SPIN EVOLUTION AND MAGNETIC-FIELDS IN CATACLYSMIC VARIABLES

A number of intermediate polars (IPs) are observed to have spin periods very close to 10% of the orbital period. We show that if they spin in equilibrium, the white dwarfs in these systems accrete essentially the specific angular momentum of the secondary star, averaged over time scales approximately 10(4) yr. Thus either no accretion disk forms or this is only marginally possible, and there is very little reduction of specific angular momentum during the accretion process. The relation between the spin and orbital periods is difficult to reconcile with accretion predominantly through disks around white dwarfs spinning at the equilibrium rate, but does not preclude their presence in observed systems; disk accretion does not significantly affect the P(spin)-P(orb) relation unless it continues for approximately 10(4) yr or more, and may well correspond to epochs when the system is brighter. The relation arises naturally if the magnetospheric radius R(mag) is of the order of or greater than the circularization radius R(circ), which will hold for those IPs with magnetic moments-mu greater-than-or-similar-to 10(33) G cm3 and orbital periods approximately 5 hr or less, except possibly for brief epochs of unusually high accretion rate. We show that some kind of disk (e.g., a nonaccretion disk) may be present in IPs if most of the matter is not accreted through it. We stress that observational evidence favoring disks, especially from optical emission-line profiles, usually sample very little of the accretion flow and cannot safely be used to estimate even the present rate at which angular momentum is accreted, and certainly not the long-term average rate. We suggest further that nonsynchronous sytems with mu greater-than-or-similar-to 10(34) G cm3 are unlikely to be able to accrete gas of low enough density to produce hard X-rays. The progenitors of the AM Herculis systems are thus effectively unobservable, and in particular they cannot be identified with the observed IPs. We conclude that observed IPs have mu-significantly lower than the AM Herculis systems and reconsider the secular evolution of magnetic cataclysmic variables.