What is the real size of the magnetosphere of an accreting star?

by Prof. Nazar Ikhsanov (Pulkovo Observatory)

Tuesday 27 Oct 2026, 14:00 → 15:00 Asia/Shanghai

Tsung-Dao Lee Institute/S4F-SW - Open Area (Tsung-Dao Lee Institute)

Host: Dong Lai
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Abstract:
The methods to evaluate the size of the magnetosphere of an accreting star are discussed. I emphasize that there is no reliable way to accurately determine the parameters of the accretion flow on the basis of currently available observational data. This introduces uncertainty into the equation describing the balance between the pressure of the accretion flow and the pressure of the stellar magnetic field at the magnetospheric boundary, rendering it incomplete. As an alternative, we propose to focus on the continuity equation for the accretion flow at the magnetospheric boundary. This opens a new approach to the magnetospheric accretion in which the radius of the magnetosphere is defined as a minimum possible distance from the center of the accreting star to the point at which the accretion flow enters the stellar magnetic field at a rate which equals to the mass accretion rate onto the stellar surface. We evaluate the size of the magnetosphere following this approach for a situation in which the entry of the accreting gas into the stellar field is governed by Bohm diffusion. The magnetospheric radius evaluated for this case appears to be significantly smaller than the conventionally accepted value of the equilibrium radius for a spherical accretion flow, known as the Alfven radius. We show that the obtained new expression for the radius of the magnetosphere allows us to solve many puzzling problems on the origin and evolution of accretion-powered degenerate stars in close binary systems.

Biography:
Nazar Ikhsanov is the Director of the Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo (Pulkovo Observatory, Saint-Petersburg, Russia). MS (the Leningrad State University in 1986) and PhD (the Pulkovo Observatory in 1989). Career milestones: an Alexander von Humboldt Fellow at the Ludwig-Maximilian University (Munich, Germany) and at the Max Planck Institute for Radioastronomy (Bonn, Germany); a Research Fellow at the Korea Astronomy and Space Science Institute (Daejeon, South Korea); Maria Curie Fellow at the Institute of Astronomy of the University of Cambridge (Cambridge, UK); NPP Program Senior Fellow at the Marshal Space Flight Center (Huntsville, AL, US). In 2011-2016 is a Head of the Stellar Evolution Dept. of Pulkovo Observatory and a Professor of Astronomy of the Saint-Petersburg State University. From 2016 is the Director of Pulkovo Observatory. Main topics of research are magneto-rotational evolution of degenerate stars, accretion in close binaries and AGNs and flaring activity of astrophysical objects.