Calibration of evolutionary diagnostics in high-mass star formation
  The evolutionary classification of massive clumps that are candidate
progenitors of high-mass young stars and clusters relies on a variety of
independent diagnostics based on observables from the near-infrared to the
radio. A promising evolutionary indicator for massive and dense
cluster-progenitor clumps is the L/M ratio between the bolometric luminosity
and the mass of the clumps. With the aim of providing a quantitative
calibration for this indicator we used SEPIA/APEX to obtain CH3C2H(12-11)
observations, that is an excellent thermometer molecule probing densities >
10^5 cm^-3 , toward 51 dense clumps with M>1000 solar masses, and uniformly
spanning -2 < Log(L/M) < 2.3.
  We identify three distinct ranges of L/M that can be associated to three
distinct phases of star formation in massive clumps. For L/M <1 no clump is
detected in CH3C2H , suggesting an inner envelope temperature below 30K. For 1<
L/M < 10 we detect 58% of the clumps, with a temperature between 30 and 35 K
independently from the exact value of L/M; such clumps are building up
luminosity due to the formation of stars, but no star is yet able to
significantly heat the inner clump regions. For L/M> 10 we detect all the
clumps, with a gas temperature rising with Log(L/M), marking the appearance of
a qualitatively different heating source within the clumps; such values are
found towards clumps with UCHII counterparts, suggesting that the quantitative
difference in T - L/M behaviour above L/M >10 is due to the first appearance of
ZAMS stars in the clumps.
