- The paper introduces a novel dust model based on new optical data to simulate the evolution of amorphous hydrocarbons under UV irradiation.
- It demonstrates how UV photo-processing and mantle accretion quantitatively shape IR-mm emission profiles, the 217 nm UV bump, and extinction curves.
- The findings underscore the importance of integrating laboratory-measured optical properties to resolve discrepancies in interstellar dust observations.
The Evolution of Amorphous Hydrocarbons in the ISM: Dust Modelling from a New Vantage Point
The paper presents a comprehensive paper on the evolution of amorphous hydrocarbons (a-C(:H)) within the interstellar medium (ISM), offering a nuanced view of interstellar dust modelling grounded in novel optical property data. The principal focus is on understanding the compositional evolution of a-C(:H) materials under ultraviolet (UV) photon bombardment, ranging from aliphatic-rich (a-C:H) to aromatic-rich (a-C) phases, and how this process influences the observable dust extinction and emission features across a broad wavelength spectrum.
The authors introduce a new dust model comprising a power-law distribution of small a-C grains and log-normal distributions for larger amorphous forsterite-type silicate grains (a-SilFe) and a-C(:H) grains. This model is firmly anchored in laboratory data, providing a robust explanation for various observed characteristics including the infrared (IR) to far-ultraviolet (FUV) extinction profiles, the 217 nm UV bump, the IR-mm dust emission, and the IR absorption and emission bands. The model achieves its strength through simplicity and an inherent adaptability to trace dust evolution throughout different ISM environments.
Key findings emphasize the dual processes of mantle accretion in molecular clouds and UV photo-processing in UV-dominated regions as pivotal in driving dust evolution. The model handles the molecular cloud's role in dust growth through accretion, eventually leading to changes in grain size and structure, while UV photo-processing in photon-dominated regions promotes transformation from aliphatic to aromatic structures, potentially coupled with photo-fragmentation. These processes affect the dust's optical properties, which in turn influence the observed interstellar dust extinction and emission features.
Quantitatively, the model explains the variation in the infrared (IR) to millimeter (mm) emission profiles, fitting observational data such as the extinction curve, albedo, and spectral energy distribution (SED) across a wide wavelength range (EUV-cm). It is also significant that the model accounts for the observed correlation and non-correlation between the FUV rise, UV bump, and the visible-NIR extinction, aligning with constraints derived from both observational data and laboratory experiments.
The implications extend to a deeper understanding of dust processing mechanisms in the ISM, especially regarding the lifecycle of a-C(:H) materials, emphasizing the importance of incorporating detailed laboratory-derived optical properties in dust modeling frameworks. This model also prompts further investigation into the coupling of interstellar silicates with carbonaceous materials in ISM conditions and might play a crucial role in reconciling discrepancies observed in current dust models, especially around the role of carbonaceous particles.
Future developments in this area could involve the integration of more detailed laboratory data on interstellar dust analogs, with particular focus on amorphous silicates, which remain a significant component yet are currently under-characterized. As observational capabilities advance, especially with space-based observatories providing higher resolution data, refining the fit between model predictions and real-world observations will further elucidate the complex life cycles of interstellar dust. The outcomes will significantly impact our understanding of cosmic dust processes and the role they play in star and planet formation processes.