Massive 70 μm quiet clumps I: evidence of embedded low/intermediate-mass star formation activity

Massive clumps, prior to the formation of any visible protostars, are the best candidates to search for the elusive massive starless cores. In this work, we investigate the dust and gas properties of massive clumps selected to be 70 μm quiet, therefore good starless candidates. Our sample of 18 clumps has masses 300 ≲ M ≲ 3000 M_⊙, radius 0.54 ≤ R ≤ 1.00 pc, surface densities Σ ≥ 0.05 g cm^-2 and luminosity/mass ratio L/M ≤ 0.3. We show that half of these 70 μm quiet clumps embed faint 24 μm sources. Comparison with GLIMPSE counterparts shows that five clumps embed young stars of intermediate stellar mass up to ≃5.5 M_⊙. We study the clump dynamics with observations of N₂H⁺ (1-0), HNC (1-0) and HCO⁺ (1-0) made with the IRAM 30 m telescope. Seven clumps have blue-shifted spectra compatible with infall signatures, for which we estimate a mass accretion rate 0.04≲ \dot{M}≲ 2.0× 10^{-3} M_⊙ yr^-1, comparable with values found in high-mass protostellar regions, and free-fall time of the order of t_ff ≃ 3 × 10⁵ yr. The only appreciable difference we find between objects with and without embedded 24 μm sources is that the infall rate appears to increase from 24 μm dark to 24 μm bright objects. We conclude that all 70 μm quiet objects have similar properties on clump scales, independently of the presence of an embedded protostar. Based on our data, we speculate that the majority, if not all of these clumps, may already embed faint, low-mass protostellar cores. If these clumps are to form massive stars, this must occur after the formation of these lower mass stars.