The evolution of hydrocarbon dust grains in the interstellar medium and its influence on the infrared spectra of dust

TL;DR

This study models the evolution of interstellar hydrocarbon dust grains under various destructive processes, analyzing how these changes affect infrared emission spectra and the aromatic features observed in the interstellar medium.

Contribution

It provides a detailed evolutionary model of dust grain size and aromatization, linking physical processes to observable infrared spectral features and their dependence on grain properties.

Findings

Small grains are destroyed in shocks exceeding 50 km/s.

IR spectra are significantly affected by dust evolution and grain properties.

High C--C bond energy ($E_0$) better explains observed aromatic bands.

Abstract

We present evolutionary calculations for the size and aromatization degree distributions of interstellar dust grains, driven by their destruction by radiation, collisions with gas particles, and shattering due to grain-grain collisions. Based on these calculations we model dust emission spectra. The initial grain size distribution play the important role in the evolution of an ensemble of dust particles. Radiation in the considered intensity range mostly aromatizes grains. The smallest grains are mainly destructed via sputtering by collisions with gas particles. There are no grains smaller than 20~\AA\ in the medium at relative gas-dust velocities more than 50~km/s, which is typical for shocks in supernova remnants. The IR emission spectrum changes significantly due to the dust evolution depending on the adopted grain properties, in particular, on the energy of the C--C bonds ($E_0$). Aromatic bands in the near-IR (2--15~$\mu$m) are absent, if $E_0$ is low, even when the medium properties correspond to the average interstellar medium in our Galaxy. As in reality these bands are observed, high $E_0$ values are more preferable. We consider dependence of the emission intensity ratios for various photometric bands on the medium properties. The aromatization degree of small grains shows up most strongly in the $I_{3.4}/I_{11.3}$ intensity ratio while the fraction of aromatic grains in the total dust mass influences the $I_{3.3}/I_{70+160}$ ratio.