We propose to conduct a comprehensive study of the diffuse and dense interstellar medium using observations of light hydride molecules. The primary data to be analysed are archival herschel…

This is a $329,171 research grant awarded by the National Aeronautics and Space Administration (NASA) to The Johns Hopkins University. The purpose of the grant is to conduct a comprehensive study of the diffuse and dense interstellar medium using observations of light hydride molecules. The analysis will leverage archival data from the Herschel Space Observatory's HIFI spectrometer, focusing on molecular absorption and emission within bright submillimeter continuum sources. The goal is to derive key parameters such as the cosmic ray ionization rate and molecular hydrogen fraction along the observed sight-lines, and to constrain the freeze-out of molecules in dense interstellar gas. This grant does not have a set-aside designation, and the period of performance runs from July 1, 2015 through June 30, 2019.

We propose to conduct a comprehensive study of the diffuse and dense interstellar medium using observations of light hydride molecules. The primary data to be analysed are archival herschel observations obtained with the hifi spectrometer toward several bright submillimeter continuum sources, which reveal molecular absorption by diffuse foreground clouds and a combination of absorption and emission within the background continuum sources. Through a careful simultaneous analysis of the spectra of the light hydride molecules HF, CH, H2O, OH+, H2O+, H3O+, and ARH+ - all interpreted with the use of state-of-the-art astrochemical models - we will derive such quantities as the cosmic RAY ionization rate and the molecular hydrogen fraction along each observed sight-line. These will be compared with estimates obtained by alternate methods and - in the case of the H2 fraction - with the predictions of previous hydrodynamical models for the diffuse ISM. Our analysis of the emission and absorption from background continuum sources will yield important constraints on the freeze-out of molecules in dense interstellar gas.