Young clumps embedded in IRDC

These clumps are found within 764 different IRDCs, 375 (49%) of which are only associated with protostellar clumps, 178 (23%) only with starless clumps, and 211 (28%) with both categories of clumps. The clumps have a median mass of ~250M_{sun}_ and range up to >10^4^M_{sun}_ in mass and up to 10^5^L_{sun}_ in luminosity. The mass-radius distribution shows that almost 30% of the starless clumps identified in this survey could form high-mass stars; however these massive clumps are confined in only 4% of the IRDCs.

Assuming a minimum mass surface density threshold for the formation of high-mass stars, the comparison of the numbers of massive starless clumps and those already containing embedded sources suggests an upper limit lifetime for the starless phase of ~10^5^yr for clumps with a mass M>500M_{sun}_.