2021ApJ...908..239C

This study assesses the contribution of neutral polycyclic aromatic hydrocarbons (PAHs) and hydrogenated PAHs (H_n-PAHs) in H₂O-ices to the 3 µm and 5-8 µm regions of the infrared absorption spectrum of the Young Stellar Object, Mon R2 IRS 3, and the Taurus dense cloud in the direction of Elias 16. We compare the astronomical spectra to the previously published laboratory data of matrix-isolated PAHs and H_n-PAHs in H₂O-ices. For the molecules in this study, the band positions, FWHMs, and integrated band strengths have been measured. For the PAHs considered here, the ratio of the experimental A(C-H) stretch (in H₂O ices) to the theoretical A(C-H) stretch (in the gas phase) is consistently about 0.10, meaning that the trove of theoretical data can be reliably scaled to compare to the astronomical spectra. We find that the fractional percentage contribution to the 5-8 µm region for Mon R2 IRS 3 ranges between 2.7 and 3.9 for neutral PAHs and 0.25-1.2 for H_n-PAHs. The best match to the observed 3.25 µm profile in the Mon R2 IRS 3 spectrum is accomplished with the laboratory-measured coronene:H₂O spectrum. Using this spectrum we estimate that neutral PAHs contribute up to 12.0% of the cosmic carbon budget for Mon R2 IRS 3 and <14.5% for Elias 16. Neutral hydrogenated PAHs contribute mainly to the 3.47 µm absorption and very little to the 5 to 8 µm region. For neutral H_n-PAHs, we estimate that they contribute up to 0.6% of the cosmic carbon budget for Mon R2 IRS 3 and <1% for Elias 16.