The survival of fossil magnetic fields during pre-main sequence evolution

Abstract

The origin of the strong large-scale magnetic fields found at the surfaces of the near-main sequence chemically peculiar (CP) stars is still a matter of controversy. The fossil theory, in which the fields are explained as remnants of fields present during earlier stages of stellar evolution, arguably is better able to explain the observed CP star magnetism. However the question of whether significant large-scale magnetic flux can survive through the pre-main sequence evolution has been much disputed, but little explored. Here we attempt to make some preliminary, semi-quantitative estimates related to flux survival in the presence of large-scale convection. We also present a simple model that attempts to quantify the fraction of flux that can survive from the top of the Hayashi track to the main sequence. A broad conclusion is that for plausible values of parameters such as turbulent diffusivity, flux can more readily survive in stars of several, rather than about one, solar masses, although the contrast is not as strong as appears to be implied by observations. Attention is drawn to the effects of uncertainties in modelling pre-main sequence stellar evolution.