  Ferrario & Wickramasinghe (2006) explored the hypothesis that the magnetic
fields of neutron stars are of fossil origin. In this context, they predicted
the field distribution of the progenitor OB stars, finding that 5 per cent of
main sequence massive stars should have fields in excess of 1kG. We have
carried out sensitive ESPaDOnS spectropolarimetric observations to search for
direct evidence of such fields in all massive B- and O-type stars in the Orion
Nebula Cluster star-forming region. We have detected unambiguous Stokes V
Zeeman signatures in spectra of three out of the eight stars observed (38%).
Using a new state-of-the-art Bayesian analysis, we infer the presence of strong
(kG), organised magnetic fields in their photospheres. For the remaining five
stars, we constrain any dipolar fields in the photosphere to be weaker than
about 200G. Statistically, the chance of finding three ~kG fields in a sample
of eight OB stars is quite low (less than 1%) if the predictions of Ferrario &
Wickramasinghe are correct. This implies that either the magnetic fields of
neutron stars are not of fossil origin, that the flux-evolution model of
Ferrario & Wickramasinghe is incomplete, or that the ONC has unusual magnetic
properties. We are undertaking a study of other young star clusters, in order
to better explore these possibilities.
