In massive stars, magnetic fields are thought to confine the outflowing radiatively-driven wind, resulting in X-ray emission that is harder, more variable and more efficient than that produced by instability-generated shocks in non-magnetic winds. Although magnetic confinement of stellar winds has been shown to strongly modify the mass-loss and X-ray characteristics of massive OB stars, we lack a detailed understanding of the complex processes responsible. The aim of this study is to examine the relationship between magnetism, stellar winds and X-ray emission of OB stars. In conjunction with Chandra surveys of the Orion Nebula Cluster and of NGC 2244, we carried out spectropolarimatric observations to determine the magnetic properties of massive OB stars of these two clusters.
In this talk, we will discuss the method to detect magnetic fields in hot stars, taking as example the detection of unambiguous Stokes V Zeeman signatures in spectra of three objects in Orion and one object in NGC 2244.
We will also present the state-of-the-art Bayesian analysis used to estimate the field strengths of the detected stars, and the dipole field upper limits for the remaining stars, resulting in a precise magnetic characterisation of two complete, co-eval and co-environmental samples of massive stars. Finally, we will show that these remarkable results bring forth new challenges for understanding the processes leading to X-ray emission in massive stars.