The structure of the Cepheus E protostellar outflow: The jet, the bowshock, and the cavity

Context. Protostellar outflows are a crucial ingredient of the star-formation process. However, the physical conditions in the warm outflowing gas are still poorly known. Aims. We present a multi-transition, high spectral resolution CO study of the outflow of the intermediate-mass Class 0 protostar Cep E-mm. The goal is to determine the structure of the outflow and to constrain the physical conditions of the various components in order to understand the origin of the mass-loss phenomenon. Methods. We have observed the J = 12–11, J = 13–12, and J = 16–15 CO lines at high spectral resolution with SOFIA/GREAT and the J = 5–4, J = 9–8, and J = 14–13 CO lines with HIFI/Herschel towards the position of the terminal bowshock HH377 in the southern outflow lobe. These observations were complemented with maps of CO transitions obtained with the IRAM 30 m telescope (J = 1–0, 2–1), the Plateau de Bure interferometer (J = 2–1), and the James Clerk Maxwell Telescope (J = 3–2, 4–3). Results. We identify three main components in the protostellar outflow: the jet, the cavity, and the bowshock, with a typical size of 1.7′′ × 21′′, 4.5′′, and 22′′ × 10′′, respectively. In the jet, the emission from the low-J CO lines is dominated by a gas layer at Tkin = 80–100 K, column density N(CO) = 9 × 1016 cm−2, and density n(H2) = (0.5−1) × 105 cm−3; the emission of the high-J CO lines arises from a warmer (Tkin = 400–750 K), denser (n(H2) = (0.5−1)× 106 cm−3), lower column density (N(CO) = 1.5× 1016 cm−2) gas component. Similarly, in the outflow cavity, two components are detected: the emission of the low-J lines is dominated by a gas layer of column density N(CO) = 7 × 1017 cm−2 at Tkin = 55–85 K and density in the range (1−8) × 105 cm−3; the emission of the high-J lines is dominated by a hot, denser gas layer with Tkin = 500–1500 K, n(H2) = (1−5) × 106 cm−3, and N(CO) = 6 × 1016 cm−2. A temperature gradient as a function of the velocity is found in the high-excitation gas component. In the terminal bowshock HH377, we detect gas of moderate excitation, with a temperature in the range Tkin ≈ 400–500 K, density n(H2) (1−2) × 106 cm−3 and column density N(CO) = 1017 cm−2. The amounts of momentum carried away in the jet and in the entrained ambient medium are similar. Comparison with time-dependent shock models shows that the hot gas emission in the jet is well accounted for by a magnetized shock with an age of 220–740 yr propagating at 20–30 km s−1 in a medium of density n(H2) = (0.5−1) × 105 cm−3, consistent with that of the bulk material. Conclusions. The Cep E protostellar outflow appears to be a convincing case of jet bowshock driven outflow. Our observations trace the recent impact of the protostellar jet into the ambient cloud, produing a non-stationary magnetized shock, which drives the formation of an outflow cavity.