Toward a precise description of the interaction between radiation and interstellar dust grains: “Interstellar dust as a dynamic environment” by G. La Mura (INAF-OA Cagliari)

Micrometric dust particles, both in the diffuse interstellar medium and in nebulae, interact with light, giving rise to many important phenomena. This interaction also depends on the shape and microphysics of the dust grains.

A galaxy like ours hosts not only stars, planets, and compact objects, but also dust and gas organized both in a diffuse, extremely low-density medium and in nebulae. It is estimated, for example, that the interstellar medium in the Milky Way contains several billion solar masses of gas and several tens of millions of solar masses of dust. Although dust is therefore present in much smaller quantities than gas, it plays a key role in various chemical and physical processes occurring in galaxies, and it is a primary agent in the interaction with optical and ultraviolet radiation emitted by astronomical objects.

When we speak of dust in the interstellar medium, we are mainly referring to nanometer-sized grains and small aggregates (1 nm = one billionth of a meter), composed of silicates, carbonaceous material, refractory elements, and ices. Their presence is mainly revealed through their interaction with radiation from astronomical objects, which can be absorbed and scattered—a combined effect that astronomers describe using the term “extinction.” While extinction is often described by simplifying as much as possible the properties of interstellar dust, it is now clear that effects related to the geometry and microphysics of dust aggregates can have significant consequences. For instance, grains with asymmetric shapes can experience anisotropic radiation pressure, which may induce rotation and, over time, even lead to the breakup of the aggregates.

It is therefore essential to introduce a detailed description of dust grain properties in order to fully understand their interaction with radiation. This is the goal of the study “Interstellar Dust as a Dynamic Environment“, led by astrophysicist G. La Mura of INAF – Osservatorio Astronomico di Cagliari. The researchers described dust grains using a specific mathematical approach known as the Transition Matrix Formalism, demonstrating that interaction with radiation—especially at short wavelengths (UV radiation)—strongly depends on the surface structure of the grains at very small spatial scales, as well as on the possible layering of material and ices. The model used is still quite simplified, and the research team plans to continue this work by analyzing more realistic models.

Astrophysicist C. Cecchi-Pestellini of INAF – Osservatorio Astronomico di Palermo also participated in the study.

The cover image (click here to view it in full) shows the model of the dust aggregate analyzed in this study, consisting of two inner grains with a radius of 68 nm, onto which 40 smaller grains, represented as spheres with a radius of 34 nm, have been deposited.