Molecules at stellar death: from the winds of massive evolved stars to supernova remnants

Licentiate thesis, 2013

Massive stars play a key role in the chemical and dust budget of the galaxy, producing heavy elements, molecules, and dust. At the end of their lives they evolve through stages of significant mass-loss which affect the subsequent evolution of the star, including whether or not it will eventually explode as a supernova (SN). To contribute to the understanding of this cycle, we aim to use molecular emission as a probe of the physical conditions and kinematics of the gas in the envelopes of evolved massive stars and supernova ejecta. This will allow us to better understand the mechanisms at work in these complex environments. For this purpose, we have studied two peculiar objects. First is the evolved star IRAS 17163, a recently identified member of the rare Yellow Hypergiant (YHG) class, and one of the few with a massive dusty envelope. To probe its mass-loss and circumstellar kinematics, low-J CO lines were observed with APEX. The observations show a complex line profile, with multiple peaks and a broad underlying component, which is significantly redshifted from observed optical line velocities. This phenomenon is as yet unexplained, and further work aims to disentangle the various velocity components and determine the circumstellar structure. Second is a knot in the supernova remnant (SNR) Cas A, the first SNR to exhibit molecular emission, in which several high-J CO lines were observed with Herschel. A large column density of warm and dense CO was found, which, along with the broad linewidths, implies that the emission originates in the post-shock region of the reverse shock. As molecules are thought to be destroyed by the reverse shock, the observed CO emission supports the predictions of chemical models that find CO can reform after the passage of the reverse shock. Studying the gas characteristics around the reverse shock also constrains dust survival in SNRs, with implications for explaining the large dust masses seen in the early universe. In both cases, the molecular emission provides vital information of the gas conditions and kinematics, allowing us to understand the evolution of these objects and the impact they have on their surroundings. Further study will require higher angular resolution, in order to resolve the complex gas structures, and hence the use of interferometers such as the exceptional new ALMA telescope.