ANOMALOUS COOLING OF THE MASSIVE WHITE-DWARF IN U-GEMINORUM FOLLOWINGA NARROW DWARF NOVA OUTBURST

We obtained Hubble Space Telescope Goddard High-Resolution Spectrograp h medium-resolution (G160M grating), phase-resolved spectroscopic obse rvations of the prototype dwarf nova U Geminorum during dwarf nova qui escence, 13 days and 61 days following the end of a narrow outburst. T he spectral wavelength ranges were centered on three different line re gions: N v (1238 Angstrom 1242 Angstrom), Si III (1300 Angstrom), and He II (1640 Angstrom). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si I II and He II absorption-line velocities versus orbital phase trace the orbital motion of the white dwarf, but the N v absorption velocities appear to deviate from the white dwarf motion. We confirm our previous ly reported low white dwarf rotational velocity, V sin i = 100 km s(-1 ). We obtain a white dwarf orbital velocity semiamplitude K-1 = 107 km s(-1). Using the gamma-velocity of Wade, we obtain an Einstein redshi ft of 80.4 km s(-1) and hence a carbon core white dwarf mass of simila r to 1.1 M-.. We report the first subsolar chemical abundances of C an d Si for U Gem with C/H = 0.05 times solar, almost certainly a result of C depletion due to thermonuclear processing. This C depletion is di scussed within the framework of a weak thermonuclear runaway, contamin ation of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably, the T-eff of the white dwarf 13 days aft er outburst is only 32,000 K, anomalously cooler than previous early p ostoutburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equili brium cooled degenerate could explain the lower T-eff.