HIFIStars: Late stages in the stellar evolution as seen with HIFI

Goal: HIFI Guaranteed Time Key Project aimed to gain deeper insight into the structure, thermodynamics, kinematics and chemistry of circumstellar envelopes and into the mass-loss history of evolved stars.

Team: V. Bujarrabal (coordinator), J. Alcolea, J. Cernicharo, L. Decin, C. Dominik, K. Justtanont, A. de Koter, A.P. Marston, G. Melnick, K. Menten, D. Neufeld, H. Olofsson, P. Planesas, M. Pulecka, M. Schmidt, F. Schöier, R. Szczerba, D. Teyssier, R. Waters.

Summary: HIFIStars is one of the Guaranteed Time Key Programs proposed by the HIFI consortium. HIFIStars is devoted to observe the nebulae ejected by late type stars, namely AGB, post-AGB, red supergiant, and yellow hypergiant stars. The excelent performances of HIFI, in particular its high spectral resolution, will allow us a very accurate study of the mass-loss processes in these objects.

Most stars, including our Sun, will end their life ejecting most of the stellar material back to the interstellar medium, from which they were formed long time ago. This material forms large shells around the stars, which can be studied in detail with HIFI. In the first steps of this mass ejection process, the red giant phase, the circumstellar shell is essentially spherical and expands at moderate velocities. However, in the very late phases of the stellar life, the nebula suddenly acquires a strong axial symmetry and much higher velocities. The mechanism responsible for this impressive dynamical evolution is not yet understood.

The precise history of mass-loss, rather than nuclear burning, regulates the ultimate fate of most stars. This ejection of material also dominates the gas and dust budget of the galaxy, including its chemical evolution. HIFI will study the mass-loss process in detail and thus the late evolution of stars and their contribution to the interstellar medium.

V band image of light scattered by dust grains in the circumstellar envelope around one of the old stars that will be deeply observed with HIFI: IRC+10216, a typical red giant (AGB) star. We can see how the mass-loss rate has been varying during the last 1000 years, but a remarkable spherical symmetry was kept.

Image of the ionized (blue) and neutral (yellow) components of OH 231.8+4.2. OH 231.8+4.2 is a young planetary nebula showing particularly interesting molecular emission. Molecular lines are known to probe the different dynamical regimes in this source, which include very fast flows reaching up to 400 km/s. The different components interact yielding spectacular shocks, whose dynamics and thermodynamics will be efficiently studied thanks to the high spectral resolution and frequency attained by HIFI.