Characterizing massive star magnetospheres at ultraviolet wavelengths
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Magnetism in massive stars is still a relatively new area of study. The last decade of research has revealed a small but significant subpopulation of massive stars that host strong, nearly dipolar surface magnetic fields that interact with and substantially alter the star’s structure and its environment. Characterizing the processes through which such changes occur is critical to our understanding of massive star evolution. ☐ Magnetic channeling of the stellar wind leads to the formation of structurally complex magnetospheres, which can be studied by examining the wind-sensitive resonance lines present in ultraviolet spectra. Since the density and velocity structure of the magnetosphere is distinctly aspherical, classical approaches to modeling these spectra fall short. Multidimensional magnetohydronamical models have been shown to agree well with observations, but are too computationally expensive for large-scale parameter studies. We have therefore adopted an alternate technique for magnetospheric modeling, the Analytic Dynamical Magnetosphere, which we couple with UV radiative transfer techniques to create computationally efficient synthetic line profiles. Our grid of models, the UV-ADM Project, constitutes the first detailed parameter study of the many factors that influence UV line formation in the winds of magnetic massive stars. ☐ Another key aspect in the characterization of magnetism, measurements of the magnetic field strength, can be obtained through circular polarization from the Zeeman splitting of (typically optical) spectral lines. The sinusoidal variation of the average longitudinal magnetic field can be leveraged to determine the stellar rotation period, which is difficult to infer for massive stars from other measurements. We have examined new spectropolarimetric data of the remarkable β Cep star ξ1 CMa, for which we report the first definite detection of a negative longitudinal magnetic field. We confirm its ultra-slow rotation using evidence from the spectropolarimetric dataset, as well as that provided by newly acquired UV spectroscopy. We also report an updated spectropolarimetric dataset for HD 54879, the most recently discovered magnetic O-type star, which hosts a complex magnetic field. ☐ Combining the UV-ADM Code with polarization techniques, we offer a first look at modeling synthetic Stokes V signatures originally from the wind (or magnetosphere) in the UV. When refined, this extension of the initial goals of the UV-ADM Project will provide important observational constraints for the new generation of UV spectropolarimeters, such as the POLSTAR mission.
Description
Keywords
Surface magnetic fields, Massive star winds, Magnetospheric modeling, Synthetic line profiles, UV line formation, Stokes V signatures