UV Chemistry in the Circumstellar Envelopes of Evolved Stars

Sammanfattning: Low- and intermediate-mass stars eject a substantial amount of their material into space during a late phase of stellar evolution, the asymptotic giant branch (AGB) phase. Therefore, they impact the chemistry of the interstellar medium. Due to the intense mass loss, a circumstellar envelope (CSE), rich in gas and dust, forms around the AGB star. Observations of molecular species and dust content in CSEs help us to broaden our knowledge on late phases of stellar evolution, mass-loss processes, the CSE chemistry, and the stellar properties. For instance, observations of carbon monoxide (CO) have been extensively used to determine the mass-loss rate and the overall CSE properties. Ultraviolet (UV) photodissociation of CO from the interstellar radiation field (ISRF) is the dominant process that determines the CO distribution and extent in CSEs. Therefore, a precise calculation of the CO photodissociation rate is crucial to determine the mass-loss rates. Subsequently, the value adopted for the mass-loss rate in further modelling of the CSEs will affect the abundances derived for all other molecules. Thus, an estimation of the CO photodissociation rate affects the estimates of the amount of all the recycled material. In this thesis, we present the most updated calculations of the depth dependency of the CO photodissociation rate in CSEs using the latest laboratory measurements. Generally, it is well known that UV radiation impacts the CSE chemistry and the influence of UV radiation from the ISRF has been considered in the models of CSEs. However, there has been little discussion on the impact of internal sources of UV radiation. Recent Galaxy Evolution Explorer observations reveal the presence of strong internal UV radiation for a large sample of AGB stars. The internal UV radiation can originate from stellar chromospheric activity, a hot binary companion, and/or accretion of matter between two stars in a binary system. This thesis seeks to address the impact of both the internal and external sources of UV radiation on the CSE chemistry. To trace the impact of UV radiation, we present two approaches. First, observations of the main UV photodissociation and photoionization products, such as CI and CII. We present, for the first time, detections of CI around a UV-bright oxygen-rich AGB star, omi Ceti. In the second approach, we investigate the isotopologue ratio of molecules with different photodissociation mechanisms. We expect variations in the isotopologue ratio of molecules that dissociate through lines. However, there should not be any variation by UV radiation in the isotopologue ratio of molecules with continuum dissociation.