Magnetic dynamos in white dwarfs – I. Explaining the dearth of bright intermediate polars in globular clusters

Abstract

ABSTRACT Recently, Bahramian et al. investigated a large sample of globular clusters (GCs) and found that bright intermediate polars (IPs) are a factor of 10 less frequent in GCs than in the Galactic field. We theoretically investigate here this discrepancy based on GC numerical simulations. We found that, due to disruptive dynamical interaction, there is on average a reduction of only half of bright IP progenitors, which is clearly not enough to explain the observed deficiency. However, if the rotation- and crystallization-driven dynamo scenario recently proposed by Schreiber et al. is incorporated in the simulations, the observed rareness of bright IPs in GCs can be reproduced. This is because bright cataclysmic variables (CVs) in GCs are typically very old systems (≳10 Gyr), with white dwarfs that almost fully crystallized before mass transfer started, which does not allow strong magnetic fields to be generated. The observed mass density of bright IPs in GCs can be recovered if around one-third of the bright CVs dynamically formed through mergers have magnetic field strengths similar to those of IPs. We conclude that the observed paucity of bright IPs in GCs is a natural consequence of the newly proposed rotation- and crystallization-driven dynamo scenario.