| Description |
Using the HIFI instrument, we will address the puzzling - but widespread - appearance of water vapour in carbon-rich stars. Following up on detections of water in ALL SIX carbon-rich AGB stars observed to date in a pilot study performed in the HIFISTARS Key Program, we will target additional water transitions in four stars already observed or expected to show the most luminous water emissions. The target stars are CIT6, IRAC 15194-5155, V Cygni, and S Cep, and the additional transitions are the 4(22)-3(31) and 3(12) - 2(21) transitions at 916 GHz and 1153 GHz. Combined with spectra already obtained for the low-lying water transitions, and interpreted in the context of water excitation models, the proposed observations will place strong constraints upon the location of the emitting water. We will therefore be able to distinguish between various hypotheses that have been proposed for the origin of the observed water: the vaporization of orbiting comets or dwarf planets; catalytic formation on dust grains; or chemical processes initiated by the photodissociation of CO. In addition, we will carry out deep integrations to observe the lowest 1(11) - 0(00) transition of para-water at 1113 GHz in two carbon-rich AGB stars: IRAS+40540 and V Hya; here, ortho-water has been securely detected but existing observations of the 1113 GHz para-water line yield weak detections that lack the signal-to-noise ratio needed to constrain the ortho-to-para ratio. |
