The CHESS Survey of the L1157-B1 Shock Region: CO Spectral Signatures of Jet-driven Bow Shocks

Abstract

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. Over the course of the CHESS Key Program, we have observed toward the bright bow shock 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 30 m and the Caltech Submillimeter Observatory telescopes. We find that all the line profiles I _CO(v) are well fit by a linear combination of three exponential laws vpropexp (- |v/v ₀|) with v ₀ = 12.5, 4.4, and 2.5 km s^-1. The first component dominates the CO emission at J >= 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 bow shock (T _k ~= 210 K), the walls of the outflow cavity associated with B1 (T _k ~= 64 K), and the older cavity L1157-B2 (T _k ~= 23 K), respectively. Analysis of the CO line flux in the large-velocity gradient approximation further shows that the emission arises from dense gas (n(H₂) >= 10⁵-10⁶ 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.