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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 (Tkin ~ 20-70 K, nH2 ~ 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 (XHCO+ ~ 0.7-3 × 10-8, XN2H+ ~ 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 (XHOCO+ ~ 10-9, XSO+ ~ 8 × 10-10, XHCS+ ~ 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

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Submitted on : Thursday, March 24, 2022 - 3:55:22 PM
Last modification on : Friday, March 25, 2022 - 4:02:14 AM
Long-term archiving on: : Saturday, June 25, 2022 - 7:34:32 PM

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L. Podio, B. Lefloch, C. Ceccarelli, C. Codella, R. Bachiller. Molecular ions in the protostellar shock L1157-B1. Astronomy and Astrophysics - A&A, EDP Sciences, 2014, 565, ⟨10.1051/0004-6361/201322928⟩. ⟨insu-03618585⟩

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