Warm gas in protostellar outflows

Context. Observations of CO rotational transitions in the 0.3-0.4 millimeter range, now possible from exceptional sites on the ground, provide the opportunity of studying the warm component of molecular outflows in star-forming regions. Aims. This study aims to characterize the role of the warm gas in high-velocity and collimated outflows from Class 0 low-mass protostars. Methods. We used the CHAMP(+) heterodyne array on the APEX telescope to map the CO (6-5) and CO (7-6) emission in the well-known Class 0 outflows L1448-mm and HH211-mm. We complement these data with (CO)-C-13 (6-5) observations and also with previous low-J CO observations. Results. The CO (6-5) and (7-6) emission was detected to be tracing the outflow lobes. In L1448, extremely high-velocity (EHV) emission was detected in both transitions. In HH211, high-velocity CO (6-5) emission was detected to be tracing the regions close to the central object, but it was also found close to the bow-shock regions seen in the mid-IR. A large velocity gradient code applied to these and the complementary low-J CO data revealed the high-velocity components to be dense (>10(5) cm(-3)) and warm (T > 200 K) gas, in agreement with previous observations of shock tracers such as SiO. Conclusions. The high-velocity emission of these mid-J CO transitions are very good tracers of the inner highly excited part of outflows, which possibly is molecular material related to the underlying jet. In addition, these transitions are also strong at the bow-shock positions, which make them a good tool for probing these environments.