| 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 will be the leading and only instrument and the observations will 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 is likely a specific tracer of turbulent dissipation, because its formation route is highly endoenergic. We propose absorption spectroscopy observations of mainly the CH+ J=1-0 line, against 10 background dust continuum sources, bright enough to allow us to sample a broad variety of galactic environments. The lines-of-sight will probe the outskirts of star-forming regions, including one InfraRed Dark Cloud, where turbulent dissipation is most intense, and diffuse gas at high galactic latitude where turbulence is milder. The primarily goal of this project is the comparison of the CH+ abundances with model predictions of turbulent dissipation regions, in which 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. As HF, CH+ is a potential sensitive tracer of diffuse 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. |
