| Description |
Seventy years after its discovery in the diffuse interstellar medium,the origin of the CH+ cation is still elusive. Herschel/HIFI offers aunique opportunity to disclose the underlying gas dynamics at theorigin of CH+ in the diffuse medium by allowing high sensitivity andhigh spectral resolution observations of the CH+ (J=1-0) transition,unreachable from the ground: it will be the leading and onlyinstrument and the observations will bring a completely new look atthis resilient puzzle.The abundant CH+ ion is not only a sensitive tracer of the mosttenuous phases of the interstellar medium but it is likely a specifictracer of turbulent dissipation, because its formation route is highlyendoenergic. We propose absorption spectroscopy observations of mainlythe CH+ J=1-0 line, against 10 background dust continuum sources,bright enough to allow us to sample a broad variety of galacticenvironments. The lines-of-sight will probe the outskirts ofstar-forming regions, including one InfraRed Dark Cloud, whereturbulent dissipation is most intense, and diffuse gas at highgalactic latitude where turbulence is milder. The primarily goal ofthis project is the comparison of the CH+ abundances with modelpredictions of turbulent dissipation regions, in which dissipationproceeds either in low-velocity shocks or intense velocity-shears.Another goal is testing the possibility that CH+ forms at theturbulent interface between the two thermally stable phases of theinterstellar medium.As HF, CH+ is a potential sensitive tracer of diffuse matter in theearly universe. Understanding its origin and the dissipative processesthat it traces will shed a new light on galaxy formation andevolution. |
