[Paper Review] Review: Magnetic fields of O stars

G. A. Wade, Collaboration, the MiMeS|arXiv (Cornell University)|Nov 13, 2014

Stellar, planetary, and galactic studies2 references3 citations

TL;DR

This review synthesizes observational and theoretical advances in understanding magnetic fields in O-type stars, demonstrating that strong, organized dipolar fields—detected via rotational modulation of wind-sensitive spectral lines—quench mass loss and drive slow rotation. The discovery of Of?p stars in the LMC and SMC, showing magnetic-like variability, suggests the first confirmed extragalactic magnetic O stars, though direct field measurements remain pending.

ABSTRACT

Since 2002, strong, organized magnetic fields have been firmly detected at the surfaces of about 10 Galactic O-type stars. In this paper I will review the characteristics of the inferred fields of individual stars, as well as the overall population. I will discuss the extension of the 'magnetic desert', first inferred among the A-type stars, to O stars up to 60 solar masses. I will discuss the interaction of the winds of the magnetic stars with the fields above their surfaces, generating complex 'dynamical magnetosphere' structures detected in optical and UV lines, and in X-ray lines and continuum. Finally, I will discuss the detection of a small number of variable O stars in the LMC and SMC that exhibit spectral characteristics analogous to the known Galactic magnetic stars, and that almost certainly represent the first known examples of extra-Galactic magnetic stars.

Motivation & Objective

• To summarize the current state of knowledge on magnetic fields in O-type stars, particularly those detected via spectroscopic variability.
• To examine the role of magnetic fields in modifying stellar winds, angular momentum loss, and mass loss in massive stars.
• To evaluate the significance of the 'magnetic desert' extension to O stars and the implications for stellar evolution.
• To present evidence for the first known extragalactic magnetic O stars in the Magellanic Clouds based on photometric and spectroscopic variability.
• To highlight the importance of future high-sensitivity polarimetry for expanding the sample and understanding the origin of neutron stars and magnetars.

Proposed method

• High-resolution circular polarization spectroscopy using the Least-Squares Deconvolution technique to detect longitudinal magnetic fields in O stars.
• Analysis of periodic spectral line variability in optical, UV, and X-ray lines to infer magnetic oblique rotator models.
• Application of radiation MHD simulations to model magnetically confined winds, distinguishing between centrifugal and dynamical magnetosphere regimes.
• Use of phased photometry and spectroscopic monitoring to infer rotational periods and magnetic field geometry in candidate magnetic O stars.
• Comparison of surface abundances in magnetic and non-magnetic O stars to assess field-driven mixing or segregation effects.
• Interpretation of UV and X-ray emission features as tracers of high-temperature, low-density magnetospheric plasma.

Experimental results

Research questions

• RQ1 What is the prevalence and strength of organized magnetic fields in O-type stars, and how do they affect stellar wind dynamics?
• RQ2 How do magnetic fields in O stars lead to rotational spindown and slow rotation, and what is the role of magnetic braking?
• RQ3 To what extent do magnetic fields in O stars alter internal mixing and nucleosynthetic yields?
• RQ4 Can the spectral and photometric variability of Of?p stars in the LMC and SMC be explained by magnetic oblique rotator models?
• RQ5 What is the significance of detecting magnetic O stars in the Magellanic Clouds for understanding the magnetic properties of massive stars beyond the Milky Way?

Key findings

• Eleven magnetic O stars have been confirmed in the Milky Way, with magnetic fields inferred to be strong (on the order of kilo-Gauss), organized (dipolar), and inclined relative to the rotation axis.
• The rotational periods of magnetic O stars range from one week to decades, significantly longer than those of non-magnetic O stars, consistent with magnetic braking via wind coupling.
• Spectral variability in magnetic O stars, particularly in Hα and C iii λ4650, is interpreted as rotational modulation due to magnetically confined plasma structures.
• The wind-magnetic field interaction in O stars produces 'dynamical magnetospheres' that are best described by radiation MHD models, with plasma confined in closed magnetic loops.
• UV and X-ray observations reveal hot, low-density plasma components consistent with magnetospheric heating and confinement.
• Two Of?p stars in the LMC and SMC show photometric and spectroscopic variability with periods of ~8 d and ~15 d, strongly suggesting they are magnetic oblique rotators and representing the first known extragalactic magnetic O stars.