Hunting Distant Ghosts: The Glowing Magnetospheres of Massive, Luminous Stars

Richard Townsend (University of Wisconsin Madison)

April 09, 2015

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Abstract: Long thought to be non-magnetic, we now know that around 10% of massive stars harbor intense, ordered, stable magnetic fields. These fields interact strongly the stars’ hypersonic radiation-driven winds, channeling and confining the wind plasma into circumstellar magnetospheres which emit across the full electromagnetic spectrum, from X-rays through to radio. With their host stars typically situated hundreds of parsecs away, there’s little chance of spatially resolving these ghostly magnetospheres — at least for the foreseeable future. Nevertheless, with techniques such as magnetic Doppler imaging we can leverage spectropolarimetric observations at multiple rotation phases into detailed maps of the stellar-surface magnetic fields. In turn, these maps allow us to extrapolate the circumstellar field topology and build detailed three-dimensional models for the distribution of trapped wind plasma throughout the magnetospheres. In this talk I’ll demonstrate how this process leads us to new models for the magnetospheres of a handful of well-studied magnetic B-stars: sigma Ori E, HD 37776 and tau Sco. I’ll likewise explore how the synthetic observables predicted by these models stack up against archival observations; and discuss how discrepancies between the two are driving further refinements to the models. I’ll finish with an update on the spin-down of sigma Ori E, the only main- sequence star for which magnetic braking has been directly and unambiguously measured.