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Low-to-intermediate mass stars end their life on the asymptotic giant branch (AGB), an evolutionary phase in which the star sheds most of its mantle into the circumstellar environment through a stellar wind. This stellar wind expands at relatively low velocities and enriches the interstellar medium with elements newly made in the stellar interior. The physical processes controlling the gas and dust chemistry in the outflow, as well as the driving mechanism of the wind itself, are poorly understood and constitute the broader context of this thesis work.In a first chapter, we consider the thermodynamics of the high-density wind of the oxygen-rich star OH 127.8+0.0, using observations obtained with the PACS instrument onboard the Herschel Space Telescope. Being one of the most abundant molecules, water vapor can be dominant in the energy balance of the inner wind of these types of stars, but to date, its cooling contribution is poorly understood. We aim to improve the constraints on water properties by careful combination of both dust and gas radiative-transfer models. This unified treatment is needed due to the high sensitivity of water excitation to dust properties. A combination of three types of diagnostics reveals a positive radial gradient of the dust-to-gas ratio in OH 127.8+0.0. The second chapter deals with the dust chemistry of carbon-rich winds. The 30-µm dust emission feature is commonly identified as due to magnesium sulfide (MgS). However, the lack of short-wavelength measurements of the optical properties of this dust species prohibits the determination of the temperature profile of MgS, and hence its feature strength and shape, questioning whether this species is responsible for the 30-µm feature. By considering the very optically thick wind of the extreme carbon star LL Peg, this problem can be circumvented because in this case the short-wavelength optical properties are not important for the radial temperature distribution. We attribute the 30-µm feature to MgS, but require that the dust species is embedded in a heterogeneous composite grain structure together with carbonaceous compounds.The final chapter considers the circumstellar gas chemistry of carbon-rich AGB stars. The recent discovery of warm water vapor in carbon-rich winds challenges our understanding of chemical processes ongoing in the wind. Two mechanisms for producing warm water were proposed: water formation induced by interstellar ultraviolet photons penetrating into the inner region of a clumpy wind, and water formation induced by shocks passing through the atmospheric and inner-wind molecular gas. A sample of eighteen carbon-rich AGB stars has been observed with the Herschel Space Telescope and offers insights into the dependence of water properties on the stellar and circumstellar conditions. We suggest that both proposed water formation mechanisms must be at work to account for the following findings: 1) warm water is present in all observed carbon stars; 2) water formation efficiency decreases with higher circumstellar column density; 3) water properties strongly depend on the variability characteristics of the AGB stars; and 4) a positive water abundance gradient is present up to at most ~ 50 stellar radii in individual star

academic collection --- 524.5 <043> --- Interstellar medium. Galactic nebulae--Dissertaties --- Theses --- 524.5 <043> Interstellar medium. Galactic nebulae--Dissertaties

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Interstellaire materie --- Interstellar matter --- Interstellar medium --- Matière interstellaire --- 524.5 --- -Interstellar medium --- Astrophysics --- Matter --- Space environment --- Interstellar reddening --- Interstellar medium. Galactic nebulae --- Congresses --- Congresses. --- -Interstellar medium. Galactic nebulae --- 524.5 Interstellar medium. Galactic nebulae --- -524.5 Interstellar medium. Galactic nebulae

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Interstellar matter --- 524.5 --- Interstellar medium --- Astrophysics --- Matter --- Space environment --- Interstellar reddening --- Interstellar medium. Galactic nebulae --- Interstellar matter. --- 524.5 Interstellar medium. Galactic nebulae

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Interstellar matter --- Congresses --- 524.5 --- -#KVIV --- #WSTE:STER --- Interstellar medium --- Astrophysics --- Matter --- Space environment --- Interstellar reddening --- Interstellar medium. Galactic nebulae --- Congresses. --- 524.5 Interstellar medium. Galactic nebulae --- #KVIV --- Interstellar matter - Congresses

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Cosmic dust --- Astronomical spectroscopy --- Interstellar matter --- Congresses --- 524.5 --- -Astronomical spectroscopy --- -Interstellar matter --- -Interstellar medium --- Astrophysics --- Matter --- Space environment --- Interstellar reddening --- Spectroscopy, Astronomical --- Absorption spectra --- Spectrum analysis --- Dust --- Interstellar medium. Galactic nebulae --- -Interstellar medium. Galactic nebulae --- 524.5 Interstellar medium. Galactic nebulae --- -Astrophysics --- Interstellar medium --- -524.5 Interstellar medium. Galactic nebulae --- Cosmic dust - Congresses --- Astronomical spectroscopy - Congresses --- Interstellar matter - Congresses