Using Galactic Plane surveys , we have selected a massive ( 1200 M _ { \odot } ) , cold ( 14 K ) 3.6-70 \mu m dark IRDC G331.372-00.116 .
This IRDC has the potential to form high-mass stars and , given the absence of current star formation signatures , it seems to represent the earliest stages of high-mass star formation .
We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array ( ALMA ) at 1.1 and 1.3 mm in dust continuum and line emission .
The dust continuum reveals 22 cores distributed across the IRDC .
In this work , we analyze the physical properties of the most massive core , ALMA1 , which has no molecular outflows detected in the CO ( 2-1 ) , SiO ( 5-4 ) , and H _ { 2 } CO ( 3-2 ) lines .
This core is relatively massive ( M = 17.6 M _ { \odot } ) , subvirialized ( virial parameter \alpha _ { vir } = M _ { vir } / M = 0.14 ) , and is barely affected by turbulence ( transonic Mach number of 1.2 ) .
Using the HCO ^ { + } ( 3-2 ) line , we find the first detection of infall signatures in a relatively massive , prestellar core ( ALMA1 ) with the potential to form a high-mass star .
We estimate an infall speed of 1.54 km s ^ { -1 } and a high accretion rate of 1.96 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } .
ALMA1 is rapidly collapsing , out of virial equilibrium , more consistent with competitive accretion scenarios rather than the turbulent core accretion model .
On the other hand , ALMA1 has a mass \sim 6 times larger than the clumps Jeans mass , being in an intermediate mass regime ( M _ { J } = 2.7 < M \lesssim 30 M _ { \odot } ) , contrary to what both the competitive accretion and turbulent core accretion theories predict .