2006A&A...454L..75V

To obtain insight into the physical structure of the warm gas in the inner envelope of protostars and the interaction with the outflow. Sub-millimeter observations of ¹²CO, ¹³CO and/or C¹⁸O in J=3-2, J=4-3 and J=7-6 were obtained with the APEX Telescope towards IRAS 12496-7650, an intermediate mass young stellar object. The data are compared to ISO-LWS observations of CO J=14-13 up to J=19-18 lines to test the different proposed origins of the CO lines. The outflow is prominently detected in the 3-2 and 4-3 lines, but not seen at similar velocities in the 7-6 line, constraining the temperature in the high-velocity (>5km/s from line center) gas to less than 50K, much lower than inferred from the analysis of the ISO-LWS data. In addition, no isothermal gas model can reproduce the emission in both the 7-6 and the higher-J ISO-LWS lines. The 7-6 line probably originates in the inner (<250AU) region of the envelope at ∼150K. Detailed radiative transfer calculations suggest that the ISO-LWS lines are excited by a different mechanism, possibly related to the larger-scale outflow. All possible mechanisms on scales smaller than 8'' are excluded. High-resolution continuum as well as high-J ¹²CO and isotopic line mapping are needed to better constrain the structure of the warm gas in the inner envelope and the interaction with the outflow.