[Paper Review] Stellar Physics with High-Resolution UV Spectropolarimetry

J. Morin, Bouret, Jean-Claude+27 more|arXiv (Cornell University)|Aug 5, 2019

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper proposes high-resolution UV spectropolarimetry as a transformative tool for advancing stellar physics, enabling detailed 3D mapping of magnetic fields and magnetized environments in both hot and cool stars. By measuring polarized UV spectra across wide wavelength ranges using advanced detectors and polarimeters, it aims to resolve long-standing uncertainties in stellar evolution, magnetic activity, and wind dynamics.

ABSTRACT

Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution. UV spectropolarimetry is proposed on missions of various sizes and scopes, from POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV spectropolarimetry.

Motivation & Objective

• Address the critical lack of understanding in the role of fossil magnetic fields in massive star evolution and their impact on structure and circumstellar environments.
• Investigate the dynamic upper atmospheres of cool stars, particularly the magnetic field structures driving chromospheric heating and stellar wind launching.
• Enable quantitative diagnostics of chemical abundances, plasma conditions, and wind properties in low-density environments such as stellar winds and circumstellar shells.
• Overcome limitations of current UV observations by developing high-resolution spectropolarimetry to access previously inaccessible polarized signatures in UV lines.
• Provide essential data for understanding stellar interactions with planetary systems and the interstellar medium across stellar lifetimes.

Proposed method

• Utilize high-resolution spectropolarimetry in the UV domain to measure the full Stokes (IQUV) spectrum, capturing intensity, linear and circular polarization.
• Implement a rotating stack of MgF2 plates followed by a polarizing beam-splitter as a modulator for wavelengths above 123 nm, enabling precise polarimetric measurements.
• For wavelengths below 123 nm, employ a reflective polarimeter using only mirrors, a novel approach not yet flown but technically feasible.
• Leverage UV spectral forests rich in atomic and molecular transitions, especially those with low-energy levels less prone to depopulation in low-density plasmas.
• Integrate advanced detectors such as CMOS with δ-doping for enhanced UV response, offering larger formats than traditional CCDs.
• Apply the technique to space missions ranging from M-size (e.g., Arago) to large-aperture telescopes (e.g., LUVOIR), enabling both detailed local studies and deep surveys.

Experimental results

Research questions

• RQ1 How do fossil magnetic fields in hot stars influence their internal structure, mass loss, and evolution over time?
• RQ2 What is the role of magnetic fields in heating the upper atmospheres and launching winds in cool stars?
• RQ3 How can high-resolution UV spectropolarimetry reveal the 3D structure of magnetized stellar atmospheres and circumstellar environments?
• RQ4 To what extent can UV polarized lines trace magnetic field geometry and plasma dynamics along field lines in stellar winds and accretion structures?
• RQ5 How do magnetic fields in stars affect planetary system formation and evolution, particularly through wind interactions and irradiation?

Key findings

• High-resolution UV spectropolarimetry can access unique diagnostics in low-density environments such as chromospheres, winds, and nebulae, where UV lines remain populated due to low-energy level stability.
• The UV domain is exceptionally sensitive to small amounts of hot gas, enabling detection of accretion, coronal heating, and magnetic activity even in cool stars.
• Polarization in UV lines provides a direct probe of magnetic field geometry and plasma flow along field lines, a capability currently unexploited due to lack of in-flight instruments.
• Reflective UV polarimeters below 123 nm are technically feasible and could be implemented on future missions despite no prior flight heritage.
• CMOS detectors with δ-doping offer a promising path toward larger-format, high-performance UV detectors suitable for spectropolarimetric instruments.
• A high-resolution UV spectropolarimeter on an M-size mission like Arago would enable detailed 3D mapping of stellar magnetospheres, while larger telescopes (e.g., LUVOIR) could extend observations to fainter and more distant stars.