2016ApJ...826L...8M

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(J=12-11) observations, which is an excellent thermometer molecule probing densities 10^{5}/cm3, toward 51 dense clumps with M\geqslant 1000 M _☉ and uniformly spanning -2 <= Log(L/M) [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 \leqslant 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 toward clumps with UCH ii counterparts, suggesting that the quantitative difference in T versus L/M behavior above L/M ∼ 10 is due to the first appearance of ZAMS stars in the clumps.