Molecular ions in the protostellar shock L1157-B1

Abstract :
Aims: 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.
Methods: An unbiased high-sensitivity survey of L1157-B1 performed with the IRAM-30 m 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.
Results: 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, ~0.5-3 km s ^-1 with respect to systemic, has a width of ~3-7 km s^-1 and is associated with the outflow cavities (T_kin ~ 20-70 K, n_H2 ~ 10⁵ cm^-3). A high-velocity component up to -40 km s^-1, associated with the primary jet, is detected in the HCO⁺ 1-0 line. Observed HCO⁺ and N₂H⁺ abundances (X_HCO⁺ ~ 0.7-3 × 10^-8, X_N2H⁺ ~ 0.4-8 × 10^-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 = 3 × 10^-16 s^-1. HOCO⁺, SO⁺, and HCS⁺ observed abundances (X_HOCO⁺ ~ 10^-9, X_SO⁺ ~ 8 × 10^-10, X_HCS⁺ ~ 3-7 × 10^-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.
Conclusions: 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.

The molecular ions line spectra shown in Fig. 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A64