| Description |
First HIFI and PACS observations obtained in the framework of the PRISMAS and HEXOS key programs have demonstrated the advantage of using absorption spectroscopy for studying the diffuse interstellar medium in the inner Galaxy. Detections of HF, OH+, H2O+, CH+, CH and the N-hydrides have significantly improved the knowledge of the diffuse molecular gas and started to open windows on the diffuse atomic gas. However the information on the diffuse ionized gas remains fragmentary. We propose to take advantage of the Herschel spectroscopic capabilities and further characterize the diffuse neutral and ionized interstellar medium along lines of sight already selected in the PRISMAS and HEXOS programs. We target the fine structure lines of ionized nitrogen and carbon, NII 1.46 THz, CII 1.9 THz,and the ground state and first excited lines of neutral carbon at 492 GHz & 809 GHz. NII is tracing is diffuse ionized gas, while the neutral carbon lines reveal the diffuse neutral gas and probe the gas pressure and CII traces both the neutral and ionized matter . Towards strong far infrared sources such as our targets, we expect that the CII line profile will present a superposition of emission and absorption features, that can only be resolved by the high spectral resolution provided by HIFI. Even for NII and CI, the profiles may show superpositions of absorption and emission features, justifying our request for HIFI spectra, since the gas and electron densities in the foreground material are much lower than in the background sources. We propose to take advantage of the sensitivity offered by absorption spectroscopy to determine the ionized carbon abundance with an unprecedented accuracy. The proposed observations will therefore bring new measurements of the abundances of neutral and ionized carbon abundances, neutral gas pressure, and ionized gas filling factor in the inner Galaxy that will provide a complete picture of the respective volume and mass filling factor of the ISM phases in the inner Galaxy |
