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Enormous progress has been made on observing stellar magnetism in stars from the main sequence through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence,in the generation and stability of neutron star fields. Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Thus we maybe at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window. We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes whichcontrol the diffusive magnetic flux transport in stars.
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Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. To investigate whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Spectropolarimetric observations were obtained during four different nights using the low-resolution spectropolarimetric mode of FORS2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. The presence of a magnetic field is confirmed in nine stars previously observed with FORS1/2: HD36879, HD47839, CPD-282561, CPD-472963, HD93843, HD148937, HD149757, HD328856, and HD164794. New magnetic field detections at a significance level of at least 3sigma were achieved in five stars: HD92206c, HD93521, HD93632, CPD-468221, and HD157857. Among the stars with a detected magnetic field, five stars belong to open clusters with high membership probability. According to previous kinematic studies, five magnetic O-type stars in our sample are candidate runaway stars.
We review the measurements of magnetic fields of OB stars and compile a catalog of magnetic OB stars. Based on available data we confirm that magnetic field values are distributed according to a log--normal law with a mean log(B)=2.53 and a standard deviation $sigma=0.54$. We also investigate the formation of the magnetic field of OBA stars before the Main Sequence (MS).
In this review, I will summarise what we know about magnetic fields in stars and what the origin of these magnetic fields may be. I will address the issue of whether the magnetic flux is conserved from pre-main sequence to the compact star phase (fossil field origin) or whether fields may be dynamo generated during some stages of stellar evolution or perhaps during stellar merging events.
About 10% of hot stars host a fossil magnetic field on the pre-main sequence and main sequence. However, the first magnetic evolved hot stars have been discovered only recently. An observing program has been set up to find more such objects. This will allow us to test how fossil fields evolve, and the impact of magnetism on stellar evolution. Already 7 evolved magnetic hot stars are now known and the rate of magnetic discoveries in the survey suggests that they host dynamo fields in addition to fossil fields. Finally, the weakness of the measured fields is compatible at first order with simple magnetic flux conservation, although the current statistics cannot exclude intrinsic decay or enhancement during stellar evolution.
We review the measurements of magnetic fields of OBA stars. Based on these data we confirm that magnetic fields are distributed according to a lognormal law with a mean log(B)=-0.5 (B in kG) with a standard deviation sigma=0.5. The shape of the magnetic field distribution is similar to that for neutron stars. This finding is in favor of the hypothesis that the magnetic field of a neutron star is determined mainly by the magnetic field of its predecessor, the massive OB star. Further, we model the evolution of an ensemble of magnetic massive stars in the Galaxy. We use our own population synthesis code to obtain the distribution of stellar radii, ages, masses, temperatures, effective magnetic fields and magnetic fluxes from the pre-main sequence (PMS) via zero age main sequence (ZAMS) up to the terminal age main sequence (TAMS) stages. A comparison of the obtained in our model magnetic field distribution (MFD) with that obtained from the recent measurements of the stellar magnetic field allows us to conclude that the evolution of magnetic fields of massive stars is slow if not absent. The shape of the real MFD shows no indications of the magnetic desert proposed previously. Based on this finding we argue that the observed fraction of magnetic stars is determined by physical conditions at the PMS stage of stellar evolution.
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