Spectral analysis of two He-enriched sdO-stars: HZ44 & HD127493

Hot subluminous stars come in two flavours: sdO and sdB stars. They display peculiar helium abundances, spanning a range between 10^-4 and 10³ times the solar value. The majority of sdO stars have atmospheres dominated by helium with hydrogen being a trace element only and, therefore, are termed He-sdO. Helium-poor sdOs exist too, though at lower numbers, but mixed composition objects are rather rare.

Both HD127493 and HZ44 are known to be of mixed H/He composition and excellent spectra at optical and ultraviolet wavelengths are available.

Given that the few mixed H/He objects that have been studied in the past tend to show peculiar atmospheric abundances, such as extreme enrichment in iron-group and trans-iron elements (Naslim et al. 2013), we decided to investigate the composition of HD127493 and HZ44 in more detail.

The origin of this extreme enrichment is not yet fully understood, but radiatively driven diffusion and therefore a highly stratified atmosphere has been proposed as an explanation. In order to understand which conditions favor this process, a bigger sample of these peculiar stars would be essential.

The available UV spectra of these stars are very useful as they give access to many chemical species that do not show spectral lines in the optical. For example lines of Ge, Sn, and Pb were identified in the FUSE spectrum of HZ 44 by O'Toole (2004).