The density structure of the L1157 molecular outflow
  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 $E_{\rm u}$ 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, $n_{\rm H_2}$ = 1--5 $\times$ 10$^{5}$
cm$^{-3}$, cavity walls produced by the episodic outflow in L1157. In addition,
the highest excitation (E$_u$ $\geq$ 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.
