Highly Accelerated Diamagnetic Plasmoids: A New X-ray Production Mechanism for OB Stellar Winds

The observed X-ray source temperature distributions in OB stellar winds, as determined from high energy resolution Chandra observations, show that the highest temperatures occur near the star, and then steadily decrease outward through the wind. To explain this unexpected behavior, we propose a shock model concept that utilizes a well-known magnetic propulsion mechanism; the surface ejection of “diamagnetic plasmoids” into a diverging external magnetic field. This produces rapidly accelerating self-contained structures that plow through an ambient wind and form bow shocks that generate a range in X-ray temperatures determined by the plasmoid-wind relative velocities. The model free parameters are the plasmoid initial Alfven speed, the initial plasma-beta of the external medium, and the divergence rate of the external field. These are determined by fitting the predicted bow shock temperatures with the observed OB supergiant X-ray temperature distribution. We find that the initial external plasma-beta has a range between 0 and 2, and the assumed radially-decreasing external magnetic field strength that scales as r^{-S} has a value of S lying between 2 and 3. Most importantly, the initial plasmoid Alfven speed is found to be well-constrained at a value of 0.6 times the terminal velocity, which appears to represent an upper limit for all normal OB stars. This intriguing new limit on OB magnetic properties, as derived from Chandra observations, emphasizes the need for further studies of magnetic propulsion mechanisms in these stars.

💡 Research Summary

The paper addresses a puzzling observational result from high‑resolution Chandra spectra of OB supergiants: the X‑ray emitting plasma shows a temperature distribution that peaks close to the stellar surface and declines steadily with radius. Traditional wind‑shock models, which rely on line‑driven instabilities and predict increasing shock velocities (and thus higher temperatures) farther out in the wind, cannot account for this “near‑star high‑ion” problem.
To resolve this, the authors adapt a magnetic propulsion mechanism originally developed for solar plasmas (Cargill & Pneuman 1984). They propose that magnetic reconnection events near the photosphere generate compact, diamagnetic plasmoids. Because the external magnetic field diverges (B_e ∝ r⁻ˢ), the plasmoids experience a magnetic pressure gradient that accelerates them rapidly outward. The acceleration depends on three key parameters: the initial Alfvén speed of the plasmoid (V_A0), the external plasma‑β at the launch point (β_e0), and the radial decline exponent of the external field (S).
Using the momentum equation from CP84, extended to include a non‑zero external β, they derive an analytic expression for the plasmoid velocity V_p(r). The relative speed between the plasmoid and the ambient wind (ΔV = V_p – V_w) determines the maximum post‑shock temperature via the Rankine‑Hugoniot relation: T_X(r) = 14