The CHESS survey of the L1157-B1 shock region: CO spectral signatures of
  jet-driven bowshocks
  The unprecedented sensitivity of Herschel coupled with the high resolution of
the HIFI spectrometer permits studies of the intensity-velocity relationship
I(v) in molecular outflows, over a higher excitation range than possible up to
now. In the course of the CHESS Key Program, we have observed toward the bright
bowshock region L1157-B1 the CO rotational transitions between J=5-4 and
J=16-15 with HIFI, and the J=1-0, 2-1 and 3-2 with the IRAM-30m and the CSO
telescopes. We find that all the line profiles I_{CO}(v) are well fit by a
linear combination of three exponential laws exp(-|v/v_0|) with v_0= 12.5, 4.4
and 2.5km/s. The first component dominates the CO emission at J_up > 13, as
well as the high-excitation lines of SiO and H2O. The second component
dominates for 3 < J_up < 10 and the third one for J_up < 2. We show that these
exponentials are the signature of quasi-isothermal shocked gas components: the
impact of the jet against the L1157-B1 bowshock (T= 210K), the walls of the
outflow cavity associated with B1 (T= 64K) and the older cavity L1157-B2 (T=
23K), respectively. Analysis of the CO line flux in the Large-Velocity Gradient
approximation further shows that the emission arises from dense gas (n(H2) >
1e5-1e6 cm-3) close to LTE up to J=20. We find that the CO J=2-1
intensity-velocity relation observed in various other molecular outflows is
satisfactorily fit by similar exponential laws, which may hold an important
clue to their entrainment process.
