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Abstract Massive-star binaries are critical laboratories for measuring masses and stellar wind mass-loss rates. A major challenge is inferring viewing inclination and extracting information about the colliding-wind interaction (CWI) region. Polarimetric variability from electron scattering in the highly ionized winds provides important diagnostic information about system geometry. We combine for the first time the well-known generalized treatment of Brown et al. for variable polarization from binaries with the semianalytic solution for the geometry and surface density CWI shock interface between the winds based on Cantó et al. Our calculations include some simplifications in the form of inverse-square law wind densities and the assumption of axisymmetry, but in so doing they arrive at several robust conclusions. One is that when the winds are nearly equal (e.g., O+O binaries) the polarization has a relatively mild decline with binary separation. Another is that despite Thomson scattering being a gray opacity, the continuum polarization can show chromatic effects at ultraviolet wavelengths but will be mostly constant at longer wavelengths. Finally, when one wind dominates the other, as, for example, in WR+OB binaries, the polarization is expected to be larger at wavelengths where the OB component is more luminous and generally smaller at wavelengths where the WR component is more luminous. This behavior arises because, from the perspective of the WR star, the distortion of the scattering envelope from spherical is a minor perturbation situated far from the WR star. By contrast, the polarization contribution from the OB star is dominated by the geometry of the CWI shock. 

Type Ia supernovae (SNe Ia) are securely understood to come from the thermonuclear explosion of a white dwarf as a result of binary interaction, but the nature of that binary interaction and the secondary object is uncertain. Recently, a double white dwarf model known as the dynamically driven double-degenerate double-detonation (D6) model has become a promising explanation for these events. One realization of this scenario predicts that the companion may survive the explosion and reside within the remnant as a fast moving (V_peculiar> 1000 km s⁻¹), overluminous (L> 0.1L_⊙) white dwarf. Recently, three objects that appear to have these unusual properties have been discovered in the Gaia survey. We obtained photometric observations of the SN Ia remnant SN 1006 with the Dark Energy Camera over four years to attempt to discover a similar star. We present a deep, high-precision astrometric proper-motion survey of the interior stellar population of the remnant. We rule out the existence of a high-proper-motion object consistent with our tested realization of the D6 scenario (V_transverse> 600 km s⁻¹withm_r< 21 corresponding to an intrinsic luminosity ofL> 0.0176L_⊙). We conclude that such a star does not exist within the remnant or is hidden from detection by either strong localized dust or the unlikely possibility of ejection from the binary system almost parallel to the line of sight.

 

Abstract Understanding the evolution of massive binary stars requires accurate estimates of their masses. This understanding is critically important because massive star evolution can potentially lead to gravitational-wave sources such as binary black holes or neutron stars. For Wolf–Rayet (WR) stars with optically thick stellar winds, their masses can only be determined with accurate inclination angle estimates from binary systems which have spectroscopic M sin i measurements. Orbitally phased polarization signals can encode the inclination angle of binary systems, where the WR winds act as scattering regions. We investigated four Wolf–Rayet + O star binary systems, WR 42, WR 79, WR 127, and WR 153, with publicly available phased polarization data to estimate their masses. To avoid the biases present in analytic models of polarization while retaining computational expediency, we used a Monte Carlo radiative-transfer model accurately emulated by a neural network. We used the emulated model to investigate the posterior distribution of the parameters of our four systems. Our mass estimates calculated from the estimated inclination angles put strong constraints on existing mass estimates for three of the systems, and disagree with the existing mass estimates for WR 153. We recommend a concerted effort to obtain polarization observations that can be used to estimate the masses of WR binary systems and increase our understanding of their evolutionary paths. 

Although we have been able to develop an understanding of many aspects of stellar evolution and formation, a few key gaps remain. One is the fate of massive binary star systems composed of Wolf-Rayet (WR) and O-type stars. In these WR + O binaries, the stellar winds surrounding these stars collide, creating a complex interaction region in which light from the stars scatters and becomes polarized. To study these scattering regions, I employ a technique that allows me to map the polarization of the light emitted from these stars and track its variation over the binary orbit. I found that although we have some models for this behavior, they do not fully reproduce the observed data, suggesting these systems are more complex that previously known. The unexplained behaviors give clues to the complexity of these systems and shows how these models can be improved upon in the future. Understanding the structure and evolution of this scattering region could be the key to understanding the lives and eventual deaths of these stars. 

https://www.bhsu.edu/Research/CUWiP#Home-1947 

Massive Wolf-Rayet (WR) stars in binary systems may produce supernovae capable of emitting long-duration gamma-ray bursts (LGRB). The canonical WR+O eclipsing binary is V444 Cygni, which is a WN5+O system that has X-ray emitting colliding winds and a well-constrained geometry. I will present new time-dependent spectropolarimetric data, collected using RSS at the Southern African Large Telescope, from several southern WN+O binary systems that may be analogs to V444 Cygni. By analyzing their polarimetric variations with respect to V444 Cygni, I investigate their wind geometries and assess the similarities among the WN subclass. Characterizing the mass loss and transfer structures within these systems will help to constrain the future evolution of these WN stars and their roles as LGRB prognitors 

https://nexsci.caltech.edu/workshop/2019/