Abstract:
White dwarf (WD) atmospheres may, at first glance, appear deceptively simple. Indeed, the majority of WDs exhibit only hydrogen (spectral type DA) or helium (spectral types DO and DB) in their optical spectra. However, both theoretical studies and multi-wavelength observations in the last decades has proven that WD atmospheres can undergo a significant change in composition on the cooling sequence, which is referred to as spectral evolution. This transformation may be influenced by an individual element transport mechanism or an interplay of several processes, such as gravitational settling, radiative levitation, mass-loss, convection, and accretion.
While single element spectral appearance is the most common configuration, a small group of WDs are distinguished with both hydrogen and helium lines in their optical spectra. These so-called hybrid WDs (spectral type DAO) comprise the majority of hydrogen dominated stars at the hot end of the WD cooling sequence (Teff ≈ 100 kK). In theoretical framework, mass-loss was established as the responsible mechanism for presence of helium in the DAO atmospheres and recent studies showed that the majority (≈75%) of WDs run through hybrid phase. However, a clear evolutionary link between DAO and DA WDs is yet to be observationally demonstrated. The main obstacle in this endeavour is the stellar atmosphere modelling and the spectral analysis, namely employing opticalspectra. This is mainly because optical analysis of hot hydrogen-rich WDs are subject to systematic effects that hinder accurate measurements of the atmospheric parameters. Fortunately, atmospheric parameters can be determined via ultraviolet (UV) spectroscopy in high precision by exploiting ionization equilibria of multiple metallic species.
Therefore, the primary goal of this dissertation was to inspect the spectral evolution of hot (Teff ≥ 60 kK) hydrogen-rich (13 DAO and 19 DA) WDs via spectral analysis in both UV and optical. By using state-of-the-art nonlocal thermodynamic equilibrium (NLTE) models, Teff , log g, and element abundances were determined from both wavelength ranges and the sample objects were accurately located on the WD cooling sequence as well as the evolutionary change in their abundance patterns. This allowed us to clearly reveal the DAO to DA evolutionary channel, to mark the transition region at Teff ≈ 75−85 kK, and to identify underlying element transport mechanisms. The abundance pattern of the DAO WDs reveals that at temperatures above ≥ 105 kK mass-loss is the dominant mechanism. Below said temperatures, combined impacts of atomic diffusion and radiative levitation can be deduced from the abundance decrease in light metals and increase in iron group elements. Below the transition region, radiative forces gradually lose effectiveness against gravitational settling. This is further discussed in the context of diffusion theory.
Moreover, radii, luminosities, and masses were derived by fitting the spectral energy distributions (SEDs) of the sample objects. The correlation observed between the helium abundances and luminosities of hybrid WDs supports the influence of the radiation driven winds. The discrepancy in the mass-derivation methods of hot WDs were also discussed, which is a longstanding problem encountered between the ones determined from the SED fitting and interpolated from stellar evolutionary tracks. Employing fully metal line blanketed atmosphere models in both spectral analysis and SED fitting was previously suggested as a possible solution. However, inclusion of metal opacities in the models does not resolve the mass discrepancy detected in this work.
Finally, six sample objects show an infrared (IR) excess, whereas four objects are identified as photometrically variable. The source of the IR excess and/or photometric variability for the majority of these objects has been already reported in the literature. However, new information has been uncovered for a few of them in this dissertation.
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