Sun-bathing around low-mass protostars: APEX-CHAMP+ observations of
  high-J CO
  We present the first maps of high-excitation CO J=6-5 and 7-6 and
isotopologue lines over 2'-5' regions at 10" resolution toward low-mass
protostars to probe the origin of the warm gas in their surroundings. The data
were obtained using the CHAMP+ 650/850 GHz heterodyne array receiver on APEX.
Surprisingly strong quiescent extended narrow-line high-J 12CO 6-5 and 7-6
emission is seen toward all protostars, suggesting that heating by UV photons
along the outflow cavity dominates the emission. At the source position itself,
passive heating of the collapsing inner envelope by the luminosity of the
source also contributes. The UV photons are generally not energetic enough to
dissociate CO since the [C I] 2-1 emission, also probed by our data, is weak
except at the bow shock at the tip of the outflow. The extended UV radiation is
produced by the star-disk boundary layer as well as the jet- and bow-shocks,
and will also affect the chemistry of species such as H2O and HCN around the
outflow axis. Shock-heated warm gas characterized by broad CO line profiles is
seen only toward the more massive Class 0 outflows. Outflow temperatures,
estimated from the CO 6-5/3-2 line wing ratios, are ~100 K. These data
illustrate the importance of getting spatial information to characterize the
physical processes in YSO surroundings, important to interpret future Herschel
and ALMA data.
