| Description |
Seventy years after its discovery in the diffuse interstellar medium, the origin of the CH+ cation is still elusive. Herschel/HIFI offers a unique opportunity to disclose the underlying gas dynamics at the origin of CH+ in the diffuse medium by allowing high sensitivity and high spectral resolution observations of the CH+(J=1-0) transition, unreachable from the ground: it is the only instrument, for the decades to come, able to bring a completely new look at this resilient puzzle.The abundant CH+ ion is not only a sensitive tracer of the most tenuous phases of the interstellar medium but it appears as a specific tracer of turbulent dissipation, because its formation route is highly endoenergic. We propose absorption spectroscopy observations of the CH+ J=1-0 line, against 7 background dust continuum sources, bright enough to allow the sample Galactic environments with highly different turbulent dissipation rates. We take advantage of the high opacity of the CH+(1-0) transition to search for CH+ in more diffuse components than previously observed: in the high-latitude diffuse medium, in gas out of the Galactic disk and in the outer Galaxy. The unknown H2 molecular fraction of these poorly explored parts of the diffuse Galactic component will be inferred from the CH absorption lines.The primarily goal of this project is the comparison of the CH+ abundances with model predictions, in which turbulent dissipation proceeds either in low-velocity shocks or intense velocity-shears. Another goal is testing the possibility that CH+ forms at the turbulent interface between the two thermally stable phases of the interstellar medium. Last, as HF, CH+ is a potential sensitive tracer of diffuse molecular matter in the early universe. Understanding its origin and the dissipative processes that it traces will shed a new light on galaxy formation and evolution. |
