Ultraviolet Spectropolarimetry: Investigating stellar magnetic field diagnostics

TL;DR

This paper explores the potential of ultraviolet spectropolarimetry, especially via the proposed Polstar mission, to detect and analyze stellar magnetic fields through Zeeman and Hanle effects in hot stars.

Contribution

It introduces the feasibility of using high resolution UV spectropolarimetry to detect stellar magnetic fields, highlighting advantages over visible wavelengths and proposing observational strategies.

Findings

UV spectropolarimetry enhances magnetic field detection in hot stars.

Zeeman effect can detect magnetospheres at high S/N in UV.

Hanle effect offers a complementary method for magnetic field inference.

Abstract

Magnetic fields are important for stellar photospheres and magnetospheres, influencing photospheric physics and sculpting stellar winds. Observations of stellar magnetic fields are typically made in the visible, although infrared observations are becoming common. Here we consider the possibility of directly detecting magnetic fields at ultraviolet (UV) wavelengths using high resolution spectropolarimetry, specifically considering the capabilities of the proposed Polstar mission. UV observations are particularly advantageous for studying wind resonance lines not available in the visible, but they can also provide many photospheric lines in hot stars. Detecting photospheric magnetic fields using the Zeeman effect and Least Squares Deconvolution is potentially more effective in the UV due to the much higher density of strong lines. We investigate detecting magnetic fields in the magnetosphere of a star using the Zeeman effect in wind lines, and find that this could be detectable at high S/N in an O or B star with a strong magnetic field. We consider detecting magnetic fields using the Hanle effect in linear polarization, which is complementary to the Zeeman effect, and could be more sensitive in photospheric lines of rapid rotators. The Hanle effect can also be used to infer circumstellar magnetism in winds. Detecting the Hanle effect requires UV observations, and a multi-line approach is key for inferring magnetic field properties. This demonstrates that high resolution spectropolarimetry in the UV, and the proposed Polstar mission, has the potential to greatly expand our ability to detect and characterize magnetic fields in and around hot stars.