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

We present a detailed 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 cloud has a serendipitous association with a B-type field star, which illuminates a reflection nebula in the cloud, allowing us to investigate infrared characteristics not otherwise discernible in such systems until a later evolutionary stage. We focus specifically on the polycyclic aromatic hydrocarbon (PAH) emission features prevalent throughout the mid-infrared range. We find that the intensity profiles do not obey any single unique scaling, for example a monotonic decrease related to the decreasing starlight toward the cloud's central regions. We note that a diversity in trends over distance is also present in the intensity profiles of the molecular (H$_2$) and atomic (Ar, Ne, and S) emission lines, which are however much less prominent in the spectrum when compared with the PAH features. All in all, our analysis reveals that (i) there is a stratification in dust sizes within the reflection nebula, with larger grains dominating the PAH emission at the outskirts of the system, and (ii) the ionization level within the reflection nebula is fairly constant, and as such independent on the amount of the ionizing UV continuum from the neighbouring star, (iii) the intensity ratios of the prominent PAH features, do not follow correlations established for the reflection nebulae with active star formation.