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

TL;DR

This work constrains the FUV scattering phase function of ISM dust by modeling halos with porous-aggregate grains; the scattering is described by $I_{sca} = \\frac{L}{4 \\pi r^2} a \\phi(\\theta) \\tau$, using theoretical phase functions rather than Henyey-Greenstein. Analysis of Spica and Achernar finds that porous aggregates with a characteristic size of $0.05\\,\\mu\\mathrm{m}$ dominate the halos (amorphous silicate toward Spica, carbonaceous toward Achernar) with forward-scattering geometries (high $g$). Dust distances to the scattering layers are typically in the range $1$–$4$ pc, and optical depths are $\\tau \\approx 0.1$ for Spica and $\\tau \\approx 0.032$ for Achernar, in broad agreement with local ISM expectations. The study demonstrates a practical method to constrain local ISM dust properties from FUV halos and highlights the need to incorporate alternative grain morphologies and size distributions in future work.

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 μm 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 (τ) of 0.032 compared to Spica which has a value of τ = 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.

Figures (17)

• Figure 1: Variation of the observed GALEX FUV intensity for Spica and Achernar w.r.t angular separation
• Figure 2: Aggregate containing 2048 monomers
• Figure 3: Representation of the position of Spica and Achernar w.r.t interface between Local bubble and Loop I Super shell , Credit: Zaninetti2020_local_bubble_shape.
• Figure 4: Diffuse sky image of FUV halos around Spica and Achernar observed by GALEX , Credit: Murthy2011_UV_halos.
• Figure 5: Extinction curves obtained for Stars mentioned in table \ref{['tab:stars-near-Spica']} near Spica using $R_V$ dependent Extinction law by Fitzpatrick1999