2014A&A...565A..64P

We perform a complete census of molecular ions with an abundance greater than ∼10^–10 in the protostellar shock L1157-B1. This allows us to study the ionisation structure and chemistry of the shock. An unbiased high-sensitivity survey of L1157-B1 performed with the IRAM-30m and Herschel/HIFI as part of the CHESS and ASAI large programmes allows searching for molecular ions emission. Then, by means of a radiative transfer code in the large velocity gradient approximation, the gas physical conditions and fractional abundances of molecular ions are derived. The latter are compared with estimates of steady-state abundances in the cloud and their evolution in the shock calculated with the chemical model Astrochem. We detect emission from HCO⁺, H¹³CO⁺, N₂H⁺, HCS⁺, and for the first time in a shock, from HOCO⁺ and SO⁺. The bulk of the emission peaks at blue-shifted velocity, ∼0.5-3km/s with respect to systemic, has a width of ∼3-7km/s and is associated with the outflow cavities (T_kin∼20-70 K, n_H2∼10⁵cm^–3). A high-velocity component up to -40km/s, associated with the primary jet, is detected in the HCO⁺ 1-0 line. Observed HCO⁺ and N₂H⁺ abundances (X_HCO+∼0.7-3x10^–8, X_N2_H+∼0.4-8x10^–9) agree with steady-state abundances in the cloud and with their evolution in the compressed and heated gas in the shock for cosmic rays ionisation rate ζ=3x10^–16s^–1. HOCO⁺, SO⁺, and HCS⁺ observed abundances (X_HOCO+∼10^–9, X_SO+∼8x10^–10, X_HCS+∼3-7x10^–10), instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the other hand, they are strongly enhanced on timescales shorter than the shock age (∼2000 years) if CO₂, S or H₂S, and OCS are sputtered off the dust grains in the shock. The performed analysis indicates that HCO⁺ and N₂H⁺ are a fossil record of pre-shock gas in the outflow cavity, whilst HOCO⁺, SO⁺, and HCS⁺ are effective shock tracers that can be used to infer the amount of CO₂ and sulphur-bearing species released from dust mantles in the shock. The observed HCS⁺ (and CS) abundance indicates that OCS should be one of the main sulphur carrier on grain mantles. However, the OCS abundance required to fit the observations is 1-2 orders of magnitude larger than observed. Laboratory experiments are required to measure the reactions rates involving these species and to fully understand the chemistry of sulphur-bearing species.