The Warm CO Gas along the UV-heated Outflow Cavity Walls: A Possible Interpretation for the Herschel/PACS CO Spectra of Embedded YSOs

Abstract

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≥slant {{10}⁶} c{{m}^-3}, log {{G}₀}/n≥slant -4.5, log {{G}₀}≥slant 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.