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We present an analysis of the mid-infrared spectra, obtained from the SpitzerSpace Telescope, of the dark globule, DC 314.8--5.1, which is at the onset oflow-mass star formation. The target has a serendipitous association with aB-type field star, which illuminates a reflection nebula in the cloud. We focuson the polycyclic aromatic hydrocarbon (PAH) emission features prevalentthroughout the mid-infrared range. The analysis of the spectra with the PAHFITsoftware as well as pypahdb package, shows that (i) the intensities of PAHfeatures decrease over distance from the ionizing star toward the cloud center,some however showing a saturation at larger distances; (ii) the relativeintensities of the 6.2 and 8.6 features with respect to the 11.2 micron featureremain high throughout the globule, suggesting a larger cation-to-neutral PAHratio of the order of unity; the breakdown from pypahdb confirms a high ionizedfraction within the cloud; (iii) the pypahdb results display a decrease inlarge PAH fraction with increased distance from HD 130079, as well as astatistically significant correlation between the large size fraction and theionized fraction across the globule; (iv) the 7.7 PAH feature displays a peaknearer to 7.8 microns, suggesting a chemically processed PAH population with asmall fraction of UV-processed PAHs; (v) the H2 S(0) line is detected at largerdistances from the ionizing star. All in all, our results suggest divergentphysical conditions within the quiescent cloud DC 314.8--5.1 as compared tomolecular clouds with ongoing starformation.

Spectroscopic Diagnostics of the Mid-Infrared Features of the Dark Globule, DC 314.8-5.1, with the Spitzer Space Telescope