Molecular ions in the protostellar shock L1157-B1^⋆

¹ Institut de Planétologie et d’Astrophysique de Grenoble, 414 rue de la Piscine, 38400 St-Martin d’Hères France
e-mail: lpodio@arcetri.astro.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain

Received: 28 October 2013
Accepted: 8 February 2014

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.