GHRS Observations of the White Dwarf in U Geminorum

Abstract

We have observed the dwarf nova U Geminorum while it was in quiescence in the wavelength range 1162-1448 Å using the GHRS on Hubble Space Telescope. The time-resolved (lambda/deltalambda=2000) spectra are dominated by the white dwarf in the system. By analyzing the Doppler shifts of the photospheric metal absorption lines that punctuate the spectrum, we have accurately determined the orbital velocity amplitude of the white dwarf to be 107.1+/-2.1 km s^-1. For an inclination of 67 deg and a K_2velocity of 294.7 km s^-1, this implies a mass of 1.12 and 0.41 M_solar for the white dwarf and secondary, respectively. A comparison between the integrated spectrum and synthesized spectral models indicates a temperature of ~30,000 K, consistent with earlier determinations, and an abundance pattern that is distinctly nonsolar and indicative of CNO processing. Specifically, the abundance of C appears to be of order 0.1 solar, while N is ~4 times solar.