Search for interstellar water in the translucent molecular cloud toward HD154368

We report an upper limit of 9 x 10(12) cm(-2) on the column density of wate r in the translucent cloud along the line of sight toward HD 154368. This r esult is based on a search for the C-X band of water near 1240 Angstrom car ried out using the Goddard High-Resolution Spectrograph (GHRS) of the Hubbl e Space Telescope. Our observational Limit on the water abundance, together with detailed chemical models of translucent clouds and previous measureme nts of OH along the line of sight, constrains the branching ratio in the di ssociative recombination of H3O+ to form water. We find at the 3 sigma leve l that no more than 30% of dissociative recombinations of H3O+ can lead to H2O. The observed spectrum also yielded high-resolution observations of the Mg II doublet at 1239.9 Angstrom and 1240.4 Angstrom, allowing the velocit y structure of the dominant ionization state of magnesium to be studied alo ng the line of sight. The Mg II spectrum is consistent with GHRS observatio ns at lower spectral resolution that were obtained previously but allow an additional velocity component to be identified.