| Description |
One of the highlights of the first year of Herschel.s science program was the discovery of warm water vapour in the envelope around the carbon-rich evolved star IRC+10216 (Decin et al. 2010, Nature). The water abundance derived for this carbon-rich AGB star is 4 orders of magnitude larger than the photospheric abundance expected under thermochemical equilibrium. This huge discrepancy had led to the suggestions of several possible origins for the water vapor. The relative strengths of the high-excitation water lines in the Herschel data indicate the presence of warm water vapor close to the star. Only two, still competing, theories are consistent with the existence of warm water vapor.Very strikingly, water vapour was later on detected in eight other carbon-rich evolved stars, i.e. every star in a small sample surveyed with HIFI. Much to our surprise, recent PACS observations did not reveal the presence of water in one other carbon-rich target (AFGL3068). So far, this is the only carbon-rich star without water detection. Currently, it is absolutely unclear which are the key physical and chemical parameters determining if water will be formed or not. The currently available Herschel observations show a very strong bias toward high mass-loss rate targets. Additionally, Mira-type pulsators with high pulsational amplitudes are favored over Semi-Regulars. We aim to extend the small HIFI sample with 10 new targets, carefully selected to cover all (circum)stellar properties thought to be r^ant for the creation and excitation of water. We will observe 5 ortho-water lines and 1 para-water line, covering different excitation energies. That way, Herschel will provide us with high-quality data being key to unravel the physical and chemical conditions prerequisite for the formation of water in the sooty outflow of luminous carbon stars. |
