Polarization simulations of stellar wind bow shock nebulae. II. The case of dust scattering

TL;DR

This paper investigates how dust scattering influences polarization in stellar wind bow shock nebulae, revealing dependencies on dust properties, optical depth, and orientation, with implications for understanding interstellar medium conditions.

Contribution

It extends previous electron scattering models by incorporating dust scattering effects, exploring dust composition, grain size, and emission impacts on polarization in bow shocks.

Findings

Polarization depends on dust composition and grain size.

Dust emission affects polarization at higher optical depths.

Increased dust between star and shock suppresses polarization.

Abstract

We study the polarization produced by scattering from dust in a bow shock-shaped region of enhanced density surrounding a stellar source, using the Monte Carlo radiative transfer code SLIP. Bow shocks are structures formed by the interaction of the winds of fast-moving stars with the interstellar medium. Our previous study focused on the polarization produced in these structures by electron scattering; we showed that polarization is highly dependent on inclination angle and that multiple scattering changes the shape and degree of polarization. In contrast to electron scattering, dust scattering is wavelength-dependent, which changes the polarization behaviour. Here we explore different dust particle sizes and compositions and generate polarized spectral energy distributions for each case. We find that the polarization SED behaviour depends on the dust composition and grain size. Including dust emission leads to polarization changes with temperature at higher optical depth in ways that are sensitive to the orientation of the bow shock. In various scenarios and under certain assumptions, our simulations can constrain the optical depth and dust properties of resolved and unresolved bow shock-shaped scattering regions.Constraints on optical depth can provide estimates of local ISM density for observed bow shocks. We also study the impact of dust grains filling the region between the star and bow shock. We see that as the density of dust between the star and bow shock increases, the resulting polarization is suppressed for all the optical depth regimes.