| Description |
With the use of the HIFI instrument, we propose to observe four simple hydride molecules - HF, OH+, H2O+, and H2Cl+ - in absorption towards five bright submillimeter continuum sources. The target sources, all located in the Galactic plane with sight-lines that intersect multiple interstellar clouds, are the massive star-forming regions W49N, W51, G29.96-0.02, W3(OH), and G330.95-0.17. This selection of sources samples sight-lines in the 1st and 4th quadrants of the Galaxy and in the outer Galaxy. The proposed observations will have integration times sufficient to obtain signal-to-noise ratios in the range 100 - 400 in a single spectral channel, providing great sensitivity to absorption by foreground material. We will thereby determine the molecular column densities in foreground clouds located in spiral arms that lie along the sight-lines to these continuum sources. The four molecules we will observe, all detected previously in the ISM in early Herschel observations, will provide critical information about the diffuse interstellar medium. In particular, HF will permit the identification and study of clouds with a very small H2 column density that may be virtually undetectable in the spectra of other molecules; OH+ and H2O+ will allow us to study clouds with a small molecular fraction (revealed by a large OH+/H2O+ ratio), and to determine the cosmic ray ionization rate as a function of Galactocentric radius; and H2Cl+ will probe the photoionization rate and its variation with position in the Galaxy. |
