Spectroscopic diagnostics of the mid-infrared features of the dark globule DC 314.8–5.1 with the Spitzer Space Telescope

We present an analysis of the mid-infrared spectra, obtained from the Spitzer Space Telescope, of the dark globule DC 314.8–5.1, which is at the onset of low-mass star formation. The target has a serendipitous association with a B-type field star that illuminates a reflection nebula in the cloud. We focus on the polycyclic aromatic hydrocarbon (PAH) emission features prevalent throughout the mid-infrared range. The analysis of the spectra with the PAHFIT software as well as with the pypahdb package shows that (i) the intensities of PAH features decrease over distance from the ionizing star toward the cloud center, with some however showing a saturation at larger distances; (ii) the relative intensities of the 6.2 and 8.6 features with respect to the 11.2 μm feature remain high throughout the globule, suggesting a higher cation-to-neutral PAH ratio of the order of unity; the breakdown from pypahdb confirms a high ionized fraction within the cloud; (iii) the pypahdb results display a decrease in large-PAH fraction with increased distance from HD 130079, as well as a statistically significant correlation between the large-size fraction and the ionized fraction across the globule; (iv) the 7.7 PAH feature displays a peak nearer to 7.8 μm, suggesting a chemically processed PAH population with a small fraction of UV-processed PAHs; and (v) the $H_{2}$ S(0) line is detected at larger distances from the ionizing star. All in all, our results suggest divergent physical conditions within the quiescent cloud DC 314.8–5.1 as compared to molecular clouds with ongoing star formation.