A Model for Protostellar Cluster Luminosities and the Impact on the CO-H2 Conversion Factor
Abstract
We construct a semi-analytic model to study the effect of far-ultraviolet (FUV) radiation on gas chemistry from embedded protostars. We use the Protostellar Luminosity Function (PLF) formalism of Offner & McKee (2011) to calculate the total, FUV, and ionizing cluster luminosity for various protostellar accretion histories and cluster sizes. We compare the model predictions with surveys of Gould Belt star-forming regions and find the Tapered Turbulent Core model matches best the mean luminosities and the spread in the data. We combine the cluster model with the photo-dissociation region astrochemistry code, 3d-pdr, to compute the impact of the FUV luminosity from embedded protostars on the CO to H2 conversion factor, X CO, as a function of cluster size, gas mass and star formation efficiency. We find that X CO has a weak dependence on the FUV radiation from embedded sources for large clusters due to high cloud optical depths. In smaller and more efficient clusters the embedded FUV increases X CO to levels consistent with the average Milky Way values. The internal physical and chemical structure of the cloud are significantly altered, and X CO depends strongly on the protostellar cluster mass for small efficient clouds.
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