Invited review abstract

Recent theories predict that high-mass star formation may occur only above a minimum surface density of the parental molecular clump. With this in mind, we have searched for OB star formation signposts in a sample of 49 massive molecular clumps ($M > 100$~M$_{\odot}$), including both IR-dark and IR-loud sources and covering a wide range of surface densities. The aim was to look for evolutionary trends and test observationally the above mentioned theoretical prediction. Each of these sources has been mapped in the HCO$^{+}$(1--0), HCN(1--0), and C$^{18}$O(2--1) lines with the IRAM-30m telescope in Pico Veleta (Spain). Molecular outflows have been detected in 75\% of our targets. Remarkably, we measure a surface density threshold of 0.3~g~cm$^2$ above which the detection rate becomes100\% and the outflows are on average more massive, lending support to the theoretical prediction. Subsequent observations in the SiO(2--1) and (3--2) transitions reveal an inverse correlation between the ratio of SiO luminosity to bolometric luminosity, $L_\mathrm{sio}/L_\mathrm{bol}$, and the ratio of bolometric luminosity to mass of the clumps, $L_\mathrm{bol}/M$, with more powerful jets corresponding to lower values of $L_\mathrm{bol}/M$. This suggests that the jet/outflow phase is more active in the earliest stages of stellar formation. The infall detection rate measured in our sample is low, but significantly higher in the IR-dark sub-sample, an indication that these objects could be associated with the onset of star formation.