How Many Infrared Dark Clouds can form Massive Stars and Clusters?
  We present a new assessment of the ability of Infrared Dark Clouds (IRDCs) to
form massive stars and clusters. This is done by comparison with an empirical
mass-size threshold for massive star formation (MSF). We establish
m(r)>870M_sun(r/pc)^1.33 as a novel approximate MSF limit, based on clouds with
and without MSF. Many IRDCs, if not most, fall short of this threshold. Without
significant evolution, such clouds are unlikely MSF candidates. This provides a
first quantitative assessment of the small number of IRDCs evolving towards
MSF. IRDCs below this limit might still form stars and clusters of up to
intermediate mass, though (like, e.g., the Ophiuchus and Perseus Molecular
Clouds). Nevertheless, a major fraction of the mass contained in IRDCs might
reside in few 10^2 clouds sustaining MSF.
