Modelling the scattering by porous aggregate dust grains in the Far-Ultraviolet halos of Spica and Achernar

TL;DR

This study models far-ultraviolet halos around stars Spica and Achernar using realistic porous dust aggregates and theoretical scattering functions, providing insights into dust properties in the local interstellar medium.

Contribution

It introduces a modeling approach using theoretical phase functions and porous dust aggregates to better understand FUV halos around stars.

Findings

Halo intensities dominated by 0.05 μm porous dust aggregates.

Achernar's foreground medium has lower optical depth ({ au} = 0.032) than Spica's ({ au} = 0.1).

Method constrains dust grain properties in the local interstellar medium.

Abstract

Far-Ultraviolet (FUV) halos have been detected around six bright stars by Murthy and Henry (2011) using GALEX observations. These halos are thought to be caused by forward scattering of the starlight by dust grains present in thin foreground clouds. The optical constants of grains producing such halos have been constrained earlier by using a single scattering model, that considered the Henyey-Greenstein empirical phase function instead of theoretical phase functions for the scattering grains. In this work, we have modelled the FUV halos for two stars, Spica and Achernar, by considering the realistic porous aggregates of different sizes and compositions. As the Henyey-Greenstein phase function is known to deviate from theoretical predictions, we have utilized theoretical scattering phase functions for modelling. The dust is placed in a double-layered plane-parallel sheet with its distance and optical depth varied to get the best fit. We find that the halo intensities are dominated by scattering due to 0.05 {\mu}m sized porous dust aggregates made of amorphous silicate and carbonaceous aggregates for Spica and Achernar, respectively. We find that the medium in front of Achernar has a lower optical depth ({\tau}) of 0.032 compared to Spica which has a value of {\tau} = 0.1. This low value is close to the optical depth of the local ISM (0.01) within 40 pc of the Sun. This study demonstrates an effective method to constrain the dust grain properties in the local interstellar medium.