HST synthetic spectral analysis of U Gem in early and late quiescence: A heated white dwarf and accretion belt?

We have re-examined two archival HST FOS G130H spectra of the prototype dwarf nova U Geminorum obtained during its quiescence 13 days and 70 days after a wide outburst. Using synthetic spectral fitting with two flux-emitting components, a slowly rotating white dwarf photosphere, and a rapidly spinning accretion belt (V_rot sin i=3200 km s^-1) significantly improves the spectral fit but does not provide a unique solution. We found clear evidence for the cooling of the white dwarf, confirming earlier results, and evidence for the cooling of the accretion belt or gas in Keplerian motion as well. If an accretion belt is really present, then for the white dwarf and belt, respectively, 13 days post-outburst, we find T_wd=37 000 ±400 K and T_belt=45 000±2500 K, while at 70 days post outburst, we find T_wd=33 500±700 K and T_belt=37 500±4000 K. These results are compared with our HST GHRS G160M observations obtained 13 days and 61 days after a narrow outburst of U Gem.