AKARI near-infrared spectroscopy of circumstellar hydrocarbon dust associated with young stellar objects

Abstract We investigate the processing of the circumstellar polycyclic aromatic hydrocarbons (PAHs) associated with young stellar objects (YSOs) through their stellar activities. We analyze the AKARI near-infrared (2.6–4.4$\, \mu {\rm m}$ ) long-slit spectral data of 10 YSOs, focusing on the aromatic feature at a wavelength of 3.3$\, \mu {\rm m}$ and the aliphatic features at 3.4–3.6$\, \mu {\rm m}$. The spectral fitting results show large variations from target to target in the flux ratio of the aliphatic to aromatic features, the shape of the aliphatic feature profile, and the central wavelength of the aromatic feature. In particular, ${\rm HD}\,\,97048$ and Elias 1 show strong aliphatic features at wavelengths longer than 3.5$\, \mu {\rm m}$, which is consistent with the hydrogenated nanodiamond features reported in previous studies. We find that the properties of the hydrocarbon dust vary under the influence of the YSO activity, where the degree of dust processing is likely to be related to the age of the YSO and the amount of the circumstellar dust. In addition, correlation is observed between the variations of the H$_2$O and CO$_2$ ice features and the aliphatic feature profiles, suggesting connection between the formation of CO$_2$ ice and the processing of aliphatic hydrocarbons. Finally, we find a significant correlation between the variations in the central wavelength of the aromatic feature and the shape of the aliphatic feature profile. These findings may be consistent with a tentative scenario in which circumstellar PAHs are converted to nanodiamonds through irradiation by high-energy particles produced by YSO activity.