2015ApJS..217...30L

A fraction of the mid-J (J = 14-13 to J = 24-23) CO emission detected by the Herschel/Photodetector Array Camera and Spectrometer observations of embedded young stellar objects (YSOs) has been attributed to the UV-heated outflow cavity walls. We have applied our newly developed self-consistent models of photon-dominated-region (PDR) and non-local-thermal-equilibrium-line Radiative transfer In general Grid to the Herschel far-infrared observations of 27 low-mass YSOs and one intermediate-mass YSO, NGC 7129-FIRS2. When the contribution of the hot component (traced by transitions of J > 24) is removed, the rotational temperature of the warm component is nearly constant with ∼250 K. This can be reproduced by the outflow cavity wall (n ≥ 10⁶/cm3, log G₀/n ≥ -4.5, log G₀ ≥ 3, T_gas≥ 300 K, and X(CO) ≥ 10^–5) heated by a UV radiation field with a blackbody temperature of 15,000 or 10,000 K. However, a shock model combined with an internal PDR will be required to determine the quantitative contribution of a PDR relative to a shock to the mid-J CO emission.