Author:

Abstract:

Mass loss is the dominating factor in the post-main sequence evolution of most stars but many aspects of the mass loss mechanism(s) are still not understood. The Herschel Space Observatory offers the astronomers some unique instruments (HIFI, PACS and SPIRE) to study circumstellar environments around evolved stars. We present the latest results obtained in the framework of the MESS (Mass Loss in Evolved StarS) Herschel Guaranteed Time Key Project (GTKP, 330 hrs, PACS and SPIRE) and HIFISTARS GTKP (205 hrs, HIFI). The main focus of this talk will be on the chemistry occuring in the envelope of evolved oxygen-rich, carbon-rich and S-type Asymptotic Giant Branch (AGB) stars. Detailed analyses of the very rich infrared and sub-millimeter spectra show that various chemical processes, as shock-induced non-thermal equilibrium chemistry, ion-ion processes, photodissociation, etc. determine the chemical fractional abundances in the circumstellar environments around these evolved stars (see, e.g., left panel in the figure). In addition, recent images obtained with the PACS and SPIRE photometers show that the energetic encounter between the circumstellar material and ISM (see, e.g., right panel in the figure) is a very important player in establishing the exact chemical composition of the circumstellar material which is injected into the ISM. Using recentHerschel spectra and images, the different chemical processes will be discussed.