Characterizing molecular clouds in the earliest phases of high-mass star formation

High-mass stars play a key role in the energetics and chemical evolution.;of molecular clouds and galaxies. However, the mechanisms that allow.;the formation of high-mass stars are far less clear than those of.;their low-mass.;counterparts. Most of the research on high-mass star formation has focused.;on regions currently undergoing star formation. In contrast, objects.;in the earlier prestellar stage have been more difficult to identify.;Recently, it has been.;suggested that the cold, massive, and dense Infrared Dark Clouds (IRDCs) host.;the earliest stages of high-mass star formation.;The chemistry of IRDCs remains poorly explored. In this dissertation, an.;observational program to search for chemical.;variations in IRDC clumps as a function of their age is described.;An increase in N2H+ and HCO+ abundances.;is found from the quiescent,;cold phase to the protostellar, warmer phases, reflecting chemical.;evolution. For HCO+ abundances, the observed trend is consistent with.;theoretical predictions. However, chemical models fail to explain the observed.;trend of increasing N2H+ abundances.;Pristine high-mass prestellar clumps are ideal for testing and constraining.;theories of high-mass star formation because their predictions differ.;the most at the early stages of evolution. From the initial IRDC sample,;a high-mass clump that is the best candidate to be in the prestellar phase.;was selected (IRDC G028.23-00.19 MM1). With a new set of observations,;the prestellar nature of the clump is confirmed. High-angular resolution.;observations of IRDC G028.23-00.19 suggest that in.;order to form high-mass stars, the detected cores have to accrete a large.;amount of material, passing through a low- to intermediate-mass phase.;before having the necessary mass to form a.;high-mass star. The turbulent core accretion model.;is inconsistent with this observational result, but on the other hand, the.;observations support the competitive accretion model. Embedded cores have.;to grow in.;mass during the star-formation process itself; the mass is not set at early.;times as the turbulent core accretion model predicts.;The observed gas velocity dispersion in the cores is transonic and mildly.;supersonic, resulting in low virial parameters (neglecting magnetic fields).;The turbulent core accretion model assumes highly supersonic linewidths and.;virial parameters ;magnetic fields in the cores have strengths of the order of 1 mG.