Submillimeter H2O masers in water-fountain nebulae

We report the first detection of submillimeter water maser emission toward water-fountain nebulae, which are post-AGB stars that exhibit high-velocity water masers. Using APEX we found emission in the ortho-H2O (10(29) -> 9(36)) transition at 321.226 GHz toward three sources: IRAS 15445-5449, :IRAS 18043-2116, and IRAS 18286-0959. Similar to the 22 GHz masers, the submillimeter water masers are expanding with higher velocity than do 011 masers, suggesting that these masers also originate in fast bipolar outflows. In IRAS 18043-2116 and IRAS 18286-0959, which figure among the sources with the fastest water masers, the velocity range of the 321 GHz masers coincides with the range of the 22 GHz masers, indicating that they probably coexist. Toward IRAS 15445-5449, the submillimeter masers appear in a different velocity range, indicating that they trace different regions. The intensity of the submillimeter masers is comparable to the 22 GHz masers, implying that the kinetic temperature of the region where the masers originate should be T-k > 1000 K. We propose that the passage of two shocks through the same gas can create the conditions for explaining the strong high-velocity 321 GHz masers coexisting with the 22 GHz masers in the same region.