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 \propto \exp ( - |v / v _ { 0 } | ) with v _ { 0 } = 12.5 , 4.4 and 2.5 \hbox { \kern 2.0 ptkm \kern 2.0 pts$ { } ^ { -1 } $ } .
The first component dominates the CO emission at J \geq 13 , as well as the high-excitation lines of SiO and H _ { 2 } O .
The second component dominates for 3 \leq J _ { up } \leq 10 and the third one for J _ { up } \leq 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 _ { k } \simeq 210 \hbox { \kern 2.0 ptK } ) , the walls of the outflow cavity associated with B1 ( T _ { k } \simeq 64 \hbox { \kern 2.0 ptK } ) and the older cavity L1157-B2 ( T _ { k } \simeq 23 \hbox { \kern 2.0 ptK } ) , respectively .
Analysis of the CO line flux in the Large-Velocity Gradient approximation further shows that the emission arises from dense gas ( n ( \hbox { H$ { } _ { 2 } $ } ) \geq 10 ^ { 5 } -10 ^ { 6 } \hbox { \kern 2.0 ptcm$ { } ^ { -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 .