Dust guaranteed - Herschel observes AGB stars

Mass-loss is the dominating factor in the post-main sequence evolution of almost all stars. On the other hand stellar mass loss is a crucial player within the cosmic cycle of matter. It is responsible for both the chemical enrichment of the star`s surroundings as well as the total mass return to the interstellar medium. This project is devoted to the study of dust shells around evolved red giant stars, especially those on the Asymptotic Giant Branch (AGB). The AGB is a phase reached by all low- to intermediate-mass stars (less than about 8 solar masses) towards the end of their evolution. Consequently, its investigation provides clues also on the future of our own solar system. On the other hand, earth-like planets could never have formed without the pre-existence of heavy elements and dust mainly produced by such AGB stars of earlier stellar generations. The mass loss of thermally-pulsing AGB stars takes place in the form of a slow (typically 5-25 km/s) winds with large mass loss rates (up to 10-4 solar masses per year). Whereas the gaseous component of the mass loss can be observed from the ground (mainly through molecular emission lines in the mm- and sub-mm radio range) the interesting dusty fraction of the wind is preferably observed with infrared space telescopes. Although mass loss is such an important process and has been studied since the late 1960`s with the advent of infrared astronomy, many basic questions remain unanswered even in the post-ISO or Spitzer Space Telescope era: What is the time evolution of the mass loss rate? What is the geometry of this process and how does this influence the shaping of the nebulae seen around the later phase central stars of Planetary Nebulae? What kind of dust species are formed at exactly what location in the wind? Within the Austrian participation in the recently launched Herschel Space Telescope PACS instrument a significant amount of Austrian Guaranteed time (GT) is invested in the GT Key Project (GTKP) MESS. It is focused on three main aims: (1) to study the time dependence of the mass loss process, via a search for shells and multiple shells around a wide range of evolved objects, in order to quantify the total amounts of mass loss at the various stellar evolutionary stages, (2) to study the dust and gas chemistry as a function of progenitor mass, and (3) to study the properties and asymmetries of a representative sample of low- and intermediate-mass (i.e. AGB) post-main sequence objects. MESS, being a joint effort of a larger European consortium also includes research on high-mass and further evolved objects which is not part of this proposed FWF project. Overall, using our unique Herschel dataset of well selected, representative AGB stars of different chemistry, evolutionary phase and main sequence mass our programme will be capable of probing the main contributors to the dust budgets of galaxies like the Milky Way. With Herschel`s improved spatial resolution, when compared to ISO and Spitzer, larger field-of view, higher sensitivity, the extension to longer and unexplored wavelength regions, PACS imaging and medium resolution spectrometer observations will lead to a significant improvement in our understanding of the mass-loss phenomenon on the AGB.

Stars, including our own Sun, lose most of their matter at the end of their life. By this they enrich the interstellar medium with heavy elements, which are a prerequisite for the subsequent formation of planets and as a final consequence - life.These enormously expanded Red Giant stars harbour an inactive Carbon-Oxygen core and acquire their high luminosity from Helium- and Hydrogen shells that burn in a quite instable way. The stellar envelopes expand further because of these burning shells, leading to stellar radii comparable to the distance between Earth and Sun! At the same time a strong stellar wind develops that is dragging away substantial amounts of gas and dust. In the case of episodic helium shell flashes, increasing mass loss may modulate the outflow by forming detached spherical bubbles of dust and gas.Already the first images, that we acquired using the Herschel satellite, showed such ring or bubble like structures that were expelled from their parent stars with velocities of typically 15 km/s some thousands of years ago. In some cases binary partners shape these outflows to even more complex morphologies, sometimes resembling Archimedean spirals. Moreover, the interaction of the outflows with the interstellar medium may change the overall appearance of the circumstellar environments.All these different morphologies of stellar mass loss with focus on its dusty component were observed by the sharp and sensitive eyes of the Herschel satellite. In the course of the FWF project we developed new data reduction pipelines, refined analytical tools and carried out a systematic study on a large sample of dying stars.Complementary to the observations that Herschel provided, we probed the innermost regions of the stars envelopes using ESOs Very Large Telescope Interferometer and studied the gaseous outflows by observations of carbon monoxide with the radiointerferometer ALMA. There again surprising morphologies become evident. The outflow from the star R Sculptoris turned out to be shaped into a long winding spiral by a so far unknown binary partner. Following this spiral it was possible to reconstruct the mass loss history over a couple of thousands of years. This is only one example of our future follow up plans. They are of wider relevance with our sun having a similar destiny in 5 Billion years from now and knowing that the chemical evolution of the Universe as a whole is strongly influenced by the mass loss processes we study.