Polarization simulations of stellar wind bow-shock nebulae – I. The case of electron scattering

Abstract

Bow shocks and related density enhancements produced by the winds of massive stars moving through the interstellar medium provide important information regarding the motions of the stars, the properties of their stellar winds, and the characteristics of the local medium. Since bow shock nebulae are aspherical structures, light scattering within them produces a net polarization signal even if the region is spatially unresolved. Scattering opacity arising from free electrons and dust leads to a distribution of polarized intensity across the bow shock structure. That polarization encodes information about the shape, composition, opacity, density, and ionizsation state of the material within the structure. In this paper we use the Monte Carlo radiative transfer code SLIP to investigate the polarization created when photons scatter in a bow shock-shaped region of enhanced density surrounding a stellar source. We present results for electron scattering, and investigate the polarization behaviour as a function of optical depth, temperature, and source of photons for two different cases: pure scattering and scattering with absorption. In both regimes we consider resolved and unresolved cases. We discuss the implications of these results as well as their possible use along with observational data to constrain the properties of observed bow shock systems. In different situations and under certain assumptions, our simulations can constrain viewing angle, optical depth and temperature of the scattering region, and the relative luminosities of the star and shock.

Characterization of a dual-beam, dual-camera optical imaging polarimeter

“…Thus, we are losing valuable information when we ignore the polarization of different astrophysical sources. Polarimetry has proven to be a powerful diagnostic tool for various science cases such as studying environment around stellar sources (Brown & McLean 1977;Shrestha et al 2018), supernovae remnants (Hoffman et al 2008), blazars (Jermak 2017), gamma ray bursts (GRBs; Steele et al (2009); Mundell et al (2013)), novae (Harvey et al 2018), and many more. In recent years polarimetric studies have taken a leading role as a key diagnostic tool of magnetic field strength/order/geometry, geometry of environment surrounding the source, and relativistic plasma dynamics in various time domain sources such as blazars, active galactic nuclei, x-ray binaries, and GRBs.…”

Ultraviolet Spectropolarimetry:Conservative and Nonconservative Mass Transfer in OB Interacting Binaries

“…The total observing time implicit in the described observing strategy is 27 days, an acceptable resource allocation for one of twelve objectives in a 3-year mission timeline, while still accounting for a 50% duty cycle, as well as potential time set aside for guest observing. The Polstar team possesses access to a suite of modeling tools, such as Monte Carlo radiative transfer (Whitney et al 2017;Carciofi et al 2017;Shrestha et al 2018), for evaluating polarization from binaries in optically thin or thick regimes. It is however useful to explore the thin scattering regime, particularly the theory advanced by Brown et al (1978).…”

Ultraviolet Spectropolarimetry with Polstar: Massive Star Binary Colliding Winds

“…Polarimetric observations are thus a vital tool for revealing details of the mass-loss and mass-transfer structures in these binaries. Both continuum emission (arising from the stars) and line emission (arising from winds, CIRs, or other shock regions) may be polarized in a CWB system via scattering from wind clumps (Harries et al 1998;Davies et al 2005), accretion disks (Hoffman et al 1998), jets (Fox & Hines 1998), bow shocks (Shrestha et al 2018(Shrestha et al , 2021, and other asymmetric distributions of material or velocities within the system (Schulte-Ladbeck et al 1992;Villar-Sbaffi et al 2005;Fullard et al 2020). Because scattering near the orbital plane tends to produce repeatable effects over many binary cycles, spectropolarimetric monitoring allows us to reconstruct the 3-D shapes of the regions scattering both continuum and line emission (e.g., Lomax et al 2015;Fullard et al 2020).…”

“…Regarding the latter, the Thom-son scattering cross-section is relatively small, and so thin scattering is often appropriate. However, for some high-density scattering regions, multiple scattering may modify the resulting polarization (Hoffman et al 2003;Shrestha et al 2018). We will employ both analytical models, described in detail below, and Monte Carlo simulations to investigate the full range of possible density distributions.…”