EVIDENCE FOR CNO PROCESSED MATERIAL IN THE ACCRETION DISK OF GP COMAE

Unique amongst the known cataclysmic variable stars, the optical spectrum of GP Com shows only helium emission lines. We have discovered emission lines of N I, N II, O I, Ne I, and possible Mg II in the near-infrared spectrum of GP Com. Modeling the spectrum as emission from a uniform, optically thin slab of gas in LTE, we find H/He < 10(-5), N/O almost-equal-to 50, and N/C > 100 by number. In GP Com, we are seeing material extracted by Roche lobe overflow from the hydrogen-exhausted core of the secondary star, after the CNO cycle has converted most of the carbon and oxygen into nitrogen. The total abundance of carbon, nitrogen, and oxygen is within 50% of the value expected if the original material had a solar composition, however, several other elements are very underabundant. In particular, lines of Ca II, Si II, and Fe II are not detected at a level which suggests abundances for the respective elements of less than 1/1000 solar. We suggest that the nitrogen, oxygen, neon, and magnesium were produced during a thermal pulse stage of the primary star and then transferred to the secondary by Roche lobe overflow. The low heavy metal abundances are then representative of the original composition of the original composition of GP Com, suggesting that it is a halo star. For this mechanism to work, the transferred material must have fully mixed with the secondary star in order now to be seen in the core. GP Com's spectrum was best-fitted with a temperature of almost-equal-to 11,000 K, roughly the temperature expected for a balance between viscous dissipation and radiative losses in helium-rich material. We estimate the mass transfer rate to be M almost-equal-to 10(14) g s-1, a value low enough for the entire disk to be on the lower branch of the thermal equilibrium curve, and therefore to be in a steady state.