The density structure of the L1157 molecular outflow

Abstract

We present a multiline CS survey towards the brightest bow-shock B1 in the prototypical chemically active protostellar outflow L1157. We made use of (sub-)mm data obtained in the framework of the Chemical HErschel Surveys of Star forming regions (CHESS) and Astrochemical Surveys at IRAM (ASAI) key science programs. We detected ${}^{12}$C${}^{32}$S, ${}^{12}$C${}^{34}$S, ${}^{13}$C${}^{32}$S, and ${}^{12}$C${}^{33}$S emissions, for a total of 18 transitions, with $E_{\mathrm{u}}$ up to $\sim$ 180 K. The unprecedented sensitivity of the survey allows us to carefully analyse the line profiles, revealing high-velocity emission, up to 20 km s${}^{-1}$ with respect to the systemic. The profiles can be well fitted by a combination of two exponential laws that are remarkably similar to what previously found using CO. These components have been related to the cavity walls produced by the $\sim$ 2000 yr B1 shock and the older ($\sim$ 4000 yr) B2 shock, respectively. The combination of low- and high-excitation CS emission was used to properly sample the different physical components expected in a shocked region. Our CS observations show that this molecule is highlighting the dense, $n_{{\mathrm{H}}_2}$ = 1--5 $\times$ 10${}^5$ cm${}^{-3}$, cavity walls produced by the episodic outflow in L1157. In addition, the highest excitation (E${}_u$ $\ge$ 130 K) CS lines provide us with the signature of denser (1--5 $\times$ 10${}^6$ cm${}^{-3}$) gas, associated with a molecular reformation zone of a dissociative J-type shock, which is expected to arise where the precessing jet impacting the molecular cavities. The CS fractional abundance increases up to $\sim$ 10${}^{-7}$ in all the kinematical components. This value is consistent with what previously found for prototypical protostars and it is in agreement with the prediction of the abundances obtained via the chemical code Astrochem.

11 pages, 10 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)

References in corpus (3)

• A Flattened Protostellar Envelope in Absorption around L1157
• The clumpy structure of the chemically active L1157 outflow
• Sulphur chemistry and molecular shocks: the case of NGC1333-IRAS2

Cited by in corpus (18)

• UCLCHEM: A Gas-Grain Chemical Code
• Seeds of Life in Space (SOLIS) III. Formamide in protostellar shocks: evidence for gas-phase formation
• Phosphorus-bearing molecules in solar-type star forming regions: First PO detection
• H2S in the L1157-B1 Bow shock
• Sulfur Chemistry in L1157-B1
• Multi-scale view of star formation in IRAS 21078+5211: From clump fragmentation to disk wind
• Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey
• Dense Cores, Filaments and Outflows in the S255IR Region of High Mass Star Formation
• Seeds of Life in Space SOLIS. IX. Chemical segregation of $\mathrm{S}{\mathrm{O}}_2$ and SO toward the low-mass protostellar shocked region of L1157
• The L1157-B1 astrochemical laboratory: testing the origin of DCN
• Molecules in the CepE-mm jet: evidence for shock-driven photochemistry?
• HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI
• A detailed temperature map of the archetypal protostellar shocks in L1157
• An ALCHEMI inspection of sulphur-bearing species towards the central molecular zone of NGC 253
• The GUAPOS project. VI: the chemical inventory of shocked gas
• Seed of Life in Space (SOLIS) XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow
• H${}_2$ mass-velocity relationship from 3D numerical simulations of jet-driven molecular outflows
• Sub-arcsecond Imaging of the Complex Organic Chemistry in Massive Star-forming Region G10.6-0.4