herschel-identified massive 70 μm quiet clumps associated with infrared dark clouds. The clumps span a mass range from 300 to 2000 M_⊙. The N₂H⁺ data show that the regions have significant non-thermal motions with velocity dispersion between 0.28 and 1.5 km s^-1, corresponding to Mach numbers between 2.6 and 11.5. The majority of the 70 μm 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 Σ of the clumps, with clumps of Σ ≳ 0.1 g cm^-2 having more asymmetric line profiles, and so are more dynamically active, than clumps with lower Σ. We explore the relationship between velocity dispersion, radius and Σ 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.