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UV spectropolarimetry with Polstar: massive star binary colliding winds

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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.

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The team will make the data obtained within the context of this mission available to the astronomical community.

Notes

  1. Some colliding wind shocks are better described by adiabatic cooling (e.g., Gayley 2009). For our purposes the solution of Cantó et al. (1996) is convenient for its simplicity and for overall characterization of the issues involved for polarimetric variability.

  2. www.astro.physik.uni-potsdam.de/~wrh/PoWR/powrgrid1.php.

  3. See the Potsdam PoWR models at the following website: www.astro.physik.uni-potsdam.de/~wrh/PoWR/powrgrid1.php.

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Acknowledgements

RI acknowledges funding support from a grant by the National Science Foundation (NSF), AST-2009412. JLH acknowledges support from NSF under award AST-1816944 and from the University of Denver via a 2021 PROF award. Scowen acknowledges his financial support by the NASA Goddard Space Flight Center to formulate the mission proposal for Polstar. Y.N. acknowledges support from the Fonds National de la Recherche Scientifique (Belgium), the European Space Agency (ESA) and the Belgian Federal Science Policy Office (BELSPO) in the framework of the PRODEX Programme (contracts linked to XMM-Newton and Gaia). NSL and CEJ wish to thank the National Sciences and Engineering Council of Canada (NSERC) for financial support. A.D.-U. is supported by NASA under award number 80GSFC21M0002. GJP gratefully acknowledges support from NASA grant 80NSSC18K0919 and STScI grants HST-GO-15659.002 and HST-GO-15869.001.

Funding

The Polstar satellite is presently being proposed as a Midex mission to NASA and is currently under evaluation.

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All authors have contributed ideas to motivate the work presented in this paper. Some have provided specific modelling, others proposed data analysis techniques. All contributed to the writing of the text to varying extents.

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Correspondence to Nicole St-Louis.

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This article belongs to the Topical Collection: UV Spectropolarimetry for Stellar, Interstellar, and Exoplanetary Astrophysics with Polstar. Guest Editors: Paul A. Scowen, Carol E. Jones, René D. Oudmaijer.

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St-Louis, N., Gayley, K., Hillier, D.J. et al. UV spectropolarimetry with Polstar: massive star binary colliding winds. Astrophys Space Sci 367, 118 (2022). https://doi.org/10.1007/s10509-022-04102-0

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  • DOI: https://doi.org/10.1007/s10509-022-04102-0

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