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Chemical evolution from diffuse clouds to dense cores

Szűcs, László

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Abstract

Star formation is linked to molecular gas, which predominantly resides in giant molecular clouds within the galaxy. The chemical composition of these clouds and the newborn stellar population within them are in an intricate relation. On one hand, the star formation rate and the mass function of the resulting stars are sensitive to the gas temperature, which is affected by molecular heating and cooling processes. On the other hand, the ionizing and photodissociative radiation from the young stars may initiate the formation of more complex molecules through ion-molecular reactions, or it may very efficiently destroy them. Furthermore, the molecular clouds are not just the birth environments of stars and stellar systems but also provide the raw material for their formation. Precursors of the large chemical diversity that we experience in our Solar System have been found in numerous molecular clouds and dense cores, heated by new-born protostars. The molecular emission also provides us with an insight into the physical conditions of the star forming material, and traces the velocity, (column) density, temperature and the dynamical history of the gas. Therefore, understanding the main chemical pathways for formation and destruction of molecular coolants, determining the chemical initial conditions of highand low-mass star formation, finding species that preserve the dynamic history of the gas and calibrating the chemical tracers of cloud properties are amongst the major interests of today’s astrophysics and astrochemistry. New observatories, like ALMA (Atacama Large Millimeter/submillimeter Array) and NOEMA (Northern Extended Millimeter Array) will provide previously unseen sensitivity, resolution and amount of data to address these issues. The new complexity of observational data requires, however, increasingly complex, multi-dimensional and time dependent theoretical models to explain and predict them. Fortunately, parallel to the advancement of observational facilities, the theoretical understanding of the relevant physical and chemical processes, as well as the numerical methods and computational resources developed swiftly, allowing us to model a variety of processes (magneto-hydrodynamics, chemistry, radiation propagation) self-consistently within a single simulation. In this thesis I present three-dimensional, turbulence-supported hydrodynamical simulations of Giant Molecular Clouds and dense cores, linked with implicit or explicit chemical models, and describe synthetic molecular emission maps to be compared with observations. First, I investigate how the active chemical fractionation and isotope-selective photodissociation affects the 12CO/13CO isotope ratio and implicitly the 13CO emission based CO column density estimates. Then I benchmark the most frequently used CO emission based molecular mass measurement techniques for a large range of cloud properties. Finally, I present a new approach for modelling the formation and destruction of complex molecules, many of which form exclusively on grain surfaces, in three-dimensional, time dependent hydrodynamical simulations, that was previously only possible with major simplifications either dynamics-wise or chemistry-wise.

Item Type: Dissertation
Supervisor: Glover, PD Dr. Simon
Date of thesis defense: 22 January 2015
Date Deposited: 30 Jan 2015 11:03
Date: 2015
Faculties / Institutes: The Faculty of Physics and Astronomy > Institute for Theoretical Physics
Subjects: 520 Astronomy and allied sciences
Controlled Keywords: Astrophysik, Kosmochemie
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