Ultraviolet spectropolarimetry with polstar: interstellar medium science

Andersson, B. G.; Clayton, Geoffrey C.; Doney, Kirstin D.; Panopoulou, G. V.; Hoang, Thiem; Magalhães, A. M.; Yan, Huirong; Ignace, Richard; Scowen, Paul A.

doi:10.1007/s10509-022-04153-3

Cited by 7 publications

(2 citation statements)

References 149 publications

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“…All of these objectives, whose realization will be made possible by Polstar, constrain key evolutionary stages of massive stars and lead to a better understanding of their fates, including the compact objects that they leave behind. Importantly, the analysis of polarization signals from all of these sources will require a detailed understanding of the polarization induced by the intervening interstellar medium; by contributing to improve this knowledge, Polstar will not only enable these topics of stellar physics, but will also yield important insights in the physics of interstellar dust grains (Andersson et al, 2022). Any of these types of systems can, in special cases, provide examples that can be studied over an orbitally modulated range of angles that are either completely sampled, or at least change considerably, over the timeframe of the Polstar mission.…”

Section: Discussionmentioning

confidence: 99%

UV spectropolarimetry with Polstar: massive star binary colliding winds

St-Louis

Gayley

Hillier³

et al. 2022

Astrophys Space Sci

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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.

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Section: Discussionmentioning

confidence: 99%

UV spectropolarimetry with Polstar: massive star binary colliding winds

St-Louis

Gayley

Hillier³

et al. 2022

Astrophys Space Sci

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“…This will be done using the well-understood wavelength dependence of the ISM component, as well as its expected time independence, contrasted with the time dependence of the intrinsic stellar component. More details are given in the white paper describing the objectives related to ISM polarization (Andersson et al 2021), as mentioned previously.…”

Section: Evidence For Structure From Polarizationmentioning

confidence: 99%

Understanding structure in line-driven stellar winds using ultraviolet spectropolarimetry in the time domain

Gayley

ud‐Doula

David-Uraz

et al. 2022

Astrophys Space Sci

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The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further evolution and eventual supernova, with consequences for ISM energization and chemical enrichment. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind generation. In the past, mass-loss rates have been characterized via collisional emission processes such as Hα and free-free radio emission, but these so-called "density squared" diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds. Furthermore, largescale structures arising from surface anisotropies and propagating throughout the wind can further complicate the picture by introducing further density enhancements, affecting mass-loss diagnostics. Time series spectroscopy of UV resonance lines with high resolution and high signal-to-noise are required to better understand this complex dynamics, and help correct "density squared" diagnostics of mass-loss rates. The proposed Polstar mission easily provides the necessary resolution at the soundspeed scale of 20 km s −1 , on three dozen bright targets with signal-to noise an order of magnitude above that of the celebrated IUE MEGA campaign, via continuous observations that track structures advecting through the wind in real time.

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