Complex organic chemistry in high-mass star forming regions

Abstract

The quest for interstellar complex organic molecules (COMs) lies at the heart of astrochemistry. One basic motivation is to figure out how the rich inventory of COMs found in meteorites and comets is connected to interstellar chemistry. While the number of COMs detected in the interstellar medium increases, we wish to understand whether chemical complexity is a natural outcome of interstellar chemistry, which degree of complexity can be reached, and when, where and under which conditions these molecules form and how their occurence evolves from interstellar clouds to planetary systems.<br /> Most interstellar COMs were first detected toward the dense and warm parts of high-mass star-forming regions, called hot molecular cores. In particular, Sagittarius B2 (Sgr B2), one of the most active star-forming regions in our Galaxy, is an excellent target to study the production of COMs under extreme physical conditions (high densities, strong radiation field, high cosmic-ray flux).<br /> In this thesis we take advantage of the high sensitivity of a new type of imaging spectral line survey made possible by the Atacama Large Millimeter/submillimeter Array (ALMA). It affords studies of the spatial structure and chemical content of active star-forming regions in Sgr B2 in unprecedented detail. We report the detection of three new hot cores in Sgr B2(N), one of Sgr B2’s main star-forming sites. In a detailed comparative study, we determine their chemical composition, density, mass, temperature, and spatial structure. We check for association with maser sources and ultra-compact HII regions, signposts of recent high-mass star formation, as well as outflows, to evaluate the evolutionary stage of the hot cores. In the second part of the thesis we analyze their physical evolution from the cold pre-stellar phase to the present time. We use results of previous radiation-magnetohydrodynamical simulations of high-mass star formation and stellar structure calculations combined with a radiative transfer model to derive the thermal history of the sources. We compute time-dependent chemical abundances using the astrochemical code MAGICKAL, focusing especially on selected COMs to investigate in detail the chemical reactions and processes involved in their formation under the extreme conditions that characterize Sgr B2(N). We compare the chemical model results to the abundances derived from the observations toward the hot cores and find that a cosmic-ray ionization rate 50 times higher than the solar neighborhood value best characterizes Sgr B2(N)’s environmental conditions. We are also able to constrain the range of dust temperatures reached during the earlier pre-stellar phase at which COMs form on dust-grain surfaces. We show that COMs still form efficiently with minimum dust temperatures as high as 15 K, but the current chemical composition of the hot cores excludes minimum temperatures higher than 25 K.