APEX-CHAMP(+) high-J CO observations of low-mass young stellar objects II. Distribution and origin of warm molecular gas

Context. The origin and heating mechanisms of warm (50 < T < 200 K) molecular gas in low-mass young stellar objects (YSOs) are strongly debated. Both passive heating of the inner collapsing envelope by the protostellar luminosity as well as active heating by shocks and by UV associated with the outflows or accretion have been proposed. Most data so far have focussed on the colder gas component. Aims. We aim to characterize the warm gas within protosteller objects, and disentangle contributions from the (inner) envelope, bipolar outflows and the quiescent cloud. Methods. High-J CO maps ((CO)-C-12 J = 6-5 and 7-6) of the immediate surroundings (up to 10 000 AU) of eight low-mass YSOs are obtained with the CHAMP(+) 650/850 GHz array receiver mounted on the APEX telescope. In addition, isotopologue observations of the (CO)-C-13 J = 6-5 transition and [C I] P-3(2)-P-3(1) line were taken. Results. Strong quiescent narrow-line (CO)-C-12 6-5 and 7-6 emission is seen toward all protostars. In the case of HH 46 and Ced 110 IRS 4, the on-source emission originates in material heated by UV photons scattered in the outflow cavity and not just by passive heating in the inner envelope. Warm quiescent gas is also present along the outflows, heated by UV photons from shocks. This is clearly evident in BHR 71 for which quiescent emission becomes stronger at more distant outflow positions. Shock-heated warm gas is only detected for Class 0 flows and the more massive Class I sources such as HH 46. Outflow temperatures, estimated from the CO 6-5 and 3-2 line wings, are similar to 100 K, close to model predictions, with the exception of the L 1551 IRS 5 and IRAS 12496-7650, for which temperatures <50 K are found. Conclusions. APEX-CHAMP(+) is uniquely suited to directly probe the protostar's feedback on its accreting envelope gas in terms of heating, photodissociation, and outflow dispersal by mapping similar to 1' x 1' regions in high-J CO and [C I] lines. Photon-heating of the surrounding gas may prevent further collapse and limit stellar growth.