He I line emission and the helium abundance in cataclysmic variables

Abstract

Time-resolved spectroscopy of a sample of eclipsing cataclysmic variables indicates that the He I emission lines are found with the Balmer lines in the outer regions of the accretion disk. Additional observations of lower inclination noneclipsing systems demonstrate that the He I emission is frequently characterized by relatively high singlet to triplet intensity ratios and an inverted line decrement, an unusual situation which can be explained in terms of emission from an optically thick (in the lines) region in local thermodynamic equilibrium. Calculations of steady state accretion disk structure show that the observed characteristics of the He I and H I lines can be reproduced in normal disk models, but only in the helium abundance is assumed to be very high, i.e., He/H> or approx. =100 by number. Enhanced helium abundances are thus implied for some cataclysmic variable secondaries, suggesting that they are either highly evolved cores or that they have an outer layer of processed material which was accreted from the white dwarf during earlier evolution of the system.