2015A&A...581A...4L

Protostellar outflows are a crucial ingredient of the star-formation process. However, the physical conditions in the warm outflowing gas are still poorly known. We present a multi-transition, high spectral resolution CO study of the outflow of the intermediate-mass Class 0 protostar CepE-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.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 (n(H₂)=(0.5-1)x10⁶cm^–3), lower column density (N(CO)=1.5x10¹⁶cm^–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)=7x10¹⁷cm^–2 at T_kin=55-85K and density in the range (1-8)x10⁵cm^–3; the emission of the high-J lines is dominated by a hot, denser gas layer with T_kin=500-1500K, n(H₂)=(1-5)x10⁶cm^–3, and N(CO)=6x10¹⁶cm^–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 T_kin≃ 400-500K, density n(H₂)≃(1-2)x10⁶cm^–3 and column density N(CO)=10¹⁷cm^–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 220-740yr propagating at 20-30km/s in a medium of density n(H₂)=(0.5-1)x10⁵cm^–3, consistent with that of the bulk material.The CepE 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.