Numerical synhesis of warm CO emissions from the UV-heated outflow cavity walls in the Embedded Protostellar Objects

Abstract

An UV heated outflow cavity wall is one of the leading candidate mechanisms for the origin of the warm CO emission with near universal 300 K rotational temperature inferred from the CO emission detected towards embedded protostars by Herschel/PACS. In this thesis, we test the UV heated cavity walls can reproduce the FIR CO ladder observed by Herschel/PACS.

We have developed a non-local thermal equilibrium line adiative transfer code (RIG) and a thermo-chemical model for treating a photon dominated region (PDR) in general coordinates (spherical, cylindrical, and Cartesian coordinate). RIG uses an accelerated Monte-Carlo method and can treat a line overlap effect, which enables to treat complex molecules with a hyperfine structure. In addition, this can solve a problem for multi-species simultaneously. PDR code can solve chemistry and gas energetics self-consistently for given UV radiation fields with different spectral shapes. The combination of RIG and PDR code provides how UV sources affect the system and observed spectra. We introduce a new adequate coordinate system, (r, \delta), for an embedded protostar having outflow cavity walls, where r is the radius in spherical coordinate and \delta is the circular paraboloid instead of a circular conical surface of \theta . This reduce a number of grid cell by an order of 1-2, resulting in reducing the calculation time significantly.

Tests with a simple 1 D PDR model and a Large Velocity Gradient radiative transfer model show that FIR mid-J (14 =< J =< 24) CO lines are radiated 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.

We have applied our models to the Herschel FIR CO observations of 26 YSOs. We find that for the UV radiation field with the black body temperature of 15,000 K, the observed mid-J CO line fluxes can be reproduced from the dense UV heated cavity walls (n >= 10^6 cm^{-3}) with -4.5 =< log G_0/n =< -2.5, where gas temperatures are higher than 300 K and CO abundances are >= 10^{-5}. In addition, the contribution of the UV heated outflow cavity wall to the mid-J CO emission in Class I obsjects is larger than that in Class 0 obsjects.