Hubble Space Telescope observations of the old Nova DI Lacertae

We have carried out a synthetic spectral analysis of a Hubble Space Telescope Space Telescope Imaging Spectrograph observation of the old nova DI Lacertae (Nova Lacerta 1910). The spectrum, obtained with the E140M disperser, reveals a rising continuum shortward of 1560 Å, a C IV P Cygni profile indicative of wind outflow associated with disk accretion, a deep Lyα profile, and strong N V (1238 and 1242 Å) and O V (1371 Å) wind/coronal absorption lines. Numerous sharp interstellar resonance lines are also present. A grid of accretion disk models, spanning a wide range of inclinations, accretion rates, and white dwarf masses, was compared with three sets of dereddened data. From the three best fits, we conclude that the most likely parameters characterizing the far-UV spectrum of DI Lac are an inclination below 18°, a white dwarf mass between 0.6 and 0.8 M _⊙, and an accretion rate between 10^-9.0 and 10 ^-9.5 M_⊙ yr^-1. The scale factors for the three best-fit disk models indicate distances between 2 and 2.5 kpc. The extreme weakness or absence of silicon features in the observed spectrum corresponds to a silicon underabundance of ≲ 10^-4 times solar. The best-fitting high-gravity photosphere model has log g = 8, T_eff = 27,000 K, V sin i = 200 km s^-1, and a scale factor distance (for a 0.7 M_⊙ white dwarf) of only 76 pc. Thus, a white dwarf photosphere cannot be contributing appreciably to the far-UV spectrum.