UV Spectropolarimetry with Polstar: Massive Star Binary Colliding Winds

TL;DR

The paper addresses how to break the Doppler–radius degeneracy in massive-star winds by exploiting UV spectropolarimetry of colliding-wind binaries observed with Polstar. It proposes a 20-target program combining high-resolution UV spectroscopy and phase-resolved polarization, guided by a semi-analytic bow-shock framework parameterized by $\beta$ and $\alpha$. The approach yields diagnostics of wind acceleration, density structure, and bow-shock geometry, with emphasis on the contrast between line- and continuum-polarization as a geometry constraint. The work anticipates tighter mass-loss-rate determinations and deeper understanding of radiative-transfer effects in optically thick UV resonance lines, informing massive-star evolution, feedback, and binary interactions.

Abstract

The winds of massive stars are important for their direct impact on the interstellar medium, and for their influence on the final state of a star prior to it exploding as a supernova. However, the dynamics of these winds is understood primarily via their illumination from a single central source. The Doppler shift seen in resonance lines is a useful tool for inferring these dynamics, but the mapping from that Doppler shift to the radial distance from the source is ambiguous. Binary systems can reduce this ambiguity by providing a second light source at a known radius in the wind, seen from orbitally modulated directions. From the nature of the collision between the winds, a massive companion also provides unique additional information about wind momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and UV resonance line opacity in the wind is strong, UV instruments with a high resolution spectroscopic capability are essential for extracting this dynamical information. Polarimetric capability also helps to further resolve ambiguities in aspects of the wind geometry that are not axisymmetric about the line of sight, because of its unique access to scattering direction information. We review how the proposed MIDEX-scale mission Polstar can use UV spectropolarimetric observations to critically constrain the physics of colliding winds, and hence radiatively-driven winds in general. We propose a sample of 20 binary targets, capitalizing on this unique combination of illumination by companion starlight, and collision with a companion wind, to probe wind attributes over a range in wind strengths. Of particular interest is the hypothesis that the radial distribution of the wind acceleration is altered significantly, when the radiative transfer within the winds becomes optically thick to resonance scattering in multiple overlapping UV lines.

Figures (12)

• Figure 1: Eclipse spectrum of $\gamma$ Velorum. Figure 2 from 1993ApJ...415..298S. Reproduced with permission.
• Figure 2: Variations in the C iii$\lambda$ 2297 profile with orbital phase. The thin line is the LWR 1316 ($\Phi = 0.534$), and the thick line is the spectrum and phase indicated at the top of each graph. Figure 4 from 1993ApJ...415..298S. Reproduced with permission.
• Figure 3: Illustration of the location of where a radial optical depth of $\tau=2/3$ occurs as a function of wavelength for an O-type and W-R type star.
• Figure 4: Illustration of how velocity shifts influence the polarized spectrum. Top panel shows how the scattered spectrum (blue) is offset to the red from the flux spectrum (red). The second panel corrects for the offset, illustrating the similarities between the two spectra. However, because spectral features arise over a range of radii, the agreement is not perfect. The bottom panel shows the good agreement between a model (red) and observation (black) for SN 2013ej. Figure from Leonard2021. Reproduced with permission.
• Figure 5: The spectrum and polarization (Stokes Q) for a WR star with an oblate stellar wind viewed edge on. The ratio of the polar to equatorial density is 0.3. Notice how the polarization switches sign -- a consequence of optical depth effects in the wind.