Magnetic fields in early-type stars

Abstract

Abstract. For several decades we have been cognizant of the presence of magnetic fields in early-type stars, but our understanding of their magnetic properties has recently (over the last decade) expanded due to the new generation of high-resolution spectropolarimeters (ESPaDOnS at CFHT, Narval at TBL, HARPSpol at ESO). The most detailed surface magnetic field maps of intermediate-mass stars have been obtained through Doppler imaging techniques, allowing us to probe the small-scale structure of these stars. Thanks to the effort of large programmes (e.g. the MiMeS project), we have, for the first time, addressed key issues regarding our understanding of the magnetic properties of massive (M > 8 M⊙) stars, whose magnetic fields were only first detected about fifteen years ago. In this proceedings article we review the spectropolarimetric observations and statistics derived in recent years that have formed our general understanding of stellar magnetism in early-type stars. We also discuss how these observations have furthered our understanding of the interactions between the magnetic field and stellar wind, as well as the consequences and connections of this interaction with other observed phenomena.

The BRITE spectropolarimetric program

“…This includes 4 B and A supergiant stars, 14 other B and A stars, 7 F, G, and K dwarfs or subwarfs, 23 F, G, and K giants or supergiants, and 4 M giants. While the detection rate of 9% in hot stars corresponds to the known ∼10% occurence rate of magnetism in these objects (Grunhut & Neiner, 2015), the rate of detections in cool stars (11%) is lower than expected for these stars (see Petit et al, 2014). This rate increases to 19% when considering only class V stars of F, G, and K types.…”

“…1). Hot stars host fossil magnetic fields, which are usually simple (mostly dipolar) in structure, stable, and rather strong (above 300 G at the poles), but appear in only about ∼10% of hot stars (Grunhut & Neiner, 2015;Neiner et al, 2015c). On the contrary, cool stars host dynamo fields similar to our Sun, which are produced contemporaneously by the star itself, are variable on short timescales, very weak on average (but can be stronger in spots), and observable in most cool stars.…”

Magneto-asteroseismology of massive magnetic pulsators

“…Magnetic fields are only detected for seven to ten percent of all studied massive hot (spectral type OB) stars, and the field occurrence does not depend on the spectral type 2 B. Buysschaert, C. Neiner & C. Aerts (Grunhut & Neiner 2015). Because these magnetic fields seem to be stable over long timescales and their strength does not seem to correlate with known stellar properties, it is assumed that they are of fossil origin and are frozen into the radiative envelope of the stars.…”

Stellar Physics with High-Resolution UV Spectropolarimetry

“…As a consequence, no global consensus has been achieved yet, on the details of the evolution and properties of massive stars. About 10% of O, B, and A stars host a magnetic field of fossil origin, usually dipolar but inclined with respect to the stellar rotation axis, with a polar field strength ranging from a few hundreds to a few ten thousands Gauss ( [4]; [5]). The ∼90% of stars that do not host such a field may nevertheless host an ultra-weak field of the order of 1 Gauss, such as those recently discovered in some A and Am stars [6].…”