Is the silicate emission feature only influenced by grain size?

TL;DR

This paper demonstrates that the shape of the 10-micron silicate emission feature is influenced not only by grain size but also significantly by grain porosity and composition, including crystalline silicates.

Contribution

It introduces a model showing how grain porosity and composition affect the silicate emission feature, challenging the idea that grain size alone is responsible.

Findings

Porosity alters the silicate feature shape similarly to grain growth.

Inclusions of crystalline silicates shift resonances and create new features.

Composite grain models can explain observed spectral variations.

Abstract

The flattening of the 10mu silicate emission feature observed in the spectra of T Tauri and Herbig Ae/Be stars is usually interpreted as an indicator of grain growth. We show in this paper that a similar behaviour of the feature shape occurs when the porosity of composite grains varies. The fluffy aggregates, having inclusions of different sizes, were modeled by multi-layered spheres consisting of amorphous carbon, amorphous silicate and vacuum. It is also found that the inclusion of crystalline silicates in composite porous particles can lead to a shift of the known resonances and production of new ones.