Evolution of grain size distribution with enhanced abundance of small carbonaceous grains in galactic environments

TL;DR

This paper presents an improved dust evolution model that better explains the Milky Way's extinction curve and dust emission by focusing on small carbonaceous grains and their unique processing in galactic environments.

Contribution

The study introduces a novel model where small carbonaceous grains are not involved in interstellar processing, enhancing the fit to observed galactic dust properties.

Findings

Better match to Milky Way extinction curve

Improved dust emission spectral energy distribution fit

Successful explanation of small carbonaceous grain behavior

Abstract

We propose an updated dust evolution model that focuses on the grain size distribution in a galaxy. We treat the galaxy as a one-zone object and include five main processes (stellar dust production, dust destruction in supernova shocks, grain growth by accretion and coagulation, and grain disruption by shattering). In this paper, we improve the predictions related to small carbonaceous grains, which are responsible for the 2175 \AA\ bump in the extinction curve and the polycyclic aromatic hydrocarbon (PAH) emission features in the dust emission spectral energy distribution (SED), both of which were underpredicted in our previous model. In the new model, we hypothesize that small carbonaceous grains are not involved in interstellar processing. This avoids small carbonaceous grains being lost by coagulation. We find that this hypothetical model shows a much better match to the Milky Way (MW) extinction curve and dust emission SED than the previous one. The following two additional modifications further make the fit to the MW dust emission SED better: (i) The chemical enrichment model is adjusted to give a nearly solar metallicity in the present epoch, and the fraction of metals available for dust growth is limited to half. (ii) Aromatization for small carbonaceous grains is efficient, so that the aromatic fraction is unity at grain radii $\lesssim 20$ \AA. As a consequence of our modelling, we succeed in obtaining a dust evolution model that explains the MW extinction curve and dust emission SED at the same time.