Spectral evolution of hot hybrid white dwarfs I. Spectral analysis

<p>Hydrogen-rich white dwarfs (WDs) comprise the majority of the WD population, but are only rarely found at the very hot end of the WD cooling sequence. A small subgroup that exhibits both hydrogen and helium lines in their spectra, the so-called hybrid (or DAO) WDs, represents the majority of hydrogen-rich WDs at effective temperatures <em>T</em>eff<em> &asymp; </em>100 kK.&nbsp;We aim to understand the spectral evolution of hot hybrid WDs. Although small in number, they represent an evolutionary phase for most (&asymp; 75 %) WDs. We conducted a nonlocal thermodynamic equilibrium (NLTE) analysis with fully metal line blanketed model atmospheres for the ultraviolet (UV) and optical spectra of a sample of 19 DA and 13 DAO WDs with <em>Teff&nbsp;</em>&gt; 60 kK. The UV spectra allow us to precisely measure the temperature through model fits to metal lines in different ionization stages, which enables us to place the WDs accurately on the cooling sequence. Here we present model fits to the UV and optical spectra in our sample. Aditionally, the <em>T</em>eff, log <em>g</em>, and abundance values of our sample objectss are compared to previous studies.&nbsp;</p>