The death throes of massive stars

Sammanfattning: Massive evolved stars affect their local surroundings as they go through phases of intense mass-loss and eventually explode as supernovae, adding kinetic energy and freshly synthesised material to the interstellar medium. The circumstellar material ejected by the star affects the shape and evolution of the future supernova remnant, and how the material is incorporated into the interstellar medium. Over time, these processes affect the chemical evolution of the interstellar medium on a galactic scale. This thesis probes these topics at different stages. First, observations of CO lines in the supernova remnant Cassiopeia~A have been used to study the effect of the reverse shock on supernova ejecta (Paper~I). We find a large column density of warm CO, which has most likely re-formed after the passage of the reverse shock. The high temperature and density implies that thermal conduction by electrons may be an important process for the evolution of dense ejecta knots, with implications for the survival of supernova dust. Second, CO observations have been used to study the circumstellar environment of a massive star, the yellow hypergiant IRAS 17163-3907. Our observations with APEX (Paper~II) and ALMA ACA (Paper~III) reveal a complex environment with several distinct components: a fast recent stellar wind of 100 km/s, a clumpy CO ring which appears to be a torus ejected by the star several thousand years ago, and a unidirectional bright spur extending from the star to the clumpy ring. These asymmetries are not seen in infrared dust observations, and demonstrate the complexity of massive evolved stars and the need for high resolution molecular observations to understand them. Finally, the contribution of massive stars to galactic chemical enrichment has been investigated indirectly with measurements of isotopic ratios in a molecular absorber at z=0.68 towards B 0218-211 (Paper~IV). We find that the ratios at z=0.68 are very different from those in the solar neighborhood, but similar to the ratios found in another absorber at z=0.89 and in starburst galaxies. We interpret these ratios as the signature of enrichment mainly by massive stars. From the evolution of a supernova remnant, to the complex surroundings of a massive evolved star, to the isotopic enrichment of a distant galaxy, this thesis addresses the role massive stars play in shaping and enriching their environments. Massive stars may not be numerous, but their death throes echo across time and space.