Polarimetric modeling of corotating interaction regions threading massive-star winds

Abstract

Context. Massive star winds are complex radiation-hydrodynamic (sometimes magnetohydrodynamic) outflows that are propelled by their enormously strong luminosities. The winds are often found to be structured and variable, but can also display periodic or quasi-periodic behavior in a variety of wind diagnostics. Aims. The regular variations observed in putatively single stars, especially in UV wind lines, have often been attributed to corotating interaction regions (CIRs) like those seen in the solar wind. We present light curves for variable polarization from winds with CIR structures. Methods. We develop a model for a time-independent CIR based on a kinematical description. Assuming optically thin electron scattering, we explore the range of polarimetric light curves that result as the curvature, latitude, and number of CIRs are varied. Results. We find that a diverse array of variable polarizations result from an exploration of cases. The net polarization from an unresolved source is weighted more toward the inner radii of the wind. Given that most massive stars have relatively fast winds compared to their rotation speeds, CIRs tend to be conical at inner radii, transitioning to a spiral shape at a few to several stellar radii in the wind. Conclusions. Winds with a single CIR structure lead to easily identifiable polarization signatures. By contrast allowing for multiple CIRs, all emerging from a range of azimuth and latitude positions at the star, can yield complex polarimetric behavior. Although our model is based on some simplifying assumptions, it produces qualitative behavior that we expect to be robust, and this has allowed us to explore a wide range of CIR configurations that will prove useful for interpreting polarimetric data.

BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant ζ Puppis unveils the photospheric drivers of its small- and large-scale wind structures

“…a closed-form expression that follows Ignace et al (2009Ignace et al ( , 2015 in which it is assumed that ϕ (r) = 0 for the purpose of solving the spiral shape, due to the complexity of nonradial force considerations and because we do not perform hydrodynamical calculations in our simulations and do not have any real guidance on ϕ (r). Also, it is worth noting that in Equation 29, the dimensionless quantity x p = ∞ /(ΩR) measures the ratio of the distance travelled by the outer boundary of the stellar wind in one rotation period with respect to the stellar equatorial diameter, thus indicative of the asymptotic pitch angle of the spiral-shaped CIR compression.…”

Polarization light curve modelling of corotating interaction regions in the wind of the Wolf–Rayet star WR 6

“…Here we summarize the approach of Ignace et al (2015, hereafter ISP) for calculation of variable polarization from CIR-like structures with optically thin scattering, and describe modifications to accommodate a WR wind that is optically thick to electron scattering. Our approach in ISP was twofold: first, the adoption of a kinematic description for a CIR structure, based on Ignace et al (2009) and second, modifications to the work of Brown & McLean (1977) for thin scattering with axisymmetry to the spiral structures of CIRs.…”

“…The parameters remaining to be fit are i 0 , ψ as well as φ , θ, β and η for each CIR. However, from ISP, it can easily be seen that the calculated values of q and u depend on the product of η and Λ 0 , which equals η cos β(1 − cos 2 β) 1 , indicating clearly that the opening angle and the density contrast in 1 Note that equation (21) of Ignace et al (2015) is incorrect. It should have been:…”

Searching for a magnetic field in Wolf–Rayet stars using FORS 2 spectropolarimetry

“…However, much of the data were obtained within two contiguous cycles. We have attempted a rough fit to this subset of the linearly polarized data using a CIR model as described in Ignace et al (2015). Ignace et al (2015) presented a kinematic description of a CIR in terms of a spiral arm morphology in shape, and in terms of a density contrast (either enhancement or deficit) relative to the otherwise unperturbed spherical wind.…”

“…We have attempted a rough fit to this subset of the linearly polarized data using a CIR model as described in Ignace et al (2015). Ignace et al (2015) presented a kinematic description of a CIR in terms of a spiral arm morphology in shape, and in terms of a density contrast (either enhancement or deficit) relative to the otherwise unperturbed spherical wind. This is clearly a gross simplification as compared to more detailed numerical simulations of CIR phenomena from massive star winds (e.g., Cranmer & Owocki 1996;Dessart 2004).…”