HCl+ in the Interstellar Medium

The identification of the key molecules in the interstellar chlorine chemistry has required nearly three decades.HerschelHIFI observations have revealed H2Cl+ in absorption in various Galactic lines of sight, with abundances at least ten times higher than predicted.Recently, the proponents have tentatively identified strong absorption signatures, observed with HIFI towards two sources, with the groundstate rotational transition 2P_3/2, J5/23/2 of H35Cl+, an ion that has long been predicted to be a key intermediate, together with H2Cl+, in the chlorine chemistry of diffuse clouds.The detection, corroborated by a good agreement between observations and models of expected opacity profiles, shows, like H2Cl+, abundances in excess with respect to the expectations.All these findings stress the need for a revision of the chlorine chemistry and of the physical parameters at play in the environments where these molecules are found. The detection of HCl+ in other sources, especially if in combination with available observations of H2Cl+, would constitute a formidable constraint for the models. The planned observations would then have an impact, not only on the chemistry, but also on our knowledge of the Interstellar Radiation Field, of the dust opacity profile and on atomic physics parameters.We propose to observe this HCl+ transition with HIFI towards strong continuum sources for which several molecular tracers are already available, allowing us to clarify the environment where this ion is found.The need for Herschel is extremely compelling, since the proposed transition, the only one reasonably detectable in the ISM for this species, is in a frequency range where a strong atmospheric O3 line would complicate any attempt of observation even for the Stratospheric Observatory SOFIA. This will thus be the last opportunity, perhaps in one decade or more, to observe this molecule that plays a fundamental role in the chlorine c tru [truncated!, Please see actual data for full text]