A PDR model for the FIR mid-$J$ CO ladder with universal rotational temperature in star forming region

Abstract

A photon dominated region (PDR) is one of the leading candidate mechanisms for the origin of the warm CO gas with near universal $\sim$300~K rotational temperature inferred from the CO emission detected towards embedded protostars by Herschel/PACS. We have developed a PDR model in general coordinates, where we can use the most adequate coordinate system for an embedded protostar having outflow cavity walls, to solve chemistry and gas energetics self-consistently for given UV radiation fields with different spectral shapes. Simple 1D tests and applications show that FIR mid-$J$ ($14\le J\le 24$) CO lines are emitted from near the surface of a dense region exposed to high UV fluxes. We apply our model to HH46 and find the UV-heated outflow cavity wall can reproduce the mid-$J$, CO transitions observed by Herschel/PACS. A model with UV radiation corresponding to a blackbody of 10,000~K results in the rotational temperature lower than 300~K, while models with the Draine interstellar radiation field and the 15,000 K blackbody radiation field predict the rotational temperature similar to the observed one.

63 pages, 22 figures, Accepted for publication in ApJS

References in corpus (6)

• [CII] 158$\mu$m Emission and Metallicity in PDRs
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• Chemical Modelling of Young Stellar Objects, I. Method and Benchmarks
• Multidimensional chemical modelling, II. Irradiated outflow walls
• Full SED fitting with the KOSMA-τ PDR code - I. Dust modelling
• Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)

Cited by in corpus (7)

• Photodissociation and X-Ray Dominated Regions
• Origin of warm and hot gas emission from low-mass protostars: Herschel-HIFI observations of CO J=16-15. I. Line profiles, physical conditions, and H2O abundance
• Herschel Key Program, "Dust, Ice, and Gas In Time" (DIGIT): the origin of molecular and atomic emission in low-mass protostars in Taurus
• Modeling Nitrogen Fractionation in the Protoplanetary Disk around TW Hya: Model Constraints on Grain Population and Carbon-to-Oxygen Elemental Abundance Ratio
• The warm CO gas along the UV-heated outflow cavity walls: a possible interpretation for the Herschel/PACS CO spectra of embedded YSOs
• Distribution of Water Vapor in Molecular Clouds. II
• Early Planet Formation in Embedded Disks (eDisk). XX. Constraining the Chemical Tracers of Young Protostellar Sources