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

TL;DR

This study uses Spitzer mid-infrared spectra to analyze PAH features in the dark globule DC 314.8-5.1, revealing variations in PAH ionization, size, and chemical processing related to the cloud's physical conditions and star formation activity.

Contribution

It provides detailed spectroscopic diagnostics of PAH properties and ionization states in a dark globule at the onset of star formation, highlighting environmental effects on PAH chemistry.

Findings

PAH feature intensities decrease with distance from the ionizing star.

High cation-to-neutral PAH ratio throughout the globule.

Detection of H2 S(0) line at larger distances from the star.

Abstract

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, which 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 pypahdb package, shows that (i) the intensities of PAH features decrease over distance from the ionizing star toward the cloud center, 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 micron feature remain high throughout the globule, suggesting a larger 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 microns, suggesting a chemically processed PAH population with a small fraction of UV-processed PAHs; (v) the H2 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 starformation.